Troubleshooting IMD and hum in a 1959SLP

[Gtr0]

Hi.

Thanks to the help of several members, I was able to get my bias properly checked and now facing a new challenge.

2000'ish Marshall 1959SLP (the pre-fx loop version) reissue.

The amp was in somewhat poor shape when I got it, just getting around to making some repairs... had some nasty IMD (ghost notes), bulging filter cans, tube sockets that seemed to disintegrate when pulling out a power tube etc.

I replaced all filter cans (6 dual cans) and while I was in there I replaced all signal caps with Synergys and a few Sozo (yes, I am one of *those* guys). In addition I replaced all of the power tube sockets.

The original heater wires were incredibly thin, so I replaced them at the power section as well. The original filter cans were all ARS 50/50 500VDC and I replaced them with the same (sourced from Watford in UK) values, also ARS.

And other than replacing the two 56k resistors in the power section as well, all else is as it was.

Started it up again today after finishing those repairs and certainly the overall tone for me had improved, but the IMD is still there and now there is a added hum :-(. Here is an audio clip - in the first few seconds you will hear the hum... then a few seconds of high notes to "feature" the ghosting.

MARSHALL_1959SLP_IMD.mp3

I will also attach some photos of the chassis, perhaps you see something I do not? A few circuit changes (small)...





Changes: The reissues weren't exactly traditional values. In lieu of a .0022 decoupling cap on v1b, there were a few in parallel. Same with the bright cap on channel 1. So I simply returned them to traditional 1970'ish values. C15 and C16 replaced with a single .0022 and C17 and C18 replaced with a single .0047 and moved to the actual volume pot.

I pulled the 4x 68k resistors out of the signal path (just clipped the wires) and attached some 68k metal film resistors at the input jacks and ran shielded cable to v1a/b.

So what are other common reasons for ghosting notes? I have read all sorts of things from output transformers to chokes to "it's part of the marshall circuit" - however this isn't the first superlead I have owned, yet the first time I have ever heard ghost notes... it took me weeks to determine that is what it was.

Thanks!

[g1]

Can you post the schematic?
And since I don't think you mentioned it, did you change the filter caps in the bias circuit?

[Gtr0]

okay - here is, AFAIK, the exact schematics for my particular amp...




I suppose they took a "template" amplifier and adjusted values to that particular template, mostly in the inner stage between v1b and v2a... the original, more traditional, values I spoke of are in this schematic...


The only difference that I know of is that I left the 330uF Ck on V1a.

I did not replace the bias caps... they were Nichicon and the voltage seemed right... BUUUUUT... yea, I also read that bias may be a ghosting issue - forgot to mention that one in my op.

I am sure I have some 350 volt F&T laying around... probably 10uF.

[Gtr0]

Semi-shameful bump .

[Tony Bones]

Ghost notes with 100W Marshalls is not uncommon and is usually due to excess ripple on the screens. You cab try increasing the capacitance in the first two PS nodes. Maybe add a second choke in series with the existing one and some capacitance to ground at the junction between the two (i.e. an additional LC filter.

I seem to recall a thread on another forum (don't remember which) where someone added an LC filter right off the rectifier (before the OT CT.) It required a choke with a lot of current capacity, probably 300mA, but it killed the ghosting completely. Not sure if it had any other affect on the sound and feel of the amp.

There has been a lot of discussion on the topic on the Metro forum.

[Gtr0]

Ghost notes with 100W Marshalls is not uncommon and is usually due to excess ripple on the screens. You cab try increasing the capacitance in the first two PS nodes. Maybe add a second choke in series with the existing one and some capacitance to ground at the junction between the two (i.e. an additional LC filter.

I seem to recall a thread on another forum (don't remember which) where someone added an LC filter right off the rectifier (before the OT CT.) It required a choke with a lot of current capacity, probably 300mA, but it killed the ghosting completely. Not sure if it had any other affect on the sound and feel of the amp.

