quad34 muting circuit

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14-Jul-2017 08:09 AM

Margus

Margus

Posts: 6

Hi,

i am now a proud new owner of quad 34 and FM4:)

all works well, except 34's output muting cirquit

T14 and T15 should close in short time after turning amp on, but they don't. Checked power supply voltages, these are ok. Voltage drop on IC21 is big enough to open T13. D33 LED seems to work correctly, lights up after few seconds. For now, i removed T14 and T15, amp works well. Cap C69 looks ok, haven't measured it.

Before starting to change components, thought to ask if this is known fault, and is it even worth to fix it. Amp doesn't produce any transcients on start/shutdown even when T14 and T15 are removed

with best, Margus


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14-Jul-2017 09:51 AM

Joost Plugge

Joost Plugge

Posts: 943

Hello,

If D33 lights up after a few seconds, it looks ok. But check the base voltage of T14 and T15, it should be low enough to close T14 and T15, during operation of the 34, T14,15 should be closed, not opened.

Joost Plugge

DaDa Engineering

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14-Jul-2017 10:26 AM

Margus

Margus

Posts: 6

yeah, it was my typo

T14 and T15 stay opened, not closed, outputs stay muted. But seems the problem is either T13 or D32 zener. Measuring on D32, there is about 6V between anode and cathode, should be about 10V i believe?

wb, Margus

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14-Jul-2017 12:05 PM

Joost Plugge

Joost Plugge

Posts: 943

Hello,

D32 limits the voltage across R103 to 12V. This means that point A is at -3.4V, enough to close T14. To close T14, the base voltage should be lower than 0.6V. Your measurement indicates that point A is 2.6V referance to ground, so T14 will conduct.

Suspects: at first T13 or C69, then the rest.

Joost Plugge

DaDa Engineering

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14-Jul-2017 04:29 PM

Margus

Margus

Posts: 6

thanks

i will give it a try, will report results

wb margus

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15-Jul-2017 06:40 PM

Margus

Margus

Posts: 6

found the problem, it was bad solder joint on D33

It caused additional resistance in chain R103-R104-T13-D33, current was enough to lit up D33, but not enough to pull down switching voltage in point A (was about 2 mA, about 5mA is needed)

wb, Margus

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