Been in a few FM transmitters in my day, had a rookie tech get mad at one and throw a wrench into the power supply not expecting it to come back and hit him in the face, took his teeth out. Yes anything can be made to work again and my philosophy is if it worked from the factory no reason it won't work again today.

So that being said the replacement tuner has resulted in little progress, it does pass a signal from the little UHF input RCA on the side thru the tubes and down to the IF input on the chassis however that's about it. All the solder connections in the tuner itself are good, the 6HQ5 RF amp and 6HB7 mix/osc check good on the tube tester but I bought some NOS tubes on the 'bay and will give them a try upon arrival. My supply of these is limited to a few in a storage tote and while they do check good on a Heathkit IT-17 something may very well be "off" enough to cause a problem. I've had the IT-17 since middle school (1982) and trust it however being little more than a "diode test" of the tube there could be something like a burnt grid wire or a spot on the cathode that wouldn't show on the meter yet could kill the local oscillator. I got the IT-17 new in the box unassembled at the Medina MN hamfest for $10.00

Until then the remaining repairs will be done using an outside IF source provided by one of those tuner units that was part of a Panasonic portable VCR combo, still need to iron out the weird phasing and chroma shading issues that I suspect might be coming from the TAC board. If you remember these had the "Total Automatic Color" feature that basically added more red & yellow giving everything an orange cast, great if you like burnt orange shag carpeting but it's not my favorite and I may just bypass the board all together.

Will post more when I can.

The RF amp seems very simple. 8200 ohm resistor from 270 B+ into tuner, then 1200 ohm resistor to the wiper contacts for the tuner drum, then the 6HQ5 plate. If B+ is not being dropped I would suspect those wiper contacts not making connection. Without the 6HQ5 plugged in, is there B+ on pin 5, measured form the top of the tuner? With set turned off, what resistance is there between pin 5 and B+ 1 source ?

I get the 1275 ohms between B+1 and pin-5 of the 6HQ5 and without the tube in place I get the full +285 volts of the power supply so there is a path to the plate. The AGC here is about 0.45V and that's consistent with the schematic, both pin-2 and pin-7 show continuity and I get about 55% on the tube tester, not stellar but it should pull some current.

According to the schematic when tuned to UHF the input from the UHF tuner is coupled directly to the grid of the 6HQ5 and from there gets passed to grid-1 of the pentode section of the 6HB7 osc/mix and that plate is coupled to the tuner IF output coil and out thru a .001uf cap. Feeding in a weak IF from an external source gets amplified and sent to the IF strip so I know things are doing what they should which doesn't make much sense considering how little current the RF amp tube is taking.

I have two NOS 6HQ5 and 6HB7 tubes on the way and once verified in the tester they're destined for the sockets, until then I have a color issue to iron out in the 3.58 chroma bandpass amp and this isn't the first time for the problem.

If tube subs don't do much and voltage checks don't give an obvious issue (if you have a SS socket saver with test points that usually is the best Best way to verify the socket) then my next step would be to look for carbon comp resistors that have drifted high enough to be out of tolerance (low out of tolerance is usually something else in the circuit in parallel with it confusing the meter).

Yeah the one plate resistor has only drifted a few ohms off the mark so I'm not to worried about that, the 8200 ohm metal film on the supply board is a few ohms high as well and the only unknowns at the moment are the tubes themselves and tracking shows their arrival on Thursday... we'll see about that.
Over the years most of my servicing was done on the RCA and Zenith sets so I have very little for Magnavox specific parts like the 6HQ5 despite being a family TV for 56 years, too bad Magnavox didn't use the Nuvisters as I have plenty of them.

Friday the 5th and the tuner is once again alive with a new RF amp and osc/mixer. My theory is they were somehow damaged internally when the IF connector brushed against the 120VAC terminal of the fuse holder and the replacement confirms this. Everything is now on the mark up to the video amp and now I have to sort out the chroma problems.

