What came in
A Marshall Valvestate S80 came in for a clean-up — reported to be in decent overall condition, just in need of some attention. It's a mid-nineties 2×40W stereo chorus combo, a bit of an odd duck in the Marshall range. The "Valvestate" name suggests tubes, and there is one — a single ECC83 in the preamp running at low anode voltage — but the power section is entirely solid state. Two channels, built-in chorus and reverb, a pair of Celestion G12T speakers. Made in Great Britain, Bletchley, Milton Keynes. A proper period piece.
On the bench it told a different story to the one reported. A loud mains hum was present from power-on, all the pots were scratchy, and both channels had an odd pulsing quality — like a tremolo effect on an amp that doesn't have one. Not what you'd call a clean bill of health.
Safety first
As always, safety checks before anything else. Earth continuity measured 1.5Ω — slightly above the <1Ω pass threshold, so that went in the book as something to investigate. Live and neutral isolation to chassis were both open, as they should be. Fuses checked out: T1A 250V slow-blow on the mains, 250mA and 100mA on the secondaries, all present and correct.
Listening test
With safety checks done and the amp powered up, the hum was immediately obvious — present on both channels with all pots at minimum, ruling out the signal path downstream of the volume controls. Both speakers isolated individually showed the same result, ruling out a speaker fault. The pots were scratchy throughout, the chorus hummed when engaged, and there was a buzz and click on power-on and a click on power-off.
The pots were the easy win first — a good dose of Deoxit through each one and the crackle was gone across the board.
Inside the chassis
The internal inspection was encouraging on first look. The PCB was clean, no burnt components, no evidence of previous repair. The ECC83 was original Marshall-branded, seated correctly. Both power amp heatsinks looked fine. The Samwha filter caps showed no bulging or leakage from above.
Less encouraging was what turned up on closer inspection: several capacitors had failed adhesive anchors and were loose on the board. Not unusual on a 30-year-old amp — the original rubber-compound adhesive used to secure tall components goes brittle and lets go. All the loose caps were re-secured.
The pulsing effect on both channels? Gone. Loose capacitors causing intermittent contact issues, coupling noise into the signal path. That one resolved itself once the caps were glued back down.
Tracking the hum
With the pulsing sorted, the main hum was still there. The filter caps were wired on the underside of the board — not the easiest access — but with some patience they were discharged and measured.
Both reading healthy: 4092µF at 0.11Ω ESR and 4410µF at 0.15Ω ESR against a nominal 4700µF. Not bad at all for caps pushing thirty years old.
So where was the hum coming from?
The answer was on the underside of the board. One pin on a main filter cap had a dry joint — the solder had never properly wetted to the lead, and over the years a shrinkage crack had opened up between the solder and the pin. Visually subtle; functionally significant. A prod with a chopstick confirmed it immediately — pressure on the cap eliminated the hum entirely.
Chasing the residual
With the main hum gone, a quieter residual hum remained. It varied with the master volume pot — louder at zero, better as the volume came up. A volume pot at zero grounds the signal, so hum present at zero points upstream, either in the ground reference itself or the PSU.
The earth continuity reading of 1.5Ω was the next thing to look at. The chassis ground screws were cleaned back, new washers fitted, and the continuity rechecked: 0.6Ω. With the master volume pot given another clean, the residual hum dropped to an acceptable level.
The reverb click
One remaining observation: a small click on power-on and power-off, present only when the reverb was engaged. A quick scan of the reverb circuit electrolytics with the ESR meter told the story — C37 measuring 3.3Ω and C41 at 3.8Ω against healthy values well below 1Ω. Aging caps.
A broader ESR survey of accessible caps across the board showed a similar picture throughout: most electrolytics running in the 2–4Ω range, consistent with a 1994 amp that's never had a cap refresh. The amp is functioning well, but these are numbers worth noting.
The reverb click itself turned out to be a known characteristic of this design — confirmed by the service literature. With the ESR values logged and the customer informed, it was accepted as-is.
Final result
Both channels clean, overdrive working, reverb working, chorus working. Power-on/off click acceptable. Hum resolved. All pots smooth and quiet.
The filter caps measured healthy, which was a genuine surprise given the board's wider cap condition — sometimes the big ones age better than the small signal caps around them.
What was done
- All pots cleaned with Deoxit — crackle resolved
- Loose capacitors re-secured — pulsing fault resolved
- Dry joint on main filter cap reflowed — mains hum resolved
- Chassis ground screws cleaned, new washers fitted — earth continuity 1.5Ω → 0.6Ω
- ESR survey completed, values logged for future reference
The filter caps measured healthy, but the wider ESR picture across the board tells the story of a 30-year-old amp that's never had a cap refresh. It's working well now. When the time comes for a full recap, the numbers are already in the file.