Marshall JCM 2000 DSL 401 power cutting in and out

[fatcharlie]

Hi all,

Sorry for the double post but not sure how to avoid it since I've been on holiday!

Anyway -

I've double-checked the reverb footswitch input socket and the tip is definitely open - not closed - in its normal position

Same as above with the actual guitar input socket - the tip is 'normally open'.

Now, I'm no expert by any means, but I can't see how, mechanically, these could have failed to that state (especially as I've never used the reverb footswitch) - hence my previous comment "I'm assuming this is normal."

To clear up a bad explanation on my part - when I plug in the channel footswitch, it was the red LED on the _footswitch_ which lit up when OD2 was pressed - not the LED on the actual amp (remained disappointingly unlit).

I treid T2 and T3 with the amp switched on and the channel buttons on the amp both out and in.

No difference in the readings from before:

T2
C 26.6v
B 0.6v
E 0.0v

T3
C 0.6v
B 1.4v
E 0.0v

These readings remain the same whether the channel switches are in our out.

While playing around with the amp I discovered C66 had one leg loose (not sure if this was due to handling the board or if it was like that before), and the HT fuse also had a loose leg.

These have been resoldered and the transistor readings above were taken afterwards.

Where are Connector 1 and Cont 2 junctions, physically?

Thanks again in antici[ation of your help.

All the best,

Roger.

P.S. - I notice the difference in voltage between T3's base and collector is 0.8v - did I read somewhere there is a limit for this difference?

P.P.S. - I also notice the following:

1) With the amp switched on, turning the Master Volume pot produces some crackle in the main speaker, also I can hear the slight buzz/hiss gets louder if I turn it fully up

2) Same with the Overdrive Volume pot - some light crackling and a definite increase ib buzz/hiss with increase in volume

3) No response from any of the pots on the clean channel

[fatcharlie]

Bump?

[oc disorder]

Bump?

You rang?

Hi again, I assume Brexit hasn't affected the "over the channel" holidays as yet although from where I'm looking
it all looks a horrible mess.. !

In your absence I thought perhaps we are going too deep and perhaps we should be looking at a break in the ground connection
around the relay and switching transistors.

You had confirmed the 24volts was there (27v) but perhaps you had your negative lead on the chassis or some remote point for ground
rather than a point near the relay e.g. the emitter of T3 or the negative end of C18 a 22uF capacitor.

Then after you wrote "Where are Connector 1 and Cont 2 junctions, physically?" got me thinking it could be this simple.

Firstly Con 1 & 2 are the only 10 pin connectors on the whole board. They connect the small switching board at the front panel
to the "electronics" at the rear of the unit via those small blue wires.

I suspect that some of those connections are open either due to dry/cracked solder joins at the bottom of the 10 pin sockets
or faulty crimps inside the plugs.

I think this has to be checked thoroughly. Without the amp in front of me it's hard to suggest a method but I'll attempt it.

You will probably need some dressmaker pins to get your meter to connect to the top of the plug at the rear of the amp if they
will fit part of the way beside the tiny crimps in that 10 wire plug.

From the diagram the two switches connect between ground (pins 1 & 2) and pin 3 for one switch SW2 and pin 5 for the other SW4.

So with your meter between pin 1 or 2 and pin 3 switching SW2 should produce a result at the meter.

You may be able to measure the continuity without removing the main board only the front switch board.
eg One side of R4 on the switchboard (10k) should connect via pin 8 in the connector to the junction of R8 (10k) and R63 (100k) on the main board.

This can be gleaned from the schematic.

"Master Volume pot produces some crackle in the main speaker"
This indicates to me the amp is ok after the master vol.

The relay even if it's not clicking should pass signal through the contacts at rest.

" the Overdrive Volume pot - some light crackling and a definite increase in buzz/hiss with increase in volume"

This suggests that the at rest contacts or the state the relay is in , is connecting part of the overdrive circuit
but I'm not inclined at the moment to figure it out from the diagram.