I have a Pioneer SX-950 that recently cooked resistor R38 (150 ohm) and the 4 outputs as the result of VR4 (100 ohm) trim pot shorting out. All trim pots were replaced with new Bourns pots, R38/R40 replaced with new 2w resistors and new outputs on right side as well (MJ21193/94). Unfortunately now for some reason the Bias on the right channel (facing front of unit) will not trim below 27mv , while correct spec is 20 mv. The only I can even get it to go to 27 mv is with the trimmer turned all the way to the right. Any turning to the left brings bias up to very high levels (200-250mv). Left channel is fine and to spec. I am running it with a DBT to bring some level of protection to outputs. When turned on I get a protection click after 5-6 seconds and DBT says no shorts or opens (bright then dim to nothing). Please note that was running fine and to spec until the R38 went open. Have checked the components in the path of VR4 and can find any open or short items. Any suggestions where to check or look. Assistance would be greatly appreciated.
Do not expect many replies
1) how many are familiar with this receiver and would have the schematic? Expecting folks to find the schematic, analyze it and responding is a stretch. If you want help then help others help you out by providing the necessary information.
2) I have and use my 950 which I bought new in 1976, so I know the design very well
3) if turning the bias pot right or cw decreases bias then you used the wrong pot, probably one with “R” in the part number.
4) changing components from original specified devices can change the bias control voltage range.
5) I answered you on AK and asked for you to measure the current in the gain stage but you failed to do so. I usually avoid folks who do not reply with info when asked for since they do not cooperate.
1) how many are familiar with this receiver and would have the schematic? Expecting folks to find the schematic, analyze it and responding is a stretch. If you want help then help others help you out by providing the necessary information.
2) I have and use my 950 which I bought new in 1976, so I know the design very well
3) if turning the bias pot right or cw decreases bias then you used the wrong pot, probably one with “R” in the part number.
4) changing components from original specified devices can change the bias control voltage range.
5) I answered you on AK and asked for you to measure the current in the gain stage but you failed to do so. I usually avoid folks who do not reply with info when asked for since they do not cooperate.
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2SD381 and 2SB536 both pulled and tested OK. Trimmers are 100 ohm and 10k ohm 3362P-1 101LF-ND and 3362P 103LF-ND from Digikey. I can not remember how to check gain stage due to some age related issues and don't want to cause a short. I used to be able to do this kind of thing in my sleep but not so much anymore. I had asked for help as to where to attach probes. Sorry to bother, have a good day.
Quick things to check just to try and prove what is happening although I suspect oscillation tbh.
1/ R35. Compare voltages between channels across this resistor.
2/ Compare voltages across the STV4H diode packs.
3/ Compare the voltage across the base/emitter junctions of the outputs.
Little fiddles to bring it down...
Try adding a resistor across the STV4. Trial and error but any resistor is only going to lower the current so its safe. I would guess by trying say a 1k and then going higher or lower from there.
Increasing R35 slightly (say to 300 ohm) reduces the current in that stage and lowers the voltage across the STV4 a little. Downside is reducing current reduces HF bandwidth of the stage but that level of change won't make any difference.
One real concern is the issue could be low level oscillation caused by the much faster replacement parts and that there isn't really a DC bias issue at all. The only easy way to prove that is to use a scope.
That is primary symptom of instability. I would urge you to use a DBT while testing. If oscillation is suspected then try adding a very small cap across R27, the 27k feedback resistor but ideally you need a scope check on this. Try something like a 22pF cap.
1/ R35. Compare voltages between channels across this resistor.
2/ Compare voltages across the STV4H diode packs.
3/ Compare the voltage across the base/emitter junctions of the outputs.
Little fiddles to bring it down...
Try adding a resistor across the STV4. Trial and error but any resistor is only going to lower the current so its safe. I would guess by trying say a 1k and then going higher or lower from there.
Increasing R35 slightly (say to 300 ohm) reduces the current in that stage and lowers the voltage across the STV4 a little. Downside is reducing current reduces HF bandwidth of the stage but that level of change won't make any difference.
One real concern is the issue could be low level oscillation caused by the much faster replacement parts and that there isn't really a DC bias issue at all. The only easy way to prove that is to use a scope.
That is primary symptom of instability. I would urge you to use a DBT while testing. If oscillation is suspected then try adding a very small cap across R27, the 27k feedback resistor but ideally you need a scope check on this. Try something like a 22pF cap.
Okay so now I see the schematic so kindly posted.
Something wrong if bias is max when pot is fully ccw which shorts out the pot and the bias is created by the voltage drop across the stv-4h diode stack attached to the heatsink. The wiper should be connected to the ccw pin on the pcb.
Do not use probes use lead grabbers or clip ons. Only clip on/off when AC power is off.
If R38 blew then maybe 2sd381 is damaged but still measures okay on meter. Might want to try changing them to ksa940/ksc2073
Something wrong if bias is max when pot is fully ccw which shorts out the pot and the bias is created by the voltage drop across the stv-4h diode stack attached to the heatsink. The wiper should be connected to the ccw pin on the pcb.
Do not use probes use lead grabbers or clip ons. Only clip on/off when AC power is off.
If R38 blew then maybe 2sd381 is damaged but still measures okay on meter. Might want to try changing them to ksa940/ksc2073
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FYI - not a fix to your current problem but some encouragement regarding the potential of these units....I've been using a Pioneer HT receiver for years with good sound. I modded my receiver to get better sound and never felt the need to upgrade it to a more expensive receiver. What I did was to add a current mirror to the LTP which improved bass and cleaned up the overall sound. Some details here (the current version starts post #13): https://www.diyaudio.com/community/threads/tgm6-modified-pioneer-ht-amplifier.230403/#post-4199133
If you compare this AWH-050 design to the basic amp figures 3.1,3.2 in Bob Cordells book, they are a like.