To back up some 40 years the set took on some lightning damage when a tree was hit and this killed both the phase detect and killer diodes, I replaced them with modern silicon small signal diodes but it was never quite the same and this was the real reason the set was replaced with a Zenith S-III and I ended up with the albatross in my bedroom. Now I want to get the set running like it used to and this has been quite the uphill battle. I found a set of what I thought were good NOS 1N270's and 1N34A's but they're all leaky passing tens of mv in reverse with no two quite the same. Plan for the time being is to "soft wire" in a 6JU8 and see how that goes, not the most elegant but they worked before in this service.

I will post my findings here so as it's been said in the past, Stay Tuned.

Try 1N914 or 1N4148 diodes. They're good to about 100 MHz. and are cheap. Radio Shack used to sell them for $1.99 for a hundred diodes.

Had the 914's in there and the drop is too high for the magnavox circuit so I wired up a 6JU8 in their places and ran thru the entire IF and chroma alignment and everything came together. Yes we have success. A few loose ends to tie up, one seems to be an issue with horiz blanking that's causing the right side of the screen to essentially go into cutoff. Bias and guns are set correctly and the cutoff remains when I run the horiz gun adjustments. Other issue is a touchy AGC and this seems to be a bad AGC pot on the rear apron, have not put an analog meter and run the control thru its range as of this evening.

Ended up replacing all of the tubes on the IF strip with new-old-stock. I sourced the tubes from AxMan Surplus in St. Paul MN which is this bazaar of electronics and other things that's been in business for almost 60 years and they have everything... and I mean everything including thousands of new-old-stock tubes in their original cartons so for $2.50/ea so I invested a good hour scanning thru them and got the everything I needed.

IF strip took a good three hours to get the response back into shape, I'm working with a Heathkit IG-57 sweep/marker generator that has seen better days. The IF sweep works correctly but for some reason the markers themselves are not so I had to basically eyeball the response curve and work from there, I used my Motorola R2670 service monitor to sweep the response of the chroma section as this has calibrated sweep/track and spectrum analyzer functions.

Once I clean things up a little I will snap a few realtime pics of the set in operation, source will be from a Sony VO-2860A editor and whatever tape I happen to grab.

Great. I'm glad things are coming together for you. I would think a silicon diode would have better performance specs than a germanium diode or... a tube.
AxMan sounds a little like WierdStuff out in San Jose, Ca.
I've got a Singer FM10CS service monitor but I use it mostly for radio work. Goes to 512MHz.
Looking forward to the pics of this Franken-set!

I'm sure the 1N914 is better overall but the problem seems to be riding the crest of the waveform with the small signals and its higher forward voltage. Once I went to the 6JU8 the chroma response was far better. This set never had what I'd call "stellar" color, it was good but maybe I'm spoiled by the sheer number of broadcast monitors. Dunno, it uses the 6HS8 as an X-Y demod and from what I remember this design was an "acceptable" lower cost design albeit not as accurate as the R-Y B-Y or I/Q with what appears to be a slower phase response. Good enough for the 1970's I guess when everything was burnt orange with splashes of primary colors here and there.

So everything was working well last night when I shut things down around 0400 central, just before this I dribbled a little hot wax down the IF coils to lock the slugs in place then checked the response curve and picture once again and everything was on the mark.
This afternoon I resumed my work as the outside temps went to the low 90s with mid-60 dewpoints, fired up the set and noticed a really bad picture, checked the IF response and found it had a very sharp peak going just above the picture carrier that wasn't there last night after I dropped the wax.
Ran thru the IF again and locked the slugs in place and everything is holding at the moment, I'm wondering if this might be a moisture issue as the humidity went from roughly 55% to almost 80% over the past day. Not much for air conditioning unless it's oppressive like this afternoon.

Did verify the AGC pot with an analog Triplett 630-NA meter and found it has several "scratchy" points along the travel so it's another trip on the motorcycle to AxMan Surplus in St. Paul as they have bins of these. Minneapolis/St. Paul area used to be a big tech hub during the early computer days with the likes of Sperry-UNIVAC, Honeywell, Control Data, ADC Telecom and IBM in Rochester and had two dozen surplus stores full of this stuff. By the mid-80's only a few remained and today AxMan is the last one standing after almost 60 years in business, they have enough inventory in that building to go another 60 years and it's always busy.

