The amplifier was producing no sound.

Curiously the HT voltage on the first pre-amp valve was only 95 VDC, way below spec.

I removed the valves and the HT on the first valve stage was still only 95 VDC. On further investigation I found that there was only 10k of resistance from the HT to ground on the first valve, ie something was shorting to ground.

Finding the problem was an interesting exercise.

The filter cap was removed, tested and given a clean bill of health.

I then traced the HT circuit around the PCB, NOT helped by the brown gunge liberally spread over the board.

After mucho careful scraping I found resistors R23 and R24, which lie over a PCB track with HT on it.

On lifting these resistors from the board, the resistance to ground was unmeasurable, and of further inspection, you could see that there was a short between R24 and the PCB track underneath.

I replaced R23 and R24 with 1W MF resistors (all I had in the relevant values), and stood them off from the PCM trace to avoid any arcing/ shorting in future.

The amp was brought in making a ton of crackling noises and was cutting out intermittently.

It had been looked at previously by someone who had concluded that the PCB had absorbed moisture (not possible), and had tried to cure this problem by removing the PCB and baking it for 5 hours! Suffice to can this didn’t cure the problem.

Anyhow, looking at the inside of the Classic 30 we can see why many techs don’t like working on them. The PCB snaps into 3 sections that are folded around, with the pots and jacks connected to the top panel and the valve sockets on the bottom.

Worse the 3 sections are connected together with jumper wires.

To change a component on the board requires that the board is removed from the chassis, and unfolded. Once the repair has been performed the PCB is refold around and inserted into the chassis. This is quite an involved process and each time you unfold the refold the PCB assembly you risk breaking the links or their solder joints.

Not a great design, although I expect this was the only way the amp can be as compact, and in their favour Peavey amps do seem to be pretty reliable.

Anyhow, on removal of the PCB I found a couple of dry joints and a broken wire link.

Dry Joint 1.

Dry joint 2.

Broken wire link.

I resoldered the joints , replaced the wire link and this cured the problems when the amp was reassembled.

Here’s the safety earth from a recent repair on a JMI era VOX AC30.

Suffice to say, if you were going to solder the safety earth to something, then the chassis might have been a better bet……

OK, that was a vintage amp with questionable earth bonding. Here’s the safety earth bond from a new Yamaha PA that had failed the PAT earth bond test.

The manufacturer had relied on the star washer piercing the power coat on the chassis to give a good bond to earth.

The power coat should have been masked off the power coating so there was bare metal under the earth bond to ensue a low earth resistance.

I scraped off the power coat before re-attaching the earth bond, and the amp passed the earth bond test.

Nathan’s now got his web-site, Brighton Guitar Workshop,  up and running so you can check out his work.

The amp is a 15W 2 x 10″ Sir Charles 15W combination.

Incidentally, the speakers are Tayden True Brits.

Portable Appliance Testing (PAT) is carried out on electrical portable appliances to verify that they are safe to use. However class III devices that are powered by an external power supply less than 50VDC (like a pedal for example) do not need testing.

The owner of the pedal was rehearsing at a studio and some there insisted that all their gear was tested, including this pedal!!

Now the regs state that PAT should be carried out be a “competent person”;  I would have thought that testing a pedal clearly demonstrats that you aren’t a competent individual.

The only redeeming feature of this episode is that the band weren’t charged for the testing!

Alfie has a SC15 1×12″ combination amp, and we aim to get some more of our range down to Brighton in the New Year.

As well as JPF Amps, Alfie has a great selection of used guitars, pedals and amps.

Although the amps are out of shot, the Flyers were all using JPF backline!

James and Patrick were using 1×12 Sir Charles 15 combos, and Dave (bass) was using a King Charles 30 head.

We’ll have more footage and sound samples coming soon.

Regent Sounds specialize in Fender and Gretsch guitars, and Fender, Vox and (of course!) JPF Amplification.

Regent Sounds are located at 4 Denmark St, London, and is the location of Regent Sounds studios, where many famous, and infamous, artists recorded back in the day, including the Rollings Stones and Black Sabbath. See the history page on the Regent Sounds web-site for more details.

Nathan has been getting requests from his customers for amplifier repairs, so has asked me if I would be interested in coming down to Brighton to repair amps.

Initially, I will aim to come down to Brighton once a week, although this may increase if demand is greater.

To book repairs in, contact Nathan on 07578 916284