Vintage Marshall JMP 100W Amplifier Repair

We often get 70’s Marshall JMP amps in for repair; these are excellent rock amps and are still very good value for money.

One the major issues with these amps are the voltage and impedance selectors which are prone to falling out.

Someone had done a really good job of wiring the mains selector in this amp!

JMPmains

 

Not only is this a total hazard, the amp is actually wired to the wrong voltage! Suffice to say I hard wired the amp to 240 V, and disconnected the unused voltage taps from the voltage selector.

The input jacks also had the wrong value resistor soldered to them; the resistor in the photo is 4k7, but should be 470k. It looks very much like a mistake at the factory.

JMPres

 

Additionally one piece preventative maintenance I always do with this era of Marshall amps is to change the caps in the bias supply. If these go leaky then the amp can go into thermal runaway.

 

JMPbias

 

 

 

Posted in Amp repair, Guitar Amps, London, Vintage amps | Leave a comment

Ibanez Tube Screamer TS9 DX Repair

Here’s an Ibanez TS9DX Turbo Tube Screamer we’ve just repaired.

 

TS9DX2

 

The pedal came in with a common fault: the switching was intermittent.

This is usually due to the foot switch wearing out, however the foot switch in this pedal was fine.

After some prodding with a high tech implement (a chopstick!) I found that moving one of the transistors in the switching circuit turned the overdrive off.

Further inspection of the PCB, showed that the solder joint on one of the legs of said transistor was broken. I’ve had to explode the photo quite a bit, but the duff joint is in the middle.

 

TS9DX1

 

As is often the case with pedal and amp repairs, it’s finding the fault rather than effecting the repair that takes majority of the time.

Posted in Effects pedal, Effects repair | Leave a comment

Better Call Saul theme by Little Barrie using JPF Regent 25 amp.

New American TV drama Better Call Saul, which premiered on 8th Feb, is a prequel to Breaking Bad.

The theme tune for this exciting new series was composed and performed by Little Barrie and features a JPF Regent 25 amp that was used both for the guitar and bass guitar.

Click here for a sound clip.

I have to admit I didn’t see any of Breaking Bad, but I heard it was very good!

You can also see Barrie demoing this amp on our media page.

Posted in Amp building, Guitar Amps, JPF Amps | 1 Response

Ashdown MAG300 Bass Amp Repair

I’ve had quite a few Ashdown bass amps in the workshop for repair.

Here’s the inside of a MAG300 I had in a recently.

Ashdownmag300.4

The amp was not powering up, and on inspection the mains fuse has blown.

On energizing the unit employing a current limiter (a 60W light bulb in series with the amp), it was obviously that something had shorted out in the amp.

The most likely candidates in a transistor amp are the output devices; however these were fine, as was the main bridge rectifier (which I’ve seen fail in these amps before), so the finger of suspicion now pointed at the mains transformer.

AshdownMAG300.3

 

Unloading the transformer and applying mains to the primary showed that the fault was indeed with the transformer and a replacement was ordered.

Interestingly the replacement mains transformer (on the right in the picture below) is larger than the one originally fitted, suggesting that the original may be on the marginal side for power rating (a larger transformer has a greater power rating).

AshdownMag300

 

As expected fitting the new transformer fixed the amp.

Posted in Amp repair, Bass Amps, JPF Amps | Leave a comment

Ampeg SVT Repair.

Here’s a rare example of a 70s Ampeg SVT that I had in the workshop recently.

The SVT has, rightly, gained the reputation of being one of the greatest bass amp ever made. Even today the SVT is the industry standard bass amp for live work.

Working on these amps is not for the faint-hearted, with an HT in region of 700 VDC.

Unusually this was a UK voltage version; most of the 70s SVTs I’ve seen have been US voltage, necessitating a (very large) mains step down transformer.

 

SVT

 

The massive transformers don’t help with the weight.

