Why don't you try three 10 Meg resistors? One 10 Meg (10 Meg) in series with two 10 Meg resistors in parallel (5 Meg). That gives you your needed 15 Meg total resistance.

I take it you still have a Radio Shack store in operation? They pulled out of Atlanta awhile ago.

These nice RS 10 meg resistors are actually 1/4 watt and the schematic calls for 1/2 watt so I'd need to double up two strings of three to increase the wattage if I wanted to do it "right". Though for a test, I'm sure what you suggest would work just fine until I put in my next mouser order and get the right one.

FK

Manufacturers didn't use 1/4 watt resistors back then. 1/2 watt was the smallest. Using the formula P=Vsquared/R. P=250*250/15,000,000= .00384 watts. 1/4 watt resistors should do just fine.

Well, I replaced the 15M resistor (R20) and made no difference -- the picture still has that noise and AGC control has no effect. I've checked and replaced all resistors and capacitors between the 1st IF grid, through the AGC control and to the plate of the AGC keying tube. I bypassed the AGC control with a low fixed value resistor and nothing different.

The DC voltages on the AGC tube and IF tubes are close enough to spec. Also verified all of the scope traces drawn on the Sams and frequencies are all correct, though P-P voltages vary a bit from the schematic but are within the right ballpark.

I don't see any visible HV arcing, and while there's an audible "pulse" that occurs with the noise, it sounds softer, more like a hold/sync issue than a sharper HV noise.

I swapped out every tube in the set last night including the HV rectifier and still no change. I can try to order up some more tubes to try (though I have at this point at least three of every one) if it's still possible it might be tube-related.

I injected an IF video signal from the VA62 after the third IF tube (with it removed) and still has noise and no AGC control.

I also saw no visible AGC control on the first power up of the set too, so I'm pretty confident that it's not related to something I did.

The only thing I've noticed is that the noise seems to go away when it's tuned a bit away from the station or the signal level from the VA62 is low.

Where could I be looking next?

Thanks!

FK

(schematic attached)

Is the 100K resistor in series with the CRT HV lead still original? If so try replacing it. I mentioned earlier that resistors in the HV can develop internal arcing, the arcing acts like a spark gap transmitter and feeds back through the RF chain as video and audio interference.

Was the 3RD IF tube installed when you injected after it? If so large RF noise spikes could still be getting through the IF and mix with the injected signal resulting in the noise remaining present. If you pulled the 3RD IF for the signal injection then it suggests a noisy component, B+ line, or connection in the stages between the injection point and the element of the CRT being driven with video.

Do you have a working o-scope? If so it may be good to ground out the signal going to the detector (a large cap ~.1-10uF from it to ground should do the trick) and if the noise persists search for it in the stages between there and the CRT. Once you find it trace it in in the signal chain to the strongest point, then look for it on any power supply lines in the area to see if it is stronger in those lines.

No, that one is original, though I tested it and value was good. I'll replace it next time I pull out the HV box (it's not a lot of fun to do on this).

It was removed, and I also removed the 1st IF and injected signal right after it. I will repeat the experiment again tonight and make detailed notes about what happens.

I do, and I'll give that a try. It's tricky because the noise either goes away or becomes imperceptible with low or no signal. With the antenna leads disconnected or IF tube pulled the screen is solid white/blank.

Do you think this is related to the AGC or do I have two unrelated problems?

Thanks for the suggestions -- will get to work on those next and let you know.

FK

If the noise goes away completely with the IF tubes pulled then the problem is either up stream of the 1st IF tube you can pull to kill the noise or it is in the AGC.

If the noise goes away with the 1st IF tube pulled, then odds are it is in the tuner, or it is local interference. If you have a signal source that operates on other channels than the one your currently using it may be wise to try different channels. There is a large gap (FM band + other stuff) between channels 2-6 and channels 7-13. So switching from low band VHF to high band or vice-versa may be a wise test (to get away from wide band noise). Also if a local DTV channel is using the same RF carrier freq as your signal source, you will likely get noise from that DTV carrier leaking into your set.