He shows what's needed to improve the design.
My AK list for Power amp has
VR1,2 (Semi-fixed,1K-B,ACP-029)
652-3266P-1-103LF offset 10k? 12 turn $4.43
72-T63ZB-10K $3.80
652-3296P-1-103LF $1.93
VR3,4 (Semi-fixed,100-B,ACP-019)
652-3386H-1-101LF bias 100 ohm 1 turn
I guess OP needed 3362R-1-101LF
He shows what's needed to improve the design.
My AK list for Power amp has
VR1,2 (Semi-fixed,1K-B,ACP-029)
652-3266P-1-103LF offset 10k? 12 turn $4.43
72-T63ZB-10K $3.80
652-3296P-1-103LF $1.93
VR3,4 (Semi-fixed,100-B,ACP-019)
652-3386H-1-101LF bias 100 ohm 1 turn
I guess OP needed 3362R-1-101LF
Will take one more serious attempt at repair next week. Will order some of trim pots as noted above. Maybe I got a bad one but it did measure ok before installed.
That is why if either repairing or reconditioning any of my OEM acquisitions , I'll build the circuit on LTspice.
Many of these classic amps designed the circuit around the gain/Ft of the devices used. That makes it a bit harder as I don't have
many believable "old transistor" models.
Using "new transistor models" will at least show you the bounds of whatever (lead/lag/miller) compensation scheme they came up with.
I've actually recompensated some older 80/90's circuits to achieve better than factory performance. Of course , I DO have a scope -
"tweaked" my square waves from slightly ringing to rounded , adjusted stage gain ....
Sx- 950 look VERY typical of the classics 80's "hi-fi store" receiver amp. Yes , 10-22pF across R27 (small lead comp.) - common....
Edit - even check the capacitance of the little ceramic's C8,10.15,17. I had 30 YO disc ceramics that measured WAY off , I replaced
with silver mica's.
OS
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Thank you , it was the simulations of these classic amps that brought me up to speed on the "old way" of vintage compensation.
While keeping the amp within it's stability "bounds" , I've found you can tweak the "voice" of the amp.
Each different OEM has characteristic (compensations) , possibly why some prefer certain brands.
The Pioneer is the most common , with a simple resistor tail current source derived from a zener reference. NO Re on the LTP ,
full beta from the pair. A cheap way to get 70-80db loop gain from 5-6 devices.
Very few classics have the (DIYA) fancy CCS's or beta enhanced super VAS's , resulting in a "voiced" .03% H2 heavy output.
Perhaps this was the goal ?
OS
I am not sure if it was a goal or that it was just the result. I lean towards it being a result of the simple topology.
This amp has been refurbished on many occasions using new semi’s without issue as demonstrated on the AK site. Using the old standby parts, ksa992, ksc3503,ksa1381,ksa940,, ksc2073, mj211994/95.
I redid mine many years ago, when I was not very power amp design savy. iirc I used nte drivers and Motorola mj15003/4 outputs.
This amp has been refurbished on many occasions using new semi’s without issue as demonstrated on the AK site. Using the old standby parts, ksa992, ksc3503,ksa1381,ksa940,, ksc2073, mj211994/95.
I redid mine many years ago, when I was not very power amp design savy. iirc I used nte drivers and Motorola mj15003/4 outputs.
I did not mean to detract from the Pioneer. "The goal" is common even among some of the forum DIY creations.
Some don't like the uber pure "clinical" or "transparent" qualities. A lot of these qualities also have to do with supply type
and size , PSRR. I can objectively differentiate a hefty supply running a EF3 versus a cheap 4700uF X2 OEM receiver.
Some of the more subtle things , like "MOSFET mist" or cap type are "snake oil" - at best. Both my PPM wolverine and
"old school" Sonance 260 have the same 40Kuf 55V rail supplies , can't tell any difference ?
OS
Some don't like the uber pure "clinical" or "transparent" qualities. A lot of these qualities also have to do with supply type
and size , PSRR. I can objectively differentiate a hefty supply running a EF3 versus a cheap 4700uF X2 OEM receiver.
Some of the more subtle things , like "MOSFET mist" or cap type are "snake oil" - at best. Both my PPM wolverine and
"old school" Sonance 260 have the same 40Kuf 55V rail supplies , can't tell any difference ?
OS
Attachments
I appreciate everyone's comments and contributions regarding my ongoing issues with the right channel but after a final attempt at repair I have decided that I obviously no longer have the skill set to solve the problem. Final note: All capacitors(all replaced with new), transistors(all replaced with new ones), diodes(all replaced with new including STV-4H, pots(new) and resistors(most replaced) check ok.
Reading through the previous posts they seem to have very little "to do" with your issues...
STV4H...I had very bad experience with this component in the past.
Blown output 😭
You can check this.
Desolder and connect an ohmmeter across this.
Place your soldering iron close to body.
If you see an ohm jump,this is a faulty part .
Blown output 😭
You can check this.
Desolder and connect an ohmmeter across this.
Place your soldering iron close to body.
If you see an ohm jump,this is a faulty part .
Yes lots of ideas but I am officially done with this receiver. I ordered a new Marantz model 50 today.
In this case, gain and fT are comparable, But Vbe is NOT. It’s much lower on the MJ21194. The D427 is the baby brother to the D424 and even it’s vbe is higher than Onsemi 250W types. Either use the parallel resistor trick on the diode stack, or replace it with 3 regular diodes and a schottky.