Technics SUV4x Power Supply Issue Help

Thread Starter

ssid

Joined Jul 7, 2020
34
Hi abrscv
IC201/2 - I checked power to these and they are similar to all the rest of the 15v rail around 1.89v
 

AlbertHall

Joined Jun 4, 2014
10,915
Voltage across D706 = -0.927 and 0.3mv with Q702 removed
With Q702 removed the voltage across D706 should correct : 16V.
If it is not, then the fault is D706, Q705, or C708.
Make sure the diode is the right way round, cathode (stripey end) connected to chassis ground.
 

Thread Starter

ssid

Joined Jul 7, 2020
34
Hi Albert Hall.

I put in a new C708. No change. As far as I can tell Q702 is OK. D706 seems to work as a diode but will check again.

Could it be a fault further down the 15v circuit. I am not sure if it works like that of course. If I unplug Board ABC on the circuit diagram, the negative voltage goes up to -20v? The voltage going to both IC201/2 is the same as the voltage going to ABC when plugged in.

This amp could be just totally shot of course - it has been repaired before I think. I was initially very pleased with myself for getting it out of protection mode! It was strange that I could remove Q702 and it did not go into protection mode. It could be because the voltage is so low. But..it did detect a short circuit when I was testing power to the jumpers on that board (ABC).

The 15v rail is critical as it affects everything including IC501 (AN7062 - voltage amp) which is why the STK does not even warm up.

I will keep tinkering ...... nothing to loose and everything to gain and will report back if I make any progress.

I appreciate all the help that everybody has given so far. If anyone has any other ideas I will give it a go. I don't want to buy any parts without finding the fault as it is vital when to know to stop digging!

Best Wishes
 

Thread Starter

ssid

Joined Jul 7, 2020
34
Hi AlbertHall

I have never removed D706 so I am assuming it is the correct way round. Q704/5 is cheap to replace so could give it a go £3 for 2. When I tested Q704/5 with a transistor tester they were OK but they are sometimes wrong. Those transistors are the sources of the 15v I suppose.
 
Last edited:

AlbertHall

Joined Jun 4, 2014
10,915
I have never removed D706 so I am assuming it is the correct way round.
The voltages you are seeing on this diode are about what I would expect if it was the wrong way round so it should be checked, especially as you know someone else has been fiddling.
Measure the resistance between the the stripe marked end of the diode and chassis ground. It should be short circuit.
 

Thread Starter

ssid

Joined Jul 7, 2020
34
The voltages you are seeing on this diode are about what I would expect if it was the wrong way round so it should be checked, especially as you know someone else has been fiddling.
Measure the resistance between the the stripe marked end of the diode and chassis ground. It should be short circuit.
Hi. Just want to be sure I am doing the right thing. Sorry - my lack of knowledge here....
Black probe to chasis.
Red probe to striped end of diode.
Diode test mode or resistance Mode?
Power on or off?

Thanks
 

Thread Starter

ssid

Joined Jul 7, 2020
34
Removed Q705 and tested OK but have ordered new ones.
I have another fully working unit which I have maintained over the years and checked the direction that D706 is fitted and it is correct unless it has failed of course.
 
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