Peavey Mark IV fault

[Jono Bolton]

I've got a Peavey Mark IV bass head that I've been having issues with recently. The sound cuts in and out and has a horrible distorted, oscillating sound. I've taken it for a service and it was supposed to have been sorted, but I used it today for the first time and it's still there. The issue is intermittent, so presumably when they tried it it was fine (it's working now). Here's a rubbish video I took on my phone earlier:

https://youtu.be/vfifHubrIV4

Any ideas what it could be? I left a voicemail for the shop as they were shut when I went to practice, but it's not the cab, cables or bass as I used the same set up with the communal head in the studio and it was fine.

Thanks!

Edited May 2, 2016 by Jono Bolton

[MoonBassAlpha]

I've never heard those sounds from an amp. Sure don't sound happy

[3below]

You could try emailing Peavey support in USA. I have found them to be excellent, to give an example I enquired about the heater current available from a 1980s valve amplifier transformer. Answer (and other useful information) came back very quickly. You could also try the Peavey forums. If you were closer I would offer to take a look and see what might be awry.

[Jono Bolton]

[quote name='3below' timestamp='1462313401' post='3042128']
You could try emailing Peavey support in USA. I have found them to be excellent, to give an example I enquired about the heater current available from a 1980s valve amplifier transformer. Answer (and other useful information) came back very quickly. You could also try the Peavey forums. If you were closer I would offer to take a look and see what might be awry.
[/quote]

Thanks, I spoke to the shop I took it to for a service and told them I still had the same issue and they said they'll look at it again. As the noise is intermittent it may well have seemed fine when they tested it out. I played through it this evening for about 15 minutes and it was fine, but I went back about 15 minutes later and it was making noise again. I filmed a 4-minute video of it to show them when I take it back, including a really weird high pitched noise (almost like a pitch shifter) when I turned it off. I turned it back on again and it was fine.

[Phil Starr]

sounds like a power supply fault to me. Maybe the power supply smoothing capacitors breaking down?? I've had to replace those before in an old Peavey

[grandad]

Agreed, a likely candidate is any/all electrolytic cap's used for decoupling. I often just changed the lot for starters in oldish amps.

[Phil Starr]

Yep the electrolytics only have a reliable life of about 15 years according to Wickipedia. The Mk IV's probably have the old liquid electrolyte in them and they dry out over time. The big ones in the power supply are usually the first to go as they get hottest in use. I had to replace the ones in a Mk III a few years back. The tech is so much improved the new ones were about a 1/4 of the size of the old ones.

[Jono Bolton]

That sounds like the case; when I took it back & showed the shop the video they said it was likely the power supply and the noise when I turned it off was a capacitor discharging. Is this an easy/expensive fix? I've already paid for a service, is this the sort of thing one would expect them to have picked up on during a standard service?

[3below]

Should be a cheapish fix, capacitors are not expensive. Well within the scope of DIY repair since the PCB tracks in the older Peavey kit are substantial.

[Phil Starr]

I'm not a big fan of a 'service' of anything with no moving parts. There's nothing to lubricate and no serviceable parts so a service is nothing more than a quick check everything is working and maybe a clean out of the innards. As you have realised an intermittent fault won't show up.

I don't think the techs have got anything wrong in this case.

I'd be careful about replacing the power supply caps unless you know what you are doing. It's a simple unsolder resolder task in these amps but there is a high voltage circuit for the amp and a low voltage supply for the pre-amp stages if I remember right so I'd only go ahead if You can read a circuit diagram for example. Only you know how confident you are. If you do then beware the capacitors, they can hold there charge for some time after turning off and you can get a really nasty electric shock off them even with the mains unplugged. DC shocks are much worse than mains shocks as I found out the hard way Power supply caps need discharging with a resistor before working on them. The other part of the replacement is thst the new ones will be much smaller than the 1970's ones you are replacing and you will need to do some modifications to the clamps to hold them in place.

[Jono Bolton]

Having spoken to the amp techs this afternoon it appears it was a problem with a leg on one of the power supply caps; they've got a replacement in and will hopefully have it on the bench today to pick up tomorrow. As I've already paid for a service I'll only need to pay the cost of the new cap (about £8) and that should hopefully be the end of it; I'll keep you all posted.

[Jono Bolton]

Picked up the amp today and all appears to be ok. I was surprised to see that the old cap was not far off the size of a Coke can, but it only cost a fiver for the replacement. It did make some weird buzzes at a couple of points but I'm putting that down to the input on my CV Precision as it was slightly rusted when I got it last week and I haven't had any problems with either of my other basses. Only time will tell if it's sorted or not but it all seems fine at the minute.

Thanks for all your input. As you were gentlemen.

[3below]

Enjoy the benefits of kit that can be repaired locally and cheaply. I like (older) Peavey kit for this reason, bombproof and IMO good tone.