repair

Marshall 2554 JCM 800 combo repair

by
Marshall 2554 JCM 800 combo repair-4

This JCM800 combo repair came to me for a full service before the user sold it. It also had a common problem with the power light illumination.

I spotted a few things during the service. A bit of preventative action has hopefully made the amp a more pleasurable experience for the user.
Replacement Marshall JCM800 Switch
There were a couple of dry joints around the potentiometers. All three preamp valves failed test.

The valve socket for the preamp valve was damaged and required replacement – I used a new Belton PT range socket, I find these to be durable and high quality with a good solid fixing. The first valve socket was

The illumination switch would have been a nice simple job, except that the part sold by Hot Rox as a “Marshall JCM800 power switch” was too wide for the chassis slot. Fortunately I found a suitable replacement from RS. That’s one to beware of in future!

The owner was good enough to write me a nice review of the service. 🙂

Replacing power switch Marshall 2554 JCM800 combo repair-1
If you need a JCM800 Combo repair please get in touch and I’ll get it sorted for you.

Bugera Repair – V55

by
Bugera repair V55

Turned this Bugera repair around quickly in time for it’s owner’s gig in Skegness. The amp is a vintage 55, or V55.

The amp was brought in for a standard 2 hour service, with a few particular points of attention. The owner had noticed that the clean channel was overdriving much sooner than previously. Also the standby switch was broken.

After testing the valves in my valve tester I discovered that the cause of the clean channel change was a very worn Shuguang preamp valve. I replaced it with a new Tung Sol 12AX7. I also discovered that one of the Bugera branded power valves was also faulty, requiring a change of both power valves to EHX 6L6s and a rebias.

Bugera repair V55Bugera repair V55 - replace standby switch

I’m not a big fan of Bugera’s bias control – a phono output that makes the bias voltage externally accessible and a little finger trim pot. The trim pot is a nightmare to adjust – screwdriver turned pots are much more accurate. And the phono socket exposes a relatively large voltage of between -40 and -80VDC. Also, the bias voltage isn’t a very good way of measuring bias (see this post by Aiken amps). Behringer use it as a way of encouraging you to buy their pre-graded valves. I use the most accurate cathode current method of biasing with a multimeter and a set of custom probes. I set the bias to 38mA, which is 70%.

A specific request on this Bugera repair was to replace the damaged standby switch. It was a quick and easy job – as seen in the image. I often hold generic parts like this in stockin order to fit a variety of amps, or I can always order specific pieces in to suit an amp’s aesthetic.

Bugera repair V55 verdigris on PCBBugera repair V55Upon inspecting the board I noticed a number of areas that were corroded – in the picture you can clearly see the Verdigris (copper carbonate) on component pins and PCB pads. I desoldered these pads and freshly resoldered them. I also spotted a couple of dry joints on valve bases and fixed these up. After this stage I refitted the PCB.The original screws were secured with thread lock, so instead I added a toothed washer and a plain washer – many manufacturers and repairers omit the plain washer, but this is very important in order to not damage the PCB surface and thus loosen the screw.

 

I was interested to note that the amp uses a ‘Turbosound designed’ speaker. This is because Bugera and Turbosound are both owned by Music Group (Behringer). The amp also uses a V1000 cool audio (I think this is anothewr behringer brand) multifx chip and a V4220M codec on a mezanine board to create the digital reverb.

If you have a bugera repair job for me, please get in touch.

 

Bugera repair V55

Bugera repair V55Bugera repair V55

Bugera repair V55

Marshall Silver Jubilee repair – Marshall model 2558

by
Marshall Silver Jubilee Repair

I was quite intruiged when a Nottingham guitarist brought this Marshall Silver Jubilee repair to my Newark workshop. These amps have taken on a bit of a legendary status, due in no small part to their association with Slash and Joe Bonamassa. I love how this amp looks, and it’s got an interesting tone control design, that deviates from the common Fender and Marshall designs.

The amp had blown a power tube, which the owner had replaced without rebiasing, tut tut! It still had very weak output, but occasionally it would blast out at full volume. It was also in need of a good pot clean.

First thing to do was to disassemble the amp. Marshall amps from this era are quite nice to work on and easy to take apart.
Marshall Silver Jubilee Repair - gutshot
The described problem was caused by oxidation in the FX loop circuit. This is a common problem with many amps and was easily fixed.

I observed some crackling on the EQ potentiometers which turned out to be slightly leaking coupling capacitors in the tonestack.

