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Pioneer SA-7500 Replacement transistors: guidance/troubleshooting

kasbah512

kasbah512

Member
I have a pioneer SA-7500 that refuses to dc bias, and i believe i know the source of the issue. I was swapping out capacitors and transistors, as provided in an ebay kit (i know... first project like this and i didn't know better), and everything works, dc bias's perfectly, however, the lowest dc offset i can achieve on both channels is ~100mv dc on both speaker terminals.

I'm confident the capacitors were installed correctly because i made sure the new capacitors went in the same way as the originals, and for the transistors i used a tester to check which pins where which before installing.

I'm 99% sure its related to the switch circuit board, as there is a variety of transistor types in a similar package, and i didnt know any better at the time. I'm reading -1v where i should be reading 0.2v at 2 junctions. (collector for Q9, and base for Q10) there are a few others that are off that i haven't recorded exact values for, but nothing inverted like the ones mentioned before.

With that being said, I was looking for viable replacements for the listed transistors, ideally in stock at digi-key:
2SC945
2SC733
2SC1384
2SA869

*** I am only asking this because I do not know which or if they need to be purchased as complementary pairs, otherwise i would have searched the existing recommendations
 
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I can help with 2: These are from my notes on my SA-8500 rebuild, I believe mouser part numbers but that will get you started.

2SC1384: 512-KSC2690AYS
2SC945: 512-KSC2383YTA
 
Livinloud said:
I can help with 2: These are from my notes on my SA-8500 rebuild, I believe mouser part numbers but that will get you started.

2SC1384: 512-KSC2690AYS
2SC945: 512-KSC2383YTA
Click to expand...
Wrong.
2SC945 > KSC1815
2SC733 > KSA1015
2SC1384 > TTC004B
2SA896 > TTA004B
 
ivandezande said:
Wrong.
2SC945 > KSC1815
2SC733 > KSA1015
2SC1384 > TTC004B
2SA896 > TTA004B
Click to expand...
Are these currently running in your sa-7500? Since it's still "functional", I don't want to order parts and tear it apart again unless they are for sure verified for this unit if possible.
 
kasbah512 said:
Are these currently running in your sa-7500? Since it's still "functional", I don't want to order parts and tear it apart again unless they are for sure verified for this unit if possible.
Click to expand...
I think it's a strange ask for them to be verified specifically for a SA-7500. I've worked on a few.

I've been doing this for almost a decade and probably close to 900 units. I'm not making stuff up here, but if you don't believe me - do your research and simply search the forum, you'll confirm my subs.
The KSC2690 would work, but it's compliment the KSA1220 is EOL and we like our pairs to match.
 
ivandezande said:
I think it's a strange ask for them to be verified specifically for a SA-7500. I've worked on a few.

I've been doing this for almost a decade and probably close to 900 units. I'm not making stuff up here, but if you don't believe me - do your research and simply search the forum, you'll confirm my subs.
Click to expand...
I'm not trying to be condescending, just trying to confirm they will work in this specific unit, as I am getting conflicting information here. Thank you for the information. I'll order them and see if that solves my issue.
 
There is no relation between your output offset voltage and these transistors. They are in the protection circuit. Changing them will not fix your offset issue.
 
kasbah512 said:
I'm not trying to be condescending, just trying to confirm they will work in this specific unit, as I am getting conflicting information here. Thank you for the information. I'll order them and see if that solves my issue.
Click to expand...
@ecluser is right. I reread your original post and I'm confused, you replaced all the transistors but you may have used the wrong ones here and there?
The power supply and the amp circuit are the only things that will fix your offset.
Q1-4 should be KSA992
Q5-6 should be KSC3503
The drivers are probably fine, but KSC2073 and KSA940 are good.
Make sure you get 37V on pin 6 and -13 on pin 10.
 
