Packard Bell AGC issue

[tom.j.fla]

If said P.B. is early to mid 60's, then look at Sams 546-1 and 614-2. I don't remember ever seeing one with individual channel lights. If VHF only then look at 546-1 first. All the best, Tom

[DaveWM]

look around for any AGC filter caps .1 ish and check for any high ohm resistors going back to the B+ from around the plate of the AGC. the grid bias looks like -10 which may be about right. If your reading 600v PP then the output should be positive not negative.

[madRad45]

Looks like 614-2 might be the correct Sams, which is for Chassis 98C6. I see the "Computer Lamp Switch" for the individual channel lamps on the schematic - which I just scanned. PM me the best way to get it to you.

[miniman82]

Cool, I'll send you my email.


Just had a real duh moment in the garage: in case the schematic don't match, follow the wires! So I followed the grid of the AGC tube to the very next pot, which happened to be on the back of the chassis instead of the tube side like my book says. That ended up being the AGC pot and now it works, so I guess the other would have to be the noise inverter? No idea, but something in vertical isn't right now so I have to tackle that next. Thanks for the push in the right direction, sometimes I guess I need to talk things out to get right. lol

[DaveWM]

IIRC there is a RC network in there that you want to check for drifted resistors, its in the feedback loop, look for where the sync pulse is fed into the vert multivibrator for a drifted resistor. You say vert issue, lack of height? sync? lin? Of course you want to check the can cap on the cathode 1st as its a likely suspect if you are short on scan.

oh the resistor I am remembering that gave me so much trouble was a 39k from the cathode (pin 3)of the 6GF7 to output from the vert integrator network (just before the sync pulse was coupled to the plate of the vert osc thru a cap). that one was pretty critical.

[miniman82]

It's got a really bad vertical fold, but I've already replaced all the electrolytics. The vertical tube was really weak, so it got replaced with an NOS one same thing with the audio out tube. Not that I care about audio at this point, but it is one of those chassis that forms a voltage divider through the 6AQ5 audio tube, so I figured why not take care of it. I don't know if the linearity pot has just gone bad or I'm not adjusting the right one, but no matter what I do right now I can't get it to look right. I'm tempted to blame the output transformer, but I don't have another I can sub in at the moment. There's also a ,001/2kv cap still on the PCB that's original, I suppose that could be bad. It looks like it feeds vertical pulses from the plate of the output back to the multivibrator, so it's probably semi critical. I'll have a look at the resistors and swap out that cap.

[DaveWM]

if it has those blue plastic caps they are prob ok, all mine were. look for that 39k resistor and any others that are 5% marked. IIRC it was pretty resistor drift that caused the issue. really doubt a vert out xformer, check the vert osc plate supply from boost, look for drifted high 2.2 meg and check pot for height.

You do have the conv board plugged in right?

[miniman82]

Conv board wasn't plugged in, does it matter? I did see a note in my fold out book for the 15 and 16 chassis that without it height won't be full. I'll give it a shot with the board plugged in.