I then attempted to rebias the amp but discovered another problem. The plate voltage of 490V requires a bias current of to give 70% bias. I couldn’t get higher than 18mA with the valves the customer has fitted. Viewing the output waveforms in the scope, the amp showed significant crossover distortion. I checked the bias supply and found it to be working correctly. Although they tested within spec in my valve tester, it just wasn’t possible to bias these JJ EL34s in this particular amp. I fitted a new set of EHX valves and was able to bias the amp properly to 34mA.

This amp sounds great, even when I play it!

If you have a Marshall silver jubilee in need of repair, please contact me

Hughes and Kettner Tubemeister repair

by

I’ve wanted to do a tubemeister repair since they first came out a few years back. I guess it’s a testament to general quality that this is the first to appear on the bench. I’m a big fan of the design aesthetic. 3 channels and a midi switching amp for £550ish is a lot of bang for your buck.

However, the owner was more worried about a different kind of bang – and a flash behind V1 when he flicked the standby switch. Afterwards the amp wasn’t making any sound at all.

Hughes and Kettner service manuals are easily the most detailed of any of the modern manufacturers I’ve seen (honourable mention goes to Roland/Boss) but this wasn’t needed to disassemble the chassis and get back to the PCB. The chassis is all metal which reduces weight and size. There’s a bent metal lid, removable end pieces and a separate metal base.

Hughes and Kettner have a power tube management system called TSC. This checks tube balance and bias. Orange amps have a similar system (see orange amp repair). The Hughes and Kettner TSC can occasionally fail and show up an error code on the TSC lights. The parts must then be replaced with these parts. But that wasn’t necessary here.

I discovered that the HT fuse had blown, which accounts for the bang the player heard. The valves all tested good, but since the TSC turns on the power tubes using a FET in the cathode I strongly suspected the power valves. After replacing the valves and the fuse the amp came back to life!

If you have Hughes and Kettner tubemeister repair, please just drop me a message and I’ll get you up and running again.

PA speaker repair – dB Arena 10

by
pa speaker repair

Last year I completed a PA speaker repair, I’ve just found the photos of the damaged crossover so I thought I’d blog the repair.

The owner had bought a pair of these nice dB Arena 10 speakers from a disreputable eBay seller. One was working, but the other had a fried HF compression driver and a damaged crossover.

Fortunately the 1 inch dB compression drivers are commonly available from the likes of Thomann so this was easily replaced.

A resistor had also burnt out in the crossover so I replaced this with a higher spec part.

Upon completion I graphed the frequency response of the crossover and compared it with a known good unit.db arena repair

If you have a Pa speaker repair, please get in touch.

Marshall Valvestate repair – Keld Ampworks, Newark, Lincolnshire

by
Marshall Valvestate repair

This Marshall Valvestate repair was performed in an advisory capacity for a friend who’s got some electronic and soldering skills, but needed a hand with the fault diagnosis. It was an interesting job so I’m writing it up alongside my normal repairs.

The amp is a Marshall Valvestate 8008 power amp. Despite the name it’s a solid state amp, intended for pairing with Marshall‘s hybrid and valve preamps. In this case it was being used as a keyboard power amp but was exhibiting an unpleasant distortion on notes. The distortion appeared regardless of input level and could be heard on pure sine wave or on clean keyboard sounds. However, when looking at the output waveform on his scope, it wasn’t visible, the waveform looked clean.Marshall Valvestate repair scope pic

First off, when looking at a the board out of the case we noticed that the reservoir caps were bulging, so I suggested some appropriate replacements. Leaking caps can lead to poor supply regulation, resulting in distortion. We also cleaned the crackly input sockets. However, none of this made a difference to the problem.

Whilst my friend’s scope hadn’t shown a problem, I looked at the problem on a different (better!) scope, the cause of the problem became more obvious. The distortion was a type known as ‘crossover distortion’ caused by a poorly biased amplifier. The first scope (shown in the pic) hadn’t had the vertical resolution to reveal the problem (although you can just about see it when you’re looking for it).

I do a lot of valve amp bias jobs, but don’t often end up having to rebias many solid state amps. This is because solid state output devices don’t often need replacing, and don’t degrade in the way that valve power amps do. Solid state amp manufacturers bias their amps at manufacture, and it normally doesn’t need to ever be adjusted. In this case, however Marshall hadn’t even biased the amp at manufacture, there was no bias pot, just a generic bias network, common to all devices. We checked another Marshall valvestate 8008 power amp, and the other amp exhibited no distortion, so this scheme obviously works for Marshall in some cases.Solid STate amp bias

I suggested adding a variable resistor to the bias network to fine tune the amp. The variable values have to be selected carefully in order to reduce the risk of amplifier damage if the bias pot fails. Once the pot was fitted, we dialed in the amp for optimum bias and completely removed the audible distortion.

