Marshall MA100C problem

[Blackwellefty]

OK then I finally have some time to test the unit as you requested. I have been working 15-16 hours a day for the last 2 weeks.
The test on the C70 .065v and then the
C79 .064v and the C71 .062v.
The reading for D64 .047v and the
R83 both sides .078v.
The test on D65 .077v
and D61 to D63 .003v
If the reading are wrong let me know.

 

[73's de Edd]

Sir Blackwellefty . . . . .

HEY ! . . . .ya know whuts ?

I keep a' seeing your very LOW voltage readings and think that you are STILL being in a diode testing . . . . . .frame . . . set of mind.

In actuality, THAT is being a GEETAR amp and some units do use a standby switch to remove the high B+ from those 'spensive power output tubes, while they might be on an extended break from a ' playing their HOT LICKS . . . .et . . .riffs.

Put your spec-tac-u-lators on, and just see if you don' t find a STANDBY switch . . . right beside the power rocker switch, flip it on ALSO, just after the main power switch, for just a silly milli second, just to see that there is being no big 'splosion . . . coming forthwith.

If not . . . then leave it on, so that you will then have B+ presence, and then be able to take those higher voltage readings, that were previously being non existant..

If not, I will consult the schematic again and set you into a proper final glide path.


73's de Edd

 

[Blackwellefty]

As you think I'm a not to smart.
Maybe so but I play with hi voltage
Um like 220 and 440 so I'm not afraid of electricity with that said I am going to
give you the readings with the unit plugged into the outlet and both switch's turned on.
C70 97 volts. C79. 78 volts C71. 82 volts. D64 77 volts R83 79 volts
And the D65 across to the D67 77 volts
I realize you are looking for higher numbers but I am not getting them
could this be the input or power transformer has no out put.?
If you would like to talk to me please feel free to call me 580 262-9482
Thank you.

 

[Blackwellefty]

Is it normal to see a brownish color on the bottom of the board. It is from the resister R71 R 97 and R 88

 

[73's de Edd]

Sir Blackwellefty . . . . . .

and BOTH switch's being turned on.

THAT was being the bottleneck . . . . and now we're on a roll . . . . and you are doing GREAT!

You have to realize that at my end I am TOTALLY . . . blind, deaf and dumb . . . heavy on the dumb . . ., nor can I smell , feel, touch nor thoroughly physically examine a person's item.
Nor, do I know their technical acumen, expertise and prowess.

C70 97 volts.
C79. 78 volts
C71. 82 volts.
D64 77 volts
R83 79 volt
With those readings up above you are dealing with the bias supplies and all are telling me of a problem over in the tube circuitry, that we will have to correct in order get that area back to normal.

One error I do see, would be that if you had your black meter probe stabbed into ground those readings displayed are being NEGATIVE voltages.


Referring back to our post # 15 pictorial and schema snippet referencing

D65 across to the D67 77 volts

Now on the above reading we are being on a different level of power supply, which is supplying DC power for the filaments of the tubes.

With your referencing to the RED CIRCLE referenced cathode of the D65 diode, that also is the same connection as the cathode of the D66 diode to its right, that also is the same connection as the + terminal of the C73 electrolytic capacitor to the right.

We are interested in THAT capacitor.

Aditionally:

With your referencing to the BLACK CIRCLE referenced anode of the D67 diode, that also is the same connection as the anode of the D68 diode to its right, that also is the same connection as the - terminal of the C73 electrolytic capacitor to the right.

That C73 capacitor is rated as 25VDC maximum voltage and the filaments are expecting in the order of 12 VDC so we need to re-check that voltage level again . . .with our fully expecting it to be down in the 14 possibly 14-16VDC range since it is now unloaded by any tubes presences..


The last voltage section info needed, was at the RED and BLACK dots associated with the D60-61 and D62-62 test points . . . . . . which I see no voltage info being given.

Finally . . .no worry . . . about the slight PCB discoloration, with time / heat from those resistors proximiy to the PC board.


73s de Edd

 

[Blackwellefty]

Dear73s de Edd
Sorry for the long delay on the readings
You are awaiting for.
I thought I would sit down and remeasure everything that you ask me to. Because it was my weekend off and could take my time with this and as dumb luck has it I had to work all weekend. I've been working long hours and after getting home the company calls me back out and fore do not get home until 10:00 - 11:30 pm so I am trying to find the time to do this please be patient I will have the readings as soon as I can. Thank you.

 

[73's de Edd]

No problems . . .just keep building up that cash flow ! . . . . standing by.

 

[Blackwellefty]

Please give me a call at 580 262-9482
Thank you.

 

[73's de Edd]

I am on CST here . . . . what is your preferred calling time hours spread. . . .in your time zone ?

 

[Blackwellefty]

Between 4-6 pm
Thanks

 

[Blackwellefty]

Here is the IC plug



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