• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

SX-950 lights working, but no sound, and no relay click. Preamp works and voltages are all good.

K

kevdubs

New Member
I know there a lot of similar posts and I've read through many of them, but I still can't figure out the problem. When I turn on the unit the relay never switches on and no sound will come out. I checked the pre-amp with a separate amplifier and it is producing good sound. I've checked all the voltages from the pins on the protection circuit, power supply and power amp, and everything checks out. I also checked the transistor voltages on the protection circuit which appear fine. I also checked them using the diode function on my multimeter and they seem to have the appropriate responses. I'm about to take out the relay switch itself to see if there is a mechanical problem, or maybe corrosion, but from the exterior it looks quite clean.

My question is other than the relay switch, what could possibly be the problem? Would it be worth it to just rebuild the protection circuit, despite having good voltages? What else can I check?

Here are the voltage readings: (The first column is pin#, second column is listed voltage from the service manual, and the third are the actual readings. The transistors (Q) have the base, collector and emitter denoted by b,c and e.)

PSZXUFZ.jpg

All the caps and transistors are original (except for the Eq board, which I rebuilt several years ago with the help of Mark the fixer).

Thanks for any input!
 
Register to hide this ad
Well, I have no idea what I did, but when I took the relay out it suddenly started working again. Maybe it just needed to be jostled around I don't know. Sorry for the unnecessary post...I guess since it's open I'll just use the opportunity to clean it. Just glad to have music again.
 
The protection voltages from the amps seem particularly strange as they are all zero volts.
One would expect at least a millivolt or so.
Check the offsets and bias settings at the amp boards(?)

Nice post of the readings, well done.
Would/could you post the Excel in a file so I can save it? Would be a big favor!
 
Its been many years since I worked on stereos (1976 - 1985) but I remember a series of Pioneers (SX-750, 850, 950?) that had an issue with a power supply transistor that was mounted to a heat sink, that was also soldered to the pcb. As the transistor heated the legs tugged on their solder joints (no place to expand). The transistor was a TO-220 package (do they still call it that?) and rather robust. If you un-soldered the heat sink and left it un-soldered, and re-soldered the transistor, the unit would work fine. You can tell, because the transistor has colde solder joints. Please don't tell anyone, I made a lot of money over the years doing that one. And I believe it was the SX- x50 series.
 
Welcome
Q5?

It would be a good idea to check the PS voltages. (And) The bottom for what new Member Steve mentioned.
 
The voltage readings above clearly indicate there is 24V across the relay (Pins 9 & 11 on the protection board). Either the relay is mechanically damaged or it is operating. If the relay coil were open the voltage at Pin 9 would likely be much closer to zero.
 
Steve March said:
Its been many years since I worked on stereos (1976 - 1985) but I remember a series of Pioneers (SX-750, 850, 950?) that had an issue with a power supply transistor that was mounted to a heat sink, that was also soldered to the pcb. As the transistor heated the legs tugged on their solder joints (no place to expand). .
Click to expand...

Welcome Aboard!

It's been many years since I worked on Lepel and Radyne 50kW RF transmitters, but I remember some of the problem spots in those units as well. Your memory serves you well. This problem also migrated into the SX-#80 series in some models.
 
You must log in or register to reply here.
Back
Top Bottom