sylvania roundie D01

[DaveWM]

did the standard startup routine, HV came up and sound, extremely weak pic and no sync lock. AGC super critical. Pulled the chassis in, checked the caps around the AGC, all checked good. replaced 5 or 6 tubes, mostly shorted 6GH8's and a 6JT8 that is part of the AGC. Replaced a few crumbling cheapo tube sockets on the chroma board (only the metal sockets remained).Tried it again, no audio or video, just some odd thermin like sounds... You could here the tuner snow start up then it would go off. Pulling the HOT would let the tuner work so I figured it has to do with the keyed AGC.

Put the old 6JT8 back in and works again (dual purpose tube, triode section is the AGC keying pentode is the video amp), but AGC is still super critical and both vert and horz sync are very weak. the orig 6JT8 was very weak, the new tube tested fine, so I am going to look there again. there is also a noise gate transistor that is the very early style, rectangle shape NPN. It looks like it would be a likely candidate for screwing up the bias of the sync separater. There is a wave form on the sams at the collector of that transistor so that will be the 1st place to check.

after the replacement tubes and socket fix the color demodulation is working. this set has a lot of the maroon drops, I expected to find a lot of bad ones but after testing several, they seem to be fine. the only weak one was in the chorma section grid coupling the 6GU7, just like on all the RCA's (this is NOT a RCA clone).

[Zenith26kc20]

You may look for a capacitor from the 3rd If or video detector to the sync circuit that goes leaky (hope I remember that correct), but I fought one of those chassis years ago for a long (!) time. Also, check to tubes for any grid leakage. This tends to play havoc with AGC tubes. This isn't the chassis with a solid state IF, is it?

[DaveWM]

I did check for grid emissons on all tubes, one of the 6JT8 subs I had was real bad and it was NOS I grabbed another NOS and it was fine, but screwed up the AGC, thats when I put the old weak one back in.

The 3rd IF tube was nearly dead, replaing it brought back a strong video signal (it was very washed out even with agc adjusted), but still has the super critical AGC.

when I have the scope out checking the collector of the noise gate I will also check other AGC stuff, generally I look for the flyback pulse, video signal, and the filtered output to the RF and IF circuits (in case the filter cap was open, but I checked that so don't think its the problem).

I will check that coupling cap you mentioned, I presume a DC voltage check would find that.

Its all tube but for that one noise gate transistor.

[DaveWM]

hmmm that cap is a 4.7pf from the sound IF detector and does not look like a voltage check would work to check it. I will check it later after scoping the keying circuit, esp the fly pulse at the plate since that was causing havoc with the strong Keying tube. there is a 68pf that couples the horz pulse that I have heard can cause issues, but nver ran into a bad one yet.

[DaveWM]

tried another 6JT8 NOS same deal, touchy sync. that's about all I can do in the short term, next up is the scope.

[holmesuser01]

Dave, I remember seeing D01 chassis in our shop. I remember the guys replacing several large resistors around the Y tubes on almost every set they serviced.

I was brand new in the shop, then, and the guys working on the older sets were seasoned veterans.

Great sets when they were running right.

Bruce

[ctc17]

I have about 8 sylvania tube color sets and they all have very touchy agc. A very small window of adjustment. Sync lock is never really a problem though.

Sylvania sets seem to have good flybacks, some of mine are super melted but work fine. Weak crts are an issue. that would suggest a tough chassis

[DaveWM]

I will be checking the AGC pot for smooth voltage division, the noise gate transistor and the sync sep tube for proper bias/waveforms. the PCB's look to be fiberglass, tough, no heat damage like seen on RCA PCB's. The tuner is very stiff, like hardened grease or something seized on the channel drum. UHF does not turn at all. focus voltage did not seem to move with the adj of the coil, stuck right around 4kv, IIRC I can see movement on other sets that use the same focus circuit when reading with my simpson 260 in the 5kv mode. Focus is soft and did not seem to change. Perhaps the focus rectifier stick.

[DaveWM]

took a quick look at the sync, pulse at plate of sync sep seemed normal enough, horz seems better, vert still touchy but working. The agc still has a very narrow range so will be looking there next. the plate voltage was a bit low 65v vs 75v but that does not seem like much. there are some 5% resistors in the grid circuit of the sync sep so may check there. Tried a brightener on the CRT helped a lot with the weak green, but the CRT is def on its last legs, the CRT has blue/brown G1 electrodes (I am guesing that is what they are, its the 1s of the cylinders you can see), looks like they just got real hot, perhaps from use or rejuv in the past.

[DaveWM]

these color sets have a very simple vert integrater, no where near as many filters as my older B&W sets. Perhaps has to do with better noise rejection on the color set. still if the vert continues to be an issue I will take a look at it. the sync pulse at the vert rate did not look very nice, but it seems each color set I work on has a different looking pulse, some are very large and well defined, some not so much. Not really sure what the driving factor is, but have a feeling it may be IF alignment and perhaps some clipping.

[DaveWM]

ok check out the keying pulse, that looked good, the video signal to the control grid fo the keying tube looked ok. I really need to see the video composite signal in better detail (not the one right at the AGC tube) and look to see if the sync is getting clipped. the AGC is just WAY too critical. I still suspect its in the sync sep tube since the CRT is not getting cut off, its just the sync that gets lost at anything above just when you go from blank raster to fairly weak pic.

I tried looking at the scope signals around that noise gate transistor, but that was inconclusive, I think I will just pull that transistor out and check it.

I still need to confirm that the AGC pot is acting right, I once had a noise inverter pot that was open so it was not acting like a proper voltage divider. I think this one is ok as it seems to function normally but for the narrow range.

have had some off line discussions with another member here, and the thought is the pot may have some issues that are hard to test for with a ohm reading, but better to test by checking the DC potentials at the center lug.

[DaveWM]

doing some resistance checks of the sync amp seems to have paid off. The control grid showed no connection to anything.tracing back there is a coil that come one of the detector diodes. it was a poor solder joint, a solder blob on the pcb,with the coil wire next to it. I suppose it could have been that way from day one, perhaps over time it opened up. Lots of flux around on these boards again looks all original. Anyhow I corrected the poor solder joint, and retested the resistance, looks good now, so will reinstall the chassis later today.

I recall I was getting old looking video to the separater, but without a working set to compare its hard to tell. The sams are really not the best for showing waveforms.

I prob would have picked up on this much sooner if I had bothered to examine the PCB up close in a strong light. the poor solder joint was pretty obvious...

[holmesuser01]

I love finding things like this.

Had an RCA CTC-38 that had intermittent color. I was the 3rd person to look at it for these folks. While it was at my shop, they found a newer set they liked better, so I wound up with this set. There was a hairline crack in the foil right beside the 3.58 crystal. When I shined a high intensity light on the top side of the board, the crack in this foil stood right out. I'm the one that needs reading glasses, and I found this about 5 minutes after starting my troubleshooting.

I ran my set for almost 15 years before the OEM CRT finally gave out.

All the best to you, DaveWM!

Bruce

[DaveWM]

thanks Bruce.

Part of the problem on the visual inspection is all the old crytalized flux, I mean its EVERYWHERE. hard to see the traces thru it. I suppose it would have been easy to clean if it had been wiped off at the factory, but now its on there for good. The PCB's on this are very solid looking, they appear to be fiber glass.

[DaveWM]

that was it, AGC works as it should now, can go from pure snow to dark pic, but the sync is now there much deeper into the dark pic. Sync is rock solid vert and horz.