I got the Motorola R2670 monitor during my days at the regional service depot, employees could buy their test equipment at cost and pay for it out of salary. I optioned this one with the everything I could and it's served me well. Other bench test equipment includes a Tek 475 scope, Motorola R1011B 40 amp power supply, along with the various Bird RF meters. TV service is done with a late 60's Tek 453 scope originally from IBM, it has nuvisters on the front end and can survive a kilovolt hit to the input jacks. I also have the Heathkit IG-57A IF sweep/marker and B&K 1246 dot/bar generator plus 466 and 467 pix tube analyzers. Yes I'm on the trailing edge of 1970s technology.

Okay so an update on the Bench Queen; it's running. I still have to iron out a few ripples in the IF section but it's a happy set after 20 years of idle time. Going to run thru the Heathkit manual for the IG-57A sweep/marker generator again and see if I can't find why the markers themselves are dead.

The big issue came down to a bum resistor on the horiz keying pulse, schematic called for a 270K and by color codes that's what I replaced the original with, turned out to be 27K on the meter. Red-Violet-Orange and 5% gold stripe so I went thru and verified ALL the replacements. Guess that's why Ohmite calls them "Little Devils". So once the correct value was in place everything started to act normally, AGC is good but still getting the horizontal bends and trails on high contrast scene details, vertical size is a little challenged despite a new tube and known correct resistor values. I suspect the Boost B+ is low.
Convergence was a challenge that had me stumped for several hours, replaced all the selenium diodes on the board but wasn't getting the correct horizontal actions until I bumped the input connector and discovered a number of traces had broken right where the Molex pins were soldered to the board and lifted the pads. Servicing the set I'd run it with the chassis out of the cabinet sitting on some tape cases then when it was time for the bench would just disconnect everything at the Molex connectors, the sheer number of cycles broke the traces. Once that was rectified the convergence fell into place for the most part, still doesn't respond to the R/G diff tilt so that's on the to-do list.

So overall it's getting there, not quite up to where it was back in the day but now an incentive to keep moving forward. One thing it still has is RF sensitivity. Growing up I remember the DX football games with this massive antenna and Alliance U-100 rotator, there were tape markings for the station positions all around the face and every fall we'd have to check all the flat cables. In the house I can run the modulator on a 1/4 wave antenna with good results.

Got loads of detail pictures of the chassis and operation, give me some time to smash them down to fit the server limitations and I will post them

I thought there might some wierdnesses going on, You should not have to do a complete IF realignment. I suspected broken grounds. Many times you can just resolder the board and that fixes it. Another thing I thought of was perhaps whiskers growing in your IF cans. I know there is at least one IF can from the schematic you sent me. And then there is the problem of carbon resistors going off value due to its age. I'd replace them with metal film.

The weirdness was the wrong value resister in disguise causing the Chroma, blanking and AGC issues and this is where I mistook the problem as being a misaligned IF strip that came from a T933. The AGC overloading was causing the strange response so I attempted to counter these and made things worse. I then went to replace all the resistors on the IF board along with by pass caps then during the alignment phase grazed the IF cable from the tuner against the little fuse block next to the input jack on the chassis and smoked the tuner. Two months later I have the front end out of a T933 and while double checking things came across the rogue resistor, once that was replaced along with the damaged AGC pot everything started to work correctly leaving me with the task of cleaning up the mess I made.

This all started years ago when I attempted to resurrect the old family set using NOS parts I had cleaned out of a storage room working at a Motorola service center. We had hundreds of those resistor kits in the little file drawers along with more in little jars and on tape & reel, I was the service manager at the time and tasked with the decision making... the decision was made to bypass the dumpster and load most everything in my truck. We had just a few weeks to clear out some 40+ years of history as the depot was closing and lease was done, once the test equipment and other capital assets were removed I got the word out to the ham radio community and they picked the place clean over a weekend.

