EBS Amp repair – HD360 Bass amp repair

EBS amp repair

Today I repaired an EBS amp repair on a 2 hour turnaround! The customer brought the unit to my Newark workshop from Nottingham.

EBS are a swedish brand. It’s the first time I’d worked on an EBS unit, though I’ve played alongside bassists who’ve used EBS gear and been quite impressed.  I contacted their technical support team for schematics and they were great – after I’d signed my life away they provided me with schematics very quickly.

The amp had an intermittent fault on the filter (EQ). When engaging the filter active switch there was an intermittent buzz sometimes, sometimes a very tinny treble sound, sometimes too much bass and sometimes normal operation!

The problem turned out to be with the bass control, which had an intermittent resistive short to ground at all times. Fortunately I was able to repair the bass amp with parts that I hold in stock. It arrived at 10am and was back after repair and PAT testing with the customer by 12:00.

If you need an EBS amp repair, please get in touch.

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

Laney VC30 Clean channel distorted

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.

Mesa Boogie Repair – Maverick (Dual Rectifier)

Mesa Boogie Repair - Dual Rectifier Maverick

This Mesa Boogie repair was one of the nicest amps to play, but one of the longest repairs of the year I’m afraid!

In this case, the Maverick had a problem with the channel switching. After a few minutes the rhythm channel would increase in volume, and the lead channel would bleed through an unpleasant fuzzy distortion. The fault was fixed by the opto-isolator replacement in the switching circuit.

It’s never a good thing to discover that your amp is faulty, but a broken amp is particularly bad news for owners of Mesa Boogie amplifiers. This is because Mesa Boogie construct their amplifiers in a way that lengthens the repair time.

The reason is that the amplifier is soldered into position on all four sides. Any changes to the board must be done after completely dismantling the amp (taking 30minutes each time). If several things are replaced in a more complex job, then the costs quickly mount up. Before I add to any myths, it’s NOT because the amplifier is PCB, it’s perfectly possible to make a PCB amp that’s easy to repair! It’s also possible to make a Point to Point amp that’s a nightmare to repair as well – I’ve seen them!

Anyway, what an absolutely great tone this Mesa Boogie has when it’s cranked. It’s completely unlike the rest of the Dual Rectifier series. The Black Widow speakers in this particular combo are brilliant speakers.

If you have a mesa boogie repair, please do get in touch and we’ll talk it through.

Vintage Fender Tweed Repair – 1953 Tweed Deluxe

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!

Laney Repair – RBX700


This Laney Repair was an RBX700 with an annoying intermittent problem that I was at first unable to replicate. The customer descibed the fault as a volume drop-out after about 10m or so of playing. On the bench at the repair shop it worked for several hours on several occasions without any issues.

When I have an intermittent fault to fix, I first of all inspect all solder joints in the amp with a magnifying glass. Then I run a high RMS signal to heat up the amplifier and attempt to dislodge any power supply or output stage joints that may be affected by heat. I’ll then use insulated tweezers to ‘poke’ likely culprits inside the amp and in PCB amps like this Laney I’ll flex the circuit board by hand. I don’t recommend anybody does any of this to a live amp unless they really know what they’re doing.

Anyway, all to no effect in this amp!

The repair issue turned out to be the FX loop jacks known as the the ‘dreaded switching jack problem’ that I’ve heard about, mostly from non-UK repairers who deal with amps in more humid conditions. The FX loop jacks in most amps are in circuit all the time, they utilise back contact switching to pass signal through when no cables are plugged in. In this Laney, the FX loop jack contacts had oxidised and a layer of insulation built up to cause the described symptoms.

Oxidised jack contacts can be fixed using simple techniques described here.

The customer sensibly said ‘but I don’t use the FX loop!’ but this issue is actually compounded by lack of use – in the other jacks on his amp, eg the input jack, any oxidation would have been rubbed off by constant use.

I like these Laney RBX amps – they make a great sound and don’t cost too much.

If you have a Laney repair, please get in touch.

Carlsbro Repair – Cobra 80

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.