rowan

Vintage Big Muff Pi repair (Triangle PCB era)

by
vintage big muff pi repair triangle

A really nice example of an early Triangle Big Muff Pi repair. From googling, I guess it is late 60s, early 70s. It came in by post from London for a 24h turnaround repair on our express service.

The owner reported weak signal, loss of volume at times, and inconsistent behaviour when engaged and tone suck in bypass mode.

Initial Inspection

The PCB showed signs of age – several dry solder joints, probably factory. A few pads were damaged or lifting, most likely from previous repairs or stress over time. Three of the five electrolytic capacitors were leaking on test. All were replaced.

A quick check of the components also revealed a 150Ω carbon comp resistor had drifted way out of spec – measuring over 2kΩ! That would’ve had a big impact on the tone and gain structure of the pedal.

Work Carried Out

  • All five electrolytic capacitors were replaced as a set. The originals were well past their best.
  • The out-of-spec resistor was swapped for a fresh 150Ω part – we used a modern replacement, chosen for low noise and stable performance.
  • Dry joints across the board were reflowed and cleaned up, and damaged PCB pads were carefully repaired to ensure good connectivity and long-term reliability.
  • The battery clip and switch were both replaced. The old clip was missing a connection and held on with sellotape! The switch was intermittent.
  • The owner asked for a true bypass mod, but didn’t want any drilling or visual changes – so no LED, and still battery-powered only. New 3PDT footswitch, but no drilling.

Testing

With everything reassembled, the pedal was powered up and tested thoroughly. Signal is now strong and stable, and the phat Muff tone is back. I tested it with some pumpkins style riffs (OK, OK I KNOW IT’S NOT AN OPAMP MUFF!!).

A really satisfying job on a classic circuit. No unnecessary mods, just careful repair and subtle upgrades to keep things running smoothly without spoiling the original vibe.

If you’ve got a vintage big muff pi in need of attention, we’re always happy to take a look.

Marshall DSL40CR weak reverb modification

by
DSL40CR-reverb-modification

One common complaint I’ve heard from Marshall DSL40CR owners is the underwhelming reverb. To be fair, Marshall have never been famed for their lush reverb circuits – that badge tends to go to Fender or certain boutique builds – but even by Marshall standards, the DSL40CR’s reverb is particularly weak and uninspiring.

Interestingly, Marshall’s JVM series, the flagship model, delivers much more satisfying reverb tones, suggesting that the lacklustre performance in the DSL isn’t down to DSP limitations, but more likely a result of how the reverb is mixed into the signal path.

I’d had a few customers mention this in passing, but recently someone brought in their DSL40CR specifically asking if I could engineer a proper fix. After studying the signal flow and DSP mix arrangement, I found the culprit – and the solution turned out to be surprisingly straightforward.

Inside the amp, there’s a resistor that sets the mix level between the dry signal and the DSP-generated reverb. From the factory, this resistor is a 6.8kΩ (6k8) unit. That value results in a very low contribution from the reverb path, hence the barely-there effect.

By swapping this resistor for a 2.2kΩ (2k2), the reverb signal is allowed to mix more strongly with the dry path, leading to a much more useful reverb tone. After testing and A/B’ing the results, the customer was really pleased – it brought the reverb into much more usable territory without overdoing it or introducing unwanted noise.

A Word of Caution

This modification requires surface-mount soldering (SMD) skills – not something to be attempted with a basic iron and a shaky hand. The resistor in question is a small SMD type, and damaging the PCB while trying to replace it could leave you with a very expensive paperweight. So unless you’re confident working at that level, please don’t try this at home.

Want This Fix?

If you’ve got a DSL40CR and find the reverb lacking, I can include this mod as part of a standard DSL40CR service – it’s a quick, elegant solution that makes a real difference. If you’ve got a competent local amp tech, there’s enough information here for them to work it out. But again: if any of this seems unclear, don’t experiment on your amp – it’s not worth the risk.

Feel free to get in touch if you’d like the mod done properly.