So during the attempt at replacing all of the resistors in the set I managed to break the original IF strip thru sheer clumsiness and made the decision to go with composite video, with the push to digital and impending demise of analog broadcast this seemed like a good alternative and I worked out a few circuits but a few bad real estate moves followed by a divorce that traded me in for a younger model forced the set into a large refrigerator box and into the barn for long term storage. That was 20 years ago.

Now the goal is to un-modify the set and bring everything back to the way it was in 1970... well the TV chassis anyway. I replaced the original amp many years ago with a Heathkit AA-100 I picked up at a hamfest around 1985 and modded the receiver section for stereo TV audio adding one of those stand alone Recoton "FRED" MTS decoders picking the 4.5 SIF up using a loop.

As it stands at the moment I have a good response curve on the IF strip, not stellar and the task for the afternoon is to get this squared away and move onto the lack of vertical height. The 1K-3W resistor for the plate side of the 6GF7 is on the mark and DC values are within tolerance, the tube itself is new from the carton and I've sub'd in a few others with the same results. I have not checked the boost B+ as of yet but it might be a bad boost rectifier as it still has the original selenium on the board.
Once this gets straightened out it's onto a convergence board issue that has the R/G tilt not responding, I've only replaced the diodes on the board so far as I don't need perfect convergence at the moment. We'll get there.

What you could do is add a switch (or relay controlled by a switch) and you could use both video and RF inputs on that set.

I could and have given thought to that but honestly I'm quite pleased with the performance using the RF input, I did leave a provision to add a composite input by soldering a two post tag strip to the top of the IF section cover shield and ran a wire to this for testing right off of TP2 and this goes directly to the grid of the 1st video amp.

The video amp is responding well and I get roughly 300 or so lines of horizontal resolution that's actually limited by the dot trio & aperture of the CRT itself, chroma response is good without the shading or fading that I was getting earlier. Convergence is what it used to be "back in the day" which is to say acceptable. It always had a slight bit of blue error on the right side that dipped down slightly and the only time it was annoying was when I was doing the ATARI 800 BASIC thing back in the early 80's.

The one annoyance is with the slight horizontal sync bending and the dark shading following the highlights and sometimes the vertical wants to follow the black bar used to adapt the 16:9 aspect ratio to the 4:3 frame in the modern DTV receiver boxes, wondering if video is sneaking into the sync somewhere but the scope didn't show anything during the last probe. I've gone thru the AGC section several times and everything is as it should be in terms of values, the 6GH8 is new from the carton and my signal strength is a measures -86db/m at the 75/300 ohm matching transformer coming from the test port of a Blonder-Tongue CAMS-60 MTS modulator with the RF drive all the way down. Thought it was an overload so I tried a different consumer grade modulator and got the same results so I'm out of ideas as I'm totally missing something here.

Vertical is still a little challenged and if I pull the height any further it loses lock and goes into this cycle of collapse then startup again only to collapse a second or so later never locking onto the sync. Supply line shows a constant +394V after the 1K/3W resistor (R112) and C5 is known good. Boost B+ is running 1196V give or take a few and I'm not seeing any horiz ripple on the supply side so my conclusion is one of the "new" parts isn't doing so well here. Plenty to keep me busy for a while.


I do have to give a big thanks to all the members who have put up with my long-o tell-o posts and given me the pointers and directions, also want to thank damen and Danny S. for their help with the replacement pieces. Without their help I'd still be at square one.

Danny if you're reading this I have not forgotten about you, just a little short on the spare change at the moment. Thanks again for all you have done.

Going to watch the set for a few more days while taking notes before I pull the back off once again, I'd leave it open but I do have three curious kitties in the house and they don't take no for an answer.

I will post a short video of the set warm-up and picture itself showing the bending and shading, this will be done in HDCAM so everything can be genlock'd together to eliminate the flicker effects.

