Marshall 2554 JCM 800 combo repair

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. šŸ™‚

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.

Marshall Silver Jubilee repair – Marshall model 2558

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

Marshall Valvestate repair – Keld Ampworks, Newark, Lincolnshire

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.Ā In this case it was being used as a keyboard power amp but wasĀ exhibitingĀ an unpleasant distortion on notes. The distortion appearedĀ regardless of input level and could be heard on pure sine wave or on clean keyboard sounds.Ā 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 aĀ the board out of the caseĀ we noticed that the reservoir caps were bulging, so I suggested someĀ 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. TheĀ 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 canĀ just aboutĀ 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 powerĀ 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.Ā 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.Ā The variable values have to be selected carefully in order to reduce the risk ofĀ amplifier damage if the bias pot fails.Ā 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!

 

 

Marshall JCM900 repair – Ampworks Newark

Marshall-JCM900-repair-2 copy

This JCM900 repair arrived after the owner found that it was lacking in volume.

It’s a 2009 unit, two channel, with a 2203 type circuit on channel A and a higher gain circuit on channel B.

The problem appears to have been with the amplifier bias. Although the valves were matched, the bias on this JCM900 repair was set very high. The bias set the dissipation at about 95% – Marshall suggest about 70%. This causes the output to compress much earlier than would otherwise occur. Fortunately this is easy to fix.

I then offered to soak test the amp which is a means of testing the amp in the conditions it would be in rehearsal or gig. This is great for revealing those hard to find problems that would otherwise missed. It revealed a minor fault with the preamp – a crackle appeared behind the note transients. Since this was a relatively minor problem, the owner chose not to have it looked into.

I sometimes have been known to complain that some amp manufacturers give little thought to ease of repair – mesa boogie have had some stick from me in other reports. Some marshall repairs – thinking the DSL range in particular – are a bit difficult because of their complexity but this JCM900 repair is a delight to work on. They’ve really thought things through both in terms of reliability and ease of repair. It’s probably the easiest channel switching amp to repair that I’ve seen.

Although the channel A had a great plexi type tone, the owner isn’t a big fan of the Drive sound in ChannelB – he’s thinking of modding the amp to acheive the fabled Silver Jubilee tone so stay tuned – this one might turn up again.

If you have a JCM900 repair, please do get in touch.

Marshall – 2205 Repair

Marshall-Repair-JCM800-Repair

This 2205 repair came to me following an unsuccessful operation by another tech.

The 2205 were a new step for Marshall – and were one of a group of amplifiers that formed the ancestors of many channel switching amps we now take for granted – including amps by Mesa Boogie, Cornford, Hughes & Kettner, Marshall’s own JCM 2000 (DSL and TSL) and too many more to mention.

Modern Marshalls use relays for switching, but the 2205 uses a transistor array. Unfortunately it was that chip in this 2205 repair that turned out to be faulty. My customer tells me that there were 3 2205 versions, of which this is the second (1987 I think).

The original switching circuit has problems with popping that the original repairer had worked on. Unfortunately in doing so they had damaged the transistor in the switching chip that shorts out the distortion channel during clean mode. This meant that both channels passed the distorted signal. Oops!

I replaced the damaged chip and added a little extra anti-pop protection.

The amplifier also had hum problems, which were fixed by a partial implementation of the 3rd 2205 version. Swapping the triodes that were used for different functions (spring reverb and gain stage) reduced the hum to a very low level.

Disclaimer/Myth avoidance: Transistors and CMOS switches are good for switching some things, relays are good for others. Transistor switches don’t affect tone any more than relays if they’re correctly implemented.

If you have a Marshall 2205 repair job for me, please get in touch.

Marshall MG Repair – MG15DFX, Ampworks, Newark

Marshall Repair

No job too small! This Marshall MG Repair was a simple job – I wouldn’t normally take a lower cost amp in to repair – with the cost of a 2nd hand replacement being around the Ā£50 mark, more than an hours labour and the repair becomes uneconomical for the customer. But this little Marshall MG had only a minor ailment – the emulated out was not working. It was a simple job to check the resistive network that links the preamp to the socket – no fault there. Cleaning the socket improved the situation, but didn’t fix it entirely so the socket had to be replaced.

Note to customers – always keep sockets that you don’t used lubricated with some cheap electrical contact cleaner – only a couple of quid spent, the pot last for ages and you can clean guitar pots and switches as well as input sockets. Never use on a live amp.

Marshall MG repair MG15DFX

If you have a Marshall MG Repair job, get in touch.

Marshall Repair – JCM2000 DSL100

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 burntĀ componentsĀ 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.