Silvertone CTC15/16: attack of the clone!
 

Well as some may be aware my Silvertone roundy has been on the fritz.

I got a chance to pull the back off today, and I now know that the HV is low. It varies between 0 and aproximatly12KV depending on what I unplug from the HV line, and how the set feels on a given power up. There is a raster (sometimes with synched video) on the brief instant after power up. The first grid of the horizontal output tube grid is running at about -48V sam's calls for -60, the the second grid is running between 85V-90V (I'll elaborate on this) sam's has it at 140V, and the third (beam power) grid is reading 44V where sam's calls for 35V.

The second grid is at 85V with the yoke connected and 90V with the yoke disconnected. The HV goes from 6KV up to 10KV when I disconnect the yoke. Also the focus is strongly diminished and boost is wiped out as soon as I connect the yoke. Disconnecting the yoke is the only thing I've tried that affects the second grid voltage (shorting across the resistors that feed the second grid had no effect on the voltage). I also tried two other roundy yokes which gave me the same result.

Aside from what is noted, and a bad resistor on the second grid that was changed I'm not finding any bad parts or anything out of the ordinary.

Does anyone have any ideas? I feel like I'm running out of things to test.

Try doing a DC ohm check on all windings of the flyback. A ring test would be best and a bk 1077 would make real short work of this. I'm not saying go timmy on the flyback, but we know these are a high failure point. I'm assuming you subbed the tubes.
Try pulling the HOT and measuring the grid and screen. The screen should come up to the supply voltage, maybe the cap there is defective. Also the caps in the boost area.

Are you getting full B+ voltage at the plate of the damper?

I did a ring and continuity test of both the yoke and the fly on my B&K 944 flyback and yoke checker and both passed fine. The fly even moved the needle higher in to the good region once connected despite me forgetting to pull the HV rect tube as reccomended. This is the first time I've used the 944 after testing and replacing all bad parts in the 944 except the transformers and meeter (I could not properly test those, and had to assume they are good) so I can't really say if my 944 is giving me a good reading or not.

I did check between the top caps of the HOT and HV rect, and got a reasonably close resistance reading to what sam's lists. I have not checked the other windings of the fly for resistance yet, but will soon. I had actually ohmed out the fly a few years back just before I found the shorted damper that had been killing the HV, and some of the readings were not as close to sam's as others yet it still worked well after that.

If I can tomorrow I will do some more checks including the ones suggested (that I have not tried) and report back.

I believe it failed due to my blocking of the cabinet bottom vents (they got blocked because I had to take the set it was sitting on out of the room and set it on the carpeted floor) which caused over heating. The video circuits were acting squirily before it failed (intermitant noise, and I think one of the signals was intermitant as well) but not frequently or long enough at the time to worry me. The picture then shrunk horizontaly, and may have ran that way for a couple minutes before I noticed. The picture was only lit on the brightest spots during that. Once I noticed I cut the power hastily and on the next power up it had a nearly if not full raster, and only had light at video peaks. It will display light on the screen with decent video for an instant after switch off (it use to do this when the boost diode was open a couple of years back). I noticed that there is an approximate pencil thickness tower of wax in the HV cage that is fairly new.....the fly does not seem to run hot though so it may be nothing.

I'm not of timmy's mind set anymore because of this set. About a year ago I was convinced that the fly was bad because all else seemed fine, and I had chased my tail on all else except the fly, but then I discovered that the damper which had tested good before was shorted, and that got me on track.....a few blown dampers later I came to realise that this set liked to blow it's damper if the horizontal frequency was not close enough to what it is in normal operation....Quite wierd.

The good news is that I have a Philco with a virtually identical chassis and circuitry (only differences are control layout, tuners, and a noise inverter stage added in to the synch sep stage in the Philco) which I can trade parts (and maybe signals if I get the two in the same room) between. Already tested the HO tube and H osc. in the Philco.

Try unplugging the crt socket. Maybe its a problem with the horz blanking if you noticed problems with the other boards. It doesnt take much to overdrive the crt and pull the hv down to nothing. I doubt thats it but its an easy check.

Its amazing how dificult some of these hv problems can be to sort out. There must have been times when shops really lost money on repairs.

it is amazing how difficult some hv problems can be to find or sort out even the TIMMY WAY.....:smoke:

Pulling the CRT socket was one of the first things I tried.....

I ohmed out the fly this weekend and it checked GREAT maybe better than the tests I did before I got it working the first time. The focus coil....not so much all three of it's windings should measure ~35.5ohms, but two came in at 17.5.