If you don't like Letterbox from your DTV converter get s Zenith DTT-900 (or 901, or the LG or insignia rebadged versions of it)... Those boxes have a zoom button on the remote (also accessible through the menus) that lets you pick whether to down convert 16x9 to 4x3 cropped (pan and scan but with pan fixed in the center), 4x3 with letterbox, or 16x9 horizontally squeezed to fit.

I have one of those Apex receivers that also has the zoom/crop/squeeze/fluff buttons on the remote, annoying in that the vertical wants to lock onto this black and not the actual blanking bar forcing me to use the ZOOM setting and cropping the sides or FILL which squeezes everything into the 4:3 box... fluff is reserved for my cats.

Just bought up a complete Magnavox T940 SAMS Set-1144 Folder-1 from stealbay for $14.00 that's going to get scanned in complete as a pdf if anyone wants a copy. My original copy is too far gone to work with anymore and for the money I couldn't go down to the main library and copy it for this price.

Give it a few days to arrive and I will get things together. It also has the (incorrect) schematic for the R228 receiver and the 8-function remote. I say incorrect as the Astro Sonic Imperial uses the R220 receiver so I will include both models plus the original Magnavox service pages making one large folder.

That's great. Show us some pictures of what you're doing. One thing to remember about using the RF input versus the a video input is that the video response is limited thru the IF strip. There's a sound trap at 4.5 MHz. that you would not have in a video input. Also there is some group delay thru the IF transformers. The video amplifier response is probably flat out to maybe 6 MHz. The sets I have converted look really sharp using the video input. Good luck!

I will get some pictures cropped & chopped to fit the vintage videokarma servers in the coming days as there's still some remaining work to be done on the chassis. Right now it looks like some video is making its way thru the sync sep and giving things the wiggles and I also have some horizontal shading of the highlights that need to be worked out, as of this post I have not verified which side of the video out amp this is on.

So for the moment things are going to stay on an RF distribution as this gives me the flexibility to run my other vintage sets without modifications. I have an early 80's Panasonic CT-1110D portable, a 70's vintage Philco B&W and Sony Watchman FD-40A plus one of those 16:9 CRT Panasonic CT34WX54 sets however that one normally runs on the Betacam inputs.

When I did have the Magnavox on composite the detail was a trife better overall so I will revisit the video input in the near future however it was more important to bring everything back to factory stock again both for technical and historical aspects as this set has been in the family since new. We were not the couch potatoes so much but shows like Dallas, this music thing in the late 70's and the seemingly endless football games were never missed and the TV got its use along with the stereo which was almost a constant growing up.

So another update to this one, June 17th.

Got the shading issue ironed out, turned out to be a bad new capacitor that was across the contrast control so that got replaced again and now all is well there. Other issue I'm still dealing with is horizontal sync bending that would normally be attributed to borderline AGC overload, AGC is set on the conservative side and there's a fixed bend to the right about 1/4 way down the raster from the top. Can't see it on the scope and it's there regardless of the source be it straight from a TBC or consumer grade DTV converter on CH-3 or 4.

Still fighting a vertical issue and this might be from well baked caps near the vert tube, resistors have all been replaced and a subsequent check shows them on the mark, Boost B+ is +1160 volts and supply is at +413V so those are good.

Did inject some raw composite video into the 1st video amp after lifting the 3rd IF amp tube out and got excellent results. When I replaced the IF strip I added a two lug terminal strip near the 1st video amp tube with the wire connecting directly to test point 2 so I could do the IF alignments with all the covers & shields in place, this is the junction of L14 and pin-2 of the 6LM8 video amp on the T933 schematic. I'm working with a T940 which is very similar however it uses a 6MU8 for the 1st video and calls for what looks like over 2V P-P signal on that grid. So the plan is to address the few remaining issues then add a tube based composite video amplifier to feed the test lug via DPDT relay which will also open the B+ line to the IF amp effectively isolating it, this way the DC integrity of the grid is maintained and the signal will be behind the 4.5 trap.

More to follow shortly.