There has been a lot of discussion on the topic on the Metro forum.

Thanks for the info. I have read lots of the Metro discussions, though not all. And I do know it can be something of commonplace for this to occur. In my experience I have never seen it having built several 100 watters myself, even using the 68 and earlier specs with much lower filtering. I suppose I can start by replacing the 50+50 at the mains with 100+100... I will have to go F&T for this... after just putting in some rather glorious looking ARS which were exact replacements.

Thanks for the ideas!!!!

[Pedro Vecino]

Noise and modulation are completely abnormal. Electrolytics look very good. Have you tried installing another choke?

[Gtr0]

Noise and modulation are completely abnormal. Electrolytics look very good. Have you tried installing another choke?

hmmmm... that's what I thought too. At least within reason. So many people contest to the fact that these amps are full of ghosting, though I have never experienced it until now, that it makes me doubt myself.

I did not try another choke, but I will - I have several laying about. Thanks.

[pdf64]

I dislike how the chassis is being used a key 0V conductor, bringing numerous opportunities for compromised, high resistance 0V to chassis connections in critical circuits. Maybe one of those is bad (exact same symptoms as a failing power supply cap)?

[Gtr0]

I dislike how the chassis is being used a key 0V conductor, bringing numerous opportunities for compromised, high resistance 0V to chassis connections in critical circuits. Maybe one of those is bad (exact same symptoms as a failing power supply cap)?

I def hear ya. I typically leave them as such when they are factory Marshall... in fact I *just* came down from my workshop... just finished a 1959 switchable 2203 thingy and it is dead silent and sounds KIIIILLLER. Of course in this case I am not using random pieces of chassis for grounding.

AND I used the same filters... all 50+50 ARS cans. Worked out great.

I will check all of those solder points, or even move some this week... if that doesn't work, I will check out a choke possibility... BUT I do not doubt that being the case, there were so many open solder joints on this amp... it must have been training day at Marshall the day this was built.... seriously... never saw anything like it.

[Chuck H]

It's very hard to follow the filter cap wiring from photo's. That or I lack the patience I did briefly compare to other photos of 1959's (including another SLPs) that I could find here and there. Yours doesn't match, but a couple of other didn't either and I think it may be that sometimes the cans are wired in a different orientation on the chassis. A voltage check throughout the power supply might reveal if something is wired in error.

[Gtr0]

It's very hard to follow the filter cap wiring from photo's. That or I lack the patience I did briefly compare to other photos of 1959's (including another SLPs) that I could find here and there. Yours doesn't match, but a couple of other didn't either and I think it may be that sometimes the cans are wired in a different orientation on the chassis. A voltage check throughout the power supply might reveal if something is wired in error.

Thanks for taking the time to look over what you could stomach - I know how that is!!

Well, what I DID manage to do was trace through the amp with a DMM following the 1970 Unicord schematic I attached above. Checking continuity at the component side (not the solder joint itself, but further past it when possible) through just about all of the schematic. I have not yet found a wiring fault.

Just about all grounds to the chassis have been redone.

I will recheck voltages today, at all points off of filtering through the power/pre amp

One thing that I noticed now is that at first it is quiet and starts humming once it is warmed up... which suggests what? I bad solder joint somewhere perhaps? I will rehit many of those later today as well.

Thanks.

[eschertron]

One thing that I noticed now is that at first it is quiet and starts humming once it is warmed up... which suggests what?

Does the hum change with a different set of tubes?

[g1]

You mentioned re-doing chassis grounds, I've had weird issues similar to this where I had to clean the lug/chassis connection from the wire off the power tube cathodes. Cleaning up oxidization as well as resoldering the wires.

[Gtr0]

I ordered a new set of tubes as all of my stock is either in use or were previously abused....

when I replaced all of the caps, I also replaced the ground lugs... I haven't had a chance to check the voltages yet, I will try to get to that this morning.

Thanks for all the thoughts!!