Technics Amplifier Repairs
2sc789 in left rear channel replaced and pots cleaned. 4.7ohm and pnp a684 driver replaced in right rear channel.
Been spending some time overhauling an SU-8080 amplifier and would like to pass my findings on for reference. One of the problems was the direct\tone switch being noisy no matter how much was squirted into it. I removed the switch, dismantled it and found the contacts on a couple of the contact blocks a bit loose. This cured the noise problem.
Something else I found was one of the main capacitors 55V 10000 µF wouldn't hold its voltage after it had been switched off. The other three held +40V for days but the bad one would read 0V after only a few hours. This showed it was discharging internally and needed replacing. Several capacitors on the phono board were looking worse for wear mainly; 33 µF 50V, 3.3 µF 50V and some 1 µF 63V, 3.3 µF 63V capacitors on the main amplifier board in the PSU circuit. The speaker terminals were goosed so I replaced those as well. The plastic inside breaks away and won't grip the cable\wire, I replaced with standard 4mm terminals.
The SU-8080 uses an unusual feedback design as in 'direct' where the gain of the main amp is increased to about 130x and in tone as the signal passes through the tone cct which has gain the main amp gain is reduced to a more normal value of 27x, I am not sure if the sound quality is affected as I have not done any comparisons. I was contemplating arranging to have the main amp gain set to 27x all the time but never did get round to it.
This was blowing one of the transformer fuses. A quick check found the o\p transistors short circuit collector-emitter. These were changed, not having the originals as a replacement I used MJ15024\25. The amp powered up but the protection was still operating. The 2SC1628 was faulty in the other channel, replaced with a 2SC1124 I had hanging around, worked okay after.
This monster worked fine but the large displays lights weren't, looked like a couple of bulbs were required, in fact there are sixteen! in banks of four connected in series. 14 V 40 mA wire ended fit okay.
From the 1980's, this would drop the speaker relay out at random intervals and some of the front input LED's weren't lighting. Ended up soldering the ribbon cable from the front panel direct to the PCB as the connector had lost its job description (or springiness!), this sorted the LED's, and there were dry joints on the current sink transistor for one channel in the voltage amp section (front of the big IC), also the connector that's at the front of the main PCB was soldered direct near the offending transistor. It's best to blanket solder the PCB's as they are getting prone to dry joints, including the class AA ic and driver transistors behind the front panel. The amp is almost totally DC coupled and any problems could spell the end of difficult to get parts. The whole main PCB, back panel connectors and front panel need to be removed and inverted to get at them. The set-up for the bias is 40 mV across the outer pins of the connector on the main PCB at the narrow end! Check for <50 mV after 15 mins.
This amplifier was taking longer and longer to energise the speaker relay, found the 820k (r413) at twice the value and the 10k (r414) burnt but correct value, replaced these and operation was back to normal.
This worked okay using the 'CD direct' facility but none of the other inputs worked, the pre\main links were missing - doh!
From the start I knew this would be either a simple repair or a bin job, as the Technics DC drive arrangement usually self destructs. This was a high power DC amp using the class AA cct. Dry joints were the order of the day on the plug in board and main board, the svi module had blown a wire off. I was correct, a bin job, the parts are not available. If anyone has one of these model series a good re-soldering will save it from the skip!
1970's 50 Watt amplifier - one channel was u\s, replacing the encapsulated o\p transistors made this good they are similar to the STK module package.