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-   -   Sansui AU 517 F2722 PCB Fail (http://www.diyaudio.com/forums/solid-state/226125-sansui-au-517-f2722-pcb-fail.html)

mcd99uk 20th December 2012 08:48 PM

Sansui AU 517 F2722 PCB Fail
 
Hello all,

I humbly beg for some assistance from the diyaudio crew.

I am currently trying to repair a Sansui AU517. It kept blowing the main fuses on the psu PCB. I know that the fuses blew for a reason that's why the replacement fuses were of the 1 Amp variety.

The fault finding so far has been to unplug all PCB power connections and then power unit up to prove the rectifiers and main caps were ok. This went well and no fuses blew and voltages measured correctly at each connector. Then I proceeded to reconnect the PCB power one by one. All ok until I tried to power up the power amp PCB (F2722) on one of the channels.

So now I know that this PCB has a problem. What would be the best way to proceed. I want to respect this amp and not start unsoldering and removing components without thinking things through first.

I'm sure there are other faults but this is a good place to start. I do have the service manual. :)

Any hints would be very much appreciated.

Many thanks


Paul

mcd99uk 20th December 2012 10:07 PM

I think I have a blown output transistor. Have been comparing the two modules. With the one that is blowing I am doing a diode test between legs and getting 0V where the hopefully working module has 0.5V showing.

jaycee 20th December 2012 11:25 PM

schematic here: Sansui AU-517 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine

jaycee 21st December 2012 12:37 AM

If you have a failed output transistor, you likely have dead driver transistors too. Getting the old parts is quite likely to be difficult or impossible. Forget eBay - you will most likely just get counterfeits.

Onsemi MJ21193/4 should make good replacement output devices. If the driver transistors (TR12 and TR13) are toast, MJE15030/MJE15031 should replace them.

When you power up the unit after replacing the transistors, it is best to insert an incandescent (old style filament) bulb in series with the mains live. This will limit fault current if there is a problem. Also make sure to re-adjust the bias current as shown on page 4 of the service manual.

mcd99uk 21st December 2012 12:30 PM

Replaced the output transistors with MJ15003 / MJ15004 and the driver transistors with MJE15031 / MJE15030 (Yes these were toasted too). Had these left over from making leach amps. The fuses no longer blow. The amp is now just totally silent can't even hear power supply noise in the speakers. The protection circuit seems to be happy enough and switches the output relay on. Wiring between amplifier pcbs and output relay is ok along with relay output pins connecting to the speaker terminals.

Suppose the next thing to do is connect signal generator to amp inputs and check it is getting through to the amplifier PCBs. Then powering up the amplifier PCBs outside of the amp chassis and applying signal generator to their inputs.

I really want this amp to work again as it sounded lovely when it did.

mcd99uk 21st December 2012 03:15 PM

Update....

Performed testing on input/preamp circuits and test wave form shows up at the connectors feeding the power amp pcbs. However, even on the (maybe undamged) pcb there is no output from the power transistors.

Have taken out damaged pcb and powered it up outside of the amp and applied signal generator to input pcb. Have traced signal through pcb and found that at TR5, TR7 that the signal levels drop off dramatically. At the bases it is strong but at the emitters it is weak. Have done diode tests on these and they don't appear to have been fried. There also doesn't appear to be any bias current.

What next?

jaycee 21st December 2012 04:43 PM

verify TR08/TR11 are good ? It would also be worth checking the resistors around the output stage in case any of them have opened or gone high

mcd99uk 21st December 2012 04:56 PM

Thank you for the help. R23 and R25 are open circuit. Checking out the rest......

TR08 and TR11 look ok with diode testing showing approx 0.5V between legs.

Power resistors are ok. All other diodes and resistors check out ok.

jaycee 21st December 2012 05:46 PM

One thing to bear in mind is that you have a working channel. You can use this as a reference for DC operating points at various places. If you then see anything wildly different on the non working channel, you know the problem is going to be around there.

My guess would be TR05 or TR07 saturated when the output stage went, and that burnt out the resistors. They may have been fusibles.

mcd99uk 21st December 2012 05:55 PM

Thing is the other channel doesn't seem to be working either. Someone had tried to repair this amp in the past and there is evidence of the protection circuit being partially disabled. I took the other module out and checked these resistors and they were open circuit too.

The amp was working after this repair but sounded weak. Prior to this repair it had been given a damn good thrashing where it cooked a tweeter. This repair was done around 20 years ago (this was before I was bitten by the DIY bug) by a local repair place.

It's all so puzzling as its a dual mono design and wouldn't expect identical faults to occur on both halves. Although only one output transistor has been destroyed.

Further research has suggested they are fusibles and are a common problem.Can I replace them with equivalent value non fusibles. I think they are 150 Ohm.

I have a couple of bench power supplies that have variable current limiting from 10mA upwards. So that may mitigate the risks of removing the fusibles while fault finding.


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