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

I have a Technics SU-V60 Stereo Amplifier. The left channel is
distorted and crackling at low or high volume with any source cd, radio
etc.
You can still get power out of it but it sounds bad. Flipped speaker
wire, problem still on Left channel.

I repair pinballs for fun so I'm not afraid to solder or replace
componets. Looking for a starting point.
I read that Technics was natorious for bad solder joints. Maybe I will
reflow all and see.

There are 2 8000 uf 66v caps at back of motherboard, is this worth
replacing?
or are the c3944 transitors with the heat sink?

Any lead would be a great help. This amp still sounds great ( in the
right channel ) I really want to save it.

Anyone have a schematic they can share?

Thanks In Advance
Dave
 
A

Arfa Daily

Jan 1, 1970
0
I have a Technics SU-V60 Stereo Amplifier. The left channel is
distorted and crackling at low or high volume with any source cd, radio
etc.
You can still get power out of it but it sounds bad. Flipped speaker
wire, problem still on Left channel.

I repair pinballs for fun so I'm not afraid to solder or replace
componets. Looking for a starting point.
I read that Technics was natorious for bad solder joints. Maybe I will
reflow all and see.

There are 2 8000 uf 66v caps at back of motherboard, is this worth
replacing?
or are the c3944 transitors with the heat sink?

Any lead would be a great help. This amp still sounds great ( in the
right channel ) I really want to save it.

Anyone have a schematic they can share?

Thanks In Advance
Dave
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
 
A

Arfa Daily

Jan 1, 1970
0
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
 
M

Mark D. Zacharias

Jan 1, 1970
0
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

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
 
A

Arfa Daily

Jan 1, 1970
0
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
 
M

Mark D. Zacharias

Jan 1, 1970
0
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.

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.
 
M

Mark D. Zacharias

Jan 1, 1970
0
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

[email protected]

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.
 
M

Mark D. Zacharias

Jan 1, 1970
0
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.

Sent the OP a PDF of the s/m.

Mark Z.
 
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