HELP Technics SU-V60 Stereo Amplifier VC-4 distorted sound

The large caps will not be the cause of the problem, so don't consider replacing them. It is also unlikely that you have defective output transistors, particularly if the amp is not going into any self protect mode. Bad joints are common on Technics kit, so it is certainly one of the first things that I would be looking at. You can get a good idea of the area of the problem by seeing if the crackle is altered by the volume control. If it is, then the problem is back up the front end, and you needn't look further into the area around the output stage. If it is not affected by the volume control, then the problem is either output stage or power supply related. Many models of Technics gear, have regulator transistors attached to the same heatsink as the output transistors, and it is very common for these to go bad jointed. Be very careful with your probing, whilst the amp is on. A slip of the meter probe can be disastrous on these DC coupled amps ...

Arfa

Volume dial has no control of crackle. Low and high behave the same amount. I noticed a small browning of the circuitboard under a C263A located behind the output transistors. Its the one located farthest left. Looks like a group of 4. If the left 2 are left channel (from front of unit) might this be it?

Thanks for the help Dave

Ok, no control of problem by vol control, says output or PSU. I would definitely start by visually examining the soldering on all power devices on the heatsink, and any areas where the board is heat-discoloured. If the joints look even slightly crystaline, wick or suck them clean, and re-do using new solder.

Arfa

Don't know what your "crackle" sounds like, or looks like on a 'scope, but I'm thinking maybe a bad transistor, either a noisy signal or pre-driver type, or if the outputs are flat-packs, which is likely, maybe an open base-emitter junction on one of them.

Mark Z.

Maybe crackle is not a good term to use. I would say distorted sounding. Almost like overdriven input to a soundboard or a blown speaker (but its not that). Well I resoldered all joints on heatsink and many of the others while I was there. No improvement or change at all. Re-checked with headphones and left and right sound great! Does that eliminate the circuitboard behind the front panel and focus us on the board with the heatsinks? By the way what do the 2 big caps at the back of the unit do? Are they a filter of sorts? In my pinball repair they have used them to stabilize voltages. Wish I had some info the the test points TP1 and TP2. I wish I knew what componets are used for the left channel, I could replace some and hope for the best. Any other leads appreciated. Thanks Everyone Dave

The two big caps are the main smoothers ( filter caps ) for the positive and negative supply rails to the output stages. You cannot repair DC coupled amplifiers by guesswork and hopeful component replacement, I'm afraid. They are a black art that can at times, drive even the most experienced of us to distraction and tears ... If you have distorted sound, particularly if it is harsh and raspy, as you say " like a blown speaker ", it is likely that one half of the output stage is not being driven. This could be for many reasons, but in your case, it may be for some very odd reason, as usually, any inbalance in the drive conditions, will result in a DC offset at the midpoint, which will be picked up by the protection circuitry, causing a shutdown. The only way really to proceed with this type of fault, is to sit down quietly with a schematic diagram, a good multimeter, and a 'scope. I would have to say that a good degree of practical experience is also needed to work on this type of amp with any liklihood of a degree of success.

As an aside, I have a Williams Cosmic Gunfight from 1982, which I spent many happy hours restoring. Originally had faults on just about every board, and some incorrect wiring on the playfield, where parts had been replaced. I became quite adept at the 7 series CPU boards, and mended quite a few of them for the dealer that I originally got the machine from for fifty quid.

Arfa

Sounding more and more like an open base-emitter junction on one output transistor. While running, carefully measure the DC voltage across the base-emitter junctions of each output transistor. Should read from about .450 volts to around .6 volts. The one that measures in the range of several volts is defective.

Mark Z.

Mark I tried to send you mail but it bouces back. I have a PDF of the amp.

Dave

You should be able to get me at

snipped-for-privacy@labolgcbs.net

and reverse the domain name.

If the manual's a PDF, send me a copy. File size may require it be split however. I'm not totally sure of the limitations on this account.

Mark Z.

hi there

Recently I've got the SU-V60 amplifier. It was in good shape,but it didn't worked. I've tried to find out what is the reason. At first, the hybrid power stage - dedicated for those AMPs, hard to find spare parts. However, there was no burned PCB close to it, so I guess its working properly. There were some dark areas near the current mirror of the first power amplifier stage. I've soldered some bad joints,but it didn't help. I also found some burned resistors, first biasing the diode in current mirror and the other series in the positive supply rail, which comes to the preamplifier mounted on the front panel. The preamplifier board is also darkened, mainly around the two power transistors and output transistors driven by the AN7062. I wonder if there is a current or a voltage signal coming from the preamplifier to the main power AMP. Unfortunately, the resistors are burned so bad, that it's impossible to read the colour markings :( The output relay is open all the time, voltage at both outputs of the power amp ~ minus (50...60) V (depending on the speaker impedance switch).

Without schematic it would be very difficult to troubleshoot this amp any further. I'd like to ask if anyone has got the schematic or service manual. I would be very grateful. Any close-up photos from the inside would be also helpful.

sorry, I forgot my email: res snipped-for-privacy@o2.pl

Sent the OP a PDF of the s/m.

Mark Z.