Marshall DSL100HR Noise/signal level/Schematic

[Mars Amp Repair]

Hello folks,
This is a newer DSL100 with HR as suffix. It has a digital board in it. I'm having some signal level issues & would like to see how the signal travels thru the amp to trace the issue. Just roughly tracing with audio tracer it seems possible that the issue may be on this piggyback pcb.
Anyone have a schemo?
Thanx, glen

[Mars Amp Repair]

OK,
Sugeda came up with schematic for the DSL100MKII which seems to match.
The 2 issues I'm having which could be separate or the same source are:
1) Generally low preamp gain in either the low or high power modes. Even with the the Gain, Vol, & Master volumes turned to full up, cannot drive the amp into clipping.
a. Signal tracing w/300mv 400Hz in the guitar input each stage up to the digital board seems to have more than adequate gain. Voltages are what I would expect. It's very difficult to figure out the input/output of the digital board as the schematic doesn't show it separate from the main board.
c. The action of the PI seems really flakey. Just connecting the scope to the different elements of the PI seems to throw its balance off. I seem to have experienced this loading by the scope before given the hi impedance of the PI.
b. I'm going to replace the digit board at this point.

2) In the hi power mode when driven to clipping, the output signal positive peak has a really weird pulse in it and after turning the volume down, the+ lower sine wave pumps up and makes a weird fizzing noise. I understand that is difficult to visualize.
a. The main B+ to the center tap of the output xformer also sags by about 80V at at full power out. I honestly never checked for this on other amps & think that seems pretty substantial at abt 20% sag.
c. As a result all the voltages to the plates of the preamp tubes likewise sag.
d. I've been thru the entire power supply & really see nothing unusual there. The amp really isn't that old & doesn't appear to have been used hard. Still has original power tubes & they look & check fine on gm & life testing on the Hickock 600A.

Guess I'll see how it all pans out after the board is replaced.
Just thinking out loud! thanx, g

[Mars Amp Repair]

The Schemo

[Delta362]

Did you verify voltages on all preamp and power tubes in steady state are correct? I had one of these in a couple months ago that signal was not making it thru. In my case, it was an open cathode resistor for V3 (R80 100k).

Under normal operation, my power tube plate voltage was around 455V, with screen grid at 445V.

[Mars Amp Repair]

Did you verify voltages on all preamp and power tubes in steady state are correct? I had one of these in a couple months ago that signal was not making it thru. In my case, it was an open cathode resistor for V3 (R80 100k).

Under normal operation, my power tube plate voltage was around 455V, with screen grid at 445V.

Yes, all the voltages make sense for a functioning preamp stage as well as output stage in static operation.
The main B+ is as you specify.
I
I'm going to get the Digital DFX board replaced and waveforms back to what they should be & then address the B+ sagging issue (if it really even is an issue).

Thanx for the info....glen

[leadfootdriver]

SOT..
Does C68 bleed off high end? What is it's purpose?

[Helmholtz]

SOT..
Does C68 bleed off high end?

Yes, but only above ~9kHz.

[Mars Amp Repair]

Could be there to prevent oscillation which at 9K for a guitar freq response could possibly be considered the same thing! g

[leadfootdriver]

Not to hijack this thread, but what's the difference then if I strapped that cap over the 100K Plate Resistor on R97 instead?

[Helmholtz]

Wiring C68 in parallel with R97 would essentially have the same effect.
Also doubling the value of C71 would have the same effect.

That is for frequencies below 300kHz.

[Helmholtz]

Yes, but only above ~9kHz.

Sorry, I made a mistake here because I didn't take into account C71.
C71 and C68 essentially act in parallel, so their values add up to 940p.
Output (source) resistance at the plate is 39k, so corner frequency is 4.3kHz.

R93 only makes a difference at RF frequencies, so might help with RF EMI.

[Mars Amp Repair]

Hey folks,
In the continuing saga of this amp:
I replaced the DFX board and resolved the rushing sound w/coming out of STBY but the low level issue was still there. Must have volumes for both channels nearly full up to get the sound level you would expect at much lower settings. If I feed the oscillator to the return jack & engage the loop, I can attain clipping with the level I would expect for the return loop injection.

I found that TR2 that follows the DFX board was reading as if the E-B junction was open. I replaced TR2 with an MPS42 instead of what's called for a MPS44. The difference between the 2 seems to be the MPS44 can handle higher E-B & other voltages. I figure if temporarily it would work fine & still checks fine.

I then realized that TR2 was actually testing as it if were reverse biased. My bahd.

I found R62 emitter resistor was open. OK, getting closer to the issue. Replaced R62 and believe it or not, not much changed. This emitter follower stage is passing signal but I guess if TR2 weren't working the signal could be sneaking thru the C39 47n cap? FYI the gain of TR2 buffer as expected is close to 1:
B: 330mv in, E:330mv out.
Here's the readings on TR2.
C- +275
B-+131
E- +134

TR3 voltages that follows this buffer read as you would expect of an NPN xistor: E:+29.2 B:+29.7 (0.51diff) C:144.0 signal in-out has plenty of gain: 500mvpp in and 3Vpp out.
I've checked all the components around this Emitter follower buffer stage & find nothing bad or caps leaking. I'm pretty much gabberflasted.

Any ideas? I need a new set of eyes on this.
Thanx, glen.

[Helmholtz]

Measure base to emitter voltage directly. Your meter will somewhat load/lower the base voltage.
Connecting a meter having an input resistance of 10M lowers the base voltage by around 3.5V
Makes your cicuit look good.

[Mars Amp Repair]

HAH! I did that for TR3 but didn't for TR2. Right you are Helholtz...TR2 is forward biased. I've been barking up the wrong tree. Time to move on.
It appears that the return jack also also feeds TR2 thru R68 above. I didn't notice this as Marshall in all their wisdom did not think to provide an interconnect diagram.
Since it appears that Return levels are not consistent between models & brands, perhaps the issue is actually after TR2 & 3 as the other preamp ckts seem to have adequate gain.
This is getting rather stupid.
Thanx Hemholtz. Another set of eyes always nice since I'm the only tech here at 12th Fret Music. glen