Marshall Arts-Reviving a JVM210

This recent iteration of the 100w Marshall head arrived with no output. I surmised it had a blown power tube and thereby hangs a tale. A couple of tales, actually.

With today's move to more solid state devices and channel switching, well, you gotta get power from somewhere to run the little devils, and it's not always obvious to the eye when something in that line fails. I've also noticed that in the time it takes for a fuse to respond, a lot of damage can be done. Such was the case with a Peavey Valveking that I worked on a while ago.

Opening the chassis up I found a bulged power supply capacitor C49, a blown out bias resistor that had scorched a nearby capacitor and relay, a resistor that had turned from blue to brown R81, and I figured replacing those items would be required in any case and might fix things as well. In addition a new fuse and a set of new EL34s were installed.

The net result was zero. A quick check plugging a guitar into the effects return jack proved that the power section was working as advertised so the problem had to be somewhere else.

I was stumped.

As usual I happened to be trolling my favorite amp tech type chat boards and found a post by the ineffable Bill Machrone, a/k/a Billm, who's more or less the god of Blues Juniors. It was entitled

"How to make an audio test probe."

I was intrigued because I'd built something similar but a lot more elaborate than it needed to be. This was cooked up with a small amplifier from Radio Shack, a chunk of coax cable, a .01 capacitor, and a bit of heat shrink tubing. I knocked it out in about fifteen minutes. I already had a Leader audio signal generator from the last project-and it's a good one. Leader is some of the best test equipment i have ever seen at any price. This one cost about 25 bucks.

I fired up the amp and plugged in the signal generator. The first thing I noticed was that the tone didn't get past the first preamp tube .A quick voltage check revealed that the preamp tubes were not getting any high voltage although the phase inverter tube was doing fine. At this point I found it very useful to draw out the B+ circuit to see where I could intercept voltage, test for continuity and thus localize the problem. It was pretty quickly narrowed down to R82, a 15k 1w flameproof resistor that turned out to have about 100 megohms of resistance. A new resistor put voltage in all the right places, and luckily enough no other damage had been done.

The images show the parts that ended up being replaced and the signal generator and tracer rig.

The takehome for me was Bill's clear and concise explanation of what his signal tracer could do and how it could be used. It is HIGHLY recommended.

Shit Happens, Part 2.

If you've been following this blog any, you already know what I think about people who won't circulate their schematics and support the trade. It isn't good, either.

Notable exceptions to this trend are Fender, Peavey, Marshall, Traynor, Rivera, and for all I know others. If you know of an amp maker that supports the product with technical services and literature I would be glad to acknowledge that. After all, the motto we go by here is Joe Hammond's Maxim:  "That crow don't taste too bad if you use enough salt and pepper."

There are two reasons, I think, for the lack of support. The first is to steer repair traffic to so called "authorized service centers". The second is, I think, to make the product as cheaply as possible and thus disposable, using a skeleton crew to distribute and merchandise the product.

This creates difficulties for people like me, because without schematics and a technical support department, the range of repairs I can tackle on such products is limited, and the amount of time figuring something out is expanded. It also creates some resistance because everyone who buys one of these turkeys (not naming names here) and has a problem past when the warranty expires ends up getting hosed and has to dump the former amp at fire sale prices. That is, by definition, someone who is a) not happy and b) will remember who screwed him or her.

I was recently repairing a Vox AC30C2, which is the latest iteration of the venerable AC30 amp. It's quite different than any Vox that preceded it because it uses a single circuit board, has a handful of ICs, no tube rectifier and the cabinet is made from MDF. Without a schematic you can't figure out what's going on in a month of Sundays.

Here's some correspondence:

I have a customer with an AC30C2 that needs repairs. Is there any chance of getting a schematic diagram for this amplifier?

Thanks

Robert Luedeman

2008 66th Street

Windsor Heights, IA 50324

Here's the answer I got.

Dear Robert,

Thank you for your inquiry. Schematics are only available for authorized Service Centers. If you would like to become a VOX-authorized Service Center, please contact our Customer Service department at (631) 390-6800 or customerservice@korgusa.com.

Best regards,

xxxxx

Product Support Specialist

P:631.390.8737

F:631.390.8738

(Monday through Friday, 1pm to 5pm Eastern Time)

So there you have it folks.