Marshall DSL40CR

by
DSL40CR repair

I don’t often see a DSL40CR come through with this kind of erratic behaviour – it was one of those jobs that kept me scratching my head until the real root cause showed itself.

The Symptoms (as described by the customer)

  • The amp would unpredictably switch from the Clean channel into Overdrive (OD) without touching the footswitch or front panel. I noticed that putting the amp in standby would instanly trigger the fault
  • Otherwise, it sounded fine: the valves were healthy, the output stage appeared to behave, and no obvious catastrophic faults were present.

Removing a valve changes the power supply behaviour, so that’s where I started.

What I found was subtle: the control supply (low voltage rail) was not rock solid. The rail that feeds the preamp switching logic and control electronics was sagging or shifting, just enough so that under certain conditions the amp would interpret it as a switch command.

Once the control rail wobbled or dipped, the channel logic would misbehave and flip the amp into OD mode. A surprising symptom, but less surprising once you realize how sensitive those digital / switching circuits can be when their supply is out of spec.

I’m lucky enough to have a friend who knows the technical issues with this range of amps better than literally everyone else in the world. He suggested I look at the power supply section on the rear board as this is the most prone to sagging.

In addition, during the inspection I noted a few design / build quirks that I’ve occasionally encountered in the DSL40CR family (and in other Marshall amps), which may be part of why these amps sometimes exhibit low output or noise issues:

  • Low power / reduced volume: I’ve seen DSL40CRs (and sibling models) drift into low power mode, often caused by conductive glue traces (flux residue or glue meant for mechanical stability) creeping between printed traces, forming parasitic leakage paths. These “hidden conductive paths” can robs bias or collapse gain subtly, especially under heat.
  • Noise issues from 1 M grid resistors: Some of those 1 M grid or grid‑stop resistors in the preamp are notorious for turning noisy over time, especially when they drift in value or have internal contamination. That adds hiss or crackle, especially on the quieter parts of the circuit.
  • Shorting via ceramic resistor bodies: I’ve even found situations where the body of a ceramic resistor is so close to a high voltage (HV) board trace that it arcs or shorts under certain conditions – e.g. if the resistor’s coating is cracked, or there’s flux residue, or simply vibration has shifted things. That can cause weird intermittent behaviour, grounding, or leakage between circuits.

So, while the main symptom in this unit was the channel‑switching, it was wise (and necessary) to check for all these known weak spots in the DSL40CR line.

After these repairs, the DSM40CR stayed rock solid in each channel: no more phantom switching. I ran the amp through its full range of channels, settings, footswitch tests, power cycling, and warm-up cycles. Then I subjected it to a soak test at gigging levels for a good hour or two – it remained stable, no drift, no switching anomalies.

I also measured output power and bias and compared it to spec – it was right in the expected ballpark (i.e. no low‑power symptoms). That confirmed that the power stages and bias were healthy.

If you have a DSL40CR (or any Marshall) behaving oddly – randomly jumping channels, volume sagging, noise creeping in – don’t assume the worst (i.e. bad valves). A disciplined service + power‑supply / control circuit check is often all that’s needed to bring it back to life. Please do get in touch

WEM Copicat Tape Echo Repair

by

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

by

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

by
AER Compact repair

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!

Line 6 Helix Ground Loop issue when using the amp channel switcher

by
Line 6 Helix ground loop issue

Here’s a fix for a Line 6 Helix related ground loop problem I did for a customer a couple of years ago. He used the HX floor unit to control the amp channel switching and it created a really bad ground loop hum. It appears that the HX amp control sleeve is ground referenced, which is a pity on such an amazing unit.

You could modify a TRS cable to break the ground loop and it would work fine. On bigger stages though so the better is to build an adapter box to break the connection so that any length TRS cable can be used.

This isn’t a particularly common problem for Helix users, but it’s a real one that people occasionally come across.

The ground loop is caused by both pieces of gear. There’s a ground path running through the audio path of the amp and helix and a ground path running through the switching path of the amp and helix.

