Technics SU X840 – Voltage Tests Main Board

Now I have a working dual rails PSU, I tested the main board to +/- 30V DC by connecting the dual PSU in series to the rectifier diodes, slowly bringing the voltage up in 5V (+/-10V) steps with the slave channel display in current mode. No current is drawn by the board at all, so there is no short on this board that could have caused the PT secondaries to short out.

But, no current at all isn’t good either..?

The only way I can test for any power amplification is to direct inject signals into pins 11 and 13 and measure AC output at pins 4 and 1 :

Pins 2 and 3 are the +/- 36V power rails resp.

Nothing definitive by injection as I can’t change the volume as it’s a rotary controller on this model, so logic levels, but by turning up the sig gen input from 0-20V I get gain from pins 1 and 4 though behaviour is alternately asymmetrical:

DUH! Dick! I forgot I removed the regulators to check them and forgot to solder them back in – just clicked them back..that’s why there’s no current flow!

Once soldered back the main board powered and drew about 4mA – same as the SA5L. Positive 30V is red on the left of the diodes, negative 30V right, black ground is on the heatsink. (Don’t always assume the heatsink or chassis IS ground though – ground may be floating on some equipment)

All front panel lights lit so there is a logic issue on the front board but probably needs the additional windings voltage from the PT..which maybe blown still unless the problem is on a front board?

I removed the front panel to find the flouroscan screen bulb blown black top left – won’t be finding a replacement for THAT on Ebay..:

The schematic says this provides 24V on some pins but it’s external feeds from the main board are 14V max on any pins:


Lessons Learned

“Assumption is the Mother of all Fuck Ups”! again…

Right tools for the job as usual – should have waited until the dual PSU arrived, but had to start somewhere. It would have saved more work and done a better job if I’d waited for all the needed kit like solder wick to do the de-soldering properly as I lifted a track but got away with the soldered connection – lucky!

The only fix is a new futaba flouroscan, then maybe a PT…doubt it. They look cool when working – shame..

Removal of it was good practice for solder wick technique at least.