noisey pic in Roundie philco

[old_coot88]

A tip worth bookmarking.. with any kind of "noisy pic" or a 'overload' symptom, try the AGC control first, and also look for an RF AGC adjustment if a set is SS or hybrid.

[DaveWM]

roger old coot. I should have just scoped the AGC line, I have chased things before that had to do with AGC one of which was hash on it from open caps, etc...

[DaveWM]

ok noise is worked out but the pic was dim. I could make it ok by adj the CRT bias, but should not need that as I was able to get all the guns to show in the setup mode at the lowest setting. I checked the CRT pins and sure enough the cathodes were at least 50v too high (was 240v shb 190v).

So looks like a video out tube. I checked and yes the plate voltage was too high, checking the Cathode and again too high (about 5v not the 2.5 it should have been). So if the video out tube plate is high, and since its directly coupled to the CRT cathodes, that explains the bias problem and the dim pic. I did some resistance checks and it looks like a 100ohm resistor on the bottom of the brightness control may have drifted. Its mount up on the pot on the control panel so I will need to pull it out to check. Hopefully this will resolve the dim pic.

I also have a intermittant blue screen issue, Its like the G2 drops down about 10% all by it self. I will be monitoring the G2 voltage and see if that is whats going on. there are 100k resistors in series with the G2 pots so maybe the blue one is getting intermittant. I will prob just pull the chassis and replace all 3 of them while I am at it. Lastly there was a .1 cap in the grid circuit of the video out coupling the delay line to the grid. I will prob replace it as well since I seem to be having bias issues in that tube.

I will also check the PCB track for any break around that G2 lead but I am just guessing that is what is causing the intermittant drop down of blue in the crt (it like someone just turns it down a few degrees, but its quick not gradual, and it will come back and then do it again). I dont feel like breaking out the cold juice just yet.

[DaveWM]

pulled the tuner out, the 100ohm was 300ohm off that pot. I need to go ahead an pull the entire chassis, will replace those other parts (the screen resistors and that coupling cap. there are some .001 disc caps across the G2 pots that I am tempted to replace while I have it out, just in case one of those is acting up causing that intermittant problem. The only thing is I never really captured it on the VTVM to confirm it was the G2 voltage that was doing it. could have been the G1 or the cathode I suppose. Just getting tired of yanking the chassis as you have to pull the tuner with it.

[DaveWM]

ah now that is much better. the grey scale setup worked as it should, where as before the incorrect bias of the video out tube resulted in too high a plate voltage, that when connected thru the serice switch in the normal mode would tend to cut off the CRT. Now with the CRT bias set at the min, I can put it in service mode, just bring up each gun, and then go to normal mode and get a nice bright pic with correct grey scale. while at it I put the old focus coil back in and now have a best focus point, before in never quite got there.

I still need to double check all my HV setup for current and voltage, but it now has a very good bright pic, great color, and NO noise in the pic.

[DaveWM]

I will play with it some more later and see if the intermittant blue in the grey scale was fixed by replacing the G2 100k resistors. I only replace the one .001 disc cap on the blue since I only had Y5U caps and not the Z5P that it had originally. Since the other guns were ok I left them be (still replaced all the 100ks just so they were balanced, the were between 108k and 120k, but I was thinking they could have been noisy).

[DaveWM]

HV is down to around 23.5 holding pretty steady. HOT current is a bit high on some bright scenes, but If I switch to serv to cut the beam curent its right at about 200ma, up to about 240ma for normal viewing. I may try some different HOT's just to see if I can get that down some. I forgot to check the resistors in the HOT feed back circuit, darn it....