What source are you using when you see bending. Three quarter and half inch machines that don't have a TBC output can cause horizontal bending. What are you using for a signal source?
I use Paint (in W7) to crop and squeeze my pictures down. Generally I have to go down to 640x480 to get them to post.
Do you think if I donate to the site, they'll let me have more space? Hmm.

The bending happens regardless of the source be it from digital or TBC analog, all of my analog tape has dedicated TBC's and everything is locked to a master BB source thru a Kramer VM-1045 distro amp.

Sources are a Sony VO2860A U-Matic that's on a NOVA-800 full frame TBC. The DigiBeta doesn't need any TBC and the Type-C Sony BVH2000/3 has the internal TBC option boards. I also source from a one of those DTV converter boxes and get the same results.

I'm seeing two types of bending, one related to the black subjects and the other bend is fixed at roughly 1/4 down from the top that doesn't move except when I push the horiz hold to the edge before lock is lost.

Going to sub in some capacitors on test probes and see how things are affected.

Having bending related to black subjects seems to indicate sync clipping at the wrong level, possibly video signal is too small peak-to-peak or at wrong DC level; or, the sync clipper itself is at fault.

That's what I'm thinking as well, going to run thru the IF alignment again and keep a close eye on the sync & burst using both the sweep/marker and actual video to see how things are interacting. I'm sure it's something I'm not doing right but can't put my finger on it just yet.

Going direct composite gives similar results however I can't get my DA to go much higher than 1.5V P-P from an unloaded output. It does however make for a very nice picture without all of the lumped bumps from the coils along the way.

You should see what my cat did to the set earlier today, it's posted here on Audiokarma... and I'm not too happy about it. https://audiokarma.org/forums/thread...#post-18377210

It's 0200 central and I'm out of ideas. A picture can be worth a thousand words so I did my best to grab a few and crunch them down, if you want larger originals they're available. I named the files to be self explanatory, if something doesn't turn out please let me know.

So what I have is endless frustration with the horizontal and to a lesser degree vertical sync. I've run thru the IF response several times tweaking here and checking for symmetry there, replaced the AGC/Sync sep 6GH8 with several NOS tubes fresh from the cartons and checked resistor values along with probing in capacitors on top of the originals with no changes to the bending.

The signal into the 1st video amp is strong, over 2.5V P-P from the detector and I'm seeing signal levels on the various test points that are very near the spec'd levels in the schematics, horizontal lock is very strong but the vertical is dependent on the size control settings, stretch the vertical to fill the screen and it wants to lock on the black band within the picture itself unless I run a 4:3 image in which case it attempts to lock onto a something near black. Tube is new and I've sub'd in several different NOS tubes with the same results.

Power supply voltages are within 5 to 10 volts of spec and I can't pick out any ripple on the scope. AGC is set conservatively however going for overload doesn't affect the bending as much as it does the overall black and contrast as expected.

Horiz out cathode is shy of 200ma at medium brightness and the 6LQ6 tests at 90% give or take.

In the group of pictures I've included the test pattern I'm using to demonstrate running in as both composite and CH-3 RF. The scope screen shot is direct from the test point TP2 using a 10:1 100 MHz probe and the test pattern itself comes from my Newtek Tricaster TC-550 which is essentially a studio control room in a box. All the output levels conform to NTSC standards and verified using a Leader waveform monitor and vectorscope.

So what this all comes down to really is what am I missing?

Just for those who want to see what the set looks like here are a few gems to safely drool over.

Sorry if I missed it, but if you haven't yet replaced C511 in the vertical circuit, do try a .03, or .033 there.

Complete success.

Found R728 had carbon tracked from what I suspect was an overvolt being it's a carbon film subjected to some 400-plus volts during the initial power on and that particular tube warming up rather quickly. My theory anyway. What I do know is there's a clear carbon burn going from end to end and being on the bottom I didn't see it right away, a carbon comp has since replaced it and the wiggles along with the nagging vertical issue are gone.

