repair Archives - Keld Ampworks Guitar Amplifier Repair

WEM Copicat Tape Echo Repair

25/11/2024 by rowan

Recently, I had the pleasure of completing a WEM Copicat tape echo repair on three different units, each presenting its own challenges. After years of use, they often require some TLC.

All three Copicats needed new tape loops, which are essential for achieving that classic echo effect. Additionally, extensive mains safety work was necessary to ensure safe operation. During the inspection, I discovered visibly leaking electrolytic capacitors in each unit, which I promptly replaced to prevent further damage.

One of the Copicats had a stiff arm that affected playback, so I carefully adjusted it for smoother operation. Another unit was extremely noisy, indicating potential issues with the tape mechanism; after thorough cleaning and adjustments, the noise was eliminated. Finally, one Copicat had a faulty channel 2, which I diagnosed and repaired to restore full functionality.

After completing these WEM Copicat tape echo repairs, each unit was tested and sounded fantastic, ready to deliver that iconic echo sound once again. If your tape echo needs some attention, don’t hesitate to reach out for a WEM Copicat tape echo repair!

Hughes & Kettner Switchblade Amp Repair

21/11/2024 by rowan

I recently repaired a Hughes & Kettner Switchblade amp that had a shorted mains transformer, causing power failure. Thankfully, I sourced a replacement transformer from a genuine parts supplier, ensuring top quality for the repair.

After installing the new transformer, I performed a full service, which included cleaning key components and checking all connections. I also replaced the old output valves with a new pair of EL34s to restore the amp operation.

Following the Hughes & Kettner Switchblade amp repair, I tested the amp at various volumes, and it delivered as expected. The owner was thrilled to have their amp back, ready for upcoming gigs.

If your amp needs attention, don’t hesitate to get in touch—your gear deserves the best!

AER Compact repair

18/11/2024 by rowan

Recently, I had the pleasure of repairing an AER Compact 60 amp that was experiencing frustrating audio dropouts. The owner had noticed that sound inconsistently faded in and out during performances, which can be a nightmare for any musician.

To diagnose the issue, I conducted a soak test—a method that involves running the amp at gigging volumes for an extended period. This technique helps reveal intermittent faults in the AER Compact that might not be evident during a brief test. During the AER Compact repair I monitored the amp, it became clear that the dropouts were linked to oxidized switches. These switches, often overlooked, can degrade over time, leading to poor connections and sound cut-offs.

After cleaning the affected switches and ensuring all connections were secure, I re-tested the amp. The result of the AER Compact Repair? A solid, consistent output with no further dropouts. The owner was thrilled to have their AER Compact 60 back in perfect working order, ready for the next gig.

If you’re facing similar issues with your gear, don’t hesitate to reach out for a repair!

Marshall 2554 JCM 800 combo repair

26/01/201826/01/2018 by rowan

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.

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. 🙂

If you need a JCM800 Combo repair please get in touch and I’ll get it sorted for you.

Bugera Repair – V55

19/01/2018 by rowan

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.

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.

Upon 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.

Marshall Silver Jubilee repair – Marshall model 2558

19/11/201706/11/2017 by rowan

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.

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

03/11/202008/09/2017 by rowan

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

09/10/201723/08/2017 by rowan

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.

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

Marshall Valvestate repair – Keld Ampworks, Newark, Lincolnshire

19/04/2017 by rowan

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.

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.

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

19/04/2017 by rowan

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