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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 18th Jan 2018, 5:19 am   #21
Boater Sam
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Default Re: Another DC coupled amp

Been caught out like that before and I always say it will be the last time, never is though.

Always check previous repairs, not everyone is as proficient as yourself.
I've had valve bases wired in mirror image, droppers fitted upside down, wrong windings on mains transformers used for rectifiers, all sorts.
There are muppets out there.
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Old 18th Jan 2018, 7:54 am   #22
Ted Kendall
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Default Re: Another DC coupled amp

Quote:
Originally Posted by Boater Sam View Post
There are muppets out there.
Yes, indeed.
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Old 18th Jan 2018, 8:23 am   #23
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Default Re: Another DC coupled amp

They would have been all bodge attempts to find the real original fault, the mis-loaded resistor from the factory. Muppets for sure.

I wonder how many amps were turned out with a similar fault?

The Blomley architecture would make a monkey out of anyone assuming an ordinary circuit.

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Old 18th Jan 2018, 4:56 pm   #24
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Default Re: Another DC coupled amp

Thanks guys, but despite this apparent fix, the DC offset of 475mV is still there

With the Mosfets still out of circuit the +/-16V rails were now back and stable with the variac at 240V,

But when I reinstalled the Mosfets the +16V rail now goes up and past 16V if I wind the variac much beyond 160V, i.e. 20V at 200V input? How can this happen, keeping in mind nothing on this amp seems to work as expected!

The drivers Q1/Q4 both read good on my tester, I haven't removed Q3 but a diode test suggests it's good? The gate/source voltage are approximately 4V on Q5/8 and 3.75V on Q2/7 (at 160V input)

So, no closer than I was in post#1, any idea guys?
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Old 18th Jan 2018, 5:13 pm   #25
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Default Re: Another DC coupled amp

The opamps will have internal diodes from their output pins to their supply rails (ESD protection)

If something tries to drive current into the opamp output pin and drive it beyond the 16v rail, the diode will turn on and route current into pulling the supply up too high (to the detriment of the zener)

Possibly one of the opamps has a damaged output stage. They've been somewhat overrun.

On that circuit, the current-mirror output stage can be isolated from the earlier stage, and with the centre of the resistor string grounded, you can look to see how healthy the output stage is.

Remember that this is a CURRENT output stage. If there is a little offset CURRENT, it can look like a significant offset voltage if there is only a high DC resistance load. The loop with the opamp should fix the offset, but if something is still dodgy you may still see the offset.

David
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Old 18th Jan 2018, 5:30 pm   #26
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Default Re: Another DC coupled amp

I wonder if it's a bad ground somewhere, might be worth measuring any voltage between transformer CT and the junction of the two 16 volt zeners when the 16 volt rail rises.

Lawrence.
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Old 18th Jan 2018, 5:33 pm   #27
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Default Re: Another DC coupled amp

David thanks, lots to consider there? I should have said the DC offset is into an 8R dummy load.

Thanks Laurence, I shall check that
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Old 18th Jan 2018, 7:30 pm   #28
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Default Re: Another DC coupled amp

Regarding the rising 16V rail - I have to admit to a schoolboy error, I used a 15V zener in series with a 1N4007 to get the 16V, but I put it in back to front

The DC offset is now the only issue, hopefully!
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