In some amps, the ground scheme will be such that noise is induced into sensitive parts of the audio path. The guitar amp designers won’t have paid much attention to this because most guitar amps with these sockets were designed before the advent of systems that share audio and switching functions, like Helix or GigRig. The amp designers were only expecting the user to plug in a latching footswitch to the jack. Latching footswitches don’t share an audio ground so no loop is created, so the designers didn’t need to control the ground currents. Thus the amplitude of the induced noise is determined by the amp grounding scheme resulting in different amps exhibiting the problem at different levels. Hope that makes sense. 🙂

It couldn’t happen if the Helix sleeve wasn’t grounded, it wouldn’t happen if the amp’s FX connector sleeve wasn’t grounded. 🙂

The opportunity to break the ground connection entirely is only available at the Helix or switcher end. Whilst an isolated footswitch design could be built in at the amp’s end, this would be awkward because it would require an industry standardised LV power source in switching systems to power a relay (or opto) in the amp. Worse, it would stop the footswitch socket working with the very common stand alone latching footswitches (which have no power). 🙁 No manufacturer is ever going to choose to do that. However it’s very easy to create a relay driver at the Helix end so that the sleeve is isolated from the audio ground (much like the GigRig remote switch options). 🙂

The noise level is also proportional to the amount of external noise as this is the external noise source that’s then picked up by the GND loop. In my workshop there’s plenty of external noise sources, just like in lots of venues. Studios, should hopefully be much quieter.

Unfortunately an ISO hum silencer box can’t fix it because this needs to respond to DC changes – the transformers in an ISO hum silencer will block DC. You could use one of those in the audio path to fix the problem by a different route (break the loop in the audio instead of the switching), but then that potentially messes with the audio path, so it’s not the best solution.

Above is a picture of the box I built for my customer. Very simple to make if you’ve got a soldering iron and a step drill. You can build one up yourself from the picture, or if anyone ever wants one building I can make one up at my usual labour rate.

Here’s the BOM to make your own:
Hammond 1590A style box.
Switchcraft 12B stereo metal jack socket
Neutrik NMJ6HCS or NMJ3HF-S or similar stereo plastic bodied socket
2 wires!

Here’s a video showing how it works:

Seymour Duncan Powerstage 170 Repair

by
Seymour Duncan Powerstage 170 repair

This Seymour Duncan Powerstage repair was a pretty simple job and was turned around next day for the customer on my express service. The customer was an American pop punk band, currently touring the UK.

Though he normally used a step down converter. Unfortunately the customer had inadvertently connected his 110V seymour duncan powerstage to UK mains (nominally 230V, really 240V) and it had blown up spectacularly.

The part that was blown up is a hifi poweramp, made by a subsidiary of Bang And Olufsen. They’re used in many amps nowadays, including Gallien Kruger, Acoustic Image, Fender Bassman 500 and Fender Tonemaster.

Icepower module replacement Seymour Duncan Powerstage 170 repair

Ironically, the unit has an internal selection for 110V/230V to allow the unit to be swapped very quickly between UK and US mains. I’m not giving details of what to do online as encouraging people to meddle with mains electricity is beyond the scope of this blog!

The band’s tour manager brought the unit in for repair and I got the new power amp module delivered on next day shipping.

I set the Powerstage back to 110V operation for the customer as had a spare for UK gigs but were touring US again immediately on their return.

If you have a Seymour Duncan Powerstage 170 that you need repairing, please do drop me a message.

Valve Amp Workshop Spec Suggestions

by

Roland RH-1 RH1 power supply for Roland JV2080 repair

by

I recently got a Roland JV2080 in for repair. The cause of the fault was the RH-1 power supply board. This is a switched mode power supply board that Roland use in a few products. Normally I’d be fine fixing a SMPS, but Roland can’t release schematics for this board as it was designed by a contract engineering company outside of Roland. That makes things difficult. Spare PCBs are also not available.

So I decided that the most sensible thing to do would be do design a replacement. I got part way through designing it when the customer decided it wasn’t something he wanted to proceed with. No hard feelings, he hadn’t actually authorised it. So it’s my lookout. But anyway. I’ve got a half designed Roland JV2080 RH1 power supply board. I’d be glad to finish the design for someone and get their unit running again!

Get in touch.