Dust seems to have retained moisture and caused rusting inside the case, this was brushed back and pickled in rust remover before being given a coat of rust neutralising paint.

 

Boards were stripped of the heatsinks and completely scrubbed down with meths, the years of fag smoke grime that came off them was unreal.

Heatsinks went into the ultrasonic cleaner, half an hour and very hot cleaning solution beat elbow grease every time as far as I'm concerned... :D

Repairs so far have been a few dry solder joints, a TX 304 bais spreader transistor that measured a gain of 2 on the transistor tester and cleaning the level pots.

There is an issue with asymmetrical clipping on one of the channels, it effectively clips earlier on the positive part of the waveform than on the negative and the effect worsens under load. 

I suspect the driver transistors, will investigate and post some photos of what I mean later.

It's a good amp, not just in terms of longevity but also it actually sounds pretty good. It is quite a fast class AB MOSFET amplifier and I have been wondering whether an upgrade to the TL072s in the signal path might be worthwhile. I am trying to work out whether the opamps need to be JFET types like the TL072, and replaced with something like OPA2134, or whether a NE5532 can be slotted in. Looking at the resistors around them, I am thinking that a NE5532 would give better performance than the originals and not cost as much as the OPA2134 to boot.

The weird clipping behaviour is as below:

Waggling R58 by chance solved the problem with the amp running out of current on the positive portion of the waveform, makes sense as one MOSFET as effectively out of the circuit and the stage sagged trying to produce the current from the remaining two.

There is an imbalance between the channels that is due to the gain pots, they will need replacing it seems.

Another small issue is that the channel B signal LED won't illuminate no matter the signal level and the overall power is a little lower than expected, clipping at around 150-160w at 8ohms rather than the spec'ed 190W. It manages 280 into 4 ohms, which is in spec, but this seems to be explained by the rails measuring at 58v at no load rather than the 64v the schematic gives.

I'm listening to it now, it's a good sounding amp through a set of Orpheus floorstanders, punchy and very airy in the top end.

Massive thanks to Colin and Jon as Cloud Electronics, they went over and above to help top find the bias figures for my very old, Hitachi lat-FET using, unit; buy British!

Did notice it doesn't like clipping at 20kHz, ugly with a bit of rail sticking. It could probably do with the refresh of the small signal semis, maybe even a opamp swap in pursuit of the almost wholly pointless goal of a nice 20khz clipping? 

What are your figures, out of interest? I tried a minimum of 33mV across each resistor for a an average of 100mA across each device at first. It sounds good but lacks... I don't know a bit of treble sweetness that good fast MOSFET amps have? I'm going to give it another day or two, try it on my dad's Quad ESL63s and maybe push the bias up to around 200mA per device. From what I tried buggering about ~300mA is where you start flirting with thermal runaway and so for a retired amp, 200mA may be ok.

I found that the current sharing at idle wasn't spectacularly good in mine, with the MOSFETs furthest away from the fan consistently showing a higher bias current flow for a given adjustment of the bias pot than the FETs closest to the fan.

Current sharing at full load wasn't too bad, though again the MOSFET furthest from the fan consistently showed a greater current across both sides and both boards. Perhaps they are getting a bit tired and could do with replacing in an ideal world but for the life this amp will lead, no fear.

Thanks again for your help, I would have endlessly buggered about with the small signal stuff if not for your tip on monitoring the emitter resistors individually. 

In my case I think it was corrosion from the dust it trapped, that fan is an 11W, 160f^3/m beast compared to the average pair of 30f^3/m low voltage fans in the average amplifier and so I think it just sucks in more crap than most. this dust then holds moisture and corrodes the tracks through the solder mask.

Mine doesn't have a dust filter behind the front grille, incidentally, does yours?

I'm not surprised with your result, I couldn't get mine to overheat full power testing either, the load (1600w continuous in theory) however did... :D 

I'm setting it to the higher bias tonight for some critical listening tomorrow. 

Nah, not this puppy...

It didn't overheat but it didn't sound any better that I could hear either. I've biased it back to factory spec and declared it finished. 

I did think about maybe fiddling with the opamps and making the input filter a little less severe but, as you say, it probably not worth it.

Cheers Kipman, I have some thin foam I will line the front vents with.

It seems that Cloud went out of their way to find low output LEDs for the front the panel and I appreciate their efforts as the new one is too bright, really. I hsvr found a special, not-as-bright LED and bought it, whilst cursing my OCD.

however the signal indication for channel B still doesn't work. I really don't feel like pulling the thing apart again but it seems the problem is with the channel B PCB...

I have to say though, new pots seem to have improved the sound more so than any other single thing I have done including recapping.

  One repair was output transistors, several rotten tracks along the front of the board and one broken rotten track found under an audio cable that's soldered across the board on the cv1000.
  One repair was replacing the transistor that helps to regulate bias with temperature ZTX304 I'm going to swap with a BC639 but swap leg position, hope this works to do the same job.
  Another one had some strange resistances where someone had piggybacked some resistors along the speaker protection circuit somewhere. Putting those and one other resistor back to normal fixed that one.
  Would love to know where to set the offset. is that just done at the speaker output terminals?
Cloud amps i own and will sell many of them:
The 8ch with multizone mixer
The 6x 120W i think.
cv500
vtx750
cv1000
vtx1200
vtx1500

Petty decent, always ran very cool, need threadlock if you want to take them on the road. Bit heavy these days.

I noticed Old Croc mentioned noise on the older cv1000 etc due to an audio ic. I'm wondering if this is an issue i had on a cloud cv1000.
  I could sometimes hear scratchy random noise when the amps were on, not enough to hear over the music but annoying just the same. Assuming this was the same issue, does anyone know which IC tended to go noisy in-case i still have that amp somewhere.
  A Mr. Carlsons super probe would help here i imagine.


Just been checking bias on a vtx 1200 it was set around 25mA and max i could get was 60mA
Did you mean 100mV?
When i set bias to 100mV i have around 50mA across / between the + & - source legs of K414 and J119.

On the VTX1200 model i ended up setting bias to 25mA and offset to each speaker output to zero V. Couldn't find a service manual with the spec.