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Pioneer sx-680 with on bad VU and with STK-0040's

Sinatora

Sinatora

New Member
I have a Pioneer SX-680 with a bad VU. The needle is to the far left and will move when tested with a battery. Opened the receiver to test the VU and to my surprise I found two STK-0040 packs in it. Is this normal? Is it safe? It's a SX-680 with switchable mains power but I do not know the exact model s/sg
It powers on and produces sound. And most of all it looks really sweet :)
 
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Welcome Aboard!

There should be a STK Darlington module for each channel. Some of the later versions of hybrid modules had combined channels within a single device, but not this vintage.

As for the vU meters, they are drive through a diode/resistor array connected to the speaker outputs for each channel. If the unit is producing sound through speakers, there is not much left in the circuit other than those few devices and the connections. It is possible that some of the wire wrap connections may have failed, so inspecting the wiring might be a good first step.
 
I was able to get a couple VU meters unstuck on a Fisher RS-2007 by applying heat to the small centering screw with a soldering iron on low heat. Only takes 2 seconds.
Didn't work on some others I've tried.
But of course, since yours is working with the battery trick, no need to go that route.
 
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Sinatora said:
I have a Pioneer SX-680 with a bad VU. The needle is to the far left and will move when tested with a battery. Opened the receiver to test the VU and to my surprise I found two STK-0040 packs in it. Is this normal? Is it safe? It's a SX-680 with switchable mains power but I do not know the exact model s/sg
It powers on and produces sound. And most of all it looks really sweet :)
Click to expand...
The STK-0039, +/- 31vdc supplies, 35W max.
The STK-0040 has a slightly higher power supply voltage capability (+/- 33v), and a slightly higher power output capability (40W) but should be perfectly safe in your SX-680.

And welcome to Audiokarma!
 
Thank you everyone!
STK 0040’s are as far as I can gather ok in this design. I’m gonna check to se if the meter is getting a signal or not.

Does anyone know if it’s normal for the FM radio signal to have less quality in stereo. Tuning in to a station works fine but in stereo it’s not clean. Background humming/static is present. If I press mono it clears up and sound is great. At this point no external antenna is connected so that might be why.
 
Ok, poked the VU meter with the DMM set to ohm. Slight rise in both meters. Switched the poles and the needle tried to go left. Measured the resistor in what I presume is the driver circuit. Both check out ok. After this the needle is moving again. Connected speakers after verifying that no DC is present at the speaker terminals. Played a CD (Tom Waits - Mule Variations) works great (slight buzzing in some places but might be the speakers). Paused the CD and turned up the volume to about 75%. Dead quiet, not a hiss. This may be a budget SX but the sound is great. I’ve used crappy speakers for now but with no audio playing the amp is silent in a way I’ve not heard before. Gonna leave it be while I study the service manual. I’ve ordered new capacitors and need to verify that correct DC is present at all test point before I use it further.

The meter seems to be mechanically slow/stuck. It just got stuck again but at least I know it’s getting signal.

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If the amp is working well, small dc offset in the right output (40mV), and everything powers up and lights go on and sound goes through there really can’t be much wrong with it? There was a small amount of distortion in one channel at high volume but need to check the speakers for that one. If the distortion is produced by the amp, is it the stk’s that are responsible for that?
 
Anything in the channel could cause it. Tone issues in the pre amp area, to the STK's.
The receivers are getting old. Life limited components.
 
Thank you for all the answers everyone. I’ll likely have more questions when I start to measure everything and need to identify faulty components
 
Started changing all the capacitors. Placed an order that was not complete but that doesn’t matter as I fried the two big caps when changing them for two pin ones instead of three pins. I think I’ll stop there for this evening :no:
 
Does anyone know the original value of the capacitor with the red mark in these photos?
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The amp is a s or sg model. Is it the 220uf 35/V?
 
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Thanks! My order was not complete. I checked and double checked a list I found in another thread and yet I’m missing a bunch of capacitors so making a new order as soon as I figure out what I’m missing including that 220uf.
I also seem to have created a hum. Hooked the amp up to check my work and there is a hum. It’s regardless of balance and tone control settings. Hooked the amp up for a few seconds with the big power caps backwards. Shut it down within seconds. Put the old ones in again. Gonna get new caps but any input on what else such a mistake might cause would be welcome. The amp runs like normal but there is a hum.
 
I do not seem to be able to identify the rectifier in the schematic. What kind of diods would be a good replacement?
 
follow red transformer wires to pins 14 & 15, in between them is D404, D405, D406 & D407

diodes between the pins are SR3AM-8 which can be quite adequately substituted by 1n4004 diodes.
1n4004 diodes will ALSO substitute quite well for the 10E2 diodes D401 and D401 next to pins 11 & 12 (green transformer wires)
 
@markthefixer
Thank you kindly! You are the best.

I’ve ripped out the old power packs in favor for the new ones. Need to modify the heat sink to make room for them. Strange that it’s so tight in this amp. Plenty of air in other places but fitting the power packs is tight. Gonna have to mount them as far up as possible in order to be able to make the connections and stay clear of the emitter resistors.
 
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