SX-780 - Counterfeit Darlingtons?

[MichaelFin]

PM him, I'd suggest that you buy 2 pcb's and the AL plates as well.

Thanks for the suggestion! I got the information about ordering them.

The one thing I want to do before I order them is check that there isn't something wrong with the voltage regulators possibly causing the issue. Aside from that I think I'll also check up on the contacts for the protection relay. But because pin 1 on one of the power packs is reading upwards of 40v I'm assuming it is the same issue that others have posted about.

While I have the guts of this thing open, is there anything else I should do for longevity? I checked the output resistors on the power packs and they're reading around .3ohm while on board. I don't see any capacitors that have blown so I'm hesitant to recap but I'd be interested to hear why/why not to recap it.

 

[Ricktptman]

I'd like to think that sooner or later, somebody will start doing runs of them again. But it probably won't happen until the prices for "quality" pieces go up as availability craters.

 

[mxthunder]

My dad picked up another 780 a couple weeks ago. Cleaned it up last night, de oxd all the switches and pots. Hooked it up this morning, played fine for about 2 minutes, then cut out. Now the relay does not click when you switch it on. Getting 40V to the post before the darlingtons, but only 19V when i probe the actual pin coming out of pin 0 or 9 of the darlington. Resister bad?

 

[merlynski]

My dad picked up another 780 a couple weeks ago. Cleaned it up last night, de oxd all the switches and pots. Hooked it up this morning, played fine for about 2 minutes, then cut out. Now the relay does not click when you switch it on. Getting 40V to the post before the darlingtons, but only 19V when i probe the actual pin coming out of pin 0 or 9 of the darlington. Resister bad?

Pins 9 and 2 are the power supply pins for the STK, +40vdc and -40vdc respectively with no signal in and no speakers connected. 0 and 1 are the input pins and should be about +1.4vdc and -1.4vdc respectively if there are no circuit faults. The outputs (8 and 3) should be very close to 0vdc (+/- 50 mvdc). The 0.22 ohm (output) resistors are difficult measure with most ohmmeters. If you remove the STK modules and put 1kΩ resistors in its place you can power up and check for DC balance at the inputs and outputs. Inputs will not be the same as working STKs, but should be ≈equal but opposite polarities centered around 0vdc, i.e. +/- 3-6 vdc roughly. See attached: