• 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

SOLVED: Sansui 3300 with faulty power amp channel

GlasgowGrip

GlasgowGrip

Sound just like a record on the phonograph
Subscriber
Hello Sansui Folk,

A friend has asked me to go through a nice, clean, unmolested and seemingly functional 'Sui Model 3300. Nice warm sound and a real attractive looking unit. I have been studying the service manual and the internals - really nice stuff - my hat is off to the Sansui engineers.

I'm ready to dive into this 3300, and first question... I don't see any nominal voltages shown anywhere on the schematic - am I missing something? Just curious, is this maybe a one-off for vintage Sansui? I own a 331, and that schematic does indicate voltages.

Anyway, I'm making semi-educated guesses that the power supply is "good enough for now". Here is what I find by measuring the pins on the F-1434 Power Block referred to chassis ground (if any 3300 experts see anything glaring below, please feel free to illuminate me...)
OA: +71.98 VDC
OB: +69.94 VDC
OC: 33.28 VAC
OD: 33.27 VAC
OE: 0V
??: +43.03 VDC (Unlabeled pin to Anode of C018, unregulated DC supply for voltage regulators)
OF: +28.82 VDC
OG: +22.17 VDC
OH: +23.54 VDC
OK: +13.15 VDC
OL: +6.28 VAC (to lamps)
OM: +8.13 VDC

Beyond that, are there any known failure points in the model 3300 besides the normal electrolytics and tantalums? I haven't found too much in my AK thread searches regarding the 3300... A couple times people seem to have replaced a pair of input FET's, and a couple of other mentions about eradicating any SC458's (of which I think there is only one pair). Also, I see some warnings about those trimmers on the Power Amp board being really fragile. I will measure Bias current and output clipping as per the service manual procedures. I will just skip adjusting them if they're close and I'm not prepared to replace the trim pots.

Any and all advice appreciated, I really want to respect this beautiful Sansui! Many thanks in advance,
Steve

IMG_8708_crop.JPG IMG_8642_crop0.JPG IMG_8683_crop.JPG
 
Last edited:
Register to hide this ad
Started the amp adjustments and I found a real problem with the Right channel output.

First, I did the current (Bias) adjustment procedure as per the manual, after a 30 minute warmup, both channels measured slightly low so I tweaked both up...
Left channel BEFORE: 11.3 mA; AFTER: 14.9mA
Right channel BEFORE: 12.1 mA; AFTER: 14.8mA

Next I did the output adjustment procedure (for symmetrical clipping into dummy loads) as per the manual...
Left channel I got 19.34 Vrms before I could see any clipping on the scope, and then it started to clip the negative peak just a hair before it started to clip on the positive - it was so close I did not attempt to adjust it. Into an 8 ohm dummy load, that calculates to over 46 Watts - nice - go Sansui!

But the Right channel quickly dashed my hopes. Everything looked clean on the scope up to about 4.8 Vrms or so, and then the sine wave starts to distort (not clip) and then goes all to hell as the volume goes up. The gory pictures:

IMG_8719_crop.JPG IMG_8721_crop.JPG IMG_8725_crop.JPG

I pulled the Pre Out/Main In jumpers and quickly confirmed the Preamp waveform looks clean, and the problem is in the Power Amp.

My first thought is a failing output transistor that can't handle the load. Also possible it's a Driver transistor. I'm just guessing based on limited experience. Will start troubleshooting but would welcome anyone's help or advice. Thanks in advance! - Steve
 
Last edited:
The Output Transistors are not the problem, they all test okay on my el cheapo component tester, so I swapped both pairs left for right, and the problem stays in the right channel. Not sure how to isolate this on the Driver board, except to start desoldering transistors to test.

I will start by just replacing the first stage transistor, the notorious 2SC458, since that seems to be a recommended step anyway. I have the KSC1815YTA and the KSC1845FTA on hand. Does anyone have a recommendation here?
 
Last edited:
Okay, it's not the input transistors either. Based on a couple of other threads (I believe following the lead of @Leestereo) I replaced both 2SC458's with KSC945CGBU. But no joy, the waveform on the Right channel still distorts as shown previously, as the output goes up above 4.5 Vrms or so.

Interesting that the NPN drivers (2SC959) have some really funky looking corrosion on the cans/heatsinks, don't know if that could possibly have damaged one of them, but I'm gonna pull them next.
IMG_8761_crop.JPG
 
Last edited:
SOLVED: It was a bad cap at C804.