One way you can eliminate AGC noise is to connect the AGC buss to an external adjustable DC supply 0-25V and use that supply to clamp the AGC line to a proper fixed voltage.

With all three IF tubes removed and the IF signal injected afterwards, same issue.

I used the VA62 to try on channels 7+ and still there. The DTV box is only 3 and 4, but same on both.

Here's what I'm seeing now with VA62 feeding channel 3 RF: https://youtu.be/q8nD70vokkE

It's odd that the audio takes hits too because it's picked off immediately after the detector which is basically right after the IF injection point that I used. Would this seem to suggest the issue is in fact HV-related since that would be about the only thing remaining that could cause this effect on both audio and video?

I used the VA62's DC output to vary the AGC voltage and that actually did what the AGC control is supposed to do. Also, with at least -5V applied, the AGC control does actually have an effect on the picture( for the first time) since the injection point is at the wiper of the AGC control. Without it, at that point, I measure a small negative voltage only in mV that varies maybe 8mV with the rotation of the AGC control. With just the VA62 connected with the voltage set all the way down to nearly nothing, the picture does become slightly more saturated - like the AGC control being turned slightly up.

There are otherwise no voltage readings called out on the schematic for that portion of the circuit so I'm confused as to where this negative voltage would be normally developed? I should expect to see that same between -9 (or so) and 0V at that point if everything was working correctly yes?

Thanks!

FK

Are the power supplies to the video output and audio output stable and quiet?
Is the external dag coating on the crt in good shape and grounded well?

jr

Excellent discussion here on how keyed (or gated) AGC works. Especially the 4th post down..

http://antiqueradios.com/forums/view...p?f=3&t=201711

I second what JR said.^

Given that with the 3RD IF pulled and the signal injection after that, that the interference remains. The problem is most likely noise in the B+ or HV arcing. There could be issues in the stages between the video detector and the CRT, but it is not as likely.

Since you are getting sound interference too, here is an idea to save you some time....Pull the H output tube or damper (to stop supply of HV) and see if the sound interference goes away after it has had a couple of minutes to run without HV (you obviously can't check picture with no HV).
If disabling the HV stops the interference then the issue should be in the HV or CRT grounding. If the noise remains without HV then check the B+ lines with a scope, and if those are clean use the scope to look at the stages between the injection point you were using and the CRT video feed.

You may have AGC issues, but if as you say the IF is passing signals properly (with the exception of the mystery noise), then I'd ignore the AGC until the noise issue is fixed. The AGC appears to only affect the tuner, 1ST and 2ND IF...So given you can inject after the 3RD IF with the 3RD IF tube pulled and still get noise, I think it is safe to say the AGC is not your noise source, and can be ignored for now.

Yeah, all new electrolytics - Nichicon 105 deg, 500V caps replacing the original 450 and 350 volt can sections. Replaced the original candohm with two 10W power resistors as well and the B+ lines are all within 3 volts of schematic. My DVM locks right on and they are stable, but I'll scope them all and look for any ripple or odd noise too.

Interesting question! The grounding spring is tightly in place and overall the coating looks in good shape, however it is a little scratched where it makes contact with the tube - see attached pic. Would that be enough to cause a problem? Recommend trying to bend the spring so that it contacts in a slightly different place?

FK

I'd try changing the contact point. I f the dag is missing (even just a whisker) under the point of contact or making poor electrical contact with the spring then you will get HV arcing (and spark gap transmitter action) at that contact.

One way to test for questionable dag grounding is tape on a grounding wire, and see if the noise disappears. Strip several inches of insulation off a piece of hookup wire and tape it to the dag, and the other end to ground.

That's assuming there's no isolated patches or 'islands' of dag lurking around, which happens occasionally.

You might want to be careful using tape to hold a wire against the dag. The dag is not known to have very good adhesive characteristics and you might pull off a big amount of dag when you remove the tape.

Okay, so pulled away the metal contact and cleaned it up a bit and then bent it to another place on the tube, and looks to be making good contact. I don't see any "islands" or places where the dag is separated from the rest. The coating looks pretty complete... just a little scratched off where the metal has been resting for the last 60 years.

