5150 evh fuse F3 f4

[Blosterc]

Ok thank you.

[Jon Snell]

Component symbols;

[g1]

ok but these are not resistors(first photo)?

Those are capacitors. The color code should give the value in pico-farads as far as I know. Any I have seen in that particular shade of green are capacitors. There are also some that have a pink body. The green ones are fairly common in Marshalls made since around the 1990's.
Sometimes you will see inductors with the same body style and color code bands. They are usually a darker shade of green.

[Enzo]

The resistors are tan, the caps are green. They are axial leaded ceramic caps. Not resistors.

COnsidering the amp used to work, it seems unlikely the factory installed a ton of resistors in place of caps years ago and yet the amp worked all this time.

[Blosterc]

Thanks at all !
Now, amp work without tonestack(i test the signal path).

the signal is interrupted, on pin 3 of vr7 ..or gets a tension from something ...

[g1]

Pin3 of VR7 is after the tone stack. So if the tone stack doesn't work, there may not be signal there.
This is confusing. The amp is all working properly, but the tone controls are not working?
Do you now have all the preamp tubes completely plugged in? There is no point doing any kind of testing with a tube plugged halfway in.

[Blosterc]

sorry for my confusion .. so:
with all tube inserted:
-only hum,
-controls(clean gain, lead gain, tonestack ,post gain clean post gain lead,)does not affect ... (but with signal on return fx, sound ok).
-presence and resonance ok.

-following the signal from j28 up to pin5 (or pin6) of k2, gain clean and gain lead works. but from pin7 of k2 (or from c52, or j89), it looks like a short circuit..

All relays ok, led ok

[g1]

Sounds like 'clamp' circuit is turned on (between K2 pin7 and C52). Look on pg.2 of schematic, right hand side. Q7 is the 'clamp' fet. It may be defective, or it may be getting a command to turn on from the "trigger' line, or CR23 could be bad.

[Blosterc]

Hi, I have already remove Q7.. problem it can be cr23 still?
i also measured all signal path .. and it is not interrupted ..
measured
v2 / ground (pin6) about 300v
v2 / ground (pin1) about 280v

not measured only the clamp / trig circuit
It's possible disconnect cr21 and cr22 for test?
thank you

[Enzo]

CR21,22? If they open, you'd never know. If one shorts you then have 24v of DC on your signal path. No need to remove them, just check with your volt meter.

Look at the circuit, all CR23 does is control Q7. If Q7 is removed, then it will have no effect.

[Blosterc]

Yes, i find 24/28v on cr22..
with voltmeter, short cr21 and cr22 80 ohm in all positions..
On pin J89, i measured 24v

[Enzo]

J89 is on the far side of a cap. 24v there is from the V3b cathode follower. That voltage belongs there. If CR22 has 24v on both ends, lift it. ANy change in the aamp?

DIodes cost a couple pennies each, if in doubt, it is a lot faster to just replace it than to test the poor thing much.

[Blosterc]

ok cr21 diode removed ... hum gone! Sound good! Last and only problem left, controls not working .. bass, tre, high, postgain lead not working .. i think post lead pot is dead ..
res and presence ok!

[tedmich]

wow I love me some 5150 fire! Exciting thread!

[Blosterc]

ok replaced two pot post gain ..... it works perfectly!!!! thanks to everyone for the precious advices