Intern Mainboard: F1 (High voltage) 1.6AT blew - Herbert

  • Thread starter Thread starter nbarts
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How do C59 and C60 look like? Normal?
If yes, could have been a spike - replace the fuse and fire it up again.
You can also use a 2A fuse - should be easier to get.
Report.
 
duesentrieb":3vxqz23l said:
How do C59 and C60 look like? Normal?
If yes, could have been a spike - replace the fuse and fire it up again.
You can also use a 2A fuse - should be easier to get.
Report.
Click to expand...

PMed!
 
Your tech needs to check the bias circuitry.
My guess is that one of the caps has a short.
C52, C53, C54 could cause the problem.
Check them visually please.

Are you familiar to use a DMM?
 
Are you familiar to use a DMM?
Click to expand...

I can't see why not, tell me what you want me to check. Thank You!

Visually they look fine. I've noticed when I insert the DMMs leads to the ground bias point & move it from one side to another that tube problem seem to come & go, like there is a loose connection a short. I think it's right above the 3 you are talking about.
 
Take out the powertubes, switch amp on, set DMM to 200V DC, black probe to chassis, red probe to the caps "-" or directly after the diode D3 (the side where the white ring is). You can leave the amp in standby . . .
Your reading should be around -30 to -70Volts.
tell me what you read on the DMM.
 
Does that change when you turn the bias pots?
 
. . . and: if you DMM can make a "beep" (set to diode check) when connecting red and black: black to ground (chassis) and red to "+" side of each cap.
You should hear three "beeps" then.
 
duesentrieb":1jnxkw99 said:
Does that change when you turn the bias pots?
Click to expand...

I've turned the pots a little. They read 32, 34, 32 now. So Yes, to answer your question. I think they were all the way up before.
 
duesentrieb":twt7d2cr said:
. . . and: if you DMM can make a "beep" (set to diode check) when connecting red and black: black to ground (chassis) and red to "+" side of each cap.
You should hear three "beeps" then.
Click to expand...

Is that the same as checking for a sharp?
 
Your bias circuitry obviously is working well.


Would you like to check the B+ as well before going further?

If not:
Put back the tubes in (you can adjust the trimmers before putting them back each to -50 Volts) and rebias.
 
High Voltage. Please don't ask me why it is called B+ . . . Marshall calls it high tension (HT fuse) and so on . . .

Set DMM to 1000 Volt DC, black to chassis (just put it where the headshell-screws are normally).
Amp (without tubes) in the "run" position.

Be careful !

Now please tell me your reading (red probe) of the hum trimmer (three legs).
And the "+" sides of C65 and C69 (if you can localize them - or if you are able to reach it with the probe).
 
Three.

Use 600V DC then.


!PT15L.jpg

two on the right, one in the middle of the left (look closely - leg is visible on the bottom of the trimmer)

If you cannot reach the third - nevermind . . .
 
Olaf, the readings for hum trimmer keep changing. The 2 on sides are around 70, ht e3rd one keeps gradually going up until 400 & than drops to 0 & then repeats the same.

I have V1.7

my c65 & 69 are 2 different things. 69 is red, the 65 looks like a capacitor.
 
ok. That sucks, I have an older schematic then. edit: mine is actually newer :D

Underneath the small board with the stuff for the loops and such you can see the big black filter caps.
Locate the "+" side (pots facing you should be on the right side, but make sure) and tell me the readings -

DMM set to 600V DC.
 
there are 6 of them. 3 of them ~254. the other 3 ~509. it goes like 254/509/254/509/....
 
ok, those are ok then.

Well - bias circuitry ok, filter caps ok - the only thing I can think of now are the screen grid resistors.
They are white, pretty big and are located close to the power tube sockets.
Check them visually please

(sorry that I cannot help more detailled, I sold my Herbie last year)
 
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