Technics SUV4x Power Supply Issue Help

Thread Starter

ssid

Joined Jul 7, 2020
34
Hi

Got the Technics SU-v4x amp as spares or repair. Fancied a challenege to see if I could fix it as I like them. Seller said it was working until it started going into protection from time to time but now full time.

Burned a cap C523 as soon as I plugged it in > replaced it > OK

Checked the pre drive transistors Q501 - 4 >OK

Checked for cold solder joints > nothing yet

Input selector does not work properly as it does not show red led selection. Only the yellow tape source selection.
Track on circuit board had been previously repaired. Cheked it

Eventually I got it out of protection mode by replacing C523 and R535 which had also blown. Also replaced relay as it was stuck.

The input selector still does not work properly - only the tape select works so no sound. I think it is a power supply issue but cannot work out what to do next.

The 50v rails are fine but the issue is with the 15v rail which supplies the various tone / selector / phono pre amp boards via jumpers is running at 1.89 volts.

All the Q701 - 5 regulator transistors seem to be OK (Not checked Q705 yet). The 4 rectifier diodes are OK I think with 2 showing 50v in one direction and the other two showing the same in the opposite direction.

I feel I am on the cusp of kind of fixing it or maybe not as it may just go into protection mode again if I get the 15v rail back!

Any help would be appreciated. Attached is the service manual.
 

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AlbertHall

Joined Jun 4, 2014
10,068
What is the voltage across D706 and R702?
D706 sets the output for the -15V.
The voltage across R702 tells us how much curretn is being drawn from the -15V.
 

Thread Starter

ssid

Joined Jul 7, 2020
34
Hi AlbertHall - I really appreciate your input as I am learning how to do this and can only get so far before I get stuck!
D706 = -0.644v and 0.3 mv
R702 = -50.8 v and -51.6v

Just an observation. If I unplug all of the boards the J connector negative voltage on the main board goes up to -18-20v.

Thanks
 

AlbertHall

Joined Jun 4, 2014
10,068
Ok. There is 50V across R702. From the parts list that is a 470 ohm, 0.5W resistor. 50V across 470 ohms means that there is 5W beIng dissipated in that resistor. That would burn up that resistor pretty quickly. There are two possibilities:
1. The resistor is open circuit (or high value) - with it switched off measure the value of that resistor.
2. There is a near short circuit on the -15V output somewhere and the only thing there to limit that current is R702. The fact that the voltage is near normal when disconnected suggests this may be the case (but still measure the value of R702).

If R702 value is correct then carefully inspect the rest of the circuit for damaged or burnt components, leaking or bulging electrolytic capacitors, or anything else that looks suspicious.
 

Thread Starter

ssid

Joined Jul 7, 2020
34
Hi. Thanks for the quick response

In circuit the R702 is measuring 0.447 k ohms but I can remove and measure properly if needed. It looks like it is in good condition.

Are there any specific components that I should check or should I be looking near R702 or the whole 15v circuit across multiple boards?

Thanks
 

Thread Starter

ssid

Joined Jul 7, 2020
34
I also replaced C201/2 on the phono sockets input board as when I first got the amp it looked like they were smoking as the amp was full of dust.
 

Thread Starter

ssid

Joined Jul 7, 2020
34
C707/8 are near where we have been looking at. They look OK but I could replace them quite easily as I have them in my toolbox? Is it worth a try?
 

AlbertHall

Joined Jun 4, 2014
10,068
In circuit the R702 is measuring 0.447 k ohms but I can remove and measure properly if needed. It looks like it is in good condition.
And when it is switched on there is about 50V across the resistor - that is with one meter lead on one end of the resistor and the other meter lead on the other end of the resistor?
 

AlbertHall

Joined Jun 4, 2014
10,068
I am a little confused.
Measure the voltage on each end of the resistor to chassis ground.
Then lift on end of the resistor and check its value.
 

Thread Starter

ssid

Joined Jul 7, 2020
34
Hi

R702 Chasis to ground -50.8 volts and -51.5v
Not sure what you mean by ("Then lift on end of the resistor and check its value. ") , but if I directly put the probes on either side of the resistor and switch to Resistance mode on the DMM it shows 0 with power on. With power off after a period of time the resistance starts to rise and now appraching 400 ohms and rising.
 

MrChips

Joined Oct 2, 2009
21,374
You are committing fundamental newbie mistakes.

1) You NEVER attempt to measure resistance with any ohmmeter (or DMM in resistance mode) in a circuit while the power is ON.
You are at risk of blowing your DMM.

2) Measuring resistance of any component while still in circuit will be affected by the rest of the circuit. In order to obtain an accurate measurement at least one end of the resistor must be disconnected. This is what @AlbertHall means when he says "lift one end of the resistor".
 

MrChips

Joined Oct 2, 2009
21,374
@AlbertHall might have misread your voltage readings taken at R702.

According to your readings reported in post #3
R702 = -50.8 v and -51.6v

Hence the voltage across R702 is 0.8V, not 50V.
 

AlbertHall

Joined Jun 4, 2014
10,068
I am suspicious of Q702 - which you say you have tested.
Can you remove Q702 and then switch on the amp and measure the voltage across D706.

Check the marking on Q702 - should be A1112 -R as below.
What do you have for testing this transistor? A DMM with a diode check range?

1594212913221.png
 

Thread Starter

ssid

Joined Jul 7, 2020
34
Hi. Had to go out.

Thanks for the advice on using the DMM and measuring resistance while the power is on Mr Chips

I used a transistor tester to check Q702, however sometimes they can be inaccurate.Q702 is as shown on the picture

bjt-pnp
123 =ecb
hfe=109
vf =604mv

With power on for Q702
c = -50.2v
b = -0.920
e =-3.722 mv

Voltage across D706 = -0.927 and 0.3mv with Q702 removed

What was strange was that it did not go into protection mode with Q702 removed. Possibly becuase the current is so low.
 
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