Hi Guys,
I am trying to bring an old sansui 331 receiver back to life with my limited tools & knowledge. I need some advice how to proceed. This is the old amp my sister bought back in the 70's and has been sitting in a shed the past 15 years. First impressions are very cheap build quality compared to other vintage amps I own, but then I read they have quite a sweet sound.
Plugged into into a DBT, all OK. All fuses are original & intact, with no signs of any mishap.Then I checked the DC at the speakers terminals & measure 9 volts or so on start-up which gradually decreases over a minute to 50mv or so. Both channels are the same. There doesn't appear to be a speaker relay on this amp. I am thinking the STK is toast and get ready to junk the amp.
Then I read this is a cap coupled amp & started to investigate. Basically I need to know if the STK IC amp is OK, before wasting time. I downloaded the service manual and made some measurements on the STK 014. Measure 45v at pin 8, and 22v at the output pin of each channel (pin 6 & 11), so absoultely spot on with the schematic diagram. Then measured both output caps (in circuit) and measure around 300-350uf on both caps in place of 2,200uf. Cross -checked the meter reading with other new caps out of circuit - the values are correct, so I guess the caps are completely shot. I don't dare plug in any speakers. I have some questions :
1) Can the DC voltage at the speakers be measured correctly like this or does it need a load (resistance) to charge the capacitors ? Doesn't the DVM itself offer sufficient resistance ?
2) Based on the output voltage can I say the STK is OK ?
3) Would the measured capacitance of the output caps be significantly different if I unsoldered them & measured out of circuit ?
4) Anything else I should be doing now apart from ordering new output caps ?
I am trying to bring an old sansui 331 receiver back to life with my limited tools & knowledge. I need some advice how to proceed. This is the old amp my sister bought back in the 70's and has been sitting in a shed the past 15 years. First impressions are very cheap build quality compared to other vintage amps I own, but then I read they have quite a sweet sound.
Plugged into into a DBT, all OK. All fuses are original & intact, with no signs of any mishap.Then I checked the DC at the speakers terminals & measure 9 volts or so on start-up which gradually decreases over a minute to 50mv or so. Both channels are the same. There doesn't appear to be a speaker relay on this amp. I am thinking the STK is toast and get ready to junk the amp.
Then I read this is a cap coupled amp & started to investigate. Basically I need to know if the STK IC amp is OK, before wasting time. I downloaded the service manual and made some measurements on the STK 014. Measure 45v at pin 8, and 22v at the output pin of each channel (pin 6 & 11), so absoultely spot on with the schematic diagram. Then measured both output caps (in circuit) and measure around 300-350uf on both caps in place of 2,200uf. Cross -checked the meter reading with other new caps out of circuit - the values are correct, so I guess the caps are completely shot. I don't dare plug in any speakers. I have some questions :
1) Can the DC voltage at the speakers be measured correctly like this or does it need a load (resistance) to charge the capacitors ? Doesn't the DVM itself offer sufficient resistance ?
2) Based on the output voltage can I say the STK is OK ?
3) Would the measured capacitance of the output caps be significantly different if I unsoldered them & measured out of circuit ?
4) Anything else I should be doing now apart from ordering new output caps ?
Attachments
Just took both 2,200uf caps out of circuit and measured their capacitance. They are both spot on !
Is there a chance there is nothing wrong with the amp and that I am not measuring the output voltage correctly ?
Is there a chance there is nothing wrong with the amp and that I am not measuring the output voltage correctly ?
Having 9 volts briefly with no load at startup may be normal. The output of the STK chip should go the half of the supply voltage. It may do that quickly or maybe slowly.
In any case, the output cap has to charge. Usually it happens slowly with the speaker attached. There will be some sort of turn on thump.
In any case, the output cap has to charge. Usually it happens slowly with the speaker attached. There will be some sort of turn on thump.
The two caps will build up DC on the output side without a speaker load, which will quickly drop once speakers are connected as the caps discharge through the speaker load.
there is a little load at the output - R33, R34 (which close the dc-path of the output cap ).
i would build in output caps with a higher voltage rating (50V......63V) instead of 35V , because these should withstand the psu-voltage (45V).
i would build in output caps with a higher voltage rating (50V......63V) instead of 35V , because these should withstand the psu-voltage (45V).
Thanks for all the advice. As the output caps are good I figure there is no way DC can get to the speakers, so I took the plunge & plugged in my suicide speakers. Good news the amp works fine !