Now here's the thing... before that, after playing a bit with the suggestion of grounding out the 3rd IF with a cap, and working with DC on the AGC circuit, the noise seems to have gone away... along with decent picture contrast. It's possible that it could have been poor contact with that rubbed-off dag and in working on the chassis the contact was intermittent.

So without a clear explanation, I seem to have a more or less clean but poorly contrasted image. As with before, it looks brighter in the video than in person.
https://youtu.be/xWOoI252aZY

There is definitely some wobble to the picture as well, which you can see with the DTV but can see it much better with the test pattern. I'm adjusting first the fine tuning then the brightness then the contrast and you can see there's not really a sweet spot.
https://youtu.be/XO5ciFJxHiM

Since the CR70 rejuv, the tube now produces a lot of light with brightness all the way up. I re-checked it again and still has responsive cutoff and gets up to about a 10.5 on the CR70 emissions when it's warm.

Still nothing on the AGC control, but injecting negative or positive DC doesn't make any large improvement to the picture's contrast so maybe that's just not going to be a big help.

I would say contrast and vertical hold tends to get worse as the set warms up. The sound (as can be heard in the test pattern video above) get very soft when the channel is fine tuned in for best picture. I know this is typical with old B&W sets and cheap DTV boxes though.

Just got the HV probe and it's measuring 15kV, while the schematic calls for 17.5k. I'll try a few different horiz damper and output tubes and see if that makes any difference.

FK

Remember, 17.5kv is at zero brightness set brite control all the way off. HV will be lower as the screen gets brighter. All the best,Tom.J

Ah, got it. Okay, that brought it up to 15.35kV. I tried all three output tubes and the one that was in there is the best HV. Changing damper had no effect.

Edit: Turning width control all the way up and brightness/contrast all the way down, I've got 17.6kV so HV seems solid.

FK

Good point. But we used to leave the grounding wire in place as a "belt & suspenders" supplementation when there was a sketchy spring contact.

So here's the update in quick bullet points.

What's working:

• Horizontal sync - strong, stays locked in at center of control rotation
• Audio - full and loud (when using BT Agile modulator)
• High Voltage (measures 17.5kV) :thmbsp:
• CRT has responsive cutoff and good emissions as tested by CR70. Output is bright
• Odd video/audio "noise" has stopped (unclear why) :scratch2:

What's not working:

• AGC control non-responsive
• Contrast very poor
• Vertical hold poor -- worse as set warms up
• Picture has a "wobble" -- especially visible with test pattern (https://youtu.be/XO5ciFJxHiM)

The Sams troubleshooting notes keep pointing at V7-6AN8 (Sync Amp), V8-6AN8 (AGC keying/Vert Mult) and V6-12BY7 (video output). Have tried four different 6AN8 tubes, and two 12BY7's -- however I've just ordered some more of both.

At this point, is there anything else this could be other than those tubes or anything else I could try?

Thanks again everyone, for all of your help and great suggestions!

FK

Given the rate of wobble I'd reckon that the power supply filtering is insufficient, your AC outlet voltage is fluctuating, or some component is shorting/leaking/arcing intermittently at a regular interval.

I've seen problems with AC utility voltage fluctuations causing picture size to change 1 a minute to ~3 times a sec. There is a business park about 1 mile away with manufacturing operations there, it was worst for the first 2 years I lived there, but still happens occasionally....I knew it was the power line since I could have 2-3 working tube era sets and one new set fed the same signal source on at once, the 3 tube sets would 'wobble' identically, but the new set would not wobble (due to having actively regulated B+ rails)...The cure was to run the tube sets through a SOLA VRT (AC voltage regulating line isolation transformer).

As for contrast check the video amplitude with a scope and compare to the schematic. If it is low there are a number of things that could be affecting it.

Your AGC may be tied to the low contrast issue....For instance the tube IF grids should be biased negative enough relative to the cathodes to prevent conducting more than their max rated plate current....If the IF tubes are weak and not amplifying enough then the AGC could be be pushing the grids as positive as the biasing design wall allows...When it hits the wall it can't make any more gain, but still it trying to.

It might be interesting to connect that external bias supply back to the AGC line, see how much more contrast you get and how the IF and tuner voltages (especially the AGC controlled ones) compare to the schematic with the external AGC voltage.

May also be good to check the sync amplitude entering the vert mult/osc system. Bad vert hold can be weak sync or osc off freq.
If you can get the vertical to roll both ways (up and then down) slowly, even 'float' without rolling but not fully synced for a while, then the problem is in the sync pulse level/cleanliness and or the osc circuits handling of it.
If vertical only rolls one way then it is off freq and the vert osc/mult/output stages are to be suspected (and sync signal can be ignored till float/dual direction roll is achievable).

I've gone through and checked through all scope traces and voltages several times and everything is generally within spec, except for a few places.

The biggest anomaly is that the grid on the sound IF / sync amp should be, according to the schematic, -1.8V however I measure between -0.3 and -0.5V. The plate voltage should be 60V but I see about 54V when the set is cold, and it falls down to about 45V. When first powered on cold, I get actually watchable contrast and fairly strong sound, however... after it warms up for about 5 minutes the contrast, sound and to a lesser extent sync gets worse (as that plate voltage goes down).

I have tried no less than six different 6AN8 tubes and all the good ones are the same. Have verified and/or replaced all nearby resistors (nearly of the originals were still within 20% spec) and replaced all but the mica and ceramic caps. And still absolutely nothing from the AGC control. The DC voltages on the IF stages are pretty much right on.

What could be the temperature sensitive components that would cause that?

FK

Resistors are usually among the more temperature sensitive parts. Usually the best way to find a thermal sensitive part is to run the set till symptoms manifest and hit sections with freeze spray (or something similar like canned air) to see what makes it clear up. Start with general coverage of suspicious areas and once you get a good reaction to the spray, target specific components in the area of sensitivity to narrow it down.

Looking at the schematic you attached earlier I'm not seeing the -1.8 volts grid or 60V plate specs on either section of the 6AN8 sound IF/sync separator tube...Are you referencing a different listing of voltages? Also on multi section tubes like that one it is good to specify which section's plate and which sections grid when you say one is not right so we know which stage to look at.

Sorry about that, I meant V7B. Attaching a close up of that part of the schematic. I picked up a fresh can of duster and will try cooling off some components to see what might be heat affected.

A few discoveries I've found:

1. By fiddling with all of the controls I can pretty much dial in most of the DC voltage measurements called out in the schematic (thought it only produces pretty much a bright white raster). The exceptions are the grid of V7B that measures -1.8V and no matter what I do the range is about -0.1 to -0.5. V11 (6CS6 SYNC SEP) pin 7 shows -10V and closest I can ever get it to is about -2V.
   The scope trace shown for the grid is somewhat confusing to but I assume I'm looking for some kind of beat on a 60Hz pulse there? (See attached)
2. By feeding in an incredibly strong signal (from the Blonder Tongue) I can get the plate voltage on V7B up to 60V, however at that point the signal is incredibly overloaded and has a negative image and poor sync (both horiz and vert)
3. The scope traces on the AGC keying (V8A) look consistent with the traces on the schematic. I cannot measure the plate on it since the voltage is beyond the range of my scope and can't measure DC since those huge pulses confuse my DVM, however visually it seems to be correct. G1 (pin 8) measures pretty much the same as the plate of V7B as the schematic suggests (see attached)
4. Adding -9V bias to the wiper of the AGC control has what would be the effect of the AGC control, and at that point the AGC control actually affects the picture. However, the control has no effect without it.
5. Sound, contrast and sync continue to degrade as set warms (will try freezing components tonight)

At this point, is it possible that some of this could be alignment-related? The multiburst bandwidth test from the VA-62 looks pretty close to the trace from the manual (see attached) and there are distinct vertical lines on screen up through 3.0MHz, so would that imply alignment is healthy?

Thx!!

FK

On V7B: Tube biasing 101 says the less negative the grid the more plate current is drawn, the more plate current the less plate voltage (at least in a common cathode circuit like V7B)....If your grid bias is off it will effect the plate.
I'd like to see more of the schematic to the right of V7B. The grid circuit wanders off the edge of your attached scans...Seeing where it goes could give some insight into why it is not negative enough.