With that being said, I'm not really sure where to go from here. The lowest I'm able to get for dc offset is 100mv on the left and 110mv on the right channel. :(
 
redk9258 said:
Are you matching the differential pair?
Click to expand...
I'm not sure exactly. I'm new to this. For now, I'm wanting to get an order list together for pretty much everything except the drivers. However, even for the transistors I've been suggested, I'm seeing different HFe ratings for the same transistor ID so I'm not sure which way to go. I've tried to find the ones closest to the original specs, but from my understanding, even that isn't sufficient. Ie: if I found transistors with the same ratings as the originals, that wouldn't be enough to know if it would perform properly in circuit.
 
ivandezande said:
Wrong
Click to expand...

Not Wrong. Maybe not your pick for a substitute but not wrong. These replacements are well documented on many sites. There may be better substitutes and your picks may be the better substitutes, I am not qualified to say. I can share what has been well documented and what I have personal experience with. My suggestions work.

https://audiokarma.org/forums/index.php?threads/transistor-substitutions.688515/

https://www.tubesandtransistors.com/pioneer-sx-680-rebuild/

To the op, hope you have success in getting it running!
 
kasbah; matching your differential pairs (Q1&Q3, Q2&Q4 on power amp board). To get the lowest possible offset these transistor pairs must match (Q1 matches Q3; Q2 matches Q4) with regard to gain or hFe. Get yourself one of those cheap Chinese component tester kits. Plug a transistor into it, wait 30 seconds for temp to stabilize then test it. You'll get a hFe#. buy 15-20 transistors (KSA992F) and match 4 of them for hFe. install them, test and set offset.
 
larryderouin said:
kasbah; matching your differential pairs (Q1&Q3, Q2&Q4 on power amp board). To get the lowest possible offset these transistor pairs must match (Q1 matches Q3; Q2 matches Q4) with regard to gain or hFe.
Click to expand...
I see, so from my understanding, there may be differences due to tolerance, and the best bet is to test the batch I recieve until I find the 2 best pairs, is that correct?

I also overturned the bias trim pots but they still work fine, however that may not be the case for the offset trim pots. (Both sets are bourns 15 turn)

I will report back with hfe values for the installed transistors, and check the trim pots for any issues. I have one of those cheap testers at home I can use.
 
Are your offsets positive or negative? I think they are positive. That is a very bastard circuit. hFE matching of the differential input pairs will not warrant low offset in this circuit. The DC resistance on the feedback side is 27k Ohms, but it is of the order of 100k Ohms on the input side. Furthermore, that resistance is not connected to ground, it must be connected to a negative voltage in order to have a low output offset voltage. The original transistors had a large hFE. If your new transistors have a lower hFE, you might need more negative voltage than the circuit provides. A solution would be to reduce R42 to 10k Ohms.
 
Just checked and they are indeed positive on both sides. the hFE reading im getting even after retesting multiple times (to heat them up i guess) is 380 on one pair. they are both pretty spot on for that side at least. I have yet to test the other. I did check the trimmer pot and it is slightly off from the 1k spec for VR3, VR4. Im reading a sum of .93k ohms. I ordered a transistor variety pack to handle the 2SC725 replacements on the output, (KSC922FA, and KSC922FB) and the others that may have been improperly substituted some of the others suggested by @ivandezande. Ill report back after testing with the new transistors.
 
You can replace as many transistors as you want. It's your call. Replacing Q5 and Q6 will have almost no effect on your output offset voltage. The best solution is to reduce R42 to something like 10k.

Don't change R7, R8, R9 or R10. They have the perfect values to give a perfectly balanced differential stage. Let me be clear on this point. Actually the collector currents in Q1 and Q3 are equal (same for Q2 and Q4) and this is the condition for minimum distortion in a differential stage. I know because I have computed these currents. You could reduce your offset if you increase R9, or R10. That is the usual method for offset correction that I have proposed on many occasions, but in amplifiers that have the same base resistance on both sides of the differential stage. That is not the situation of your amplifier. If you increase R9, R10, you will also increase the distortion in the differential stage. It's not a good idea. You need a larger negative voltage on the "lower side" of the 82k resistor at the input side in order to pull down the base voltage of Q1, Q2. Just a single resistor to change (R42) and you will be able to adjust the offset in both channels.
 
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