Technics SU-V303 vintage amplifier fault, stuck in Standby

Thread Starter

Itkovian

Joined Dec 1, 2020
37
Hi all

I have a faulty Technics SU-V303 amplifier, a lovely piece of kit which I bought new back in 1983. It worked great for many years, then was stored in an attic for almost 20 years until I took it out to use again. It was a bit erratic at first and I cleaned all pots and switches with contact cleaner, then it worked great again but recently went faulty. It started with one channel going silent and then the output "standby" light came on and it refuses to go beyond that. I have the service manual and I bought myself a DMM as I would really love to have my amplifier back in action.

I took a good look around inside the amp. No obvious signs of damage and no smell of burning. It has been a very long time since I did design and troubleshooting but I am ready to start the hunt to find and hopefully repair the fault and I would be very grateful for any advice and help the members could offer.

Many thanks in advance
Les
PS I have now uploaded the Service Manual
 

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peterdeco

Joined Oct 8, 2019
356
I see a PC mounted trimmer resistor for at least 1 channel. Clean them and as many as there are. They could dirty and offsetting the bias.
 

Thread Starter

Itkovian

Joined Dec 1, 2020
37
I see a PC mounted trimmer resistor for at least 1 channel. Clean them and as many as there are. They could dirty and offsetting the bias.
Many thanks for your suggestion PeterDeco, I will make that my first task and report back. I noticed today that sometimes the amp will switch on fully then after a second revert to standby. Maybe that has some significance.
 

Thread Starter

Itkovian

Joined Dec 1, 2020
37
Because the amp sometimes switches on after preheat (relay clicks and Standby light goes out, then reverts to standby after a second) should I also check out the DC detection circuitry, Q703 and Q704 - and if so what should I expect to measure ?
Many thanks
 

Dodgydave

Joined Jun 22, 2012
9,951
IC702 pins 7, 22 are the overload protection , check those pins to see what voltage they are, then work back to IC 701 inverter, ..Q703, Q704.
 
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Thread Starter

Itkovian

Joined Dec 1, 2020
37
So I measured IC702 pin 7 is 5V and pin 12 is also 5.2V. Pin 26 is 0.07V (Low I guess) and turning off RLY301, same as power off muting. So it seems I may have a DC level where it should not be ?
 

Dodgydave

Joined Jun 22, 2012
9,951
Ok what voltage is pin 22 of IC702?
Check Q701, Q705 these are checking the Zero Voltage at the speaker outputs.
 
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Thread Starter

Itkovian

Joined Dec 1, 2020
37
What does that behaviour suggest ?
Ok, More info. With no speakers connected, when I crank the Volume knob up to full, the output relay clicks on and stays on for about 8 seconds, then reverts to Standby. However, this only happens when Input Selector is set to Phono. For all other Input settings the output relay does not switch in and the amp stays in Standby when I crank up the volume. The behaviour is consistent regardless of the settings of the Speakers select knob or the Rec Select knob. Is this helpful ?
 

Thread Starter

Itkovian

Joined Dec 1, 2020
37
Ok, More info. With no speakers connected, when I crank the Volume knob up to full, the output relay clicks on and stays on for about 8 seconds, then reverts to Standby. However, this only happens when Input Selector is set to Phono. For all other Input settings the output relay does not switch in and the amp stays in Standby when I crank up the volume. The behaviour is consistent regardless of the settings of the Speakers select knob or the Rec Select knob. Is this helpful ?
While the Output relay is switched on briefly, Pin 26 reads 2V, then reverts to 0V when the Standby lamp comes on
 
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