There is a slight chance the video detector diode is going bad (I've seen it in some TVs from time to time) and reducing video amplitude.

The AGC wiper appears to basically be the start of the AGC bus that biases the tuner and IF grids. If the control is dead till a voltage is injected it probably means the AGC circuit is not working. If all the tube voltages look right I'd be looking that fly winding between the plate of the AGC tube and the AGC control.

Unless you can get proper/much better contrast by injecting -9V at the AGC control (and adjusting the control) I think the AGC is not the main issue....Even if you can I'm still a bit more concerned about V7B.

With what I see I'm not inclined to think it is an IF alignment issue....And even if it is part of the problem, you need functional AGC and to work out the issue with V7B first before any alignment can be attempted. If you try to align circuits with defects the results often are not an improvement over the defective circuit as it was.

I'm running on ~3 hours of sleep presently so take this advice with a grain of salt...

Attached the second page of the schematic. Makes sense that the issue could be in the detector since the DC voltages on the IF stages are spot on. I assume I'm going to need to dig up some germanium diode or equivalent, but worth trying.

Yeah, checked that -- the flyback winding is 5 ohms, right on the schematic value. Glad it's not that at least! Since I am seeing those 15,750KHz pulses on the plate of V8A, would that imply the AGC is at least partly working?

I've replaced every passive component on the AGC circuit between the grid of the 1st IF and the flyback. The control works perfectly too... no jumps and smooth resistance all the way across. I suppose I could try changing out some of the small mica and disc caps in the adjacent circuits.

I've tried at least six different 6AN8 tubes that have all tested way-off-the-charts-good and all produce the same results, so not a tube. Adding the bias certainly affects the contrast but not really better. It's more like if you turn the AGC too far where it whites out the picture.

No problem at all! I really appreciate all of your help and thoughts!

I'll try changing out that diode and see what that does. I think I have some low forward voltage drop diodes... somewhere...

FK

I pulled out the old diode and tested it and it was showing a 0.7V voltage drop one way and 0.1V drop the other way, so it was definitely suspect. I changed it out with a new 1N60 and it seems to have made a difference in terms of how the signal degrades as the set warms. Contrast and sound seem more stable as the set warms. No change to the grid or plate voltages on the V7B tube though.

What I do see is that there is a slow brightness increase over 5 mins or so when starting from cold. As in, having adjusted the bright/contrast the night before, it will start very dim - almost not visible. Could this just be the age of the CRT and it's poor life test?

I can set all of the controls so that all of the voltages on the CRT are exactly on spec, and this is how the picture appears https://youtu.be/jzV57jRqrcI. I can adjust for better picture, but it changes the cathode and g1 voltage by 50-60V away from the schematic spec. Does it make logical sense that I would want to keep the controls set to produce the schematic voltages on the CRT because that will always be it's optimum operating point? As in, this should be the best picture, and there's something else causing it to be whited out like this that still needs to be found.

FK

When a CRT is weak or old it can have a long warmup time to it's stable operating point. I'd guess that if you put that CRT on a tester and watched it's emission/cutoff readings it would probably change over that same 5 minute interval.

I've got a color set with a weak CRT with poor life. All guns will start dim and the weakest one will take the longest to warmup and stabilize (which makes the colors look weird for a bit).

Generally speaking the CRT gun voltages listed in the schematic are the correct settings for a new CRT (because that is what they would have had when recording that data). The optimum CRT grid and cathode voltages are the ones that correspond to the contrast and brightness settings that look best. Those settings will drift as the CRT ages from use.
Keep in mind that you want to NOT* adjust the picture for the brightest possible image with good contrast (doing that drives the CRT hard and given it seems to be a tired CRT that will kill it faster than normal). You want to adjust for the dimmest picture (with good contrast) you deem comfortably watchable (at least if your concerned about CRT life).

*Unless you plan to ride the CRT hard till it dies, and then replace it...Some people do that when they have a replacement on hand.

