Pioneer SA-500A amplifier - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Diabolical Artificer · 27th Feb 2017, 6:58 am

Well done for sussing out the dry joint. Find out the pinout of your suspected iffy transistor's, turn on and put your DMM probes on the emitter and base, DMM set to DC volts,doesn't matter which way round. You should get 0.5 -0.7v. Be careful it's easy to slip and short out something. If you can trace where the emitter or base joins a resistor, cap or whatever, put your probes there.

Either that, desolder and test with your diode checker. NPN = negative, positive, negative, so red probe to base, black to collector, emitter in turn. Opposite for PNP trannies.

Andy.

ms660 · 27th Feb 2017, 10:16 am

In general, sometimes wiggling some old Japanese transistors gives the effect of a dry joint when in fact sometimes it isn't, the surface of the transistors encapsulation where the leadout wires are can have a conductive film on it, wiggling the transistor makes the symptoms come and go (a bit like a dry joint) so the person re-solders the leadout connections and all is well, that's because the heat generated by soldering interupts the conductive film, often later on it will start conducting again so the fault returns, I have had countless examples of that during my troubleshooting days, not saying this is the case here but it's something to be aware of.

Lawrence.

michamoo · 27th Feb 2017, 4:32 pm

Lawrence is correct there. Often there is a sooty like deposit around the lead out wires. I find removing the transistor and giving a good scrape between the connections with a sharp scalpel clears the problem for good

starone · 28th Feb 2017, 12:10 pm

That's very interesting about the transistors, I've not removed the suspect ones yet as I didn't want to cause too much damage to the board by taking things out and putting them back in etc. I did check the continuity to earth on the faulty channel but couldn't find a problem. Unfortunately my meter gave up the ghost before I could recheck the voltages, so for now I've come to a standstill until my new one arrives. I've also ordered some replacement transistors and the caps to complete the job. I'll still plod on with the cabinet until the parts arrive, but in the meantime a big thankyou to everyone who's contributed so far.

Martin

starone · 6th Mar 2017, 1:34 pm

I've now replaced the suspect transistors and it's now working really well. Also checked the voltages and both left and right channels are equal, and the sound is nice and balanced.

There's still a slight mains hum which I thought would be cured by replacing the 3300uf can in the power supply, however it's still present. Initially it was only noticeable on the left channel but since replacing the power cord it's now only present on the right side. The voltages check out OK at the rectifier, so I'm not quite sure on this one.

It's all cleaned up and back together as I don't like to have stuff in bits for too long - things always end up going missing! Also fitted another bulb holder as the original had been removed, and raided my scrap bits for knob inserts. They don't match but I think it looks better than it did before.

Here's a pic.

Regards
Martin

Diabolical Artificer · 7th Mar 2017, 6:26 am

Good to hear you got it sorted Martin.

A.

27th Feb 2017, 6:58 am	#21
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,669	Re: Pioneer SA-500A amplifier Well done for sussing out the dry joint. Find out the pinout of your suspected iffy transistor's, turn on and put your DMM probes on the emitter and base, DMM set to DC volts,doesn't matter which way round. You should get 0.5 -0.7v. Be careful it's easy to slip and short out something. If you can trace where the emitter or base joins a resistor, cap or whatever, put your probes there. Either that, desolder and test with your diode checker. NPN = negative, positive, negative, so red probe to base, black to collector, emitter in turn. Opposite for PNP trannies. Andy. __________________ Curiosity hasn't killed this cat...so far.

27th Feb 2017, 10:16 am	#22
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Pioneer SA-500A amplifier In general, sometimes wiggling some old Japanese transistors gives the effect of a dry joint when in fact sometimes it isn't, the surface of the transistors encapsulation where the leadout wires are can have a conductive film on it, wiggling the transistor makes the symptoms come and go (a bit like a dry joint) so the person re-solders the leadout connections and all is well, that's because the heat generated by soldering interupts the conductive film, often later on it will start conducting again so the fault returns, I have had countless examples of that during my troubleshooting days, not saying this is the case here but it's something to be aware of. Lawrence.

27th Feb 2017, 4:32 pm	#23
michamoo Hexode Join Date: May 2006 Location: Cambridge, Cambs. UK. Posts: 469	Re: Pioneer SA-500A amplifier Lawrence is correct there. Often there is a sooty like deposit around the lead out wires. I find removing the transistor and giving a good scrape between the connections with a sharp scalpel clears the problem for good __________________ If smoking is so bad for you, how come it cures kippers?

28th Feb 2017, 12:10 pm	#24
starone Triode Join Date: Feb 2016 Location: Nottingham, Notts. UK. Posts: 43	Re: Pioneer SA-500A amplifier That's very interesting about the transistors, I've not removed the suspect ones yet as I didn't want to cause too much damage to the board by taking things out and putting them back in etc. I did check the continuity to earth on the faulty channel but couldn't find a problem. Unfortunately my meter gave up the ghost before I could recheck the voltages, so for now I've come to a standstill until my new one arrives. I've also ordered some replacement transistors and the caps to complete the job. I'll still plod on with the cabinet until the parts arrive, but in the meantime a big thankyou to everyone who's contributed so far. Martin

6th Mar 2017, 1:34 pm	#25
starone Triode Join Date: Feb 2016 Location: Nottingham, Notts. UK. Posts: 43	Re: Pioneer SA-500A amplifier I've now replaced the suspect transistors and it's now working really well. Also checked the voltages and both left and right channels are equal, and the sound is nice and balanced. There's still a slight mains hum which I thought would be cured by replacing the 3300uf can in the power supply, however it's still present. Initially it was only noticeable on the left channel but since replacing the power cord it's now only present on the right side. The voltages check out OK at the rectifier, so I'm not quite sure on this one. It's all cleaned up and back together as I don't like to have stuff in bits for too long - things always end up going missing! Also fitted another bulb holder as the original had been removed, and raided my scrap bits for knob inserts. They don't match but I think it looks better than it did before. Here's a pic. Regards Martin Attached Thumbnails

7th Mar 2017, 6:26 am	#26
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,669	Re: Pioneer SA-500A amplifier Good to hear you got it sorted Martin. A. __________________ Curiosity hasn't killed this cat...so far.