Technics SU-V4 no output - UK Vintage Radio Repair and Restoration Discussion Forum

Frankcal · 12th Dec 2019, 4:36 pm

Hi all, (my first post)
This amp just cut out while playing a record through it not too loud.
I replaced ic301 stk8050 (a direct short on pins 2&3). Protection circuit would still not kick in. I jumped the relay points on both channels ( bypassing the protection circuit )& get an output hum on the RH side (driven by ic3] but the LH side is still dead from newly replaced ic3.

On checking v readings The main 48v + & - are ok, however pins 12 to 14 on both stk8050 ic’s are incorrect with pin 14 at 14v. Also ic303 a duel opp amp pin 1 is reading 14v even when isolated, p2 is 0v & p3 is 0.13v power to pins 4&8 is correct.

Any help appreciated.
Frankcla

Frankcal · 13th Dec 2019, 3:54 pm

Hi
I’d just like to update my last post, more info, for technics amp su-v4
This amp just cut out while playing a record through it not too loud.
I replaced ic301 stk8050 (as there was a direct short on pins 2&3). Protection circuit would still not kick in. I jumped the relay points on both channels ( bypassing the protection circuit )& get an output hum on the RH side (driven by ic 302] but the LH side is still dead from newly replaced ic301.

The main 48v + & - are ok, however pins 12 to 14 on both stk8050 ic’s are incorrect with pin 14 at 14v. Also ic303 (an6552) a duel differential amp pin 1 is reading 14v even when I isolated it. p2 is 0v & p3 is 0.13v power to pins 4&8 is correct. Looks like one side of the ic is faulty?

How does the protection circuit work & what components are linking to it from the output amps STK8050
Would appreciate any help here

Thanks
Frankcal.

FIXITNOW · 13th Dec 2019, 4:47 pm

the one note I have is to check Diode D311 if s/c or leaky will damage stk8050

John_BS · 14th Dec 2019, 5:27 pm

It's not the easiest circuit to follow!

Do you have a schematic of some kind for the innards of the stk8050?

If you have +14v on both 8085 inputs could it be the -16V line is down? Power supplies common to both halves?

John

Frankcal · 18th Dec 2019, 3:57 pm

Thank you both for replies
All supplies are OK 16V and 48V both + and -

I now suspect IC303 is faulty on the pin 1 to 4 side as pin 1 reads 14v even when it’s disconnected and does not change state when I tie the input pins 2 & 3 togeather. When I repeat this on the other side (2nd dif amp output pin 7 changes state. This ic seems to control both IC301 & IC303. I will order a replacement and hope it works?

12th Dec 2019, 4:36 pm	#1
Frankcal Diode Join Date: Dec 2019 Location: Galway, Connacht, Ireland Posts: 3	Technics SU-V4 no output Hi all, (my first post) This amp just cut out while playing a record through it not too loud. I replaced ic301 stk8050 (a direct short on pins 2&3). Protection circuit would still not kick in. I jumped the relay points on both channels ( bypassing the protection circuit )& get an output hum on the RH side (driven by ic3] but the LH side is still dead from newly replaced ic3. On checking v readings The main 48v + & - are ok, however pins 12 to 14 on both stk8050 ic’s are incorrect with pin 14 at 14v. Also ic303 a duel opp amp pin 1 is reading 14v even when isolated, p2 is 0v & p3 is 0.13v power to pins 4&8 is correct. Any help appreciated. Frankcla

13th Dec 2019, 3:54 pm	#2
Frankcal Diode Join Date: Dec 2019 Location: Galway, Connacht, Ireland Posts: 3	Re: Technics SU-V4 no output Hi I’d just like to update my last post, more info, for technics amp su-v4 This amp just cut out while playing a record through it not too loud. I replaced ic301 stk8050 (as there was a direct short on pins 2&3). Protection circuit would still not kick in. I jumped the relay points on both channels ( bypassing the protection circuit )& get an output hum on the RH side (driven by ic 302] but the LH side is still dead from newly replaced ic301. The main 48v + & - are ok, however pins 12 to 14 on both stk8050 ic’s are incorrect with pin 14 at 14v. Also ic303 (an6552) a duel differential amp pin 1 is reading 14v even when I isolated it. p2 is 0v & p3 is 0.13v power to pins 4&8 is correct. Looks like one side of the ic is faulty? How does the protection circuit work & what components are linking to it from the output amps STK8050 Would appreciate any help here Thanks Frankcal.

13th Dec 2019, 4:47 pm	#3
FIXITNOW Octode Join Date: Feb 2007 Location: Ely, Cambridgeshire, UK. Posts: 1,976	Re: Technics SU-V4 no output the one note I have is to check Diode D311 if s/c or leaky will damage stk8050

14th Dec 2019, 5:27 pm	#4
John_BS Octode Join Date: Dec 2008 Location: Wincanton, Somerset, UK. Posts: 1,780	Re: Technics SU-V4 no output It's not the easiest circuit to follow! Do you have a schematic of some kind for the innards of the stk8050? If you have +14v on both 8085 inputs could it be the -16V line is down? Power supplies common to both halves? John

18th Dec 2019, 3:57 pm	#5
Frankcal Diode Join Date: Dec 2019 Location: Galway, Connacht, Ireland Posts: 3	Re: Technics SU-V4 no output Thank you both for replies All supplies are OK 16V and 48V both + and - I now suspect IC303 is faulty on the pin 1 to 4 side as pin 1 reads 14v even when it’s disconnected and does not change state when I tie the input pins 2 & 3 togeather. When I repeat this on the other side (2nd dif amp output pin 7 changes state. This ic seems to control both IC301 & IC303. I will order a replacement and hope it works?