Interesting and got it. This CRT is clearly high hours since it is the original and was pretty dead when I got it. I zapped it with the rejuv on the CR-70 and it brought it back to life with good emissions and cut off, but yes the life test was poor. I consider it borrowed time and I don't plan to make this a daily use set. It sounds like this can explain some of the remaining warming up issues though.

FK

(attaching bonus pic of original Ratheon diode)

Well, this set is just full of surprises. I took the cover off of the tuner to double check all of the resistors and voltage measurements, but it's really tight in there and wasn't able to safely probe everything. Resistors were all good and voltages seemed a bit high and I wasn't really watching the picture and so put it back together and turned it back on.

Good news:

So... what happens next is that for no explainable reason, the picture has massive contrast, sound is loud and clear and has flawless sync all the way across both controls. What?? And... the AGC control now actually does something. It doesn't go all the way from no picture to white picture/no sound, but it does have a noticeable affect on brightness.

Odd news:

The voltages that were previously low or in question are now massively high. The 130V B+ supply is now measuring about 175V -- despite the fact that the two other B+ voltages right above it in the power supply are still right. The V7B plate that was supposed to be 60V and was previously measuring as 45V is now measuring 95V, and the plate and grid voltages on the IF tubes which are supposed to be 115-120 are now measuring 160V. The grid 3 on V11, that's supposed to be -10V, that was previously measuring -1V is now measuring -10.5V. Oh, and now adjusting all of the CRT voltages to the schematic is right about where the best, brightest picture is.

This is how it looks, though I was able to dial it in even better with the focus, fine tuning and width controls later. The video doesn't even do it justice -- this thing looks like a brand new modern TV in person.
https://youtu.be/dKXksyLwGqs

Note the sharpness and clarity of the text:
https://youtu.be/6pDbjJgIk1Q

Bad news:

Of course it doesn't like to be run with those voltages. After about 5-10 minutes, the picture starts to take hits like this. And yes, I turned it off as soon as I recorded this:
https://youtu.be/rYIgDQ-g3ZE

The only thing that was actually changed recently was that detector diode, and this was not happening right after I did. Previously the B+ supply voltages were all dead-on correct with the schematic and the IF's being fed from the 130V line were all correct while the sync circuits fed from the same 130V were all too low. I've never seen a situation where only one single part of the power supply divider goes from being right to being 30% high, unless a significant load was removed. If that were the case, and a tube that was drawing a lot of current got disconnected why in the heck would the set all of the sudden start playing WAY better?

Has anyone ever seen something like this before?

Very befuddled,
FK

Have you cleaned the contacts in the tuner? I've seen dirty tuner contacts do stuff like that.

If you have not cleaned the tuner contacts make sure to get both the ones involved in changing channel and the tube socket pins.

I recommend Deoxit, or the cheaper comparably effective De-ox-id as the cleaning solution. If you can rub the contacts with a pencil eraser (the soft white ones are especially good) while they have the cleaner on them it will be better still. Those erasers are almost good enough contact cleaners by them selves to not need the spray.

I had cleaned the channel changing contacts earlier with WD40-branded spray electrical contact cleaner (I ran out of deoxit), and have worked and rotated around the channel clunker quite a bit since and never saw anything like this happen. I've wiggled the OSC and RF tubes in the tuner and didn't see any reaction, but who knows. I can try cleaning the tube pin contacts next, but now I'm afraid if I touch it that it might go back to the lower voltages and sub-par performance!

Clearly getting the sync and AGC circuit voltages up directly affected all of the remaining problems so at least there's some confirmation there. I'll trace out everything from the 130V on and see what's up...

FK

My guess is dirty contact or loose connection in the tuner. The issue with AGC and such is probably being caused by that connection and the issues in the 3rd video are probably it trying to relapse.

I've had noisy carbon resistors cause intermittent grief. They check o/k with a meter but break down when passing current. Since it was the tuner that you disturbed before it came good it might be worth replacing the carbon resistors in it. There shouldn't be many in it but access will be tight. Be careful not to disturb coils and feed through caps etc. The turret will need to be removed to gain access. I'm not a fan of WD40 for electronic work. Use Deoxit as it leaves minimum residue. If you have a spare tuner from another similar vintage set even better. Doesn't have to be physically the same just electrically close enough.

