Oh no! It's an intermittent fault!

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brettl-s
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Oh no! It's an intermittent fault!

Postby brettl-s » Sun 12/01/19 11:43 am

Hello

First post so forgive any newbie mistakes.

I'm hoping for some help in dealing with an intermittent fault on my Marshall 1974X (ser no M-2009-33-0304-1). The amp doesn't get used much, only a couple of rehearsals per month for the last year or so. The faults have been happening for 12/18 months, I was kind of hoping it would permanently fail so I could give it to a repairer - I've had the experience in the past of giving a valve amp to a engineer only to have it behave impeccably for the two months he had it....

Symptoms:

Occasional loud crackle (mp3 attached)

Occasional buzz like a badly earthed guitar. Switching inputs can sometimes stop this.

Very occasionally a 90% loss of volume.

I replaced the valves in January, but not sure if this has improved things.

So my questions are can anyone suggest what might cause the crackle the mp3 shows? Or any general ways in which to deal with an intermittent fault. Can it be 'stress tested to failure? I've got a gut feeling that there may be more than one issue at work here...

Any thoughts much appreciated.

Cheers from London, Brett.
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Daviedawg
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Re: Oh no! It's an intermittent fault!

Postby Daviedawg » Mon 12/02/19 3:26 am

Welcome Brett. As you already know intermittent faults are the worst kind. You may have done this already but first thing is to eliminate outside elements such as the guitar leads and issues with electricity supply. Not easy when the fault is intermittent.

It sounds like the fault could be related to "dirty" inputs especially with limited use. So If it were me I would start there. Try cleaning the jacks with switch cleaner and a q tip as a starter. Then repeatedly move the jacks in and out. See if you can still generate the faults.

After that it is a case of removing the chassis and "chopsticking" looking for a bad connection somewhere. We can guide you through that if necessary. In any case let us know how it goes and we are here if required.

Dd
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brettl-s
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Re: Oh no! It's an intermittent fault!

Postby brettl-s » Mon 12/02/19 6:22 pm

Many thanks Daviedawg. Checked the power lead and tried different guitar leads, all okay. I'll give the input jacks a clean and check them for any signs of dry joints or damage. I'd be up for trying the chopsticking thing, can you advise? Just so you know, I've got a multimeter and I can wield a soldering iron, but no experience beyond minor repairs....

Cheers from a southern jessy
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Daviedawg
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Re: Oh no! It's an intermittent fault!

Postby Daviedawg » Tue 12/03/19 3:26 am

It is good to do your own fault finding. It gives you close ownership of the amp as it were.

If you get to the point of removing the chassis, some pictures will help us to envisage the action and advise as well as possible.

But chopsticking will involve working with a live high voltage chassis still connected to the speaker. Essentially trying to create noise and isolate the source. So using a non conductive stick like a chop stick (but not a lead pencil) or plastic stalk you methodically tap each component and joint and move them to try to find a noise through the speaker. Keep one hand behind your back and the active one out of the chassis as much as possible. Dirty sockets, bad joints, broken wires or the like are potential causes of the noise you described.

Good luck.

Dd
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brettl-s
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Re: Oh no! It's an intermittent fault!

Postby brettl-s » Tue 12/03/19 5:29 am

Thanks Db. I'm up for that, couldn't agree more about doing you own repairs, might even get me to the point where I'm confident enough to have a go at building.

I'm getting the amp back from my rehearsal studio in the next couple of days so will get to work then. Should I post photos of the chassis before I start investigating?

Cheers, Brett
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Daviedawg
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Re: Oh no! It's an intermittent fault!

Postby Daviedawg » Wed 12/04/19 3:26 am

The pics are up to you Brett. But if you need more help it will make it easier for us to see what the layout and style is like.

Dd
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brettl-s
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Re: Oh no! It's an intermittent fault!

Postby brettl-s » Wed 12/04/19 4:48 am

Thanks Db, I'll be back in a couple of days. B
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brettl-s
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Re: Oh no! It's an intermittent fault!

