Marshall MA100C problem

[Blackwellefty]

A friend gave me this amp and the thing does not turn on. The main fuses are good. I believe something else is bad causing the problem. Any info would be very helpful. I took it to a certified dealer and they would not even touch it because they are having issues with Marshall Company. So I am like stuck. I may have to scrape it if I can not find schematics. Thanks

 

[73's de Edd]

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Sir Blackwellefty . . . . . .


Inspect and confirm:

First how's about some info refresh, doesn't that unit use 4 larger power output EL34 tubes and 3 smaller support tubes and then the rest of its electronics is a mix-match of some 4 or so integrated circuits and the rest is transistors.
Considering my looking at the the base model M100 and with it probably being basically 98% like your "C" version.


73's de Edd

 

[Blackwellefty]

Thank you for your response. Yes it has 4 EL34 power tubes and 3 preamp tubes.

 

[Blackwellefty]

I just talk to the guy that gave me the amp he forgot to mention that the tubes where red plating and he tried to test it himself and shorted something out. He said he heard a pop and then amp went dead.

 

[Blackwellefty]

OK thanks.

 

[Blackwellefty]

Amp went out in the trash this morning thank you for all your help.

 

[73's de Edd]

I hope it was / is retrievable as I just got schematic in hand and was was just about to help you.
THAT is quite a piece of electronics to TRASH !

73s de Edd

 

[Blackwellefty]

OK I just retrieved it. Thanks

 

[73's de Edd]

.


Sir Blackwellefty . . . . . . .Hmmmmmmmm . . .now . . . shouldn't that be Lefty Blackwell ?


First operation at hand will be to get those tubes wrapped up in towels and put in a shoebox. You will need DEEEEEP pockets, to replace those EL34s if rolling off the bench and to the floor.

I believe that you will find all of the fuses intact in the fuse holders that you have shown in your pic.
In a no power applied to the amp condition . . . .take DVM in hand and place OHMS mode and lowest range and short its leads together to associate the meter reading viewed to the shorted condition and then probe across each of the fuses and expect a like reading.
I also believe that the prior person's error was in probably getting his metering probes across some high voltage level . . . . over in the tube section . . . . which can present a big BANG but with its most adverse
effect being on the erose pitted gaps thereby produced in his probe.

INITIAL TEST PROCEDURE:

This will be in a powered up situation, but with no tubes installed.
Initially plug in and switch on and observe the components side to watch out for any adverse affects, ready to immediately power off, is so experienced.
If all is well you initially have the tube's filament supply circuitry power source to measure.
Using metering set to DC volts in a range level greater than 15 VDC
Your meter negative test lead goes to either NON silver banded lead of D67 or D68 diodes.
The meter positive lead goes to the silver banded lead of D65 or D66 diodes and be expecting in the ~15 VDC range.

Next power supply section test will be the high voltage section at the D60-61-62-63 full wave bridge cluster.
It will also involve the DCV mode but could approach up to 500VDC unloaded.
Meter negative lead goes to the unbanded lead of D62 or 63 diodes and the meter positive lead goes to the silver banded lead of either D60 or 61.
Come back with that voltage level that you read.
The last supply of our immediate interest, which your first photo did not encompass, will be the negative bias voltage supply for the EL34s.
Seek this out on the chassis top, in your exploration on the numbering, but I think that you will find a D64 as the designated rectifier diode and some C71-22ufd/160V, C70-10 ufd / 100V and C79-10 ufd / 100V electrolytic capacitors should be nearby, also being immediately associated with that circuitry, would be trim pots TRIM 1 and TRIM 2.
A close up photo of them / that area would assist, should it be that they are deviating from that numbering assignments scheme given.
If you do find them like that , then you want to use either C71-70-79 electrolytic capacitor's POSITIVE lead as your ground for your metering.
Your meters free positive lead goes to the NON banded lead of diode D64, where you might the read on upwards of up to a NEGATIVE hundred 100 volts DC.
If possibly seeming in error to you . . . . this is because in this sole power supply we are dealing with a NEGATIVE supply vice the normally encountered POSITIVE polarity of supply.
Then we need to see if the voltage set up to feed the two EL34s was set up correct, to be ascertained by the feedback of the NEGATIVE voltage settings found on the NEGATIVE leads of C70 and C79.
Analysis time :
Feed back the voltage readings encountered.
OR any fallacies encountered in acquiring them.

73s de Edd

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[Blackwellefty]

 

[Blackwellefty]

Here is the rest of the board you requested. Thanks

 

[Blackwellefty]

I tested the FS1 and FS2 fuses 00.4
The test across D65 D66 and D67 D68
The reading was 0.00
I tested the D64 0.00
C79 00.3
C70:0.00
I think I have to remove the board to test the C71

 

[Blackwellefty]

It has been a long day
Update my readings
FS1&FS2 reading 00.4
The reading across D65 & D67 .290
And D66 & D68 also.290
The reading across D60 & D62 .946
And D61 & D63 also .946
The reading on D64 .525
C79 & C70 reading 00.1
I will have to remove the circuit board to test C71

 

[Blackwellefty]

OK the test on c71 0.00

 

[73's de Edd]

Sir Blackwellefty . . . . . .

