Marshall SE-100 DOA

[JustMike]

I've attached the schematic. It appears to be dead. The 15VAC wall wart is also dead. Someone soldered a burnt land between the Bridge rectifier and the speaker out sleeve (shown on page 2). I'm guessing maybe somebody plugged their amp to the speaker OUT instead of the IN?
 Anyway, How can I start troubleshooting this without the 15VAC pwr supply? And is there a risk of shorting out another wall wart? I do have a 9 VAC wall wart lying around...

[PRR]

I would start with a 12V AC 1A transformer *and* 100 Ohms 5W series resistor. That may not get full +/-12VDC internally, but close enough to pass signal; and won't risk more major damage.

> a burnt land between

Pictures.

[JustMike]

Hope this helps.

[JustMike]

This shows where on the schematic.

[JustMike]

So, I  replaced the 79L12 regulator (the only one I had lying around) and using a 9VAC wall wart with a 7w 120 ohm resistor, it turns on and I'm getting plus and minus 3.4ish volts on the op amps. I noticed that after a minute or two, my inline resistor is getting a little warm. I assume this is probably  to be expected?

[JustMike]

I've tried a 12vac wart with no resistor and I'm getting DC rails of -12v and +14.8v. I assume that with a 15VAC wallwart, my voltages will be higher. I replaced the 79L12, the neg. regulator. With a supply voltage of 12v giving me 14.8v I assume the +regulator needs to be replaced. Correct? Or do these things either work or die?

[shooter]

do these things either work or die?

typically yes, I have had a couple put out 1/2 desired volts an lots of heat.
I'd swap it
also check that AC isn't sneaking through on the DC side

[JustMike]

also check that AC isn't sneaking through on the DC side

Is this measured right across the output of the regulator?

[shooter]

ya, put your black lead at the C30/C32 junction, then probe the red lead at the Regulator output.  you should have your +/-12vdc and I suspect < .5VAC, hopefully closer to <.1

[JustMike]

With 12VAC supply, after I replaced the 78L12, I'm now getting 19VDC and .2VAC on it's output. This makes no sense to me.

[PRR]

Let's get all the numbers. I've marked-up the schematic. Some points should be DC, some AC, but since it "makes no sense" let's check both at all nodes.

[JustMike]

Thank you PRR. I've checked the voltages and everything appears to be normal. I don't know what was going on last night. I found a 13VAC wall wart and I'm burning it in now. In fact I've hooked it up and I'm testing all functions. It appears to be working fine with the 13v supply. Apparently 19v going into the regulator is within tolerance?
Thanks again!

[shooter]

Apparently 19v going into the regulator is within tolerance?

I think they're good to about 35v IN
glad you're up n workin

[PRR]

You need 15V to have the 3V headroom for the regulator to do its job. 18V is ample for your current wall voltage. 15VAC would make 21VDC, a bit more ample in low wall voltage situations.

[JustMike]

Good to know. Thanks again.

[JustMike]

Back from the dead...I put this to rest after I replaced the regulators. I plugged it in recently and found what's probably a related problem. The emulation circuit intermittently works. No line out. WHat I've found is that if I remove power (unplug the 15VAC supply from the back of the unit a few times) I can get signal. Sometimes distorted and when this happens, if I replug the power cord it comes back and works as it should.
 I suspect the 15V regulators, but that's what I replaced.
 How should I approach this?

[shooter]

How should I approach this?

unplug the 15VAC supply from the back of the unit a few times

I would just monitor the +/- rails for ~~ 1hr, just hook up meter, power up, check it as you're putzin.  If it's still stable, plugging/unplugging a few times tells me something's not making contact.

I'll look at the schematic some later, it's still SUNNY!

[JustMike]

 Exactly how would I monitor the rails for AC? I don't have a scope...you said meter,,,?

[shooter]

+/- rails

the DC, if it's flaking out you should "catch it" just watching, if it's good giggle/wiggle the input cable at jack to see if the jacks wonky. 

[JustMike]

More updates, It seems I can consistently get it to work when I power it up thusly; power up-nothing happens. Power down and wait about 5 seconds until I hear a pop thru the speaker (XLR out is plugged into my DAW) power back up and then it works as it should.
 All the while I'm monitoring the +rail across c30 where I get 12.4v and I see no fluctuations other than the obvious when I power down.

[shooter]

power up-nothing happens.

when nothing happens you still have the +12, correct?

I'm monitoring the +rail across c30

when I am monitoring a +/- rail I put the + lead at C30 (like you did), But I put my - lead at the C32 - 7912 junction.  Then test.

I don't have a scope

That's gonna be the game stopper if the -12 rail is solid also.
Do you have a constant signal source, 1khz sinewave kinda thing?
Does your meter measure frequency?

[JustMike]

OK, after moving my leads to the C32-7912 Junction things are a little different. When I first power it up from cold I get 11.3v. When I turn it off the voltage starts to drop at a rate of about 2v/sec. When it pops after a few seconds, the voltage spikes back up to 11.something and then continues to roll down. I repower via the switch and now I have 24v and it works.


And I don't have a scope.

 

[shooter]

the voltage spikes back up to 11.something and then continues to roll down.

sounds like an Ecap that's cranky. wholesale swapping is an excepted troubleshooting method 

[JustMike]

sounds like an Ecap that's cranky. wholesale swapping is an excepted troubleshooting method 

My thoughts too. Thanks.

[Sennen]

Not sure if anyone is out there still on this one but my SE100 has exactly the same problem - i.e. the speaker simulator section doesnt work but if I turn it off, wait for the pop noise then turn on again it comes in fine.

JustMike.......did you ever get to the bottom of this issue with your's????? Was it a bad electrolytic as suspected?