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

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Old 8th Nov 2019, 6:10 pm   #1
FERNSEH
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Default Sherwood AX-7030R amplifier.

I'm repairing a Sherwood AX-7030R amplifier for a client. It's dead, no standby light and all the fuses are OK.
Removed lid and did a few initial tests, check the driver and output transistors and for anything obvious. Found a 2SC2168 driver transistor with a short circuit between emitter and base. Out with it and test it again, it's OK! Check the PCB, no short circuit. So the transistor has been refitted, again no S/C.
Anyway, it turns out the standby transformer has an open circuit primary.
The secondary voltage will have to be determined. The 330uF reservoir capacitor is a 35 volt component so I'd guess it's a 25 volt transformer but I'd better make sure before a new one is fitted.
The third attachment shows that the standby transformer doesn't look too good.

Anyone familiar with this amplifier?

DFWB.
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Old 8th Nov 2019, 7:25 pm   #2
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Default Re: Sherwood AX-7030R amplifier.

If you are confident that you correctly measured a fault in that transistor, then I would replace it anyway. It may have developed an intermittent fault, and suitable replacements only cost pennies.
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Old 8th Nov 2019, 7:27 pm   #3
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Default Re: Sherwood AX-7030R amplifier.

Hopefully a reasonable assumption- the small blue relay in the third picture appears to be marked "12VDC", presumably coil voltage and presumably driven more-or-less directly from the standby supply by the small plastic transistor, though the small, slightly cooked and stood-off adjacent resistor may be dropping some voltage. I'd do a bit of track-following for guidance here,

Good luck, Colin
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Old 8th Nov 2019, 9:28 pm   #4
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Default Re: Sherwood AX-7030R amplifier.

The service manual is available foc from https://elektrotanya.com/sherwood_ax...wnload.html#dl
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Old 8th Nov 2019, 10:48 pm   #5
Michael Maurice
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Default Re: Sherwood AX-7030R amplifier.

Standby transformer should be approx 8 - 12V, no more, I'd go for 10V.

Circuit attached.
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Old 9th Nov 2019, 2:41 pm   #6
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Default Re: Sherwood AX-7030R amplifier.

"Standby transformer should be approx 8 - 12V, no more, I'd go for 10V."

Hi Michael,
many thanks for the information. I've found this unused 8 volt transformer which I'll try out later today.

DFWB.
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Old 9th Nov 2019, 11:59 pm   #7
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Default Re: Sherwood AX-7030R amplifier.

That's the amplifier fixed. The "8volt" transformer is delivering slightly under 10 volts to the bridge rectifiers. There is 10.5 volts across the reservoir capacitor C502.
That short circuit transistor was in fact caused by the collector tab of the 2SC2168 coming into contact with the safety resistor R1711, see second attachment.

DFWB.
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Old 10th Nov 2019, 1:13 am   #8
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Default Re: Sherwood AX-7030R amplifier.

OK, fair enough.
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