Of the few amps I have tinkered with I haven't come across a cap coupled one so was confused about seeing such high voltage at the speaker terminals. I feel a bloody idiot about this but I guess that's part of the learning curve !
BTW, this amp is nearly 45 years old - What's the concensus about recapping the power board ? It's a really simple board with just a handful of caps, and recapping shouldn't cost more than 15Euros or so. Is it worth it from a sonic point of view ?
I understand for the tuner side, the board should be left well alone, especially as it works well.
One more question all 0.7V 300ma fuse bulbs are dead. I guess I will put LED's in there, but can anyone confirm if the correct colour for these xx1 Sansui receivers is blue or warm white - I am confused because the signal VU lamp bulb has a kind of blue film around it, while the ones on the main dial screen do not
Of the few amps I have tinkered with I haven't come across a cap coupled one so was confused about seeing such high voltage at the speaker terminals. I feel a bloody idiot about this but I guess that's part of the learning curve !
BTW, this amp is nearly 45 years old - What's the concensus about recapping the power board ? It's a really simple board with just a handful of caps, and recapping shouldn't cost more than 15Euros or so. Is it worth it from a sonic point of view ?
I understand for the tuner side, the board should be left well alone, especially as it works well.
One more question all 0.7V 300ma fuse bulbs are dead. I guess I will put LED's in there, but can anyone confirm if the correct colour for these xx1 Sansui receivers is blue or warm white - I am confused because the signal VU lamp bulb has a kind of blue film around it, while the ones on the main dial screen do not
Put warm white LEDs, with an appropriate resistor.
But 0.7 Volts?
You need at least 1.4 for green, and warm white at least 3 or so...blue around 4
Recap if you will, as capacitors do have internal corrosion.
But Japanese capacitors were good quality.
Change them if the seniors with practical experience here advise that.
Some idea of your location, its average temperature and current use conditions would help.
Sansui was reputed for for sound quality, and the Japanese designs are almost Zen like in their simplicity, as they had very good component quality.
If you compare with British designs, you will understand.
But 0.7 Volts?
You need at least 1.4 for green, and warm white at least 3 or so...blue around 4
Recap if you will, as capacitors do have internal corrosion.
But Japanese capacitors were good quality.
Change them if the seniors with practical experience here advise that.
Some idea of your location, its average temperature and current use conditions would help.
Sansui was reputed for for sound quality, and the Japanese designs are almost Zen like in their simplicity, as they had very good component quality.
If you compare with British designs, you will understand.
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All three lamps on this model are 7v fed from an (ac) winding, so if converting to LEDs you'll have to rig up a diode, cap and a suitable current limiting resistor.
The two dial lamps are sidelighting the length from either end so you may have to use two LEDs each end, or turn them away from pointing down the glass to get a good even spread of light (because the LED will obviously be a point source of light unlike the original festoon lamps.)
The two dial lamps are sidelighting the length from either end so you may have to use two LEDs each end, or turn them away from pointing down the glass to get a good even spread of light (because the LED will obviously be a point source of light unlike the original festoon lamps.)
So maybe better to stick to incandescent bulbs, which is fine for me, but what about the blue film on the tuner sensitivity meter. Would putting a whaite bulb ruin the effect ?
Blue LED, sand it slightly (all in fact) to diffuse the light. Warm white LED (2700K color temperature) in place of festoon bulbs.
Each LED can be run at 3V/20mA, and a protection diode will drop 0.7. Can go to 3.5 without major issues.
So you have to drop 3.3 volts from 7, and the LEDs will take 10 to 20mA each.
Calculate it, based on current draw. Ohm's Law works.
Use a 4000 series diode on on side, and the resistor on the other side of the circuit, all the different LEDs are to go in parallel. Any small electrolytic, up to 47 uF/25 V , as a smoothing cap after diode if you feel it is needed.
Each LED can be run at 3V/20mA, and a protection diode will drop 0.7. Can go to 3.5 without major issues.
So you have to drop 3.3 volts from 7, and the LEDs will take 10 to 20mA each.
Calculate it, based on current draw. Ohm's Law works.
Use a 4000 series diode on on side, and the resistor on the other side of the circuit, all the different LEDs are to go in parallel. Any small electrolytic, up to 47 uF/25 V , as a smoothing cap after diode if you feel it is needed.
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