• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Sansui driver transistor alternative

R

Rhod

New Member
Friends,

I wonder if I can ask for some help. I'm repairing a Sansui 310. Have replaced blown power transistors (2SC1061 replaced with MJE15034G). There was also a blown driver transistor - one of two 2SA678s (TR 807 & 808) - which I replaced with some 2SA1015Ys, noting the different pinout.

The resulting audio was distorted on both channels. Subbing the original (good) driver transistor back in sorted the problem on that channel.

What do you think is going on here?

Dodgy replacements? (I'm unsure of the provenance of the 2SA1015Y)
Wrong substitute?
The substitute results in some other adjustments needed e.g. bias? (No bias settings in the service manual)
What would you substitute for the 2SA678?

Thank you!
 

Attachments

Register to hide this ad
You could use a BC212 for the 2SA678 but check for hFe ranking (A, B, or C). Keep in mind that the pinouts will most likely be different.
The 2SC1061 is still for sale.
 
zaibatsu said:
Did you confirm the pinout on both parts with a tester or DMM?
I found the original Sony datasheet for the complement 2SC1364 here and from what I can see the orientation shouldn't change?
Click to expand...
Yes: pinout for the SA678 is ECB and for the SA1015 BCE like on your datasheet - so it's just reversed or the same if viewed from the other side!
 
You'll want to check the bias for sure, you need to check it every time to change transistors. It will change. You can measure across the emitter resistors and compare to average values. Usually somewhere around 20mA. So around 20mV across each emitter resistor.
 
Rhod said:
Yes: pinout for the SA678 is ECB and for the SA1015 BCE like on your datasheet - so it's just reversed or the same if viewed from the other side!
Click to expand...

SA1015 has pinout ECB according to the ones I have.


20220518_175303.jpg

Have you checked voltages at both collectors, emitters and base? Left and right?
 
Raccoon1400 said:
You'll want to check the bias for sure, you need to check it every time to change transistors. It will change. You can measure across the emitter resistors and compare to average values. Usually somewhere around 20mA. So around 20mV across each emitter resistor.
Click to expand...

That's what I would like to do, and what I'm used to doing on other amps - and your guidance is valuable as usually I'm just following instructions i.e. put meter leads on test points A and B and reading should be between x and y!
 
Is there a response to my potential schoolboy error above - I'm thinking that if I replaced a driver PNP transistor with an equivalent I should have also replaced the complementary NPN, as leaving the old one there will mean that they're not complementary! Am I right here? Apologies for a basic question. The power transistors btw don't have complements.
 
Rhod said:
Is there a response to my potential schoolboy error above - I'm thinking that if I replaced a driver PNP transistor with an equivalent I should have also replaced the complementary NPN, as leaving the old one there will mean that they're not complementary! Am I right here? Apologies for a basic question. The power transistors btw don't have complements.
Click to expand...

You could try to replace them too.
complementary of 2SA1015 --> 2SC1815
complementary of MJE15034(G) --> MJE15035(G)
 
Last edited:
Roger_88_K said:
You could try to replace them too.
complementary of 2SA1015 --> 2SC1815
complementrary of MJE15034(G) --> MJE15035(G)
Click to expand...
Thanks, Roger. Just to note that their are no complementary power transistors in this circuit to the MJE15034s - just a bank of NPNs (TR 809-812) unless I'm mistaken (entirely likely!). The MJE15034s that I've put in seem to be just fine.

The driver transistors, though - the 2SA678s that I have replaced with 2SA1015s (TR 807 & TR 808) - do have NPN complements in this circuit: 2SC634A, TR 805 & TR 806.

So I am minded to replace the 2SC634As with 2SC1815s if you guys think that this could be the cause of my distortion...which makes sense to me!
 
Rhod said:
Thanks, Roger. Just to note that their are no complementary power transistors in this circuit to the MJE15034s - just a bank of NPNs (TR 809-812) unless I'm mistaken (entirely likely!). The MJE15034s that I've put in seem to be just fine.

The driver transistors, though - the 2SA678s that I have replaced with 2SA1015s (TR 807 & TR 808) - do have NPN complements in this circuit: 2SC634A, TR 805 & TR 806.

So I am minded to replace the 2SC634As with 2SC1815s if you guys think that this could be the cause of my distortion...which makes sense to me!
Click to expand...

Yes, I noticed. But just in case ;)

The only two differences that may cause the issue is the transition frequency, i.e. the SA1015 has 80mHz and the SA678 70mHz or the hFE rating. The 1015 is in between 25 and 400.. while the 678 has around 250, according to the datasheet.
Have you tried your new SA1015's with a simple hFE tester? It may overload the complementary type causing the distortion.
When replacing the SC1815 that shouldn't be a problem anymore but may cause problems further on in that circuit.
 
Well, particular thanks to @Roger_88_K for pushing me on the orientation of the replacement transistor! I was misinterpreting the spec sheet when orientating the replacement - aaah! - and this didn't dawn on me until I had sat staring at the transistor and the sheets for some considerable time. Many apologies, all, for my foolishness and thank you for your patience. Amp's sounding good on the workshop grotboxes - will test on proper speakers tomorrow...

Voltage drop across emitter resistors btw read 20.1mV, 20.2 mV, 20.1mV, 20.1mV. Sounds alright, but can't say I know what to do with those readings or which variable pot to adjust if it needed it!
 
I spoke too soon!

Amp powered up with the "proper" speakers, worked fine initially, then intermittent loss of volume on left channel. Thought it might be the tape monitor switch as this has sometimes been a bit dodgy, cutting volume to one channel if it isn't located properly.

Tried swapping the replacement driver transistor i.e. taking the original from the right channel and putting it in the left channel and the opposite for the replacement. Symptoms the same - by this time there is no output from the left channel. Right channel is fine.

Back on the bench, found one of the power transistors on the Left channel, TR809 is getting MUCH hotter than the others.

What might cause this overheating do you think?
 
Roger_88_K said:
collector current too high.
Have you checked voltages on the transistors?
Click to expand...

Here's the scores on the doors, Roger, thank you. Seem a bit all over the place to me! Figures in V wrt ground. TRs 809 and 811 are left channel (not working) with TR 809 getting particularly hot. TRs 810 and 812 are right channel (sounding awesome)

TR ----- B ----- C ------ E
809 -- 3.96 -- 43.8 -- 3.49

810 -- 23.9 -- 43.4 -- 23.3

811 -- 1.03 -- 3.23 -- 0.36

812 -- 0.54 -- 23.3 -- 0.01
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom