Marshall DSL 100H - wires instead of fuzes - weird behavior

[Spunko]

Hello, a friend of mine bought this Marshall DSL 100h kinda cheap, not working, and acting weird. These are the series of events for the initial test:

1.- First power-on was using a bulb limiter that inmediately light at max bright, so I suspected a short somewhere.

2.- Took the chasis out, visually inspect and noticed FS2 (500mA) was lifted from one clip, I pushed it back to its clip, tested again and the bulb limiter didn't light bright this time, that was weird. Kept checking and noticed mains fuze (4A) and FS1 (1A) where blown and soldered a wire between their terminals

3.- Replaced the fuzes with the correct ones, using a bulb limiter powered the amp and when engaged the standby switch the amp didn't sound and the pilot light from the power swicth went off (no light), the channel switching didn't work either, but as soon as I swicthed stand by off, the pilot light went on again. Also tested the pre amp using the FX loop with another power amp and no sound. All this time the power limiter bulb didn't light bright.

4.- Took out the 4 power tubes and tested only with preamp tubes, this time the amp did work, I had sound on the preamp sent to another power amp, and also the channel switching was working. So I tested with another quad set of EL34's but the same happened in point 3, no sound, no channel switching, no pilot light.

5.- This time I tested with 2 of the original power tubes and the amp seemed to work fine again, then took those out and put the other pair and again it worked in the middle or in the ends, but when the 4 power tubes are installed the amp behaves like point 3.

6.- One day have passed. The amp is sitting without power tubes, only pre amp tubes. Always with the bulb limiter, I powered on the amp today just for my pleasure and channel swicthing didn't work, and I'm pretty sure I didn't hear the relays acting. Then I powered the amp off and on again and this time the channel did swicth normally. So this time it failed without the power tubes.

Does anyone have experince with this type of fault, or with this amp, that can help me?
Thanks.

 

[TheKman76]

First thing to note is that a bulb limiter is going to effect the voltage available to all the control electronics. So, power indicator lamp, channel switching, even the muting circuit may or may not work correctly under this condition. Almost nothing will work predictably with a bulb limiter on this amp.

@Dogs of Doom Move to the Workbench please, sir?

 

[Jon Snell]

I would visually inspect the board and components for obvious signs of stress and heat issues then replace all fuses with the correct type and value.
Power it on and observe.
Check for HT on W5 and other test points.

From your description it sounds like a new set of matched output valves with the bias voltage set to the correct value, (see service manual for values) and it will be a working amplifier.

 

[PelliX]

First thing to note is that a bulb limiter is going to effect the voltage available to all the control electronics. So, power indicator lamp, channel switching, even the muting circuit may or may not work correctly under this condition. Almost nothing will work predictably with a bulb limiter on this amp.

This ^. The logic section of the amp has to function in order to actually pass signal, etc.

You mentioned the pilot light - those amps have an illuminated power switch, no? If so, disregard it, it's just a little filament that is fed straight from the mains. When/as they get older they may not fire up without sufficient voltage (so a slight decrease may cause this) or fail alltogether and light up from time to time just to remind/annoy you (like mine). It's cosmetic though.

1.- First power-on was using a bulb limiter that inmediately light at max bright, so I suspected a short somewhere.

Could also be the caps taking an initial charge from being completely empty, just saying.

5.- This time I tested with 2 of the original power tubes and the amp seemed to work fine again, then took those out and put the other pair and again it worked in the middle or in the ends, but when the 4 power tubes are installed the amp behaves like point 3.

You're probably best off approaching these problems separately; the logic section requires a given voltage (and it has to be fairly stable) for it to function. That's the unmuting of the amp initially, channel switching and so on. The power section with all 4 valves installed doesn't trip any fuses or redplate? Good, then verify the bias in a safe/sane range and proceed with the rest.

The DSP board in the last pic is probably not the culprit, but do check the Vcc on the IC beneath labeled as "DSL40". The schematic you posted is of a JCM2000 DSL100, though the amp looks to me like a DSL100HR (newer gen). You can find the schematics here on the forum/Internet or I'll post them later on. IIRC there's a cap and a Zener diode supplying the Vcc for the IC that can go south (seen this before).

Best of luck.

 

[Pete Farrington]

First power-on was using a bulb limiter

Good decision to use a light bulb limiter, but what’s the bulb wattage in it?
For the amp to have any chance of operating normally (and that’s without the logic circuits), it needs to be twice the amp’s rated power output, so 200W here.
Once the amp has proved good on the limiter ie no shorts, it either needs taking off the limiter, or the limiter needs a really high power bulb fitting, eg 300W here, if you’re still concerned about something shorting.

 

[rocker68]

All the images seem to be from Vietnam build DSL 100 H ? ( mk 2 ) . ( Not JCM 2000 or DSL hr ) . Internal bias test points on a DSL 100 HR ? Does it even matter?

 

[Spunko]

First thing to note is that a bulb limiter is going to effect the voltage available to all the control electronics. So, power indicator lamp, channel switching, even the muting circuit may or may not work correctly under this condition. Almost nothing will work predictably with a bulb limiter on this amp.

@Dogs of Doom Move to the Workbench please, sir?

Thank you very much, that was the problem all along. I was trying to be safe but I have learned my lesson.
The amp is working great without the bulb limiter. I'm pretty sure what blow the fuze was a poorly biased amp. One side was reading 40mA and the other 120mA, also those 2 power tubes have signs of over heating. Now both sides are at 86mA and 84mA, couldn't get to 90 mA tho.

The amp has been ON for like 5 hours without any problem at all, no blown fuses =)

 

[TheKman76]

Now both sides are at 86mA and 84mA, couldn't get to 90 mA tho.

This seems a little high, but I'm unfamiliar with the DSL100C. Do check your HT voltage and aim to bias at 60-70% max dissipation.

And you're welcome.

 

[Spunko]

Now both sides are at 86mA and 84mA, couldn't get to 90 mA tho.

Each value is for a pair, it issupposed to be between 40mA to 45mA per tube. I got those values from the test point on the amp.
I also have tube probes on the middle tubes. HT is at 437 VDC. Tubes are not that matched, and the probes show one tube is running at ~39mA +/-, the other at 42mA +/-

 

[TheKman76]

Each value is for a pair, it issupposed to be between 40mA to 45mA per tube

That's a little hotter than I'd run them. 45mA is around 80% dissipation. I'd be aiming for 35mA.