Yamaha TX-802. Not working so a few questions... SOLVED!!!

MuBase · Last edited by MuBase on Wed Mar 20, 2013 11:51 am; edited 1 time in total

Hi all.
I have recently acquired a Yamaha TX802 FM synth module. I live uin the UK.
This TX802 was sitting in the electronics Lab storeroom for over 15 years. It is an American 120V version. I bought a 240 - 120 V stepdown transformer and tried pluggin it in.
The machine appears to be dead. Its not the lead because on testing I get 120V AC.
I have also tested the rectifier and get 120 V or 240 V depending upon where I put the test leads so it is recieving voltage.
But there seems to be no voltage at the DC end of the PSU... I have tested the +/- 15 V regulators and got no reading... Has anyone had a problem with the PSU of a TX802 before?? There are no obvious burn marks or damaged components on the PSU board. I read on a couple of sites about the ripple capacitor dying after a number of years... Has anyone experienced this?
If anyone can help. I'd be very appreciative. I would love to get this synth working. My partner and I are learining about chowning Fm synthesis and have a copy of Chownings book. we'd love to be able to use this module...

Oh yeah, the internal battery measures 2 volts but like I say. it seems to be the PSU board...
Cheers,
Steve.

Uncle Krunkus · Posted: Wed Mar 20, 2013 4:27 am Post subject:

Definitely replace the ripple caps.
On something that old, re-capping is a must if you want to bring it back to good condition. Even when working, replacing as many electrolytic caps as possible (start with the biggest ones and work your way down) can actually improve the sound quality quite dramatically.
The only thing I'd keep in mind is that, assuming the ripple caps have gone belly up, they may have taken something else out with them. Especially if they failed short circuit (which they often do).
Worst case scenario is if they've burnt out the secondary of the (onboard) transformer. Still fixable, but a bit of a pain.

MuBase

Hi Unkle Krunkus. Nice to hear from you.
I am replying with good news.
After replacing the ripple cap I tried her again and still no joy...
Much scratching of head and then more systematic MM testing.
I had a suspicion about a 10 ohm cement resistor right next to the replaced cap so I de-soldered it. Where the component was supposed to be 10 Ohms it gave a reading of a whopping nothing!
Did a quick test by replacing with a 1 watt 10 ohm standard resistor and
she turned on lights, LCD and all.
(Don't worry, I have ordered some 10 ohm wirewound flameproofs and my girlfriend insists I don't turn it on again until I have replaced with the proper one..
I am very chuffed. Can't wait to start working this synthesizer out.

All the best,
Steve.

Uncle Krunkus · Posted: Wed Mar 20, 2013 2:51 pm Post subject:

Sounds like a 5W load resistor in the PSU. That's just what I was talking about with the caps going short. It could have even been another cap etc. gone partial short further down the line. That would slowly cook the resistor which has a tendancy to fail open.
I'd still recommend replacing as many electrolytic caps as you can be bothered with. Remember, same value (uF), same or higher voltage rating, depending on what will fit. Check polarities.
Glad it's going well! Cool