Ashdown ctm15 loud hum, no signal

[MoonBassAlpha]

Just turned it on yesterday and there's a loud 50Hz hum and no instrument sound. 

Haven't got round to swapping any valves yet,  but  could a faulty phase splitter valve cause this? It gets louder as the master volume is turned up, the gain control makes not difference. 

[Reggaebass]

It’s worth giving Dave Green a call at Ashdown, really helpful guy, I’ve met him many times and I’m sure he’ll tell you what it is 

[BassBunny]

3 hours ago, Reggaebass said:

It’s worth giving Dave Green a call at Ashdown, really helpful guy, I’ve met him many times and I’m sure he’ll tell you what it is 

+1 for. Dave Green. Pretty much the stock answer for Ashdown problems. Speak to Ashdown, best customer service going.

[MoonBassAlpha]

Thanks chaps, seems a bit like cheating though!

I'll try the phase splitter valve first,  then  call in the big guns!

[MoonBassAlpha]

Swapped the phase splitter valve,  still the same. 

[Pow_22]

if I remember rightly these were advertised as all valve but turned out to actually be hybrid, as such they may not need 'valve specialist' type repairers and maybe a local radio tinkerer could take a look?

[MoonBassAlpha]

I wonder if anyone on here has a circuit diagram for this  or the guitar version 'MoFo'? My Google fu isn't working on this one.

There's a short to ground on the preamp psu -5V, present with the tl071 and 072 op amps removed and with the -5V regulator leg lifted. -5V present on the leg,  so the regulator is OK.

I'm thinking one of the smoothing caps may have gone short,  not easy to remove. There's a couple of 2n7000 fets in there too, not sure what they do, phase splitter drivers? Clipping circuit?

 

[sandy_r]

1 hour ago, MoonBassAlpha said:

I wonder if anyone on here has a circuit diagram for this...

 

 

[MoonBassAlpha]

Thanks,  that's reassuring. 

[MoonBassAlpha]

Update,  I  sent a request for schematics last Sunday,  got a reply Monday morning 9.45 from Dave Green enclosing schematics and layout,  and a few words more  or less confirming my diagnosis,  and advising on a preventative upgrade.

The  preamp has a split +/- 5V supply,  the negative was shorted to ground,  turns out to be the filter capacitor failed short. I  was advised to upgrade all the 1000ųF 16V caps on the preamp supplies to 25V as these had failed before "and not just on this product "

Duly bought and replaced (and the little regulators) and the amp is running perfectly again. The humming sound I'd always assumed was inherent in the design is also  massively reduced,  so I  think this cap was on the way out since I  acquired the amp (second hand).

Kudos again to the  Ashdown customer support,  they have been excellent,  as many others here have noted.

[MoonBassAlpha]

Now, if I could just find the thread that mentions a  component change that stops the Treble boost when you turn up the gain. It was years ago though...