DSL40 bad PT?

[LarBal]

This is a weird one, no sound, opened up to find HT fuse fs2 open(HI power secondaries). Seemed routine, replace fuse , new tubes.
Cranking the bias pot all the way gets me only 4ma per El34. So the amp IS producing sound, in high power mode. Low power mode i got nothing.
This amp appears to use different windings for lo and hi power.

Plate voltage is only 300VDC. I disconnect the RED and BROWN (hi power secondaries) from the board, and only getting 220VAC between them.
I disconnect ALL secondaries, to double check, and now I get no reading between red and brown secondaries. Put the other secondaries back, and I get 220VAC again.
Then I pull the lo power secondaries (white and blue) from the board, along with the red and brown pulled, and I get 110VAC across the lo power secondaries, and nothing across the hi power secondaries.
I reconnect lo power secondaries to the board, and 220vAC comes back across hi power secondaries.

So basically with everything connected high power mode gives me 300VDC plates, and lo power gives me nothing, but I'm not sure where to start with this.
I think the secondary voltages are very low for a bridge rectifier, but also something else is going on that is preventing lo power from getting any voltage to the plates.

Could this PT be bad, but still put out half the voltage it needs to, to produce some sound out of the hi power mode?

[DrGonz78]

Sure more than likely some induced voltage might still occur through the wires. So you will see voltage but under load it might collapse. In your case it sounds like it is holding up but the symptoms are pointing towards a shorted turn or something.

[Helmholtz]

If something excessively loads the PT (this includes shorted turns) a fuse should blow.
So my question: Does the amp use the correct value primary and HT fuses?

[LarBal]

Well, the fusing calls for 630ma , I put a 1A in there to get er goin. I'll switch them around and see. That is a good point though....if shorted turns is the issue, it should blow another fuse eh?

[Helmholtz]

Shorted turns should blow the primary/mains fuse. What value is it?
Seems it's integrated in the IEC receptacle.

[g1]

What resistances do you measure for the 2 HT windings?

[Helmholtz]

While HT secondary wires are disconnected from circuit, please measure no-load voltages.

[Helmholtz]

Well, the fusing calls for 630ma , I put a 1A in there to get er goin. I'll switch them around and see. That is a good point though....if shorted turns is the issue, it should blow another fuse eh?

Each HT winding has its own 630mA fuse (FS2 and FS3). Did you check if FS3 is good?
An open FS3 could explain your symptoms.

[LarBal]

While HT secondary wires are disconnected from circuit, please measure no-load voltages.

That's the thing, the voltages change/disappear depending on which HT secondaries are connected to the board(or not) .
With both HT secondary sets pulled, I get no reading across the HI power set, and 120VAC across the LO power set .When I reconnect the LO power set to the board, the 220VAC comes back, across the HI power set .
This seems to be the biggest clue, as in the schematic, they are somewhat connected.
I've attached a screen shot of that section of the schematic.

FS3 and all other internal fuses in the amp are good. I need to check mains.

I'll check resistances today if I can get to it.

What should voltages be across HI HT secondaries, to get 444VDC on plates , given a bridge rectifier? Probably 330 ??

[Helmholtz]

There are 2 separate HT windings. As they are not connected, the AC voltages don't add.
Winding voltages should be measured at the PT wires before the fuses, i.e. between CN3 and CN4, and between CN5 and CN6.
For no-load voltages both windings should be disconnected from the board.

Each HT winding has its own rectifier. The DC output of the lower supply should be around 170VDC.
The floating upper supply by itself might give around 280VDC, to be measured between CN7 and CN8.
For High, both DC supplies are wired in series, so DC voltages add.

[SoulFetish]

I've swapped a few PTs in these amps before. However, in most of the DSL40s we get in, they need work done on the board to repair heat/damage from the heater bridge rectifier and other cracked joints. Plus, the way Marshall designed the chassis in most of these, the only tube that will fit is the JJ EL84s (This is such a pain in the ass that Marshall did that, by the way). So, if the customer wants more options, we disassemble and drill those holes out to clear all makes of EL84s.
As much as I hate pulling the board on these, I don't even bother hoping I can get away with not doing it in these amps. So, I've pulled all the crimp terminals off the board enough times to have screwed up the reassembly. Luckily, the service manual has a very good schematic drawing showing color codes, terminal numbers, and the circuit board is pretty cleanly labeled. Still, they double up on wire colors a lot in the primary and secondary connections leading to and from the switches, board, and transformer, so it pays to double check.

[LarBal]

There are 2 separate HT windings. As they are not connected, the AC voltages don't add.
Winding voltages should be measured at the PT wires before the fuses, i.e. between CN3 and CN4, and between CN5 and CN6.
For no-load voltages both windings should be disconnected from the board.

Each HT winding has its own rectifier. The DC output of the lower supply should be around 170VDC.
The floating upper supply by itself might give around 280VDC, to be measured between CN7 and CN8.
For High, both DC supplies are wired in series, so DC voltages add.

For the HT windings, Between CN3 and CN4 , I have 220VAC
Between CN5 and CN6 I have 140VAC.

So right there, that should tell me the PT windings are good, eh?
This is with the HT secondaries pulled from the board .

Now, when I plug in ONLY the LO secondaries(blue and white, back to their board spots(CN5 and CN6), I get only a weird reading across R6, around .5VDC
When I plug in ONLY the HI secondaries(Red and Brown) back to the board, I get proper rectified DC voltage at CN7, around 300VDC .

So essentially , the lo side rectification is not working.

Anything else I'm missing before I replace these 4 diodes?

[g1]

So essentially , the lo side rectification is not working.

Anything else I'm missing before I replace these 4 diodes?

It could still be a connection or fuse issue between the PT winding and the diodes. Verify that the AC voltage is getting to the diodes. One probe to D12 anode, one probe to D13 anode.