HELP ME SAVE 74 SUPERLEAD!!

[neikeel]

Having lower preamp voltages than expected and a high voltage on the presence suggests a partial short to ground somewhere between V2 and the PI. I would have suspected the PI tail cap (the one hooked up to a 1 meg and the 10k tail resistor that goes to the presence pot, rather than the cap on the pot itself.

Trace back from the presence pot to the purple wire to the board, then to the tail of the 10k resistor, then the 1Meg and compare.

I think Plan A was to check all the resistors to be in spec on the first pass (plate resistors do drift up and can become noisy, jhave seen a few V1 resistors open circuit, typically the odd Iskra on a hard working amp.

[heckitar66]

That was definitely plan A, I must have missed some. Also I'm getting some odd resistance across s few resistors which I attributed to them still being in the circuit. For instance my 100k nfb reads 4.7 in circuit but when I removed it and checked it I got 100k.

I've been all over the PI multiple times and seem to have voltages that are correct. One of the leads actually broke off the presence cap last night so I have to replace it now. I'll keep checking for a partial ground someplace around there. I don't know what else it could be.

[bigsmitty]

Get it goin yet?

[heckitar66]

Not yet. I ordered a .1 cap for the presence pot but I'm stumped about the low DC at v2. I've been looking online for a schematic that includes voltage but I can't find one.

Thanks for asking. Any suggestions?

[Doug H]

I assume you tested the 1k cathode resistor? I think you can actually measure the R on that one in place.

[heckitar66]

Yep. I got .971. I'm looking for a schematic with voltages on it but I can't find one. Seems like it would help alot.

[heckitar66]

I found a Marshall service manual online with some voltages listed!! Hopefully it will lead me to something tomorrow. I'll post what I find asap.

[neikeel]

http://www.kbapps.com/audio/schematics/ ... pchart.gif

[heckitar66]

Thanks, Neikeel. The service manual I downloaded has some additional voltages listed that might help pinpoint the problem. That is, might help you guys pinpoint the problem. I checked my amp against the service manual last night and it made me think even my PI readings are still off.

I'll post my readings as soon as I can tonight.

[heckitar66]

Okay so here's what I have at my preamp tubes.

Pin 1. 3. 6. 8.

V1. 135. .76. 167. 1.5

V2. 66. 1.9. 254. 78

V3. 204. 33. 187. 33

These readings are with tubes installed. Would it be helpful to know readings without tubes?

[neikeel]

Why are the plate voltages so low on V2, they are fed by the 100k resistor that sits on the socket?

You are dropping 190v across it (should be half of that voltage drop). What is its value (marked and measured are the connections ok, have you tried a new tube in that position?

This may have been a problem with the amp before (someone replaced all the 10k carbon comps at some stage (wonder if they wired in the filter cans properly (half the preamp can is tied to the same point as the feed to V2) but then your V1 voltages are quite close so it has to be downstream of that resistor that sits on V2.

[heckitar66]

It's marked 100k and it read 99.9 when I installed it. I'll check it again this afternoon. I have not tried different tubes since I changed the resistor. I'll do that today too as well as checking the filter cap wiring. Thanks!

I did notice some upward drift in the 10k's, around 10.12 ohms.

[neikeel]

It's marked 100k and it read 99.9 when I installed it. I'll check it again this afternoon. I have not tried different tubes since I changed the resistor. I'll do that today too as well as checking the filter cap wiring. Thanks!

I did notice some upward drift in the 10k's, around 10.12 ohms.

Maybe check the link between the blue wire of the board to the pot as well and the quality of solder joint where the wire folds back through pin 1 to 7 (a duff tube might do it or a fault in the V2 wiring/components dragging the plate voltage down)

[Doug H]

When was the last time you tried a different tube in v2?

Since the CF is direct coupled, could something wrong there ripple backwards and effect the voltage on the other half of v2?

What if the V2 grids were wired backwards?

edit: guess that would be possible if there were two grid wires on v2 in the first place lol.

makes no sense, I'm really curious to find out what the deal is here.

[Doug H]

There has to be something wrong with the grid of the CF, maybe the socket? If the 100k r there was fried who knows what kind of spike went through there. If the tube is good I'd suspect the socket next. Cheap to verify but the heaters are a PITA.