Wasn't regular WD40 lubricant, it was their electrical contact cleaner that's just typical electrical contact cleaner with a fancy nozzle. And yeah, I'm very careful around those wire coils that had been hand tuned by carefully forming and pinching in the factory.

I spent some time last night cleaning the tuner contacts and tube pins and so far what I'm seeing is that when all wafer switches are making good contact I'm getting great performance but with the over spec voltages. I tested tube voltages and everything that is fed of the 240V B+ is right about on, and everything fed by the 130V B+ (the one measuring 170V now) is proportionately high. It doesn't seem like it's a bad contact on the tube socket since all voltages are present and no glitching at all if I wiggle the tubes as would normally be associated with dirty connections.

Meanwhile though, the thing is still producing a beautiful picture and sound (though I know at those voltages I'd be cooking tubes alive). This is with CRT pin and high voltages dead-on schematic measurement, so it's not even being pushed out of spec. Not bad for a tube that was found with a brightener (which is of course no longer on) and had virtually no emissions and zero cutoff. That CR70 rejuv zap is incredible!

https://youtu.be/MYtxY-Eanrk

I'll keep cleaning and maybe try replacing some resistors best I can without having to pull the tuner totally apart.

FK

So I might have just hit a "duh" moment here. Reading the barely-legible scanned print at the bottom of the schematic it says "channel selected shown in channel 13 position" and "measurements taken with no signal applied". And well, I had previously been testing voltages with a signal connected. Now, with no signal I get pretty close to all of the voltages on the schematic -- of course with a signal they jump up to those much higher ones.

Based on that, would it be safe to say then that what I'm seeing is in fact the correct, normal working voltages (despite being quite a bit different) when tuned in?

FK

As a rule voltages are +/- 20%. Service data at one time would have that noted in the info bar/box. All the best,Tom.J

Yeah, with no signal I'm getting pretty darn close to the schematic which says they were taken with no signal. With signal, many of those B+ voltages jump up 40% or more, but it sounds like that's actually fine because what was published wasn't measured under actual use conditions. The set works really darn well and so if this is in fact how it is supposed to run and I won't cook the tubes then I'm more than glad to move on from it!

Thanks!

FK

Generally speaking the only tubes you ought to be concerned with are power rectifier, H/V/Audio outputs and maybe a stray 6CG7/6FQ7 or 12AU7/12AX7....The rest are usually $1.50 bottles that only TV folks want (and unlike audiophiles/radio collectors) we're cheapskates. :D
I've got ~8 caddies full of mostly TV tubes in my work shop (a good portion used pulls in unknown condition), and I've been known to grab a used one smash it open and dissect it on the spot to explain how a tube works to an interested guest..."Hey Tom, might that have been a good tube worth keeping intact?"
ME:"who cares? It's a $1.50 6xx#....The visual aid/savings in explanation time we got out of it is worth it." :D

Ha ha, yeah! Well, I've bought so many tubes for troubleshooting that I've easily got enough for two more sets just like this, so I should be in good shape just in case. This whole voltage issue seems like a rookie misunderstanding on my part, so really a non-issue. The data sheets on the tubes in question (6CB6 and 6AN8) have a max plate voltage of 330V so running them at 160V instead of 125V doesn't sound like it would be any problem either way.

Otherwise my last issue here is that pesky picture glitching (and this), possible HV arcing noise that was bothering me a while ago went away, and now has come back which seems to happen after the set has warmed up about 5 mins or so. I did have to repair the grid cap wire on the horiz output tube because it had frayed at the point where it exited the cap and eventually broke off. Perhaps my solder job wasn't quite perfect enough. If not, will go back to dag. Will keep pluggin' away.

Also... let me just say huge thanks to everyone who helped out on this. I feel like this project a little bit of all kinds of things that could go wrong with an old, unrestored set and there's probably enough useful information in here to help many more restorers in the future.

Thanks everyone!

FK