If you have a Marshall valvestate repair, get in touch!

 

 

Trace Elliot GP12 Repair – Keld Ampworks, Newark

by
Trace Elliot GP12 Repair

The symptoms exhibited by this Trace Elliot GP12 repair were a loud hum and distortion evident during note decay. Unfortunately I wasn’t able to replicate the customer’s problem which makes repair difficult, so I took a number of steps.

I left the amp on soak test for 2 hours, which involves heavily driving the output stage – this normally shows up faults that are temperature or power supply related.

What I did find was that one of the internal IDCs was causing audible noises when manipulated. I’ve seen this before with another Trace Elliot GP12 repair Mostly crackles, although there was a slight increase in hum sometimes which matched the symptoms described. Cleaning the IDC fixed this and that connection is now stable. I also cleaned the FX loop sockets because unused FX loops are a prime candidate for noises such as those described.

As part of a general service I also cleaned the Graphic sliders as there were a few little crackles – nothing to do with the described fault.

The next step in hum busting would have been to replace the reservoir caps. I avoided this because it adds significant cost and I had no concrete reason to suspect them. The description of the distortion on the note tail does sound a bit like caps, but the soak test didn’t reveal any problems with them.

The amp was returned to the customer. When I contacted him again the next month, the amp was still working well, another successful repair!

Note that the image is an old image, not the amp in question, it’s actually

If you are in need of a Trace Elliot GP12 repair, please get in touch via the contact page

HH amp Repair 

by
HH amp repair

I’ve seen and played most amps in my career in music retail and amp repair, but this HH amp Repair was a bit of a revelation. The clean channel is a thing of beauty.

It’s a full solid state oversized 1×12 combo with two independent channels. The tube emulation circuit seems to involve distorting a signal transformer. It’s OK but nothing special. The clean sounds on both channels are really great.

The amp needed repair because it was making a strange ticking noise when the reverb was turned up.

The reason for this turned out to be a lack of local decoupling on the reverb board. This is a fairly major design flaw, but the amp obviously ‘got away with it’ for the majority of its life, the problem only appearing as central filtering components aged.

Here’s a quick video of the HH amp repair, showcasing the crackling and ticking issue then the lovely clean sound when fixed.

If you have an HH amp repair job please get in touch.

Dual Rectifier Tremoverb repair – Mesa amp repair

by
Dual Rectifier Tremoverb Repair

This Tremoverb repair was  for a customer who was attempting a combo to head conversion. It’s a lovely lovely sounding amp, but the 2×12 speaker box in which it was housed was too heavy for the owner. The owner performed the conversion himself but in doing so, the hand wired input jack became disconnected so the he brought it in for repair and basic service.

The Tremoverb input was easily and quickly fixed. My minimum fee is the 1 hour diagnosis period, so I proceeded to give the amp a full valve test in order to give best value for money.Mesa weird PCB layout

The Valve test revealed that one of the power 6L6 valves was faulty and the others were out of spec. Most Mesa amps require a specific set of valves and this Tremoverb Repair is no different so I fitted a matched quad of Mesa STR-440s.

Whilst giving the amp a look over I noticed a very wierd bit of PCB layout. The Opto isolators that Mesa always used on amps (pre-late nineties) were stacked one on top of the other. I hope I never have to replace the bottom opto on one of these. As I’ve said before, Mesa really don’t think about repair when they’re designing. Pity, when they sound so awesome!

 

If you have a Mesa Tremoverb repair please get in touch.

MXR Dyna Comp repair

by
Dyna Comp Repair

The Dyna Comp is possibly the most famous of the compressors used on guitar. This Dyna Comp repair was similar in nature to the ESD fix that I’ve documented elsewhere in relation to the Carbon Copy pedals. 

The issue is with the switching circuit – the FX does not switch in when the pedal is pressed.

There was a small complication in the Dyna Comp repair however – this unit uses through hole components which are now obsolete. It took a very steady hand to fit the 1mm wide body of the modern equivalent part into the massive space left by faulty component, but fortunately I’ve done this before many times in R&D work.

If you’re interested in the Dyna Comp, check out my analysis of the Behringer DC9, a clone.

UPDATE: 2017

The MXR ESD repair kit is now available for sale at www.rsdsound.co.uk. If you’re an experienced repairer and skilled with SMT repairs then you can purchase this kit. If in doubt, just post it to me and I’ll sort it for you. Note that some DynaComp are not SMT.

http://www.rsdsound.co.uk/product/mxr-carbon-copy-esd-repair-kit/

If you have a Dyna Comp repair with problems, please let me know.