Pioneer SX780

Hi,
I have a Pioneer SX780 amoung many other receivers. My protection relay has been giving me a lot of greif on it. What happens is that when I first turn the receiver on if I dont wait at least 10 minutes until I turn the volume up it will turn it self off, and backon again! Anyway sometimes too for no reason it will just turn it self off and back on again. The last thing it does is that it will randomly turn it self of completely and I will have to turn the power swich off, wait exactly 15 minutes and I will be able to turn it on again. If I try to turn it on before 15 minutes I can flip the power swich of and on and it acts like its not plugged in. Any help would be grealy apreciated! Thanks

Peter

Typically the relays are designed to delay engagement of the speakers till circuit chargup. A simple circuit is as the one below..

The 220µf cap and the 56K resistor provide the startup delay...it must charge to about .65V before the D669A can turn on and engage the relay. The relays are usually 24V jobs, so the R? resistor is there to drop the proper voltage to the relay coil, resistance value depending on the supply voltage and the coil rating. The 1N4004 across the relay is to prevent inductive spikes from damaging the D669A on disengagement of the relay.

The C2547 is there to remove base drive from the D669A if positive going DC is detected at the amp output. If negative DC is present, the two 1N4148's are forward-biased, and remove base drive from the D669A.

The 10µf cap and the 1K 2W resistor are tied to the unrectified AC coming out of the transformer. The cap has a lot of ripple current across it, and only has the one 1N4004 diode (half-wave rectified) to supply current. The cap and resistor are chosen to collapse the voltage quickly, and since the current supplied is from the unrectified AC comng in, this cap discharges very quickly on power-down. This gives the 220µf cap a fast discharge path through the 1N4004 diode tied to its positive end, so the D669A turns off very quickly when power is removed (the big PS filter caps could keep the relay on for a long long time and send nasty turnoff transients to the speakers).

Under normal conditions, the C2547 is off, and the D669A is on (Duh!)

Candidates for failure in this circuit are: the 10µf cap (it takes a beating), the D669A (killed when the 1N4004 across the relay failed and allowed inductive spikes to take out the collector/base junction), or a shorted/leaky C2547 that is somtimes removing drive from the D669A.

This is the basic style of circuit used in vintage solid-state gear, and there are many varients. Find a schematic and dig in there.

I guess the schematic might help. :arrow:

EW, I am assuming that all units don't have a protection relay? My H/K 930 doesn't have that relay click when I turn it on. It just grabs the woofers and sucks them in as it charges then they settle back out when it is done charging up. I've been told my my new best old school ex-audiotech that it is normal, but I'll admit it is a bit disconcerting to see my Klipsch woofs move in such extreme ways. Though, the sound is very nice, so it must be normal operation.

No, not all do, although I'd admit I prefer ones that do have them. I'd rather have a relay between the amp output and the speaks than rely on an electronic circuit to disable the output pairs in case of catastrophic failure...my speakers are just too damned expensive.

(BTW, increasing gain on the amp input pair with a current source can drastically reduce that turn-on woofer excursion).

PS. Kenwood Man, are you sorry you asked yet?? :boink:

Not sure if this helps, but I found it on another forum.

>>Doing as Mr. Burrill advised prior to actually reading his suggestion, I came across the previous threads addressing this problem, as well as the song devoted to it. I noticed several areas on the PCB where the coating was discolored, particularly around the 3-prong power supplies on the main board and the multiple pin units (SK-500s, or something to that effect) mounted at the bottom of the receiver. I resoldered the power small power supply nearest the back of the receiver, as the factory solder appeared to have pooled in two concentric rings around each lead - an inner ring around the lead, and an outer ring around the hole in the PCB. There did not appear to be any discontinuity, and the one lead I could get to checked out fine with the ohm-meter, but I dumped some more solder over the leads, anyway. I also did half the pins on one of the two larger units on the bottom, then decided to test the receiver under power. The "source" light came on, so I got the speakers and connected them, and I was delighted to find that my Pioneer receiver was back to its old self. Many posts mention the larger voltage regulators (I believe that they are also known as "Darlington Power Packs"), but fail to mention the smaller power supplies, one of which I believe was the main problem, and could potentially be such again, which brings me to my next point.
After operating the receiver for an hour or so at moderate volume, I noticed that the C-shaped aluminum heat sinks of the 3-prong power supplies were extremely hot, and that the rear one was especially so. Is there anything that can be done to allow these to operate at a safer temperature? I was considering the possibility of installing a cooling fan similar to those used for PC processors. If I were to do this, what would be a good place to take power off without interfering with the function of the receiver? Also, is it possible to replace the regulators with something which will not generate so much heat?
I acquired my SX-780 at a garage sale after haggling the owner down from $3.00 to $0.00. It did not work at the time, and it seems that the only problem was in the tuner section, which mysteriously began to function after I brought it home. I did not appreciate what fine receivers these are until I heard mine for the first time, and immediately remembered the mellow, warm sound of an old Magnavox receiver my father used to keep in his office when I was very young, something I had not heard for years. The Pioneer has these same qualities, but with much better definition and features. These old receivers are a frequently overlooked treasure, and I wish that modern consumers had the discretion to demand that manufacturers build audio equipment of this quality today.
Thanks, Mr. Burrill, for your suggestion.

Not sorry yet!

Hey,
EchoWars I'm not sorry that I posted that question! Every additional responce adds to my knowlege, which is always a benifit! Thanks for all you info on the relay! I'll try and tear into in myself we'll see what happens! :withstpd:

Thanks again!
Peter AKA Kenwood Man