Pioneer SX-434 No output, just hum after fixing blown power transistors

Zack512

New Member
Hi there,
I have been fixing the phono stage on a Pioneer SX-434 and whilst testing it without a heatsink on (very naively I know..), I have blown the power transistors and a couple of the transistors preceeding them in the signal chain. I have replaced these no problem, expecting the receiver to now work. However:

When I turn the amp on, no matter which input is selected, I just get a 100Hz hum with no audio output whatsoever, level and tone is irrelevant of the master volume or any other pots for that matter. Power supply caps and diodes seem ok. This has me stumped.

Does anyone know if there is anything else that may have blown when the power transistors blew and shorted out?

Thank you for any replies,
Zack
 
Without heat sinks, the transistors can go into thermal runaway within just a minute or even seconds.

Sounds like you may not know too much about electronics so don't go shot gunning (guessing) until you can learn more about your unit. Learn to use a meter if you don't already know how to use one.

Also make sure that today's replacement transistors are oriented the correct way. Sometimes new one's have BEC connections instead of BCE.
 
Thank you for your reply,
I am a masters electronic engineering student, good with electronics theory, been doing this to gain experience with audio equipment repair.
I tested the transistor configuration to make sure it was correct before installing.
If you've got any other ideas please let me know,
going to double check over the power circuitry because it seems like there is AC at the supply line, will get back with any advances,
Thanks,
 
Have you tested the driver transistors? Also make sure the emitter resistors are still good.
 
Yeah, driver transistors were bad, replaced them before. Thanks for your replies guys, any other ideas keep them coming, not working as of yet. All the power circuitry seems fine which is very odd.
Couldn't possibly be a problem with the power transformer could there? I doubt it would cause this issue though
 
Yeah, driver transistors were bad, replaced them before. Thanks for your replies guys, any other ideas keep them coming, not working as of yet. All the power circuitry seems fine which is very odd.
Couldn't possibly be a problem with the power transformer could there? I doubt it would cause this issue though

Are your voltages correct at the filter caps, should be plus and minus 24v DC, any imbalance will cause DC offset at the outputs. Have you tested the main rectifier? Also what DC voltage are you seeing at the speaker outputs.
 
Will re-assemble and double check, could I use 1N4007s for the rectifier just incase? I have some lying around, they might be too small though
Edit will also test speaker dc
 
Will re-assemble and double check, could I use 1N4007s for the rectifier just incase? I have some lying around, they might be too small though
Edit will also test speaker dc

I believe a 1N4004 is a good sub for these diodes so a 1N4007 should be more than enough.
 
They test fine to be honest
EDIT: I'll just go for putting the originals back in, it's just they're a pig to get in and out because the legs are so short
 
They test fine to be honest

Not your culprit then, test voltages on filter caps and you may want to look into building a dimm bulb tester if you haven't already.

DimbulbSketch.jpg
 
Yeah may well do that actually, thanks for your help by the way. Is that to limit the current from the wall?
 
Yeah may well do that actually, thanks for your help by the way. Is that to limit the current from the wall?

No problem :thumbsup:. Yep It'll stop anything else from burning up and it's also a great identifier for when something like an output or a rectifier is shorted.
 
Thanks, will definitely build that. I'm getting 12VDC on the top 3300uf cap, with 2.8V AC. and -15VDC on the bottom 3300uf cap, with fluctuating measurement AC voltage on the bottom cap.
Left speaker has -4VDC across it (+4v at black relative to red). Right speaker is at -1.4VDC respectively. Seems that AC is getting through the rectifier, but the diodes seem fine. Speakers are still humming at 100Hz
 
Sounds like PWR SUPPLY PROBLEMS. You should get BTWN 20 & 24V on C5 and -20 to -24V n C6. (read the notes on the schematic for explanation of the voltage in the bubble. and set the 434 for those parameters and test again.) You should not have more than a 1/2v or so AC riding on the DC after the cap. Check all the diodes and doublecheck orientation AGAIN. If it were mine (and I have one I bought in 1975) I'd just go ahead and replace the DIODES (D-1 thru D5). Check the associated caps and make sure they aren't shorted. Replace if necessary.

Take Voltage readings on the PWR SUPPLY and verify they are within +/-10% of nominal listed on Schematic. Post the voltages.
 
Diodes and associated caps are definitely good and oriented correctly (caps aren't shorted and diodes are the correct orientation). Will check all the caps on the power supply board. What do you mean by set the 434 for those parameters? Test with a supply? (I don't have a DC supply unfortunately, but I may be able to take the unit into uni if it comes to it). I shall test the other power supply voltages and post them. Thanks for your help, glad to hear you have this unit yourself

EDIT: Could something be shorting and dropping supply voltage? It seems fishy that one of the speakers has 4VDC across it?
EDIT2: Would you think using 1N4007 diodes for the rectifier to be sufficient should I swap out the diodes? I am currently collecting a list of pwr supply voltages, will post when I've got them
 
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There is a LEGEND at the bottom of the Schematic that shows what conditions (parameters) each symbol represents.

Plug the unit in to the Wall, and take voltage readings (mainly DC or Ground) on ALL of the PWR SUPPLY PINS. The pins with the Fuses are AC voltages. You need those to be within +/-10% before anything downline will behave. If the voltages are out of spec, you'll get erroneous readings or non working sections.

The 1n4007's are fine for the 434. Check the pwr supply transistors for shorts/opens (use the 6way test outlined here..... http://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/ ) and the Zener Diodes for correct voltage. replace any bad or out of spec parts.
 
Ah thanks, yeah I did get that, do you mean the key regarding voltages under no load and voltages under 15W into 8ohms etc?

I actually have a transistor/components tester which has been extremely useful. Will check Zener voltage. Got quite seriously wrong voltages across the board.
Here are some:

C5 (Top cap): 12VDC 2.8VAC
C6 (bottom Cap): -15VDC, fluctuating AC reading (unreadable by meter)
Pin3: 17VDC
Pin12:8.4VDC
Pin11:7VAC
pin2: 8.68VDC 4.08VAC
Q1 Collector: 32.3VDC, AC fluctuating again

Haven't done all the pins yet but will do, just the ones i've done so far.
 
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