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Sansui 661 Idle Current Issue After Recap

T

T Somerville

Active Member
Well to be honest I never checked the idle current pre re cap so it may have always been like this.

Amplifier was working good pre recap though it had a faint haze/fuzz on both channels on any setting (just enough to be annoying between songs)

So I thought I would give it some love with a re cap, deoxit and replace problematic transistors. After this the amp sounded lovely though the fuzz was still there. I thought maybe after dc offset/idle current adjust it might fix the annoying sound.

The only transistors I replaced were the 2sc1845 equivalents ( I think there was 4 in the pre amp section)

DBT did the usual bright at start then quickly down to a dim glow

Dc at speaker terminals was showing around 0v

When connect DMM to F01 for idle current after 10min warm up it started at 9mA then over the next 5 minutes it gradually rose to 70mA and then I turned the amp off.

When I connected to F02 the DMM it showed no reading at all. The DMM just showed a 1 as if it was a fault code or something.

Not sure whats going on here and was wondering if this has happened to anyone before?

Im a hobbiest and have done a few restorations on my own amps and have never had this happen before. I would be grateful for some expertise here as Im still learning. (wish I tested the idel current before recap!!)
 
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This post would be better served in the Sansui forum. The resident experts there can help.
 
Well to be honest I never checked the idle current pre re cap so it may have always been like this.

Amplifier was working good pre recap though it had a faint haze/fuzz on both channels on any setting (just enough to be annoying between songs)

So I thought I would give it some love with a re cap, deoxit and replace problematic transistors. After this the amp sounded lovely though the fuzz was still there. I thought maybe after dc offset/idle current adjust it might fix the annoying sound.

The only transistors I replaced were the 2sc1845 equivalents ( I think there was 4 in the pre amp section)

DBT did the usual bright at start then quickly down to a dim glow

Dc at speaker terminals was showing around 0v

When connect DMM to F01 for idle current after 10min warm up it started at 9mv then over the next 5 minutes it gradually rose to 70mv and then I turned the amp off.

When I connected to F02 the DMM it showed no reading at all. The DMM just showed a 1 as if it was a fault code or something.

Not sure whats going on here and was wondering if this has happened to anyone before?

Im a hobbiest and have done a few restorations on my own amps and have never had this happen before. I would be grateful for some expertise here as Im still learning. (wish I tested the idel current before recap!!)
 
You removed the fuses for testing bias? Did you have you meter set to measure current? Should be seeing ma not mv.

Rob
 
T Somerville said:
When connect DMM to F01 for idle current after 10min warm up it started at 9mv then over the next 5 minutes it gradually rose to 70mv and then I turned the amp off.
Click to expand...
Don't you mean mA? Are you checking idle current with the setting on volts?
T Somerville said:
When I connected to F02 the DMM it showed no reading at all. The DMM just showed a 1 as if it was a fault code or something.
Click to expand...
It may indicate that the reading is too high to measure on the selected scale. It could also signify a blown fuse.
What DMM are you using?
 
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eiraved said:
Don't you mean mA? Are you checking idle current with the setting on volts?

It may indicate that the reading is too high to measure on the selected scale. It could also signify a blown fuse.
What DMM are you using?
Click to expand...
Sorry yes I’ve edited it. I meant mA.
 
A "1" may indicate that the reading is too high to measure on the selected scale. It could also signify a blown fuse.
What DMM are you using? Won't know exactly what the issue is without looking at the user manual.
 
eiraved said:
A "1" may indicate that the reading is too high to measure on the selected scale. It could also signify a blown fuse.
What DMM are you using? Won't know exactly what the issue is without looking at the user manual.
Click to expand...
Im not too worried about the multimeter, im more looking for advice on where to start looking for the fault. What would be causing the idle current to slowly rise that high?
 
Robisme said:
You removed the fuses for testing bias? Did you have you meter set to measure current? Should be seeing ma not mv.

Rob
Click to expand...
YesI removed the fuses and DMM set to mA. What would be causing the current to slowly rise that high? Almost looked like it wasn't going to stop, hence turning off the amp.
 
Oh, I thought you were concerned as you stated your DMM just showed a 1 and you didn't know what that meant. I'm still curious about that, as it may help.
Since you had the amp running after recap and sounding lovely, there's no need to fear the current exceeding the fuse rating. Otherwise the fuse would have blown during the time you were running it.

Have you done what @quadmatrix82 suggested in post #5?
 
eiraved said:
Oh, I thought you were concerned as you stated your DMM just showed a 1 and you didn't know what that meant. I'm still curious about that, as it may help.
Since you had the amp running after recap and sounding lovely, there's no need to fear the current exceeding the fuse rating. Otherwise the fuse would have blown during the time you were running it.

Have you done what @quadmatrix82 suggested in post #5?
Click to expand...
I think if i just play the amp as is with the rising idle current it will cause over heating then component failures. I feel like the constant static is from this issue. I will replace the 2sa726 transistors tonight and hopefully that shows improvement. I was hoping for some help with diagnosing where the fault is. Could it be faulty output transistors or even the trimpot itself, im not sure where to start?
 
T Somerville said:
Does @Hyperion have any tips for me? I see your very knowledgeable with the Sansui's
Click to expand...
You say you replaced maybe 4 transistors in the pre-amp, what was the type number of the originals? 2SC1313 ? (as per the schematic). If it was, you know that KSC1845 has a different pinout? 2SC1313 = BCE from left to right (legs down) flat face towards you, whereas KSC1845 = ECB under the same conditions. So you would have to install the replacements the opposite way round from the originals for them to be correct.

BTW there are 4 transistors in the Phono/EQ amp, and 2 in the pre-amp - all are 2SC1313 according to the schematic.

It has been suggested to replace some 2SA726, with KSA992 which is fine, (TR01,03 & TR02,04?) however, did you know they need to be (Hfe) gain matched pairs?

Finally, there is a question mark about your DMM, it is possible that you may have blown the current range fuse, which obviously affects the operation of the current ranges, and with some meters affects the voltage ranges as well. So I think you need to verify correct operation of your DMM as a priority, before proceeding.
 
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T Somerville said:
I think if i just play the amp as is with the rising idle current it will cause over heating then component failures. I feel like the constant static is from this issue. I will replace the 2sa726 transistors tonight and hopefully that shows improvement. I was hoping for some help with diagnosing where the fault is. Could it be faulty output transistors or even the trimpot itself, im not sure where to start?
Click to expand...
Your outputs aren’t the trouble I’d start with replacing the 2sa726 if you still have static next step is to isolate if the noise is from the pre amp section or output section to track it down noise is normally a transistor failure or bad joint.

If idle current isn’t stable with new trimmers it’s possible the driver transistors are not matched or one is leaking and should be changed however first try isolate the noise issue
 
Hyperion said:
You say you replaced maybe 4 transistors in the pre-amp, what was the type number of the originals? 2SC1313 ? (as per the schematic). If it was, you know that KSC1845 has a different pinout? 2SC1313 = BCE from left to right (legs down) flat face towards you, whereas KSC1845 = ECB under the same conditions. So you would have to install the replacements the opposite way round from the originals for them to be correct.

BTW there are 4 transistors in the Phono/EQ amp, and 2 in the pre-amp - all are 2SC1313 according to the schematic.

It has been suggested to replace some 2SA726, with KSA992 which is fine, (TR01,03 & TR02,04?) however, did you know they need to be (Hfe) gain matched pairs?

Finally, there is a question mark about your DMM, it is possible that you may have blown the current range fuse, which obviously affects the operation of the current ranges, and with some meters affects the voltage ranges as well. So I think you need to verify correct operation of your DMM as a priority, before proceeding.
Click to expand...
Was a while ago when I worked on it but yes I would of corrected the pin out when installing. I will go back and double check though as sometimes I worked on this amp late at night (which is never a good idea for me)

I will replace the 2sa 726 and I have a atlas tester to match the hfe. I have ksa 992 fbu and fta on hand.

The DMM is working fine and I think the mA where out of range and that is why it wasn't reading correctly

I will open the Sansui back up tonight and have a tinker and find out what the original transistors were I replaced. I hope I can get it going as its a beautiful looking and sounding receiver
 
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