Technics SU-V5. Output stage 'fault'. - UK Vintage Radio Repair and Restoration Discussion Forum

samjmann · 10th Apr 2023, 10:37 am

This 41 year old integrated amp appeared with both output stages midpoints varying from -20v to +40v. All the output transistors were intact, no burnt resistors etc. A check across the emitter resistors revealed barely any current draw. It then got put on the 'to do' list... A week later I switched on again without a lamp limiter, and it instantly destroyed one output transistor.

Next step was to disconnect the collectors of both outputs and drivers on the damaged channel. At this, both stages powered on with the lamp limiter in, midpoints both the same at -37v.

The only common factor was a servo amp as Technics calls it in the feedback line to the power amps differential amp. The inverting input was varying wildly again, it should be 0v. The 100k resistor to ground off both inverting inputs were fine, but there was no ground connection. In fact there was no ground on this section of the main board.

This 'servo' amp ground was ran via the tone control pcb to the main pcb again. The wired connector that was on the main pcb had been refitted at some point the wrong way round. Correcting this restored the ground, midpoints now stable!

So not really a fault, but a good head scratcher!!

vinrads · 11th Apr 2023, 10:18 am

I wonder why Technics did that ! well done finding the man made fault . Mick.

samjmann · 11th Apr 2023, 8:01 pm

Thanks for that Mick

On a similar vein, whilst at the Co-oP, I once had a Hitachi integrated amp with bad crossover distortion on one channel only. The base bias resistors to each driver transistor had been replaced to a value 10x what they should have been. I never found out the background of it. Perhaps it was a trade test unit
that got sold by mistake. It was still under warranty at the time.

Regards, SJM

10th Apr 2023, 10:37 am	#1
samjmann Heptode Join Date: Feb 2014 Location: Nottingham, UK. Posts: 648	Technics SU-V5. Output stage 'fault'. This 41 year old integrated amp appeared with both output stages midpoints varying from -20v to +40v. All the output transistors were intact, no burnt resistors etc. A check across the emitter resistors revealed barely any current draw. It then got put on the 'to do' list... A week later I switched on again without a lamp limiter, and it instantly destroyed one output transistor. Next step was to disconnect the collectors of both outputs and drivers on the damaged channel. At this, both stages powered on with the lamp limiter in, midpoints both the same at -37v. The only common factor was a servo amp as Technics calls it in the feedback line to the power amps differential amp. The inverting input was varying wildly again, it should be 0v. The 100k resistor to ground off both inverting inputs were fine, but there was no ground connection. In fact there was no ground on this section of the main board. This 'servo' amp ground was ran via the tone control pcb to the main pcb again. The wired connector that was on the main pcb had been refitted at some point the wrong way round. Correcting this restored the ground, midpoints now stable! So not really a fault, but a good head scratcher!! Attached Thumbnails __________________ It's never been right since we've had it...

11th Apr 2023, 10:18 am	#2
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,762	Re: Technics SU-V5. Output stage 'fault'. I wonder why Technics did that ! well done finding the man made fault . Mick.

11th Apr 2023, 8:01 pm	#3
samjmann Heptode Join Date: Feb 2014 Location: Nottingham, UK. Posts: 648	Re: Technics SU-V5. Output stage 'fault'. Thanks for that Mick On a similar vein, whilst at the Co-oP, I once had a Hitachi integrated amp with bad crossover distortion on one channel only. The base bias resistors to each driver transistor had been replaced to a value 10x what they should have been. I never found out the background of it. Perhaps it was a trade test unit that got sold by mistake. It was still under warranty at the time. Regards, SJM __________________ It's never been right since we've had it...