SVT1

 

This amp needed a couple of the 6550 output valves changing. Fortunately I was able to match a couple of valves with the existing valves in the amp, avoiding the expense of a complete new set of 6.

SVT3

 

The amp achieved well in excess of 300W on the bench.

 

 

Posted in Amp repair, Bass Amps, JPF Amps, London | Leave a comment

Fender Blackface Deluxe Reverb repaired at JPF Amps

We’ve repaired hundreds of vintage Fender amps, and it’s always a pleasure to get these classic amps back in tip top working order.

It’s a testimony to Leo Fender that so many Fender amps from the 1960’s are still being used today.

Here’s a picture of the inside of a vintage 1960’s Fender Blackface Deluxe Reverb that we repaired recently.

 

 

FenderBFDchassis1

 

The amp was working, but at higher volumes would intermittently cut in and out.

On further inspection, we found that one of the ground wires from the eyelet board to the brass ground bus had become unsoldered.

FenderBFDeluxeChassis2

 

Not the best photo!

Anyhow, the mechanical vibration when the amp was being played disturbed the ground wire and hence the intermittent signal.

I’ve seen this problem on several Balckface and Silverface Fenders, so I now routinely check the soldering to the ground bus whenever I’m looking inside a Fender chassis.

Resoldering the wire solved the problem.

 

Posted in Amp repair, London, Vintage amps | Leave a comment

Two EH Memory Man Deluxe in for Repair at JPF Amps

Here are a couple of Electro Harmonix Memory Man Deluxe delay pedals we’ve had in for repair.

 

 

EHMM1

 

The pedal in the right was distorting badly with an ES-335 when set to unity gain; a problem with the Memory Man is that they don’t like hot guitar signals. This can be reduced somewhat by calibrating the pedal for optimum performance, which I did with this pedal. However, the problem can only really be cured by modification.

The pedal on the left was not passing signal at all, and on investigation the leg to the gain pot had broken off.

EHMM2

 

This is a very common problem with EH pedals (not just the Memory Man) as the PCB is often secured solely by the legs from the pots.

Posted in Effects pedal, Effects repair, London | Leave a comment

Two Marshall JCM900 Amps Repaired At JPF Amps

Here are a couple of Marshall JCM900s guitar amplifiers we’ve had in our workshop for repair in the last few weeks.

 

JCM900

 

The one on the bottom simply need new output jack.

Unfortunately the one on the top needed a new mains transformer and power valves, a significantly more expensive repair.

These are original early 90’s amps, and I suppose will soon regarded as vintage amps….

Posted in Amp repair, Guitar Amps, Vintage amps | Leave a comment

Orange Crush 15R & 30R Guitar Amp Repair

Here’s some photos from a pair of Orange Crush guitar amps I’ve had in for repair: a 15R and a 30R.

Both had a very loud hum when the amp was at idle.

In a solid state amp this is almost always due to poor power supply filtering. and further inspection showed that this was caused by the same fault in both amps, namely one of the electrolytic filter caps had failed catastrophically. Below are photos of the offending items (almost!) in situ.

 

OrangeCrush

 Orange 30R filter cap failure

 

 

OrangeCap1

 Orange 15R filter cap failure

 

And here’s the rest of the filter cap from the 15R, which I found in the bottom of the amp.

OrangeCap2

 

 

I replaced both filter caps in each amp with better quality units rated for 105degC  and this sorted the problem (as expected!).

Posted in Amp repair, Guitar Amps, JPF Amps, London | 1 Response

Edwyn Collins’ Girl Like You Barnes & Mullins Fuzz

Here’s a piece of history.

This is the actual Barnes and Mullins Fuzz used by Edwyn Collins to record his classic track Girl Like You.

GLYfuzz

 

 

The pedal was in the work shop for repair; nothing seriously wrong with, a broken solder joint on the PCB.

 

 

Posted in Effects pedal, Effects repair | Leave a comment
%d bloggers like this: