rowan

Marshall JCM 2000 Repair – TSL 100

by
Marshall-jcm2000-repair

Marshall JCM2000 repair

I get quite a lot of the JCM2000 series in for repair – both DSL and TSL. I think it’s because they were so popular, rather than any endemic issues. This particular JCM2000 is a TSL100, the triple super lead model.

I’m writing the blog post quite a while after the repair now, so I’m looking at my notes to remember the issue. IIRC, the amp needed new valves and the owner mistakenly paired valves 1+2, 3+4, instead of the correct 1+4, 2+3. He knew how to bias his own amp and knew about pairing valves, but had always replaced for a matched quad in the past. When he paired the valves wrongly things had become hot and smokey – the valves red-plated For more information on valve amp bias, check my bias FAQ.

The amp had become so hot that a screen grid resistor (R78) – mounted upside down in the amp – had melted its solder connection and fallen out of the amp, melting some cabling where it landed. I replaced this resistor (with a lighter weight, but equally rated component). I also replaced the burned out suppression capacitor that can be seen in the picture.

The lesson to be learned: read my bias FAQ before replacing power valves yourself. If in doubt, ask!

If you have a JCM2000 repair, do get in touch.

Vintage Fender Tweed Repair – 1953 Tweed Deluxe

by
Vintage 1953 Fender Tweed Repair

This Vintage Fender tweed repair is the oldest amp I’ve worked on to date. I’m informed it was made in 1953, its a 5C3 design.

The 5C3 deluxe tube complement is 2x 6SC7 for preamp and phase inverter and 2x 6V6 running a push pull output stage to about 12W. The amp uses a valve rectifier – the stock is 5Y3. The customer was running the whole amp run from a nice meaty 240V to 110V transformer from maplins to provide the US mains voltage.

The customer brought the amp in as not working and requested an HT capacitor refit.

I noticed that the HT voltage gets to 500V at inrush before dropping to below 450V (the rating of the existing caps). Since the amp doesn’t have a standby switch this means that the caps are subject to significant stress at turn on. I experimented with a 5V4 rectifier valve to reduce the inrush current, however the 5V4 results in a higher HT with not much headroom before the 450V rating. I recommended 600V caps for the repair to allow for a good safety margin.

600V 15u/16u caps aren’t that common in these days of low voltage electronics – unfortunate for those attempting a vintage fender tweed repair! So we used 350V electro caps in pairs. This didn’t look as pretty, but functionally provides the same performance. I never recommend NOS electrolytic capacitors as electrolytic capacitors degrade even when out of circuit.

I removed the low value bleed capacitor on the primary side of the mains transformer as this is considered unsafe by today’s standards. If the cap becomes faulty and passes DC then the amplifier chassis can become live. Unlikely, but not nice! I always insist upon following mains safety procedures – even on vintage amplifiers.

The fault with the amplifier was actually a loose ground connection underneath the eyelet board – simply fixed.

One last problem – these 6SC7 valves in this particular period of vintage fender amps have a horrible tendency to be microphonic. The speaker vibrates the cabinet, the cabinet shakes the rather old valve base, which shakes the valve and the whole thing takes off in LF feedback. There’s a probably a reason Fender dropped them after this model! I first assumed that the fault was with a worn out valve but on ordering a replacement I now believe that this is likely to be a feature of all 6SC7s. The solution I found was to replace the valve base with a more mechanically rigid Belton one, to stop the valve moving in its base. Interested to hear from anyone else who’s experienced this!

MXR Repair – Carbon Copy

by
MXR Repair

This poorly delay pedal repair had the same problem as this MXR repair job from last year. This is a known ESD fault with Carbon copy delay pedals ‘of a certain age’.

As you can see on the photo, this Delay pedal uses surface mount technology (SMT). This can be a little fiddly to repair, but I often prefer it to through hole!

The faulty items are JFETs Q10-13 (the little 3 pin triangles. As you can see I’ve fitted a tiny Transient suppressor between two legs of the replaced JFETs. This should guard against recurrences of the same problem.

I’ll take another moment to thank the guys at MXR/Dunlop (partic. Derek Jones) who provided excellent tech support the first time I came across this issue.

The owner of this pedal plays harmonica through it with the Whitefern Mountain String Band. If you’re passing through South Wales, then check them out!

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.

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

If you have a delay pedal repair or a Carbon Copy Repair please get in touch.

Carlsbro Repair – Cobra 80

by
Carlsbro Repair

Carlsbro Repair – Cobra 80

The owner of this Carlsbro repair complained of an intermittent crackling and occasional dropout. The fault was found to be within the power supply – a not uncommon source of trouble in bass amp repairs.

One of the pins on the bridge rectifier had developed a hairline crack through the pin. It was mounted to the rear panel, and when the panel flexed, the power connection arced, creating the described noises.

This repair was easily finished within the diagnostic period. When I find a quick fix for customer’s amps I always perform a routine ‘mini service’ to ensure the customer gets their money’s worth.

They’re not bad little amps these old Carlsbro Cobras – I gigged for many years in a jazz band using a Cobra 45!

If you have a Carlsbro Repair job that needs doing please, get in touch.

MXR Repair – Carbon Copy

by
MXR-Repair

This MXR repair, a Carbon Copy, was a simple fix. ESD (static) had destroyed 3 switching transistors used in the bypass circuitry. After identifying the fault I contacted MXR to find the part numbers of the damaged items. MXR are aware of the issue and recommended a modification to significantly reduce the likelihood of a repeat problem. I’ll take this moment to thank the guys at MXR/Dunlop (partic. Derek Jones) who provide excellent tech support.

UPDATE: 2016

I’ve since repaired many of these and I now keep the parts in stock ready for the next MXR repair.

As you can see on the photo on the other delay pedal repair page , this MXR uses surface mount technology (SMT). Some find this a little fiddly to repair and many repairers won’t touch it, but I actually prefer it to through hole!

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.

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

MXR Repair – Carbon Copy

If you have a an MXR Repair or a Dunlop Repair job that needs doing please, get in touch.

Marshall Repair – JCM2000 DSL100

by

Marshall Repair

This Marshall Repair was brought to me with one of those sneaky ‘hard to find’ problems. The amp was blowing fuses when the customer was playing it. He’d taken the amp to another repairer who hadn’t found any problems. Unfortunately the amp continued to blow fuses.

Guitar amplifier repairs of this kind often have to be treated in a different way. After an initial examination, opening up the amp and checking for any burnt components or loose joints I placed the amplifier on soak test. This involves playing a representative signal through the amp into a dummy load in the conditions that the amp was in when it was blowing fuses. During this soak test I observed that under certain conditions one of the power valves was sparking internally. I swapped the set of valves, re-biased and then placed the amp back on soak test. Since the amp wasn’t blowing fuses or sparking any longer, I decided that it was ready to go.

If you have a Marshall repair job that needs doing, please get in touch via the contact page.

Laney Repair – VC50

by
Laney Repair

Laney Repair

This Laney Repair was a mixed bag of several issues! The amp is a VC50 – 5 12AX7 valves and 2 6L6s in the output. This particular specimen has a strange an whine present whenever channel A’s spring reverb was used, also a faulty gain control on channel A, an intermittently working footswitch and a rather noisy pre-amp stage.

Since the amp is old and worth under £300 in used condition I advised the customer that an in depth repair might not be in his best financial interests. We agreed a price limit, within which I was able to fix the faulty gain knob, the reverb oscillation and the intermittent switch. This then allowed the customer to sell the amp in working condition and he put the money towards a new amp that suited his purposes more closely. Everyone was happy!

Forgot to photograph this one – apologies for the stock image!

If you have a Laney repair job that needs doing, please get in touch via the contact page.

Dual Rectifier Repair – Mesa Boogie – Ampworks, Newark

by

This Mesa Boogie Dual Rectifier Repair is one of the more unusual problems I’ve come across. The Mesa was brought in with very low (and unpleasantly distorted) volume and blowing fuses. I tracked the issue to the output circuitry and checked for all the usual failures with no result. The output transformer looked to be the faulty component but the issue turned out to be a faulty component that was loading down the 4 ohm input with a low impedance. Component changed and the amp springs back to brutal METAL life!!

UPDATE 2016:
Mesa Boogie repairs are among the most difficult jobs to do, partly because of the complexity of the amp (which makes for a fantastic product) but also because they design their PCBs in a way that aren’t very friendly to repair. I’ve done many mesa boogie amps though (you can read about others here) and they are always worth fixing!

Mesa Boogie Repair

If you have a Mesa Boogie Dual Rectifier repair job that needs doing, please get in touch via the contact page.