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?