marshall dsl 401

[Valvehead]

cant find this one -- i need the DL40-60-00 version , dsl401


email is 493dart at gmail com

[doctor]

cant find this one -- i need the DL40-60-00 version , dsl401


email is 493dart at gmail com

I have only the DL40-60-02 schematic, if it will help you...

[Enzo]

As far as I know, the only difference between the -00 and the -02 is the addition of a couple mute transistors, T15 and T16 plus a couple small parts at their gates. So unless your problem involves them specifically, the same schematics will work.

[Valvehead]

ok-- the 02 scem shows 2? pots for the bias. my amp has only 1 . and thats just one of the problems im having with the amp (red plating)

[Enzo]

No, the -02 drawing has only one bias control, PR1, top middle area of page 2 in the power supply.

The "2" pots you see are VR12a and VR12b. They are the same control, it is a "stereo" or dual control. In fact it is the master volume control on the front panel. The bias voltage does run THROUGH them to the power tube grids, but they do not adjust it. If one of them opens, the tubes on that side will lose bias of course.

If you are red plating, start at the start. Pull your power tubes, fire up the amp and probe each power tube socket. Is there good bias voltage on every pin 5?

Are all four tubes redplating? Or just one? Or just two? Is it always the same socket or sockets no matter which tube is in it? Or does the red plating stay with the particular tube if you move it socket to socket?

You do have a bias test point inside the amp, but please note the sense resistor is 10 ohms not 1 ohms, so the voltages will be 10x what they are on a 1 ohm.

[Valvehead]

0k---bias is good at the sockets

its only 2 tubes red plating--- V5/ V6. ive swapped tubes.---its in the circuit for sure. v7/v8 are fine.

im measuring at the OT (shunt)---the good pair will sit at 35ma----the red plate pair will start normal, and gradually rise from 35ma up to 110ma and more in about 2 minutes , as the amp idles...i shut it down when i see the red plates. Its not the tubes. The bias pot is set at full " lowest bias" i can get .

[Enzo]

And the bias voltage is the same for all four sockets? What voltage is on those pins 5? While t it, verify you also have B+ on BOTH pins 9 and 7 of every power tube.


You are watching the current rise over a couple minutes. Is your meter clipped on there the whole time? May I assume the tubes will red plate even without the meter attached?

Got a scope to check for RF oscillation?

[g1]

Have you resoldered the sockets?
When you checked the bias voltage with the tubes out (pin 2, not pin 5, Enzo is probably thinking 6L6 or EL34) did you wait as long as it usually takes before the tubes red plate? The voltage may be there for a while before it falls off.
A bad coupling cap might do this.

[Enzo]

Oh, DUH, I am so used to saying pin 5, I just said it automatically. Sorry. I knew they were EL84s I just lost focus..

[Valvehead]

I might add the amp has 2 other problems--not sure if they are related : low freq hum and almost no volume. Ive pulled the board and checked solder joints, cleaned and tested jacks.I have new filter caps on the way and Im thinking the OT might be bad(which i cant find) .

So anyway, yes the 2 tubes red plate without the meter.

with no tubes, pin 2 starts at -4v and creeps up to -9v (this is at the 2 suspect sockets)

i stuck the tubes in and pin 2 starts at -6v and creeps DOWN to -3v over 2 minutes and thats when they red plate

the other sockets stay stable at -16v

[g1]

Ok, so the bias at the sockets is NOT good, it should be around -16V like the other side. Try lifting one end of C79 and see if you then get proper bias at the socket.

[Valvehead]

ok--i have to pull the board to do it, so i will throw a new cap in

[Valvehead]

changed that cap --- no difference. would that 47p thats near it cause this if its open ?

[g1]

I don't think the 47pf would affect the bias. But as Enzo mentioned, the bias runs through the master volume pot. So check for bad solder or a bad side on that dual pot. Looks like the master pot should have the bias voltage at all 4 outside legs.

Edit: Sorry, error. The master pot should have the bias voltage at the 2 legs connected to vbias and the 2 wiper (center) legs.

[Valvehead]

joints looked ok--i re-flowed them. there are 7 pins. i do see both of those voltages on some of the pins. possibly a bad pot ?? maybe thats the cause of the no volume issue if so


edit -- i found a factory marshall replacement but it has 4 pins. maybe im not seeing some jumpers or something--ill have to pull the board for the 57th time to double check