I was able to make it so that the HV drops when the yoke is disconnected by disconnecting the lead from the focus coil to the damper (and a couple of other things on that line), but the max HV was only about 10KV. I was getting 11KV before that without the yoke and about 7-8 with the yoke. Connecting the HV reg cap drags the voltage I'm getting down some even with only 11KV to start with.:scratch2:

The drive looks good, but the entire wave form is negative (even the peaks are below 0V) on my scope when switched to DC mode. Also sam's calls for 150V signal and I only get slightly less than 100VP-P. Could this be what is causing weak HV?

I can post the drive wave forms and some fly pics later when I have time.....Man, have I ever been busy lately!

yes 100v pp is too low. should be more like 175-200vpp

there are two caps from the plate of the HOT, scope there see if one is getting weak.

check the cathode current of the HOT before you do any real damage.

make sure the 100ohm resistor has not opened up on the grid of the HOT.

what is the -DC voltage reading on the grid of the HOT? a VTVM is best to read that if you have one.

check the screen resistor on the HOT

try subbing the HOT/Damper/shunt tube/HV rectifer

The DC at the grid is around -44 to -49 on my DMM (it varies between different power ups/warmup times) sam's calls for -60DC at the grid pin of the HOT. I don't have a functioning VTVM, and don't have time to take on fixing my busted one.
I recall that all caps and resistors between the HOT grid, and the osc. tested good on a DMM and an eye type cap checker.

I replaced the screen resistor a while back, and if I run the set with the plate lead removed the screen resistor will lightly smoke(it gets hot otherwise). The plate lead of the damper is at B+ regardless of the screen voltage.

The screen shoots up to B+ when the HOT is removed from the set.

It is a pain to measure the cathode current in my set, and my VOM has a bypassed blown fuse (I know it is bad for the meeter, and is temporary until I can grab a new one somewhere) so measureing the cathode is likely to smoke it if it is supplying all that current to the screen. I did insert my meeter in series with the HOT plate lead and got somewhere around 50mA (This is from memory, but I'm sure it was MUCH lower than the cathode current spec).

This set was given a complete lytic, white tubular ceramic, and plastic molded recap. I also replaced all maroon/orange drop types between from the osc. onwards through the the rest of the horizontal/HV stage. Also most of the maroon drops on the video board. All of this recapping predates the HV issue by months. I left all the original disc ceramics and all that I tested were good. There are some drops left in the horiz AFC stage though.

"there are two caps from the plate of the HOT, scope there see if one is getting weak."
I don't understand this statement can you please clarify. Also I believe it is verboten to measure the plate lead of the HOT?

Here is an old picture of the osc schematic for you all to digest.
http://i1095.photobucket.com/albums/...y/DSCN0697.jpg

I would suspect the coupling cap going to the output tube grid, or some malady with the cathode circuit. The tube is too 'turned on', which is what's dragging down your screen voltage and causing the low HV production.

Quote: "I replaced the screen resistor a while back, and if I run the set with the plate lead removed the screen resistor will lightly smoke(it gets hot otherwise). The plate lead of the damper is at B+ regardless of the screen voltage."

That shouldn't be happening in this situation. You have other 6JE6's to sub?
As miniman82 suggested about the coupling cap also see if there's any voltage on the grid pin with the tube out.

sorry mispoke ment the two caps from the plate of the horz osc, C92 and C93

I would be nice if you could put a 4000 ohm resistor from the plate cap to ground (100ma about), then measure the plate voltage with the HOT removed to see if you have good conductivity through the damper and flyback. You can measure the voltage there with the tube removed but that doesnt present a load.

You should have enough drive there to get HV.

The caps C90, C92, and C93 were replaced with those modern yellow plastic 630V caps that we all buy and install in our restorations, but despite that I removed C92 and C93 and tested them on my eye tube type cap checker and they both tested good both for value, and leakage (my tester can only do leakage testing up to 450V). Aside from those three caps all parts shown in that piece of the horizontal schematic are original.

The grid goes to (if I remember right) +9VDC with the HOT out of the set.

I have another working set with a virtually identical circuit (the horizontal/HV section IS identical), and I subbed the HOT and osc. tubes from the bad chassis into the good one and got healthy operation in the good chassis so I have little (if any) suspicion left of those tubes.

CTC17, I'll see if I can scratch up a 4K ohm resistor to try your suggestion.

Would it be a stupid idea to put a battery or an adjustable DC bias supply of some sort on the grid of the HOT to see if adjusting the bias helps things?

If I remember output circuit theory right, low drive would lead to higher bias voltage on the grid since the bias is developed in part because of the waveform going through the cap, and the coupling cap itself. So Dave might be right about the drive being a little off.

Keep this in mind, the whole thing is kind of a loop. IF the fly were shorted the voltages behind it wouldnt all be perfect because the HOT would be overloaded, the feedback pulses would be missing and the phase correction would be missing.

I was talking to Dave about this and he mentioned that maybe the screen resistor has increased in value. On a 16 its a 13K at 7watt.

You did sub the HOT??

