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Old 4th July 2013, 06:17 PM   #1
amptech is offline amptech  Scotland
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Default sherwood ax-7030r

This old hifi amp came in with total dead fault, the owner said he'd opened it and all the fuses are ok but it's still dead? oK leave it and I'll get back to you....

Sure enough a full check of all fuses even in the mains plug tested ok, so it was down to tracing out the fault, I started looking at the standby circuit pcb as this allows the main feed to the power transformer to turn off and on via relay circuit linked with a s/by transformer and it turned out to be open circuit to it's primary winding so a quick temp psu was linked in and up it came all ready to go...

All in all half an hours tracing out plus fitting a replacement part and now this old amp is back to working order
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Old 31st May 2014, 08:08 PM   #2
RoryN is offline RoryN  United States
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Hi Amptech, any chance of a bit more detail on this repair?
I have one of these amps and it sounds like exactly the same issue - no standby light, but all the fuses are ok.
So, if I'm reading your post correctly, I've basically got to get mains voltage to the input of the transformer - possibly bypassing/replacing the small circuit board in the corner where the mains lead comes in?
I don't mind having a "proper" mains switch on the amp instead of standby mode...
Any suggestions you could make would be appreciated!
Thanks!
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Old 2nd June 2014, 07:14 PM   #3
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It seems he found that the small standby transformer had an open primary winding (randomly failed thermal fuse?). That should be easy to check with a multimeter in ohm mode - do you get any conduction across the mains plug?

He ultimately installed a matching replacement based upon the circuit he'd traced out - as a hint I would look at the relay that switches power (should be somewhere there in the power supply section), read off the nominal coil voltage (typically 12 V or 24 V), measure coil resistance and look for any additional resistors in series with the coil. Relay drive voltage has to be about Vcoil * (Rcoil + Rseries) / Rcoil. Then you only have to check what sort of rectifier you find connected to the standby transformer secondary, so you can estimate which one of the typical standard AC output voltages you need. (For a bridge rectifier, Vdc ~= 1.4 * Vac - 1.5 V.) You can eyeball the VA rating by size (sanity check: it would have to be greater than relay drive voltage times relay current).

Of course you could also simply bypass the relay (taking care not to bypass the fuse as well...) and use an external power switch, though this strikes me as the less elegant solution. The microprocessor should normally be powered by the big transformer then.
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