Picture quality? Absolutely stunning for 1970 and I've known this set for the past 56 years. Remaining issues? None. A couple of loose ends with the add-on MTS decoder I use in place of the sound discriminator and a mountain of cat hair that's trapped in the cooling fan is really all that remains. What a ride this has been!

So at this stage I'm going to run it for a while and construct the Phil Nelson CT-100 triode based video input adapter that uses the 12AT7 and go from there. It's not a need so much as I have the IF working quite well but now that all the variables have been worked out I can move to the next chapter.

Give me a day or two and I will post more pictures of actual operation and some of what I feed the set with.

Again want to thank everyone for all their help and support. A special thank you to damen and Danny S. for help with the IF board and tuner. Without you I would still be at square one.

Okay so as I promised here's a short clip of the Magnavox in action with playback from my U-Matic of a tape from 1985. Contrast is a little crummy from the ambient lighting but I wanted to do all of this as in the moment at the moment. Anyone know how to repair or restore the woodgrain on that left side slider?

see for yourself.

https://youtu.be/5nUg7hVFQmo

:thmbsp:

Looks pretty good! Is that a 25 or a 23 inch CRT?

It's a 23 inch (25BHP22) and the third tube so far. First was a frosty with bad cataracts and low on emissions, second one I swapped in the summer of 83 that came from a C-II and it went another 20+ years before the emissions died off, this was around 2006 and was replaced with an RCA Hi-Lite from an unknown CTC before the set was parked in a big refrigerator box and stored in my parents barn.

It's been quite the journey but now it's back to its former glory and working like it did, most of the resistors and a number of capacitors were replaced along with the tuner after the accidental shorting of the IF cable to the 120V primary fuse. The picture is still vertically challenged but I'm going to let that go for now while I focus on the composite video input interface. One idea right now is to use a DPDT relay in conjunction with the rear audio selector switch that will cut the B+ feeding the IF amp itself and at the same time switch in the composite to that test terminal strip I soldered to the cover tin. Years ago when I added the Recoton MTS decoder I also wired in a 3P3T rotary which selects between the internal MTS and external R/L "simulcast" TV audio input so that way I could have the stereo audio from my U-Matic deck or the satellite we still had going, the 3rd contact set or position was never used so this could handle the switching of a relay as I need to keep the lead length to the 1st video amp very short to avoid smear.


I also received the SAMS 1144 folder this morning and will scan in the Magnavox as a complete set including the TV chassis, stereo receiver & amp chassis and the remote if you want to add these to your collection. Everything will be in a high-res pdf format and will overlap the large pages so things don't get cut off. Give me a few days and I will email it, if anyone else wants a copy drop me a private message and I will get it out to ya.

Here are a few of the audio details. The 45_Coil is exactly how I did it back in 1986 and it feeds a "probe" input on the Recoton FRED MTS decoder, originally they included this adhesive loop to grab some stray 4.5 MHz but that didn't work so I made the pickup instead and have it set to grab some from the 4.5 MHz trap.
Input switch selects between the MTS decoder and external R/L feeding the TV position of the receiver selector.

The hairy fan needs no explanation, the set doubles as a vacuum cleaner of cat hair.

Hi to all,
Hi ARC Tech-109,

The Phil Nelson composite video adapter for the CT-100 is here :
Original RCA design + Pete Deksnis improved version.

https://antiqueradio.org/VideoAdapte...Television.htm

I purchased the Pete Deksnis modified version of the original RCA design, it works well and is easy to install (on the CT-100) on other TVs, i don't know.

Best Regards
jhalphen
Paris/France

A few years ago I built an improved version for a restoration customer. We wanted to still be able to use the tuner in a switchable capacity. Running the detector video through coax to a switch degraded it so I put a mini double throw relay on the socket extender so the tuner signal path lead dress would be virtually unaltered when not in composite video input mode, I added an audio injection circuit too and copied the preamp circuit from an RCA 45RPM record player for the audio line input... It worked great and I need to build a couple more for me.