Postby brettl-s » Fri 12/06/19 11:10 am

Ok amp's home, photos h/w. Any guidance on how to 'chopstick' much appreciated.
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Re: Oh no! It's an intermittent fault!

Postby JMPGuitars » Fri 12/06/19 1:37 pm

Scroll up to Dd's post above for how to chopstick.

He mentioned it sounds like a dirty input. It does, but that could also be the guitar jack, the cable (which I think you ruled out), or any other solder joint on the signal chain.

First, try a different guitar or input device if you haven't already.

Then start chopsticking.

One thing I see from your pictures is that the solder joints are ugly. Solder joints should be smooth, and some degree of shiny.

One other thing that bothers me is how close those two resistors are (to the right of that fat cap, and the three red caps). I would separate the resistor leads better.

Thanks,
Josh
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Daviedawg
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Re: Oh no! It's an intermittent fault!

Postby Daviedawg » Sat 12/07/19 3:13 am

The bent lead gets very close to the other one. Well worth moving that away whatever else comes out.

Dd
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brettl-s
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Re: Oh no! It's an intermittent fault!

Postby brettl-s » Sun 12/08/19 12:06 pm

Ok, so I've had a poke around. Some crackling when pressure put on the circuit board pins to the left of the three red caps, particularly the pin with two resistors attached. I removed the circuit board and had a look at these pins but could not see any obvious problem. On replacing the board the crackling had mostly gone away. Some noisy pots, so gave them a good clean/lubricate.

The only other thing I have noticed is that on the normal channel, one input jack is much more prone to hum than the other when an open (i.e. nothing connected at the other end) jack lead is connected??

I have read various opinions on whether the two inputs on each channel are different, anyone got the definitive answer on this?

Thanks for all you thoughts/help.

Cheers Brett
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geoff 1965
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Re: Oh no! It's an intermittent fault!

Postby geoff 1965 » Sun 12/08/19 6:46 pm

hello Brett,
the difference between the inputs on the channels is on the normal channel is single/parallel i.e. one jack sends signal into one half "triode" of V1 preamp tube and the other jack sends signal into both triodes of V1 "parallel"
the tremelo inputs are High/Low i.e. the high input passes signal from both jacks through 2 resistors of 68K value in parallel "which gives you a resistance of 34K"into one triode of V3 preamp tube, the low input passes signal from one jack through one 68K resistor and the other 68K goes to ground acting as a voltage divider i.e. 2 resistors of the same value in a voltage divider will half the input signal.
the other triode of V3 is used for the tremelo and V2 preamp tube is your phase inverter.
here is the schematic for your amp so you can study it and trace the signal path of the channels
Marshall-1974X-18W-Schematic.pdf
good luck with your trouble shooting
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brettl-s
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Re: Oh no! It's an intermittent fault!

Postby brettl-s » Mon 12/09/19 3:48 am

Thanks Geoff. I did trace the wiring from the jack inputs on the normal channel and saw that they seemed to be different. I'm not knowledgable about amp theory so would be grateful if you (or anyone!) could explain why the difference? I can't hear any change to my sound if I swap inputs on the normal channel, so am puzzled as to why single/parallel?

Thanks in advance, Brett.
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geoff 1965
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Re: Oh no! It's an intermittent fault!

Postby geoff 1965 » Mon 12/09/19 7:51 am

quite a few people say that about the single/parallel inputs but there is a difference,Josh recently posted a link from rockmumbles with a very good indepth explanation but i'm struggling to find it.
how is your confidence on taking some voltage readings? there is a chart in the downloads you can use.
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Daviedawg
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Re: Oh no! It's an intermittent fault!

Postby Daviedawg » Tue 12/10/19 3:27 am

When I built my 18 watt I wired the inputs to have one single and one parallel input in the normal channel. There is a distinct difference in volume and tone, especially as you increase the volume.

I looked at the schematic posted above. That appears to me to show two singles as was in the original. For there to be a parallel one of the switches would have to be connected to the other input at some point. Ian's Reference schematic ( "Pedal Monkey" schematic) shows this arrangement while Colin's Original shows two singles. Both are in the Downloads section.

Dd
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