The thing that stumps me here is the lack of smoke. If the drive is missing the HOT would be red plating unless it failed, if the fly was bad the HOT would be red plating or the fly would be smoking.

If the damper was bad, the effency coil open or the screen supply resistor bad it could cause this but were assuming you subbed/checked all that stuff.

If you dont have a 4k resistor you could put 4 25watt bulbs in series and see if they light up fully. (can you even buy 25 watt incandescent bulbs any more?)

If the yoke or the focus coil was twacked it wouldnt develop full HV but that should result in high cathode current.

Low cathode current equils, bad hot, low screen voltage or low supply to plate.

If you dont have an adapter you really need to break into the cathode ground lead and check it.
You can check the plate current if you have a stand alone plate current meter. There are huge pulses on the plate (or should be) that will creamate a multimeter.

You can check the boost diode and focus rectifier with you cap tester using leakage test. With the tester on 450v the diode should read full leakage one direction and no leakage reversed.

Those are a bunch of random thoughs, im looking the the 16 sams in a trance so take what you can from it.

Just wanted to say, I'm paying close attention here. I'm currently fighting an earlier bw set with no HV and the suggestions are very helpful.

My 4k resistor or light bulb test is based on the ohms law of the plate voltage and the cathode current so dont apply that to your bw set:D
Im going on 400 volts at 220ma.

One thing I coulldnt recommend enough is a bk 1077 for these issues. It pretty much instantly isolates the problem.

Not to hijack the thread but I would be interested in hearing about your bw set.
We really cant have enough discussion on HV issues. It has always been the biggest issue with sets and looking through old trade mags verifies that.

Well I have 100mA on the cathode with the yoke connected and ~170 with the yoke disconnected.

I probably don't have an exact 4K ohm resistor, but should have some 2 watt units that are close. I'm too tired to do the wattage calc right now to see if that will work.

The screen is at about 90V, and seems to jump up if I disconnect the .1 cap on it, but the HV dies when I do that.

I'm going to call it a night.

double check the screen resistor value, both the large one and the 47 ohm one.

The closest replacement I could make up for the 13K was a 12.5K and it still reads at approximately 12.5K as of last night. The 47ohm is down around 41 I want to say.
The voltage is at about 90V both at the screen and at the junction of the 47 and 13K resistors. There is voltage across the 47, but it is only around a volt or so.

That shure sounds like
a weak hot
bad damper
low b+
open in the flyback

I'll post a new thread in B/W.

It is working again!:banana::yippy::banana:

I had subbed all the tubes except the damper (which tested good the first time around) well I subbed all the horizontal stage tubes from the bad set in to the good set and the good set would not light. So next I went and put the horizontal tubes from the good set in the Silvertone and it got TONS of HV (I for got to connect the top of the HV reg.:stupid::withstpd:) very fast. I then started plugging the original tubes in one by one until it went bad again. I retested the bad damper and it is in the "?" zone on my tester unless I let it sit for a while.

I really need to keep a spare damper with that set with a sign that reads "In case of HV issues: don't do anything until you try this." :o

I still need to readjust the horizontal stage, and the deflection in general. As well as fix the no color issue (I'm hoping I did not blow anything when I accidently shorted one of the color pots to ground). Here is a picture of it in action.
http://i1095.photobucket.com/albums/...y/DSCN1599.jpg

I fixed the color issues there were two. The one that killed the color was a lead breaking loose from the color level pot. This set's leads tend to break of their terminals easily (especially the ones going to the tuner sub chassis). The second issue, which has been there since I knocked off the color osc coil during a previous post restoration repair, is that the color lock occasionally gets disturbed. The effort I put in to getting it right after discovering the slug was stuck almost deserves it's own thread (meaning I don't have time to go into it now).

I had little time after fixing the color lock issue, but I managed to do a quick and dirty static convergence, and readjust the horizontal stage, but totally for got to check the purity and rotate the yoke to a give a more perpendicular raster. Hopefully it is good enough that I won't be tempted to mess with it again for awhile.

Well as I was writing this I powered it up for a purity check and lost color synch again (this time it is clear I need to carefully redo the color osc alignment).

This is a picture of what it did when it worked right last night.
http://i1095.photobucket.com/albums/...y/DSCN1602.jpg

Well I finally redid the color osc. adjustment last night, and this time it held synch for over 2.5 hours without issue. It also has good color lock from power up onwards which it lacked before this iteration of adjustments. I decided that the position it was in from the first botched readjustment (the slug half sticking out the top) did not look right so I kept turning it in to the coil until it it was on the edge of lock with the burst reference disabled (if I turned it closer to lock the screen would go green) then I re-enabled the burst reference and and checked if it would lock to a weak signal which it did. Aside from yellows and pinks being a tad weak all seems like it has returned to normal so I'm going to focus on other things for the present.