Boogie - cuts out

[oc disorder]

Try the faders at extreme settings ie all up and all down.
They could have got a bump as they stick out slightly .
Bit of Deoxit cleaner perhaps think they use a darlington in there
MPSA63.
Heck of a lot of gain - I'd still be suspicious of the eq faders.

[txstrat]

You were too fast for me boys. I edited my post and the pic while you posted. Please read again.

Try the faders at extreme settings ie all up and all down.
They could have got a bump as they stick out slightly .
Bit of Deoxit cleaner perhaps think they use a darlington in there
MPSA63.
Heck of a lot of gain - I'd still be suspicious of the eq faders.

The oscillating comes and goes with the settings of the EQ faders. So I assume they're OK. Will check them again.

[oc disorder]

Whoops I didn't read that you had already been to the MPSA63..

[txstrat]

There's another issue coming up.
The MPS Axx transistors seem hard to find. Does anybody know a source or a replacement part?

[oc disorder]

Welcome to rswww.com
Your search for MPSA63 matched 1 product MPSA63 N
PNP darlington transistor,MPSA63 1A
RS Stock No. 157-7163 Manufacturer Fairchild Semiconductor Manufacturers Part No. MPSA63 Catalogue page 2 - 2787
Your search for MPSA70 produced zero results
Your search for MPSA20 produced zero results

well one out of 3

http://de.farnell.com/
3 gefundene Suchresultate für “MPSA63”
0 gefundene Suchresultate für “MPSA70”
ON SEMICONDUCTOR - MPSA20G - RF Bipolar Transistor
Hersteller: ON SEMICONDUCTOR
Farnell Best.Nr.: 1155814
Herst.Bez.: MPSA20G

ah 2 out of 3

MPSA70 Digikey and Mouser List them but appear to have none in stock.

[txstrat]

Found a retailer and ordered the transistors. Will take a few days.

[oc disorder]

It may not be the transistors.
Do the ground points look good?
Maybe that 10uF cap on the power rail has drifted considerably
and is helping to make the eq unstable.
Its a lower value than normally used for power filtering.

[Robert M. Martinelli]

There's another issue coming up.
The MPS Axx transistors seem hard to find. Does anybody know a source or a replacement part?

Some suitable replacements:

MPSA20---> BC167, BC183, BC237, BC547

MPSA63---> BC516, BC876, BSR60...62

MPSA70---> BC213, BC257, BC307, BC557

Check the replacements' pinout, as it might be different.

( of course, as oc said, it might not be the transistors, but at least you will be able to rule them out for dirt cheap )

HTH

Best regards

Bob

[txstrat]

It may not be the transistors.
Do the ground points look good?
Maybe that 10uF cap on the power rail has drifted considerably
and is helping to make the eq unstable.
Its a lower value than normally used for power filtering.

I'm gonna have a look at that 10uF. The ground points, as far as I can see them, look OK.
Problem is, the conductor paths are partly on the underside and I can hardly see them, far less touch them, unless I take out the whole PCB.
And I don't really want to do that since there are lots of cable connections to desolder and resolder again to get the PCB out. If I can I would spare me the work.
I fear it might be necessary to resolder the new transistors. Wish me luck...

[txstrat]

Changed the transistors to no avail. Well, sort of.
If I turn the EQ off there is a slight hum from the open input (guitar cable plugged in). But when I turn the EQ on (all faders full up) the amp is dead quiet. Even with a guitar in the input.
When I hit the strings, the signal gets amplified (so far that's what the amp ought to do) but when the signal looses strenght the amps starts to sound like a machine gun until the signal fades out. Then it's dead quiet again.
As if the signal needs a minimum strenght to get through.
EQ off and the amp plays fine.
Changed the 330k resistor at LDR2 (the one that made noise when I nudged it) to no avail. Next I thought to change the caps after the base of Q3 (10pF parallel to the 3.3K and the .1uF after them).
Any other ideas?

[txstrat]

The 1uF cap leading to the FX loop wasn't it. But I found that the amp shows less of a problem and when I turn down the mid pot OR one of the first three faders of the EQ (i.e. 60Hz, 240Hz or 750 Hz). Provided the EQ was on.
Amp plays a lot better that way. (alright, I could leave it that way and tell the guy not to turn up all faders )
b.t.w. No change when I turn down the other two faders. Can it be frequency related?

[Robert M. Martinelli]

Hi Matt,

Things would be easier with an o-scope, but, given the symptoms you're describing, I'm starting to suspect that you have some HF oscillation going on, due to unwanted coupling / positive feedback occurring somewhere inside the amp, and the phenomenon gets worse with the EQ because of its boosting effect.

HTH

Bob

[txstrat]

I had a similar thing happen in an amp that was missing a ground connection of a master vol. pot.
Have checked the board so far but only found some minor stainings over some pcb paths. Cleaned it up but nothing changed.
Think I'm gonna go over the first tube and it's surroundings again, since there was the only spot where I could "hear" something when I poked around with the chopstick.

[oc disorder]

I just remembered something I hope it hasn't been brought up already.
That is the main input jack socket is the main earth for most of the front end.
I would suggest undoing it you will probably need a socket spanner
then cleaning the inside of the chassis with emery paper and if you have it
a light smeer of deoxitbefore refitting the socket tightly.
They always have a star washer so check for any corrosion on that as well.
Did you try replacing the 10uF ?
If its hard to get to you could try paralleling a ...say 4.7uF with a similar voltage rating as a temp measure and see if it makes any difference.

The first tube is so sensitive you can hear the capacitors around that are partially microphonic for want of a better explanation.

[txstrat]

I just remembered something I hope it hasn't been brought up already.
That is the main input jack socket is the main earth for most of the front end.
I would suggest undoing it you will probably need a socket spanner
then cleaning the inside of the chassis with emery paper and if you have it
a light smeer of deoxitbefore refitting the socket tightly.
They always have a star washer so check for any corrosion on that as well.

I'll definitely have a look at that next. Thanks.

Did you try replacing the 10uF ?

If you mean the 1uF electrolytic cap after the EQ, yes I replaced it to no avail.

The first tube is so sensitive you can hear the capacitors around that are partially microphonic for want of a better explanation.

Yeah, even moving the first grids wire causes severe buzz.