Technics SU-V65A Troubleshooting please

iefes

Member
2016-04-23 8:36 pm
Hello Folks,

I have a Technics SU-V65A HiFi amplifier which has been working until a few months. I have not been using it for a while now and when I tried to hook it up again it wouldn't work anymore. There are two status bars on the front panel, one is named "Voltage Control" , the other one is called "Current Drive". When the amp is ready to operate both of these bars should be illuminated but in my case there is just the "Voltage Control" illuminated, the "Current Drive" stays switched off. I read that this is probably caused by the protection circuitry of the Chip. So it seems like the amp gets power and everything works but the protection circuit engages and prevents the main power amp chip from working. I wasn't able to upload a full handbook but I found this link, where the protection circuitry is firmly described: TECHNICS SUV65A - Service Manual Immediate Download

Now I'm wondering what I could do to find the error and probably solve it. I attached some photographs. On the pictures you can see the part with the protection circuitry for example (labelled on the PCB).

If somebody could point me towards some ideas about what I could try, I would be very happy. I have experience with mainly guitar effects pedals and some tube-related projects, so I think I know what I'm doing, but I'm not yet familiar with HiFi-electronics.
As the amp has been working fine until a few months ago I don't think there is a big issue with it, I just don't know where to start and what to try.

Thanks a lot!

And yea, it really is quite dusty inside, I should take a vaccuum and a cloth and clean it.

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Welcome

Member
2013-03-11 10:52 am
Technics units are generally unrepairable when they finally break down (because they do). This is because there isn't much in there that can fail... except the various ICs Technics used. And these are all impossible to find replacements for (very bad fakes are abundant on Ebay, though).

But it can also be something really simple. Try re-soldering the main IC, it tends to run extremely hot and over time ruin its soldered connections. Optimally use a liberal amount of flux.

Cleaning is best made with a long bristled paintbrush. Obviously ESD damage is a concern, but there aren't really any particularly sensitive devices in an old amp like this.
 
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Welcome

Member
2013-03-11 10:52 am
That's the one.

But while you're in there, take a look for other suspicious looking solder joints. Particularly around the driver transistors on that little heat sink. The ones on the left look a bit toasty. Might even be blown up. Check that with a meter.
 
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iefes

Member
2016-04-23 8:36 pm
Hey again! Actually I found the reason the Amp wasn't working. By conincidence I stumbled upon a german forum entry where somebody had the same problem: Wer kennt sich gut mit Technics aus, Problem mit einem SU-V65A, Hifi-Klassiker - HIFI-FORUM

He said that the Voltage Regulators on the main PCB behind the front panel got quite hot and produced failing solder joints. I found the units he mentioned and resoldered them and now the beauty is working again! :)

Only problem is, that almost all of the potentiometers crackle and that on the right channel sometimes theres not the full range of sound coming out of the speaker. At the moment its working fine and I'm using the "Remote" speaker outputs because on the main outputs the left channel wouldn't work. I assume this could be solved by cleaning the contacts etc, what do you reckon? I think this whole amp could really need a good clean to work properly again. I have some deoxit spray laying around which I use to clean tube sockets of my guitar equipment. Can I use this for the potis on the Technics amp as well?

Thanks a lot!
 
@Welcome .... Some things will never change ....

Technics amplifiers of that era and IC type indeed fail because of the IC Actually fail at a rate 40-50 % if abused ...The other 50-60% fail from other reasons while techs like Welcome reject them thinking that the IC is at fault so ...Let us not bother with it ....

---Soldering is one issue check that with caution
---Power supling is one other issue
Verify all voltage present according to the schematic
---Do some basic checking
---Visually inspect small blue electrolytic capacitors ... if you see green marks around the pins either on the PCB or on the lower body of the capacitors then you also have capacitor issues ( quiet common for Technics gear of that era ).

Play with subsonic switch to see if something is changed ( it is quiet hard and will take too much writing but it might be involved )

---Locate outputs of the amplifier located before the relay Measure to check if there is DC there that triggers the protection .

---If not your fault lays elsewhere
---If yes see how much this is If full rail the chances that the IC is gone increases
---If less start verifying voltage at the pins of the IC according to the schematic
---In the voltage amplifier section exist Q401-402 that might be able to fail and drive the amp to DC in the output ( very common 2USD fix)
---There might be other small things like that in fail before the IC fail ....


If the above is not English for you then seek expert help ( Clearly not Welcome )

Kindest regards
Sakis
 

Anorak

Member
2019-05-28 10:20 pm
Hey again! Actually I found the reason the Amp wasn't working. By conincidence I stumbled upon a german forum entry where somebody had the same problem: Wer kennt sich gut mit Technics aus, Problem mit einem SU-V65A, Hifi-Klassiker - HIFI-FORUM

He said that the Voltage Regulators on the main PCB behind the front panel got quite hot and produced failing solder joints. I found the units he mentioned and resoldered them and now the beauty is working again! :)

Only problem is, that almost all of the potentiometers crackle and that on the right channel sometimes theres not the full range of sound coming out of the speaker. At the moment its working fine and I'm using the "Remote" speaker outputs because on the main outputs the left channel wouldn't work. I assume this could be solved by cleaning the contacts etc, what do you reckon? I think this whole amp could really need a good clean to work properly again. I have some deoxit spray laying around which I use to clean tube sockets of my guitar equipment. Can I use this for the potis on the Technics amp as well?

Thanks a lot!

This was the cause of my "Current Drive" light staying off as well. Even with my very amateurish soldering skills I managed to carry out a repair and when I switched the amp on the light came on after a few seconds. My joy was short-lived however, as there is still no sound :(. I say no sound, but if you turn the volume to maximum, you can hear music at a very, very low volume (on all inputs). The bass and treble controls have the expected effect, but the balance control makes no difference when turned.

I have tried both the main and remote speaker connections with no success. Does anyone have any suggestions what to do next?