That's what I had in mind, DPDT relay to feed the 1st video amp while cutting the DC to the IF amp, composite goes in after the 4.5 sound trap so I still maintain the DC integrity of the grid, did some tests just poking a 1V Hi-Z video signal in and got excellent results.

I've found it unnecessary to cut power to the IF, it doesn't really change the opperation of the Deknis circuit on the CT-100 or 21CT55... If you build it with the socket adapter and use a clip lead to harvest the B+ that goes to one side of the rear most contrast pot next to the HV cage you can make the video mod a completely bolt on/plug in mod...
...With audio that becomes more involved. On the customer set I added a stereo headphone jack to a bracket that screws to the tuner... the jack is configured to short the detector output to the audio input (return the circuit to stock) if the plug from my preamp module is pulled out, but pass both lines to that module in isolation if the plug is in.

If life wasn't so busy I'd build a few more and sell them. I think I did a really good job with it.

The reason for doing it this way is so I can mechanically isolate the video input line from the grid of the 1st video amp when using the tuner and then cut the IF amp off when using the video input so I don't get swamped with mixer noise. Not cutting the grid out of the existing circuit as I see no need to and this way the modifications are kept to a minimum.

I haven't seen mixer noise doing it the way I have. Neither circuit noticably interferes with the other when the video amp grid is switched.

Thread Revival Time... long post

Been running my Magnavox T940 on a steady diet of both composite video and real RF for the better part of the summer and aside from a few unrelated issues with the focus circuit I'd say it's a win.

Okay so how this all works is on the rear of the set is a 3-position 4-pole rotary selector switch that I installed the summer of 1985 on the existing tape I/O strip and the function of this was to select between internal detector audio that was actually provided by an internally mounted Recoton "FRED" V622 MTS decoder that picked the 4.5 SIF using a coil of wire wrapped around the detector tube, not exactly elegant but it worked for over 20 years. On the internal stereo receiver it was a matter of cutting the two resistors feeding the original detector audio to the main selector switch to get a left & right which was brought to the wipers of the new rear selector switch and the second set of selected contacts were wired to a new set of RCA jacks on the I/O strip to provide a simulcast function, RF video and external stereo audio from a laserdisc or stereo capable VCR which at the time was a Sony VO-2610 U-Matic. This was during my high school years and the set has been following me through life since then.

Now over 40 years later with the everything back in working order a composite input has successfully been introduced. basically its a SPST relay that's been wired to the video test point TP-201 on the IF board to switch the input without loading the 1st video amplifier from the detector with shielded coax. I've found that even a 3 inch piece of wire or coax is more than enough to smear the details and chroma.

So what's happening here now is the 3rd position of the rotary selector switch is not only handling the input audio but also controlling the +200V source to the 3rd video IF plate and closing the relay to feed composite from a rear mounted BNC connector, the video amp works best with about 2.6 volts P-P so I'm using an output from a Kramer VM-1045 distribution amp that's been turned up to provide this and that gets fed from a Leitch A/V switcher that I funnel my sources into.

I constructed the RCA CT-100 video adapter circuit following everything to the letter but for some reason it would not play nice with the Mag no matter what I did and found it worked well with the video provided by the Kramer so this is what I'll use in the house.

The reason for the switched +200V is the IF would leak into the video detector even if the diode was shorted or lifted so shutting down the stage was the best solution.

So viewing results.
Both the RF and IF alignments are very good after spending some 8 hours going thru the procedures and making little compromise adjustments along the way and honestly it looks nearly as good as a composite signal with only a slight bit of color beat on the fine details while composite seems a little better despite a full 5 MHz of bandwidth from the digital betacam, type-c is just as good coming from a NOVA 800 digital full-frame TBC. Biggest limitation seems to be the coarse RGB trio count of the delta gun CRT and it only shows just under 300 lines properly focused.

Now my biggest complaint is videokarma is not allowing me to upload any images no matter how small.

You'll have to do what I did and that is delete some of your pictures from earlier posts to make space for you new pictures. Yeah, I found they don't give you an unlimited amount of space for your attachments. I'm not sure if you donate to the site if they'll give you any more space.