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Cloud VTX repair and restoration

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Randy Bohannon View Drop Down
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    Posted: 28 October 2022 at 1:54pm
The latest instalment in the resurrection of my teenage amplifier collection, Ole Faithful... the Cloud VTX 750, the amplifier that took more abuse than any other and kept going...Big smile





This thing has a cooling fan which put the average Dyson to shame and as a result barely even needed recapping despite having been on for most of the late nineties/early noughties.

I thought it might need brushing and hoovering, it actually needed sandblasting and acid pickling. 



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.
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kedwardsleisure View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 28 October 2022 at 6:02pm
Lovely! The VTX were essentially an improved version of the CV series with more modern mosfets and a few other tweaks, designed I think by Andrew Colley and then morphed into the VTX by Julian Usher.

They were an expensive amp in their day, very few foibles really, the earlier habit of going DC and melting the speaker relays was mostly cured by a factory mod to the offset circuit and fully incorporated into the VTX boards. The clip limiter was also modified from a VCA based one to an optical one which is more reliable, the old VCA chips sometimes developing weird behaviour as they aged.

Nice to see them being loved!



Kevin

North Staffordshire

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Randy Bohannon View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post Posted: 28 October 2022 at 11:09pm
Thanks for the information Kevin, always appreciate hearing the history of these things. 

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:

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kedwardsleisure View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2022 at 12:20am
I wouldn't touch the op-amps. nothing to gain. I would disable the clip limiter using the onboard selector and see if it still clips which might help to narrow it down. I assume you've checked the zeners are balanced and the droppers are in tolerance. You might also have a gate-source leak on one of the output fets which is loading the drive current down. Measure the idle current by checking the mV across the emitter resistors to make sure they're balanced. Use those j-hook test leads rather than multimeter probes, just to avoid an explosive slip of the probes. I can't think of anything else off the top of my head apart from the odd fractured tracks around the hot parts of the circuit.
Kevin

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Randy Bohannon View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2022 at 6:36pm
Thanks for the reply Kevin, in short I found the issue with the help of the nudge in the direction of the emitter resistors, R58 had developed a invisibly dry joint, found only by measuring a very weird high voltage across the source resistor of an ice cold MOSFET:



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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 30 October 2022 at 10:58pm
the leds can fail, just sayin!
Kevin

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Randy Bohannon View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post Posted: 31 October 2022 at 2:12pm
I hope it's not that, I've already embarrassed myself with the test points on the crown as it is...LOLLOL

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!Clap


Edited by Randy Bohannon - 31 October 2022 at 2:15pm
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Randy Bohannon View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post Posted: 31 October 2022 at 2:18pm
I tried a square wave test on it, it seems that the inherent bandwidth of the amplifier has been, sensibly given it's a PA amp,  been band limited.

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? Evil SmileLOL
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 01 November 2022 at 8:37am
you could have asked on here, I have the figures
Kevin

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post Posted: 01 November 2022 at 11:53am
Thank you for all your help Kevin, I certainly would have but as it turned out I ended up calling them up in the heat of the moment and was pleasantly surprised to be put through to the actual engineering team at the company. they said they thought it was 25ma (turns out this figure was for the later IRFP240/9240 revisions) but emailed me the next morning to say they'd looked through old notes and the figure for the Hitachi Lat-FETS (100mA per device). Really great service and getting hold of people like that is a great reason to buy British if at all possible, I have to say. :)

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. Beer


Edited by Randy Bohannon - 01 November 2022 at 11:57am
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kedwardsleisure View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 01 November 2022 at 12:01pm
Yes that's right, 100mA each for the hitachi fets and 25mA each for the International Rectifier ones.
Kevin

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Randy Bohannon View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post Posted: 01 November 2022 at 12:22pm
I suppose the considerations are different for a PA amp, reliability over all things, but my Hafler DH200 seems to have a sweet spot at around 200mA per device, be interesting to see if this responds the same.


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