Audio Engineer Report 'Snippets'

So... How's it Looking...

Oh....

Just what I needed!

 "The left power amp has had work done on it previously. Some transistors had been changed to a cheaper low quality type (probably just what the tech had on them at the time) it looks like there was a blow in the driver stage which had blown out a few transistors in the past. This is why I decided to do a full rebuild. It’s got to be reliable and the sound on both amps needs to match so I rebuilt the whole board and even replaced a lot of resistors that were looking very tired.
The right channel however, was in almost original condition but again, rebuilt to the same extent to match the left in terms of components and reliability."
"There was damage to a number of traces again which I have tried to repair. The idle current fault turned out to be poor soldering on the output transistors which upon inspection simply just broke off."
"I'm afraid now, I bring some bad news. I'm not sure if I had yet mentioned that there was a problem with the speaker protection circuit. This was the last thing I needed to check.
The protection system on the SX-1980 is mostly condensed into a small Pioneer chip. A PA3004. It looks like the chip has shorted out between the power supply and the relay. This means the relay triggers both too early and too late and with the chip shorted, it means that right now, there is no protection circuit operating." A replacement PA3004 is required. A rare chip! A suitable SX donor unit may be required to gain the chip.

Excellent!

"As for the soft start, yes. I checked it over with a few other techs and the inrush, although increased, shouldn't strain the soft start circuit. I will, of course, check it myself once everything is finished to ensure its going to cope well but having started it a few times myself now, it seems to be happy enough, no issues or excessive heat etc.
It's quite a simple robust circuit and is very safe. If there are any thermal problems the whole power system is shut down via a thermal resettable fuse."

Sensory!

lovely Jubbly!

"The touch sensor issue was quite strange. The circuit behaved as though there was a dead short in it. I removed some components to do testing which seemed to test good and on replacing the components the circuit came back to normal and is now working very well. Perhaps there was a poor solder joint as there are a few damaged traces in that area which I’ve repaired."
"Yes, the 80V regulator was replaced, along with almost every other component in the power supply as a necessity if it is to remain reliable and stable. Most regulators have been migrated to large heatsinks.
Not only did I replace the driver and small signal transistors, every transistor on both amp boards, excluding the big output transistors, has been replaced with a new high quality transistor with appropriate properties to keep it running as new, if not better."

Great!

"I fitted the new chip into the SX today and now it looks like the protection circuit is doing what it’s supposed to so we are nearly there now! I had a small issue with one of the signals to the chip which was another broken trace so I have connected a wire from point to point which as resolved the issue. It’s almost ready for a test drive and then one of the last things I want to do is clean out the tuning capacitor which I have to do in a warm dry environment ie. my house."