Marshall 1987X crackling distortion

[Littlewyan]

Can you measure the plate voltage on pin 3 of both EL34s please? And check that the cathodes are grounded ok.
Do you have an oscilloscope?

[pompeiisneaks]

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I don't really understand the tone of this reply, but it seems a bit rude/hostile.

My primary goal on this forum is to keep it civil.

~Phil

not rude just right question. if the guy has a repair shop he is charging other people knowledge to me this is apsolutečly wrong. i saw also before here this situation. from my point of view apsolutelly wrong.if for you is ok then ok proceed. in our side of sea this is rude what the guy is asking. maybe diferent mentality of nation.

I know you've been a member longer than I have but multiple regular commenters here build amps for their primary trade, and are not only happy to share their knowledge but also regularly ask for assistance when stumped. That's been the case for as long as I've been the admin of the forum. Just because your specific geographic location won't help other professionals in the trade as a courtesy doesn't mean others don't. I've seen this kind of help given on MANY forums related to things that are both hobby and business related. I've regularly frequented beer brewing forums that had professionals helping and asking for hints when a problem arose, I've also started hobby machining recently and found forums with many expert machinists asking for help on complex issues that others happily helped with. This is pretty much the reason forums exists... for sharing knowledge and information and helping people that like what you like. Again, you don't ever NEED to help, but trying to imply your personal beliefs about his issue are what everyone else should believe, seems a bit of a stretch. Let's leave the thread hijack if you'd like to discuss more, PM me.

~Phil

[bepone]

i did beer brewing too.. in my area this is not really the case , if you are professional then it is your bussiness, dont cry arround for help, you should know this how to repair things, how you open then . if you are diy all doors are open you are getting and prividing all, this is normal for me.
but ok, no hard feelings.

[cassiddidy]

Can you measure the plate voltage on pin 3 of both EL34s please? And check that the cathodes are grounded ok.
Do you have an oscilloscope?

I forgot to write those voltages down... I'll post them in a little while.

And yes, I have an oscilloscope. It's a cheap hantek USB scope, but it works

[pdf64]

... the bias current doesn't seem to stop rising. It's very slow, but I haven't found the top point and I'm not sure that it's related to this distortion. Only thing I can think of at the moment.

How about if the amp is operated with the valve in V3 is removed? ie does the output valve anode or cathode current stabilise?

[Colossal]

My primary goal on this forum is to keep it civil.

~Phil

ALL: Please be respectful of one another. Let's keep things on topic and try to help the OP out. Thank you.

[dorrisant]

i did beer brewing too.. in my area this is not really the case , if you are professional then it is your bussiness, dont cry arround for help, you should know this how to repair things, how you open then . if you are diy all doors are open you are getting and prividing all, this is normal for me.
but ok, no hard feelings.

There are too many different things that can go wrong in a tube amp. With a higher parts count that possibility goes up more. There is hardly anyone who could repair everything that comes across their bench without a bit of outside wisdom. If you find that in your case the last statement is not true, then you probably haven't repaired enough to find that limit. I would not be able to have had a viable business without the help from the members here. In particular Martin Manning. He is one of several who drink morning coffee and instead of a crossword he likes to come here and solve puzzles just like this. It may not be your thing to help like this, but there are others here who consider it a part of there daily requirements, solving issues and stimulating their own brain as well as healthy conversations. Sometimes these dicussions lead to greater solutions for general problems. And from personal experience, I can tell you that posting here to get guidance on any amp issue is the best way to learn that answer. Those that learn this way are usually happy to share what they have learned as well.
In summary, a lot of the guys that try to help out, do so because they want to see resolution (because they have been in that exact situation.) Also, a lot of the guys that try to help have their own repair/build buisness and are trying to give back to the forum that helped them get where they are. If you don't have something helpful to contribute, silence is the best suggestion.

[cassiddidy]

Can you measure the plate voltage on pin 3 of both EL34s please? And check that the cathodes are grounded ok.
Do you have an oscilloscope?

The plate voltages are 476 and 477

[cassiddidy]

... the bias current doesn't seem to stop rising. It's very slow, but I haven't found the top point and I'm not sure that it's related to this distortion. Only thing I can think of at the moment.

How about if the amp is operated with the valve in V3 is removed? ie does the output valve anode or cathode current stabilise?

Seems like it's doing the same thing with or without V3. Only it's 2-3 mA lower without it

I almost wanna say "inconclusive"? The rising has slowed down a little since yesterday. Maybe I need to run the new capacitors for a little bit...

[Littlewyan]

Now this is a puzzle! Just to clarify, do you use two bias probes? One per socket? If so, have you tried swapping the probes? And can you please measure screen voltages.

[cassiddidy]

Now this is a puzzle! Just to clarify, do you use two bias probes? One per socket? If so, have you tried swapping the probes? And can you please measure screen voltages.

It sure is, isn't it?

I actually only have one bias tester, I just power down and alternate when checking different sockets or tubes. Screen is pin 4, right? I'm getting 473/474V on the screen, in relation to the 476/477V I'm getting on the plate

[Littlewyan]

Ok that all sounds good.
This issue started happening after changing the transformers, are the OT primaries definitely hooked up ok? Wondering if the centre tap has been connected to one of the EL34s by mistake. And have you checked continuity between the EL34 cathodes and ground?

[cassiddidy]

Ok that all sounds good.
This issue started happening after changing the transformers, are the OT primaries definitely hooked up ok? Wondering if the centre tap has been connected to one of the EL34s by mistake. And have you checked continuity between the EL34 cathodes and ground?

This was something I was worried about initially, because I had in fact gotten the blue and white wires mixed up. When I first powered it on there was immediate oscillation. When I swapped them, the oscillation was gone but this noise was there. Before replacing the transformers, the volume levels felt like they were cut in half, so it's hard to have a reliable reference point.

So take a look...

PXL_20220424_012517187.jpg

After trying to troubleshoot the problem for a couple days, I disconnected the OT to give it another short test and see if there were any faults. Everything appeared to be fine, so I don't think I burned it out somehow. It was only on for a couple seconds anyway

I've checked over the connections to ground and there don't seem to be any problems there. But maybe you notice something that I missed?

...also attaching the schematic for the 1987X and the Hammond replacement OT...

[dorrisant]

If that is the OT you have in there now (Hammond)... the white wire is the CT. Blue and red are plates. Swap the white and blue wires. That should restore full volume and get rid of the crackle. If it squeals, swap the red and blue to opposite plates.

[cassiddidy]

If that is the OT you have in there now (Hammond)... the white wire is the CT. Blue and red are plates. Swap the white and blue wires. That should restore full volume and get rid of the crackle. If it squeals, swap the red and blue to opposite plates.

...sigh... Really? Oh boy...

Just so you all can see what threw me off, this is the diagram that was printed on the box it came in...

PXL_20220424_064941708.jpg

After getting the oscillation, I second guessed myself and thought the little dots were the CT and the big were the A side of the winding. Mostly because that's how the big dots are used on the power transformer! What the hell...

PXL_20220424_071853638.jpg

But I guess that's not the case, because I just swapped the white and blue wires and the distortion is gone! I guess the lesson is...go with the pdf?

Thanks for the help, dorrisant. And everyone. We finally got there

Cass