rowan, Author at Keld Ampworks Guitar Amplifier Repair

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.

Bass preamp repair – Yamaha RBX JP4

08/09/201708/09/2017 by rowan

This recent bass preamp repair was for a friend, bass player for professional party band the soul thieves. The preamp came from one of his backup instruments.

Identifying the circuit was a bit of puzzle due to conflicting information on the web. The bass preamp has bass and treble controls, a blend for the pickups, a master volume, two trimmers for gain, a mid frequency select knob and a mystery mid switch.

The model later became the John Pattitucci signature model. Certain other builds have piezoelectric inputs.

The repair was relatively simple – the unit was producing no sound on either pickup which turned out to be a loose connection between the two board layout. It’s worth noting that later units were a single board design.

The graphs of the response show the range of bass and treble control as well as the performance of the mid frequency control with bass Andy treble at max and minimum.

If you have a bass preamp repair, please get in touch and I’ll get it sorted for you.

Wireless mic repair – Line 6 XD-V30

08/09/201708/09/2017 by rowan

This wireless mic repair came in from the singer of Newark soul band the Acetones.

Both units powered up but no sound came from the receiver when the mic was used.

Upon investigation I discovered that the transmitter and receiver were able to communicate. They use 2.4GHz. The unit has 6 wireless channels, all of which were communicating, but not passing audio.

I discovered that the connection between the capsule and the transmitter was broken. The connection was made by 3 sprung pins on the transmitter and 3 concentric gold plated rings on the capsule. It’s a nice idea to allow easy swapping of capsules and easy manufacture, but here it wasn’t working properly! I soldered the two modules together permanently to ensure a good connection.

If you have a wireless mic repair, please get in touch.

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.

Dunlop MC404 CAE Wah pedal repair

23/06/2017 by rowan

In October last year I fixed this wah pedal repair for a lincoln customer. The pedal would not turn on, possibly as a result of the user plugging the wrong power supply in.

Often pedals aren’t economical to repair, but a unit like this is normally worth it. In this case it was a fairly simple repair, completed within the first hour and the completed wah pedal repair cost only £35.

if you have a wah pedal repair, please get in touch.

Laney VC30 – Clean channel distorted Repair – Keld AMpworks, Newark, Worksop

21/06/2017 by rowan

My customer brought this Laney VC30 from Worksop to my Newark workshop complaining that the clean channel distorted. It’s a pity, because he’d kept it in pristine condition!

Two other attempts had been made at repairing this fault. One by Laney themselves, and one by a local electronic engineer. Both had fixed the problem temporarily but it had re-appeared. Upon testing, the clean channel was very weak and distorted.

For this reason I took a different tack. At least one of the previous repairs had simply been replacing V1. This is a logical choice since V1A deals with the clean channel, but the valve tested as working and I suspected that the clean channel distorting problem was probably with an invisible dry joint in the valve socket. I reflowed all the joints on the socket and the problem dis-appeared. Of course I’ve guaranteed my work and in the unlikely event that the problem re-appears again I’ll take the amp straight back in.

I also noticed that the output valves were very poorly matched. This wasn’t causing the problem clean channel distorted problem, but it was worth rectifying to alleviate future issues.

I noticed some interesting design changes in the VC30 – compared to other Laney VC amps that I’ve repaired. Most interestingly, Laney have swapped their traditional spring reverb for a digital reverb based around the SPIN ASM reverb chip that they’ve used in other recent ranges, such as the Cub.

If you have a Laney VC30 or a clean channel distorted repair, please get in touch.

Smoke machine repair

03/05/2017 by rowan

A smoke machine repair, brought to my Newark workshop by a Grantham based DJ.

This is a fairly simple unit: A power supply, a thermostat and a relay to control the power to the smoke heater. There is also an external remote on a 6.5mm 3 pole jack cable.

The customer was finding that the relay sometimes did not switch on the heater. The problem was intermittent, meaning that it would sometimes happen, but sometimes it would not! Despite filling my workshop with smoke on many occasions, I couldnt replicate the fault.

Upon investigation, I discovered that the jack socket had a slightly loose sleeve connection so I replaced the socket with another. After a bit more smoking the problem still didn’t manifest itself, so I returned the unit to the customer.

If you have a smoke machine repair for me, please get in touch.

LED lighting repair – Unbranded LED Parcan

03/05/2017 by rowan

LED lighting repair - Safety Earth repair

This LED lighting repair was a bit of a horror story! Aside from fixing the problems with the two lights brought in, they needed several modifications to improve the unit safety.

They were brought to me by a Grantham based DJ. One of the lights wouldn’t power up, whilst the other had a flicker on the red LEDs.

The powerup issue was caused by a faulty power board. The unit has a cheap SMPS that the customer informed me had blown up before and been replaced by the manufacturer. Unfortunately they couldn’t provide spare this time so I swapped in the PSU from the Red flicker unit.

I then repaired the red flicker unit by reflowing the red joints on the front face. Although I found another off the shelf internal supply to fix this, the replacement boards were judged to be too expensive to warrant this course of action. Instead, I fitted a DC socket to allow the customer to power the unit from a cheap external power supply. This also gets around the safety issues detailed below by removing mains from the unit.

I was quite alarmed by the breaches of safety good practice in these LED lights.

The first thing that I spotted was dubious safety earth connection – shown in the picture. The manufacturer had simply twirled the safety earth round a mounting pillar and screwed a nut on top. Unsurprisingly, The earth connection was already loose when I opened the unit. I fixed this by soldering the wire to a solder tag and refixing to the pillar using a Nyloc nut. These are special nuts that do not loosen in the same way as traditional nuts. To comply with safety regulations, the safety earth should have its own fixing point, but this was impossible to modify without compromising the unit function.

The next safety issue that I noticed suring the LED lighting repair was that the board’s PSU was next to a grille with large holes, allowing a person with small fingers (a child) to inadvertently touch the AC mains through the chassis. this was particularly worrying, so I fitted a mesh with 3mm holes inside the existing grille. I used Nylon fixings to prevent any connection of the mesh to the mains board.

If you have an LED lighting repair, please contact me.

DJ Mixer Repair – Kam KAP1500 Audio Pro

03/05/2017 by rowan

This DJ Mixer repair was a bit of a departure for me, but I did it for an old customer.

The mixer is getting a bit long in the tooth: the send and PFL (pre fade listen) buttons were intermittent, the faders were scratchy and an intermittent fault with the echo unit.

The faders were easy – they simply need contact cleaner spraying inside. However one fader (mic 3) exhibited crackle even after cleaning. It didn’t sound like dirt, so I investigated further.

Upon investigating the issues with the echo and the mic3 fader I discovered that the board had several dry and cracked solder joints. Some joints just had very fine hairline cracks. I repaired the broken joints which fixed the mic3 and echo issues and I was able to identify that a certain pad to pin hole ratio was particularly problematic. This allowed me to take preventative action on similar sized joints across the board.

The next task was to fix the faulty intermittent PFL and Send switches. This required complete disassembly to allow the spraying of contact cleaner into the switches. The biggest challenge with a unit like this isn’t technical – it’s just the time it takes to remove the 50+ knobs and switches!

The image is a stcok image as I didn’t take a photo this time.

If you have a dj mixer repair, please get in touch via the contact page.

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!