Technics SUV4x Power Supply Issue Help

AlbertHall

Joined Jun 4, 2014
10,908
Remove the diode from the circuit and test again - with the leads both ways round.
If Q705 is still out of the board, then the problem is either D706 or C708.
 

Thread Starter

ssid

Joined Jul 7, 2020
34
Hi

C708 already replaced

Just heating up the soldering iron. Am I doing a diode test on the diode in both directions. Not using ground any more I presume?

Thanks
 

Thread Starter

ssid

Joined Jul 7, 2020
34
Diode removed from circuit

Diode test

Red to stripe = 0.367v
Black to strip = 0.195v

Is this a failed diode test?
 

AlbertHall

Joined Jun 4, 2014
10,908
Yes, it is.
The original is an MA4160 but that might be difficult to get. Any standard 16V, 400mW, zener diode will do the job.

Incidentally the -15V supply is used as the reference for the +15V output, so no -15V will cause no +15V supply. With a new diode they should both spring to life.
 

Thread Starter

ssid

Joined Jul 7, 2020
34
Wow that is fantastic. I have learned so much. Please can you direct me to the correct component on RS or CPC or another good supplier. There seems to be numerous options for zenere diodes!
 

Thread Starter

ssid

Joined Jul 7, 2020
34

MrChips

Joined Oct 2, 2009
22,886
Get extra 16V zener diodes while you're at it.

I tried simulating the original circuit and could not get the +15V section to work (perhaps my not having the correct transistors).

If replacing D706 does not fix the supply problem, I would be inclined to remove Q703, Q704 and Q705 and add D1, R3, and R4 in their place as shown in this simulation. The other components in the drawing are left over from the simulation and can be ignored.

Technics SU-V4X power supply.jpg

Original circuit:

Technics SU-V4X  16V power supply.jpg
 

AlbertHall

Joined Jun 4, 2014
10,908
I tried simulating the original circuit and could not get the +15V section to work (perhaps my not having the correct transistors).
Or maybe because the junction of R706 and R707 is not connected to 0V. That is the path where the +15V gets a reference from the -15V.
 

Thread Starter

ssid

Joined Jul 7, 2020
34
Great

I will get those zeners and report back after the weekend now I suppose. Will need to buy some extra stuff to avoid shipping charge!
 

Thread Starter

ssid

Joined Jul 7, 2020
34
Thank you all. Getting the 15v will hopefully bring it back to life or unearth another issue! Either way I am absoloutely delighted by progressing just a little further. These are great sounding amps from 1984/5. I have 4 working ones which I have maintained and repaired (and sometimes almost accidentally destroyed) over the years which is why I thought I would go for a difficult case!

I love doing this but don't always fully understand the schematics or what I am looking for.

Best wishes

I will report back once the diode is fitted. Should I fit a new Q705 or just put back the original?
 

MrChips

Joined Oct 2, 2009
22,886
I would agree with @AlbertHall by suspecting D706, Q702, Q705 initially.

If you have more of the same amp to fix and you are looking for extra parts to fill the order it would not be a bad idea to order these parts, as well as Q701 and Q703.

Also don't forget 1kΩ ½W resistors. I have not done a thorough calculation on the resistance and wattage required since I don't know how much current the +15V and -15V supplies have to deliver.
 

Thread Starter

ssid

Joined Jul 7, 2020
34
I would agree with @AlbertHall by suspecting D706, Q702, Q705 initially.

If you have more of the same amp to fix and you are looking for extra parts to fill the order it would not be a bad idea to order these parts, as well as Q701 and Q703.

Also don't forget 1kΩ ½W resistors. I have not done a thorough calculation on the resistance and wattage required since I don't know how much current the +15V and -15V supplies have to deliver.
Hi

Can you suggest modern replacements for Q701/3? Preferably from CPC! Q701/3 are around £5 each from littlediode for example.

Thanks
 
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