Marshall DSL100 Humming

[Alex R]

Thanks Enzo, clear and concise as always. Somehow it hadn't twigged that 'W' might stand for 'winding'... I guess I spend too much time looking at those beautifully drawn old Fender schematics that are so easy to understand.

Why do they feed the bridge through caps? I guess at 22u they have to be non-polarised electros (can't bring myself to write bipolar, brings up a whole new scenario).

And in that case, bajaman, I guess it might be worth looking at C43 and C44? And even the diodes, though I guess they will be ok.

This amp I think doesn't have the little line-out feed cap that can leak bias to ground? That's the TSL?

[John G]

Hi, I have experienced the hum and bias drift problems in many of the DSL/TSL series amps.
The hum I believe (front panel deep/boost ) is caused buy magnetic induction from the closely positioned mains transformer ?? maybe earth loop ?? Pull the con that runs from the front board to the o/p board and see.
The bias drift I believe is caused by leakage fron the Splitter anode resistors(hi positive volts ) close proximity to the bias tracks and components ( neg volts ).
Cleaning can help but I have found that if I either lift the spitter anode resistors and coupling caps of the board or leave them in place and disconnect the bias tracks at the gids (cut ) and run new grid stoppers up in the air and run hard wires to the bias adjust pionts, like you would do in and old poit to point Marshall.
Seems to work had no returns, makes for not a pretty sight........but.
Just trace the tortured path of the bias and signal........designd by an artist a tech never went near it.
John G

[rf7]

The hum I believe (front panel deep/boost ) is caused buy magnetic induction from the closely positioned mains transformer ?? maybe earth loop ??

How do you fix the problem if it is magnetic induction? Do you add some shielding?

Pull the con that runs from the front board to the o/p board and see.

Which connector are you referring to, for that?

[John G]

Hi,
I'm afraid that none of my customers have been willing to pay me to research the " Hum " problem, and I havn't had the time to persue with it myself.
Sure, some sheilding would be a good place to start, at least two of my customers have had me disconnect the con12 plug at the power amp end and tie it out of the way as they didn't use those two controls.
The others have just put up with the hum and found that under live playing situations it wasn't a big issue.
Sorry I can't help more at this stage.
John G

[wakeskater]

Hi, I have experienced the hum and bias drift problems in many of the DSL/TSL series amps.
The hum I believe (front panel deep/boost ) is caused buy magnetic induction from the closely positioned mains transformer ?? maybe earth loop ?? Pull the con that runs from the front board to the o/p board and see.
The bias drift I believe is caused by leakage fron the Splitter anode resistors(hi positive volts ) close proximity to the bias tracks and components ( neg volts ).
Cleaning can help but I have found that if I either lift the spitter anode resistors and coupling caps of the board or leave them in place and disconnect the bias tracks at the gids (cut ) and run new grid stoppers up in the air and run hard wires to the bias adjust pionts, like you would do in and old poit to point Marshall.
Seems to work had no returns, makes for not a pretty sight........but.
Just trace the tortured path of the bias and signal........designd by an artist a tech never went near it.
John G

I've got a DSL100 that the bias drifts up on. I am not real familiar with the tech terms though. Can you tell me what the splitter anode resistors and coupling caps are? I have a schematic I can check out, but I dont know what the parts are called.

Also are you saying you raise these off the board and hardwire the connection between the board and bias adjustments, or do you only hardwire in the bias adjustment if you cut the bias track.

[John G]

Hi,
V4 is the phase splitter R45/R46 are the anode resistors, the coupling caps are C22/C23.
The problem is not with the schematic it's with the PCB layout, in that the Hi voltage tracks are positioned too close to the grid negitive voltage circuitry.
I lifted all the grid components and wired as you would a point to point, cutting the PC tracks at the Tube grid pin.
Cheers
John
PS If you are inexperienced then I suggest you take it to your Favourite tech.

[wakeskater]

Hi,
V4 is the phase splitter R45/R46 are the anode resistors, the coupling caps are C22/C23.
The problem is not with the schematic it's with the PCB layout, in that the Hi voltage tracks are positioned too close to the grid negitive voltage circuitry.
I lifted all the grid components and wired as you would a point to point, cutting the PC tracks at the Tube grid pin.
Cheers
John
PS If you are inexperienced then I suggest you take it to your Favourite tech.

Thanks for the reply.

On the amplifier I have the bias drifts up on one side and the other side stays steady.

The longer the amp is on the higher the voltage goes from around .050V when first turned on, and up to over .400 if it is left on long enough.

Do you think your bias modification would help this amp, or is it likely that this amp has other problems?

[John G]

Hi,
Yes ! more often the leakage only effects one pair, but I have seen both drift.
If you trace the grid circuit back from tube to the Splitter anode resistor you will find that there are several points that come close to the Hi volts but the most obvious is close to where the anode resistors sit.
John

[j45]

Just got one in,with similar problems.bias resistor open(R9) .C22 burned.
Replaced tubes,el34s,new.

Runaway bias, one side will climb to 250mv!!, shut amp down at that point.
Esr'd all caps, check like new. Resistors good. Did not solder board yet.

Any suggestions?

Thanks, Marv

[Enzo]

Hi Marv, welcome to the forum.

My first suggestion is to start a new thread for your amp. We don't want to get the discussion going with two amps involved in one thread because pretty soon no one will now which we are talking about.

If your resistor burnt, then first thing to suspect is a shorted power tube caused the damage. Pull the power tubes, power up, and monitor pin 5 of the power tube sockets and see if any of them drift away from the bias voltage.

Likewise try a different set of power tubes in the amp. DOn't have to be new, just ones that work.

Again monitor the bias voltage at each tube, especially the tubes that drift hot.

[j45]

Forum,Sorry

Hi Enzo,

Obvious I am new to this sort of communication, I will "clean up my act" Ha.

Thanks for your quick response and comment.I am now testing the board with tubes removed.Originally this unit had a blown tube as well. I am now leaning towards a badly arced tube socket or trace, as I am getting strange readings from V7.Will let you know.

Thanks again ,
marv

[j45]

Enzo,

Removed V7 and V8 sockets,mesured pinouts,all checked good.

Re-installed, and powered up without tubes,

Pin5 readings,

V5- 61ma V6-60ma V7- 520ma V8- 535ma.

ouch!

[Enzo]

Let's start that new thread now.

[j45]

Marshall fixed

Sorry Enzo,

I do not know how to start a new thread.

Four brand new el34s.One was bad! only when it got hot.Measuring pin 5(which I did not on previous attempt) I did not have the bias board plugged in!!! I am aTV tech,...Anyhow by switching tubes and monitoring voltage at pin 5 , I found the bad tube.

Please accept my apology for not posting a thread, I dont understand how!!
Thank you for yor help!!!!

Marv

[Enzo]

It is easy, when you start at the menu for a particular forum, for example this Maintenance and Repair forum, ther is a button upper left that says New Thread. CLick it, and a posting box opens. Post your opening post and don't forget to fill in the title.

TVs? My condolences. I work on video monitors for the coin-op arcade games. They are like TVs without the RF parts or the damn plastic cabinet. I know what it is like when you forget to connect the ground strap from the outside of the CRT. SO don't feel bad about the bias board. WOn't do it again, will you?