fatdarryl

hey guys, looking for some advice on this one; I opened up my dsl 100 head to investigate some intermittent noise (hum) problems, to find one of the 5w, 1k grid resistors falling out into my hand! By the looks of it, the solder had never really taken to it in the factory. So, I checked it visually, and checked the resistance on the meter, all good. So I went ahead and soldered it back in. i gave it a final check in situe.. 220ohms.. so i popped it out again, checked across the connections at the board with no resistor.. around 390ohms. I've looked at my schematics, it doesnt look like that resistor should be in parrallel with anything else... Do i have a cooked board?

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plexifier

Without that resistor installed, those points should read "open" (infinite resistance) so that's not good.

Check for burn marks or other signs of arcing on the PCB (both sides). You may be able to cut them or scrape them off with a razor. Test resistance before and after re-installing the resistor to be sure.

Also, the tube that was in that socket may be damaged.

ghostdriver

+1 everything plexifier said. Can't say it better.

RickyLee

Is that on the last power valve V8? I am wondering if the C46 is shorted/fried? That is why I asked. If so, that would put the choke resistor across the two points you are measuring if I am figuring correctly for this amp.

plexifier

It's late and I'm tired so please forgive me if I'm wrong, but RickyLee has a good point here...

IF the screen grid resistor in question is R76 (on V8) AND C46 is open to DC ("shorted"), there is a potential resistive path through one half of the OT primary and R71 ("choke") as Ricky mentioned.

What is the purpose of C46 anyway?

(cue Jon for sarcastic comments here )

RickyLee

C46 is supposed to help keep parasitic's away. I snipped out the one in my TSL100. Did not have any issues with constant running to the bathroom or vomiting after that.

fatdarryl

Bang on guys.. C46 is toast! I think this is an overlooked issue from a few months back when v8 got killed.. it was somewhat of a chicken / egg situation I'm not sure if component failures caused the blow or vice versa, already changed out a few burnt out resistors around v8, but its been working fine since then on a fresh set of tubes, except for the odd bit of hum. Must have missed this as the burning was on the side nearest the valve socket. I guess I'll wait and see if It's caused any probs for the new v8 tube. Components ordered! Cheers everyone!