Marshall 1959HW 100W Plexi Oscillation

[Chuck H]

I would definitely suspect the lead dress. The "HW" aspect certainly allows for more possibility of error than a board does. I haven't looked into it, but I wouldn't be surprised to find that it's a problem with more "HW" amps than just the one your working on. The more photos you can post, the better. From all angles. I, and others here, have dealt with lead dress problems with Marshall layouts and circuits. Maybe something problematic will get recognized.

[nevetslab]

The grid stoppers (four) are connected directly to the sockets, yes?

Yes....though with the component leads at near full length between terminal posts and socket terminals....probably ~ 30mm on 3 of them. One the body is ~ 5mm or less from the body. I'll post more wiring close-ups in the AM, as I begin probing with the scope in the preamp stages for the 100kHz oscillation. It appears to be pre-volume control.

[Chuck H]

Yes....though with the component leads at near full length between terminal posts and socket terminals....probably ~ 30mm on 3 of them. One the body is ~ 5mm or less from the body. I'll post more wiring close-ups in the AM, as I begin probing with the scope in the preamp stages for the 100kHz oscillation. It appears to be pre-volume control.

The thing about preamp oscillations is that that the actual cause isn't usually isolated to the preamp, but rather an induced positive feedback loop from radiant field crosstalk. That's what I suspect because of the HW (hand wired) nature of the amp. Marshalls can be sensitive to this sort of thing. Output leads need to be kept from preamp leads. Grid leads should be raised up and plate leads pushed down. Preamp lead slack should be pulled toward the input and PI and output lead slack should be pulled away from the input. Remember that he "presence" lead in coming straight off the impedance switch. OT leads should enter the chassis close to the sockets and be twisted as mush as lead slack allows. Especially true of the secondary leads. Grounding can also be important. Small positive feedback loops can float there and be amplified enough from early stages to cause problems in later stages when like phase circuits are daisy chained, etc. I'm sure you know all this. I guess I'm just reminding you to look for it I don't think it's unreasonable to disconnect the feedback loop just to see if it may be causing the problem. Remember that phase errors can occur with different operating conditions and the feedback loop is really almost never truly 180* out of phase anyway. I've had an OT reverse wired that caused the symptoms your having. Usually a reverse wired OT will howl at instantly, but not always.

[nevetslab]

Some good sound advice. As the amp wasn't working correctly as I received it, with the failed output transformer, I didn't get a look in it's original state. How much I upset things in elevating the component board to gain access to the O/T lead bundle, and cutting those at the end bell exits, and opening up that bundled set of secondary leads, to join with the new leads...using the in-line splices as can be seen on the primaries, all this may indeed have opened the door to the oscillation issue. I'd rather have had soldered those in-line, but the wire population and access from the component board being totally in the way would have made for a lot of insulation getting burned.

I haven't begun todays' troubleshooting session yet. I did get more photo images, as seen below.



I looked at replacing the crimped in-line splices on the primary lead bundle sitting below the tone controls. I can't un-crimp them, having folded the smaller AWG wires over to fill the crimp terminals, so surgery will be more tedious, but feasible. The Secondary leads would be a nightmare. I don't know if I could wind copper tape over the output lead bundle and ground it or not. Should be a painful day.

I haven't opened the feedback loop yet. I left that alone, having verified phasing of the output transformer before cutting it loose, and that of the new one.

[pdf64]

Consider moving the 68k input grid stoppers / mixers on to the socket terminal.

[nevetslab]

I hadn't thought about moving those input grid stoppers down there.

What I did just do, and seems successful, was to first remove the cable ties from the O/T primary wires' in-line splices, replacing them with spot ties (lacing tape), then wrap that bundle as far as I could go in both directions with 1/2" copper tape, made some solder bonds to add to the continuity of the short shield, then grounded that shield to the ground buss running along the back of the pots.

Then, after listening and watching the behavior of the input stage wiring, working with the lead dress of the grid wires & plate wires, I ended up taping the grid wires to the floor with the same copper adhesive tape, and ran copper strips out along the grid wires. it wasn't until after seeing the results that I thought about unsoldering the grid wires from the sockets, and wrapping the two input leads with the copper tape, then lay them back down again so the effective shielding would be much further than it is at present. I think I'll do that with the grid wires.

Still, the results was successful. I don't see the oscillation anymore, and with the foil shield in place over the chassis bottom, it completes the isolation from hand capacitance being picked up Plugging into it and cranking it up, I'm not getting the nastiness I had when this story began.



Now, I still have the power tubes to deal with. I'm working with a successful 'borrowed' set of EL34 tubes from a working amp of our inventory. Now I wonder if the first alternate set of tubes I matched up will now work, having cured the other issues. Still have a vendor to deal with on the one tube failure, leaving me with only 3 matched tubes from his lot.

Added copper tape down most of the length of the grid wires from both input circuits, then re-taped them to the chassis floor. This further improved the RF susceptibility of the input stage. Grounded flexible coax would also have done this well.