Good evening folks, long time lurker here but never had much reason to post until now.
I finally got enough funds to start getting into vintage audio. I happened to pick up the following units:
Now I haven't powered these units up. Knowing that any unknown unit should not be powered until at minimum performing a visual inspection of the circuitry.
I started with the SU-A80 as it seemed it be the easiest of the two to inspected. I figured there isn't much you could damage on a (pre-amp)? Well I was dead wrong with that ***-umption. During my visual inspection of the circuit board I found two transistors that experienced a major thermal event. Both of the transistors are on the power supply section.
See images below:
Now Ive scoured the internet for any and I mean any information on this unit. Outside of parts here and there I can not find a user manual nor repair manual for this unit. I believe this is one of those pieces that could only be acquired if you were in the armed forces at the time. I say that due to the fact that its power supply can be set to 110v, 120v, 220v, and 240v.
Here is my question:
I used NTE Quick Cross to try and find replacement transistors and was recommend the following replacements.
Have a wonderful day and thank you.
I finally got enough funds to start getting into vintage audio. I happened to pick up the following units:
- Technics SU-A80 control amplifier
- Technics SU-A70 power amplifier
I started with the SU-A80 as it seemed it be the easiest of the two to inspected. I figured there isn't much you could damage on a (pre-amp)? Well I was dead wrong with that ***-umption. During my visual inspection of the circuit board I found two transistors that experienced a major thermal event. Both of the transistors are on the power supply section.
See images below:
Now Ive scoured the internet for any and I mean any information on this unit. Outside of parts here and there I can not find a user manual nor repair manual for this unit. I believe this is one of those pieces that could only be acquired if you were in the armed forces at the time. I say that due to the fact that its power supply can be set to 110v, 120v, 220v, and 240v.
Here is my question:
I used NTE Quick Cross to try and find replacement transistors and was recommend the following replacements.
- Original - 2D9 AN78M18. Recommend replacement - NTE958
- Original - 5N5 AN79M18. Recommend replacement - NTE959
Have a wonderful day and thank you.
Hello,
Those are 500 mA +18 V and -18 V respectively, voltage regulators. Available anywhere, just make sure they're genuine. Your NTE findings will work fine - they are 1 A rated. You should consider increasing heatsinks' surfaces. Personally, I would place them on that chassis bar/reinforcement because I wouldn't trust pcb's insulation in that area. Also, I would replace surrounding small electrolitycs (in fact, all of them).
Service manual is available for 5$ here . Worked with them, they're ok. Or on paper, here .
Make sure that voltage selector is set right (I'd set it on 120 V, even if mains is 110 V).
Are you sure about power amplifier model? Isn't SE-A70? If so, I think is the same with SE-A5 mk II.
Regards, good luck,
Cosmin
Those are 500 mA +18 V and -18 V respectively, voltage regulators. Available anywhere, just make sure they're genuine. Your NTE findings will work fine - they are 1 A rated. You should consider increasing heatsinks' surfaces. Personally, I would place them on that chassis bar/reinforcement because I wouldn't trust pcb's insulation in that area. Also, I would replace surrounding small electrolitycs (in fact, all of them).
Service manual is available for 5$ here . Worked with them, they're ok. Or on paper, here .
Make sure that voltage selector is set right (I'd set it on 120 V, even if mains is 110 V).
Are you sure about power amplifier model? Isn't SE-A70? If so, I think is the same with SE-A5 mk II.
Regards, good luck,
Cosmin
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Thank you very much for the response Cosmin!
You mentioned mounting the voltage regulators to chassis bar. I take it that would require running jumper wires from the PCB to the voltage regulators? Or is there a more elegant solution? I agree I was planning to increase the heat sink size but didn't consider relocating the regulator. I take it I would have to insulate the voltage regulator from the chassis to prevent it from grounding out 🤔
Ill check mouser and digikey for the voltage regulators. Unless there is another reputible source for silicon components?
Ill double check the model number of the power amplifier once I get home, but I'm pretty positive its a SU-A70. Both units came from an ex armed forces member.
You mentioned mounting the voltage regulators to chassis bar. I take it that would require running jumper wires from the PCB to the voltage regulators? Or is there a more elegant solution? I agree I was planning to increase the heat sink size but didn't consider relocating the regulator. I take it I would have to insulate the voltage regulator from the chassis to prevent it from grounding out 🤔
Ill check mouser and digikey for the voltage regulators. Unless there is another reputible source for silicon components?