I'm chasing transistors and it was a bad cap all along. I saw that waveform getting whacked out and just assumed it was a failing semiconductor. Pulled all the pre-driver/driver transistors on the Right channel and they all tested good. Only THEN did I check the electrolytics, and sure enough... C804 is a nominal 10uF/16v electrolytic, but only measured out at 615pF. I substituted with a Panasonic FC 10uF/25v electrolytic and we're back in business.

Hopefully this might help someone in the future.
 
GlasgowGrip said:
Hello Sansui Folk,

A friend has asked me to go through a nice, clean, unmolested and seemingly functional 'Sui Model 3300. Nice warm sound and a real attractive looking unit. I have been studying the service manual and the internals - really nice stuff - my hat is off to the Sansui engineers.

I'm ready to dive into this 3300, and first question... I don't see any nominal voltages shown anywhere on the schematic - am I missing something? Just curious, is this maybe a one-off for vintage Sansui? I own a 331, and that schematic does indicate voltages.

Anyway, I'm making semi-educated guesses that the power supply is "good enough for now". Here is what I find by measuring the pins on the F-1434 Power Block referred to chassis ground (if any 3300 experts see anything glaring below, please feel free to illuminate me...)
OA: +71.98 VDC
OB: +69.94 VDC
OC: 33.28 VAC
OD: 33.27 VAC
OE: 0V
??: +43.03 VDC (Unlabeled pin to Anode of C018, unregulated DC supply for voltage regulators)
OF: +28.82 VDC
OG: +22.17 VDC
OH: +23.54 VDC
OK: +13.15 VDC
OL: +6.28 VAC (to lamps)
OM: +8.13 VDC

Beyond that, are there any known failure points in the model 3300 besides the normal electrolytics and tantalums? I haven't found too much in my AK thread searches regarding the 3300... A couple times people seem to have replaced a pair of input FET's, and a couple of other mentions about eradicating any SC458's (of which I think there is only one pair). Also, I see some warnings about those trimmers on the Power Amp board being really fragile. I will measure Bias current and output clipping as per the service manual procedures. I will just skip adjusting them if they're close and I'm not prepared to replace the trim pots.

Any and all advice appreciated, I really want to respect this beautiful Sansui! Many thanks in advance,
Steve

View attachment 2263810 View attachment 2263811 View attachment 2263812
Click to expand...
GlasgowGrip said:
Hello Sansui Folk,

A friend has asked me to go through a nice, clean, unmolested and seemingly functional 'Sui Model 3300. Nice warm sound and a real attractive looking unit. I have been studying the service manual and the internals - really nice stuff - my hat is off to the Sansui engineers.

I'm ready to dive into this 3300, and first question... I don't see any nominal voltages shown anywhere on the schematic - am I missing something? Just curious, is this maybe a one-off for vintage Sansui? I own a 331, and that schematic does indicate voltages.

Anyway, I'm making semi-educated guesses that the power supply is "good enough for now". Here is what I find by measuring the pins on the F-1434 Power Block referred to chassis ground (if any 3300 experts see anything glaring below, please feel free to illuminate me...)
OA: +71.98 VDC
OB: +69.94 VDC
OC: 33.28 VAC
OD: 33.27 VAC
OE: 0V
??: +43.03 VDC (Unlabeled pin to Anode of C018, unregulated DC supply for voltage regulators)
OF: +28.82 VDC
OG: +22.17 VDC
OH: +23.54 VDC
OK: +13.15 VDC
OL: +6.28 VAC (to lamps)
OM: +8.13 VDC

Beyond that, are there any known failure points in the model 3300 besides the normal electrolytics and tantalums? I haven't found too much in my AK thread searches regarding the 3300... A couple times people seem to have replaced a pair of input FET's, and a couple of other mentions about eradicating any SC458's (of which I think there is only one pair). Also, I see some warnings about those trimmers on the Power Amp board being really fragile. I will measure Bias current and output clipping as per the service manual procedures. I will just skip adjusting them if they're close and I'm not prepared to replace the trim pots.

Any and all advice appreciated, I really want to respect this beautiful Sansui! Many thanks in advance,
Steve

View attachment 2263810 View attachment 2263811 View attachment 2263812
Click to expand...
 
Hyperion said:
Two quotes of the same thing, but nothing to say? :idea:
Click to expand...
Sorry, but I had attempted to reply once and the process ceased up. I'm quite certain it was due to my ISP which went offline for several minutes right in the middle of me replying to the OP's message.
Hopefully it is working now.
EDIT: Seems to be working. Will post another reply re: Sansui 3300 Receiver
soon.
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom