marshall dsl 401

[g1]

Resistance checks of the pot should tell you if the one half is bad.
Have you tried cleaning it?

[Valvehead]

yes--cleaned. ive never done this but how do you check a 4 pin pot ? pins to ground ?


lets assume the pot is good , what would be the next step? maybe i can do some more checking before pulling the pcb again

[g1]

Not sure what else it could be. You have correct voltage at vbias (V7 & V8 pin2 voltage proves this), but it is not getting to pin2 of V5 & V6. All there is between there is the master pot (VR12B) and R84 & R85, plus solder connections for those 3 items.
By the way, are you saying that the master pot on that board only has 4 legs?

[g1]

Ok, here is the pot: Marshall® Dual-Pot, 2x A200K, PC Mount, D-Shaft - Marshall style and Pots,
It has 7 legs, one of which is apparently a tap that goes to the "screen d" connection which goes to VR11.

[Valvehead]

r84/85 are good. the pot does have 7 pins ! The replacement i found shows 4 . ?

link to new pot : Marshall Amp Parts


heres a pic of mine -

[Enzo]

Isn't it a 5-pin pot or even 7-pin? As g said, an ohm meter will tell you if the pot is bad. Don;t know which oin is wwhich? Then use the other parts as test points. If you want to know if the VR12b section is open, measure resistance between the bias supply and R115. Want to know if the wiper works, then measure from the bias supply to R84 or R85, then turn the knob up and down and see if it goes from very low up to about 200k. That way I don;t have to know which pin is which on the pot.

Don;t guess and throw parts at it. If that 47p cap shorted all it would do would be put a 2M2 resistor across the end terminals of the pot.

Bias voltage should be on ALL the pins of the pot. It comes into the center, and exits via the wipers to the tube grids, but the voltage nonethe4less will be on the outer ends too. You changed C79, yes? But did you lift C80 just to see?

You are losing the bias voltage to the red plating tubes. APparently it sits there doing that, so it is not a fleeting momentary problem, you can sit there and watch it. So track it down. SOunds like your bias is -16v. SO there ought to be -16v coming into the center between the pot halves. What voltage is on the top end of VR12b, the end connected to R132. Now if we suspect the C79 route (maybe not the cap itself, but SOMETHING up there), check the voltage at BOTH ends of R115. If nothing is coming through C79, then you will have the same voltage at both ends of it. Whether it is -16v or -2v, if it is the same at both ends of the resistor, then nothing is coming through the resistor to fight the bias supply. On the other hand if we have something like -6v at one end of R115 and -2v at the other, then clearly the fight is on within it.

With power off, pull the red tubes, and measure resistance from pin2 of V5 and pin 2 of V6. Should see 20k. COmpare to the good side if you like. We just verified that not only are both R84 and R85 good, they are also making complete circuits to the sockets. Now if that is OK, power up without the tubes, and see that both those pins 2 have the SAME voltage, whatever it is. If they differ, then one socket has something funny about it. Now turn the volume control all the way up to put the wiper up at the R115 end. Measure voltage on pinsw 2 AND on R115. They ought to be pretty much the same. Are they?


And one other warning. Make sure the tubes we are calling V5,6 really are the same ones in the schematic. IN other words if you take a resistance reading from pin 2 of V5 to R115, it ought to read 10k with the volume maxed. If you get instead more like 400k, then we should be looking at C80 and friends. These tests take a lot longer to describe than to do.

[Enzo]

Aham you guys posted a few while I was writing that.

[Enzo]

Here, check this out:

Bourns - PTV142B20E120AA204 - Variable Resistors - Potentiometers - Allied Electronics

[g1]

What he said ^^^^ (post22)
I linked the correct pot in post #19, none of the ones you linked are dual pots.
Can you not somehow run this thing without having to keep taking the board out? Prop it up somehow and insulate it from touching anything? You may have to connect the ground to the chassis but that should be the only connection you have to make.
Do all the resistance checks Enzo mentioned, that should tell you if the pot is bad or not. However, here is another way to check the pot: find which leg of the pot connects to R84 and R85. Jumper that pot leg to the 2 legs on the left in your pic (the ones soldered together) with a short wire. Now see if you get your bias at pin2 of V5 & V6.

[Valvehead]

ok---thanks in advance guys !!

resistance between bias supply and r115 is ok.

lifted C80 = no change.

volts at R132 , -14v and -17v .

r115 has -17v and -16v


resistance from v5/6 pins 2 , 20K

volts at pin 2 v5/6 is -5v , volts at pin2 v7/8 is -17v

R115 to pins 2 of v5/6 is OPEN , r115 to v7/8 is 378K ohms


testing from bias pot lugs to R84/85 = OPEN





hope i didnt miss anything enzo

[Valvehead]

Here, check this out:

Bourns - PTV142B20E120AA204 - Variable Resistors - Potentiometers - Allied Electronics

[ATTACH=CONFIG]23602[/ATTACH]

ok---the Marshall is 11mm according to that link i posted , also there is a linear and an audio taper version of the same pot.

[Valvehead]

G-one--

if you look at my pic, pin 3 goes to r86/87

pin 4 goes to r 84/85


this might be stupid but, why cant i just run a jumper from r84/85 to r86/87 ??


and why in the hell do they have the bias running thru the master volume ??

[Enzo]

R115 to pins 2 of v5/6 is OPEN

And there it is. Pin 2 to R115 is a route through R84 or 85 through VR12b and to R115. That path is open. Do pin 2 of V7,8 to R116 and compare. Not sure just where the open is, but it is not doing you any favors. Sounds like the wiper on that section is bad. But it is also conceivable the traces from R84/85 to the wiper is broken somewhere.

Why can't you short the R84/85 to R86/87? because it is a push pull amp, you'd be shorting the push to the pull. You'd be connecting the two sides of the phase splitter together.

Why run the bias voltage through the pot? Eh, why not? You are struggling with the procedure of troubleshooting, if the master volume was separate from bias, because this is a post PI master volume you'd have to add more caps to isolate it. You could just as easily have a bad master volume in some other amp, and the one side would lack drive, and you'd be fussing with trying to find why it sounded funny. It is the procedure that is confusing, not the circuit. At least that is my assessment.

g-one
The pot you linked in post 19 is 14mm according to the web page. I think it is just the single pots that are 11mm. The one I just linked is 14mm. I put that part up not because it is a drop in replacement, I believe it is so I stock them, but mainly just to show and tell the type pot it is. The ALlied site should have a data sheet to download which would show the internal connections of the legs.

[Valvehead]

ok--i have the board out--testing the traces --and im getting continuity from the pot lugs all the way to all 4 resistors--- 84/85/86/87 .

sorry man--im getting confused here......so it would have to be the pot itself ??

[Enzo]

Sounds like it. SOunds like your wiper is not making contact with the resistive track inside.