Ill double check the model number of the power amplifier once I get home, but I'm pretty positive its a SU-A70. Both units came from an ex armed forces member.
Stupid question, as I'm still very green here. How do I go about matching up/finding the proper output transistor? Given I dont have a data sheet yet for the output transistors in question. I appreciate the help.
On the voltage regulators...
It is normal for paxolin PCB material to discolour badly with heat. There may well be no issue at all with the regulators beyond you needing to check the soldering is OK. Measure the output voltage.
Obtaining genuine output transistors for a power amp may be a problem as there are so many fake parts around. Matching between NPN and PNP types is not normally needed, however if there are multiple parallel pairs then all the NPN's and all the PNP's normally need matching.
It is normal for paxolin PCB material to discolour badly with heat. There may well be no issue at all with the regulators beyond you needing to check the soldering is OK. Measure the output voltage.
Obtaining genuine output transistors for a power amp may be a problem as there are so many fake parts around. Matching between NPN and PNP types is not normally needed, however if there are multiple parallel pairs then all the NPN's and all the PNP's normally need matching.
Thank you very much for the information and input Mooly. The power output transistors Im in need of are for a pre-amplifier unit.
We would need to see a circuit really. Preamps are usually low power stuff 🙂 Are you sure they are faulty?
Hello,
I think we're starting at the wrong point. So, using a dim bulb tester - schematics/discussions everywhere, turn on the unit. I have a feeling that will be fine. Then, will go from there.
Regards,
Cosmin
PS: You might want to check if schematics of SU-A6 mk II is what you need.
I think we're starting at the wrong point. So, using a dim bulb tester - schematics/discussions everywhere, turn on the unit. I have a feeling that will be fine. Then, will go from there.
Regards,
Cosmin
PS: You might want to check if schematics of SU-A6 mk II is what you need.
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The link to the schematics you provided match what I'm seeing on the unit! Thank you very much for that.
I have this unit I purchased for slowly bringing up the unit. I'm going to build a dim bulb tester tonight. Due to not having access to a wiring diagram I wasn't comfortable with powering the unit up due to what I found during my visual inspection. In my limited experience and from reading around the forums. Due to having a variable power input and lack of history on the unit I did not want to potentially damage/cause further damage to the unit.
Given the information you've provided I will attempt to power it up with a dim bulb tester and monitor the voltage at the output transistors. That being said, due to the amount of thermal heat that was produced in that area I feel it be best to install new units for longevity purposes.
Unless I'm completely off my rocker and overthinking this entire situation
Glad to help.
Excuse me, what are you calling 'output transistors'? IC901 & IC902? They are linear integrated circuits. There are no power transistors in this unit. All the 'heavy lifting' is done by these 2 (+/-)18V @ 500mA voltage regulators. And IC903: + 5V @ 500mA regulator for logic ICs.
If you really want to change them, consider 1A variants with apropriate heatsinks from M... or D... you mention earlier. I still have my doubts regarding the charcoal made between the pins. But is just my opinion and I'd be happy to be wrong.
Best regards,
Cosmin
Excuse me, what are you calling 'output transistors'? IC901 & IC902? They are linear integrated circuits. There are no power transistors in this unit. All the 'heavy lifting' is done by these 2 (+/-)18V @ 500mA voltage regulators. And IC903: + 5V @ 500mA regulator for logic ICs.
If you really want to change them, consider 1A variants with apropriate heatsinks from M... or D... you mention earlier. I still have my doubts regarding the charcoal made between the pins. But is just my opinion and I'd be happy to be wrong.
Best regards,
Cosmin
Hi Audio_Kintsugi,
Those are voltage regulators. Just search for 1 ampere, 18 volt three terminal regulators. These are the standard 78xx (positive) series and 79xx (negative) series regulators. Digikey, Mouser, any reputable authorized distributor will work for you.
It wouldn't hurt to use larger heat sinks that would lower the operating temperature. As Mooly noted, the discoloured PCB is normal, not great but normal.
They probably still work fine, so you can power it up.
-Chris
Those are voltage regulators. Just search for 1 ampere, 18 volt three terminal regulators. These are the standard 78xx (positive) series and 79xx (negative) series regulators. Digikey, Mouser, any reputable authorized distributor will work for you.
It wouldn't hurt to use larger heat sinks that would lower the operating temperature. As Mooly noted, the discoloured PCB is normal, not great but normal.
They probably still work fine, so you can power it up.
-Chris