RCA 1983 EGR338WR (CTC107) Tuner problems

[probnot]

Hi all,

I've lurked for a while, so this is my first actual post.

I've got what seems like a tuner issue on a little CTC107 I picked up recently (EGR338 WR).

The set worked great for the first few days. Nice bright CRT, I even had to turn the screen down a bit to set the greyscale since the bias controls were at their lowest.

However, I noticed it wouldn't tune any UHF stations. It would tune a VCR on channel 3 or 4, but I just got normal snow on any UHF station I tried - I have about 6 UHF stations going through the coax in my house. It wouldn't even pick up the noise of the digital OTA stations.

After a few days, the picture suddenly went dark on it. I thought at first something in the HV went out, but I could still hear it. After turning up the screen ever so slightly I was able to see retrace lines. There was audio, but it was just static even when tuned to a channel with a signal present.

Weirdly, some stations showed snow in the picture, while others were blank. I can't really see a pattern to this as it doesn't seem to line up with any bands.

I've heard the PLL tuners on these older RCA sets were prone to issues with cold solder joints. I verified the voltages coming from the MSC board look ok:

CATV - Good, Switch Off = 5v, Switch on = 0v
5.1v - Good
19v - Good
VLO - 17.2v to 17.6v on channels 2-13 and CATV 17-22, 0v everything else
VHI - 18.3v on channels 17-22, 7-13, 0v on everything else
MID - 18.1v on channels CATV 69-73(A5-A1) and CATV 14-16, 0v on channels
U/S - 18.4v on channels 14-83 and CATV channels 23-53, 0v on 2-13
TV - this voltage seems to make sense as it varies between ~1.5v and 22v as you tune across a band

This seems to match what I found in this document (except for the MID/CATV band, since the documentation only covered VHF/UHF):

https://worldradiohistory.com/BOOKSH...1-20-08-20.pdf

I don't have any equipment that can provide an IF signal to verify that the problem is in the tuner stage. I do have an old signal generator I use on AM radios, and by tuning it to the RF frequency of some VHF low stations (~55MHz for ch2 for example) I was able to see the tone from the generator appear as bars on the screen. This was just with a coil of wire acting as an antenna nearby. This makes it seem like the frequency it's tuning into is correct, but may be blocked somewhere?

Where I'm at right now:
- Reflowed solder on the tuner board (MST) - this seems liked it helped at first as a bunch more channels showed snow, but after the set warmed up it went back to normal
- More channels appear with snow upon a cold startup and several fade away as the TV warms up
- Checked electrolytic caps in MSC - all seemed to test fine
- Checked AGC voltage in IF - 11v with no signal, down to 8.5v when picking up the signal generator4
- Checked supply voltage to IF stage - checks ok at 11.2v

I'm hoping someone might have some experience with these and be able to point me in the right direction, as I'm stumped right now.