In checking out the progress, I now see the confirmation of all good fuses by your readings.
With no blown fuses, I was intending on you then passing right to a power up to confirm the units
3 power supplies output voltages.

BUT, I now see that you went on and checked the diode junctions Vf specs of those 9 diodes.

Your readings confirm all good and sound diode junctions as well as . . .at no extra charge . . . .
that the low Vf readings on the four that feed the filament supplies, are telling us that they have upgraded to more efficient and lower loss Schottky diodes being used for those units.

That just might be one aspect of the latest " A " version upgrade of your units chassis.

Proceeding on, for a voltage check of the supplies, initially I have extracted the points of major interest from your first photo and placed its relevant schematic snippet just below it.

If you are a little antsy in probing ~500VDC . . . .like the other guy . . .you can use two of the clip leads that I have shown.

In which case, you clip solidly into a diode or resistor and the other wire ends clip goes to your probe’s tip, then you replicate, for making the other probes connection.

RED and BLACK circles are placed at the voltage test points.

Here are the points of major interest from your second photo and its relevant schematic snippet just below it.
Now if you will take careful note and compare against the schematic to top board components
you will see replicated common connections.
If you trace the brown foil path for interconnects, it will be seen that several resistors make interconnects to the electrolytic capacitors.
Using metering connects to those points preclude having to lift the board to get bottom access
to a capacitor..

Thasssit . . . unless further procedural info is needed

The next relevant info to be needed, will be a photo showing the area around vacuum tube V3 on the PCB.


73s de Edd

 

[Blackwellefty]

Here is the V3 photo you ask for.

 

[Blackwellefty]

I do not want to sound stupid,
but not understanding how you want me to test the last photo. Thanks

 

[Blackwellefty]

I have tested the D65 pos lead and D67 neg lead .209v same on D66 and D68
The D60 and D62 00.4mv and D61 and D63 00.5 MB
Now for the last test on the last photo
I'm still unsure what some of that means. Sorry but I do not understand the negative side points.

 

[Blackwellefty]

Poster model MS8233D

 

[73's de Edd]

Sir Blackwellefty . . . . . .


OK on the meter . . . it should absolutely more than is required to accomplish all of your needs.

I am expecting it to already be set up with its :

BLACK TEST LEAD in bottom center terminal.
RED TEST LEAD in right bottom terminal.
METER FUNCTION one click clockwise from left off position, up to its DC-Pulsating DC voltage function.
With the ONLY need for the Red lead ever being moved to the left bottom terminal, being for a series connected measurement of the 10 amps AC or DC function. (Which we will not be doing.)

OPERATIONAL PLAN:

I have placed both photo information and additional schematic snippets of them just below on your initial info that I posted.
Do you have preference of mastering one over the other in the future or need I include both ?
The first readings now needed from the unit, is the NEGATIVE voltage coming out of the tube bias power supply.

AC POWER now applied to the unit and being turned on.

You will notice that at the bottom there is the BLACK CIRCLE and off from it, I have given you 3 options of making your BLACK PROBES connection to . . . pick your easiest to find.
Then there will three readings taken from the RED PROBE to the marked in RED CIRCLE and its marked three offshoots from it.
Initially we need the voltage being metered at either the anode of diode D 64 OR the lead of R 83 that also connects there OR even the lead of R 124 that ALSO connects there.
All are being the same . . . .just use your easiest to find, and log down and pass that voltage being found.
Next voltage to find would be the voltage being on the OTHER lead of R 83 and log it.
The last voltage would be the one at the OTHER lead of R 124.
There is a high probability that all of these readings may be the same since we do not have any tubes plugged in and presenting their loading, yet.

Now we move to the filament supply for the tubes of the unit.

It is a simple placing of the BLACK METER PROBE to any of the same component connections to a common ground
Which are being either two capacitors or the common anodes of D 67 or D 68.

The RED METER PROBE goes to either D 65 and D 66 diode cathodes or a shared 5 other resistors and capacitors that also connect there.
You log down that voltage to pass on.
Final voltage test for now will be the high voltage being used on the tubes, at this time we will only be needing the right RED highlighted voltage .

The BLACK METER PROBE goes to any of the BLACK DOT connected ground connections like D 62 or D 63 anodes connect to.

The RED METER PROBE goes to any of the RED DOT connected supply connections like D80 or D 61 cathode connections.

We need that voltage reading.

If you are squeamish on probing that voltage level , use my mentioned remoting of your test probe leads via the two clip tipped test leads referenced to.
That will be the worst that we will be encountering, and really, will probably be working on down in the 50 VDC range.

But we gots to knows what we gots !

Standing by for incoming readings or for assist if any procedural questions arise.

BOARD / SCHEMA REFERENCING:

73s de Edd