Hi,
Thanks for letting me join your forum.
I have a basic knowledge of electronics.
I took it upon myself to repair this amp of a friend and learn a thing or 2 about amps on the way.
I left another forum because i got bullied for not knowing right a way what they were asking me.
I downloaded a service manual and can find the components if referred by partnumber. I know how to solder components.
(Love the smell too😁)
About the amp. Must be about 45 years old, it looks very well designed.
The power light is on. No sound, no humm of the trafo, no hiss in the speakers.
While i was looking around the relay clicked, which is there to protect the speakers, i read. So there is some current where it shouldn't be i presume. I read that faulty capacitors is a thing in vintage audio. I can't However see any bulging capacitors.
I'd like to know where to start looking for faulty components.
Hope some of you audio freaks like to share your knowledge.
Greetings, Henri
Thanks for letting me join your forum.
I have a basic knowledge of electronics.
I took it upon myself to repair this amp of a friend and learn a thing or 2 about amps on the way.
I left another forum because i got bullied for not knowing right a way what they were asking me.
I downloaded a service manual and can find the components if referred by partnumber. I know how to solder components.
(Love the smell too😁)
About the amp. Must be about 45 years old, it looks very well designed.
The power light is on. No sound, no humm of the trafo, no hiss in the speakers.
While i was looking around the relay clicked, which is there to protect the speakers, i read. So there is some current where it shouldn't be i presume. I read that faulty capacitors is a thing in vintage audio. I can't However see any bulging capacitors.
I'd like to know where to start looking for faulty components.
Hope some of you audio freaks like to share your knowledge.
Greetings, Henri
This relay is part of the protection circuit but in an unusual way. Instead of being in the amplifier output stage it's in the power supply. Under normal operation the relay is de-energized just as it would be if the power is off. When the protection circuit detects a large enough DC voltage on the output of either power amplifier the relay energizes and removes power from the power amp stage. To see if the protection circuit is activated check for DCV on the base of TR603 and the emitter of TR602, it should be zero if no problems exist. If you can't get to those points any place connected to those points will work such as R608 and R609. You can even check it at the speaker fuses, F3 and F4, check both sides of the fuses in case they're blown. Another perhaps easier way is to check for voltage on the big capacitors, C3 and C4, should have approx. 37VDC on each, C4 will be a negative voltage. If there's voltage on both the amplifier is NOT in protection.
Craig
Craig
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Hi Craig, thanks for your reply!
I found no DcV on the fuses, they're intact. No oltage on R 608 and R609.
There's 38,8V on C3 but 0V on C4
I appreciate your help and your clear instructions.
Now i'll do some work on my snowthrower engine, bit messier ��have a nice day!
Greetings Henri
I found no DcV on the fuses, they're intact. No oltage on R 608 and R609.
There's 38,8V on C3 but 0V on C4
I appreciate your help and your clear instructions.
Now i'll do some work on my snowthrower engine, bit messier ��have a nice day!
Greetings Henri
Hi, i'm sorry i made a mistake. I measured dc over the fuses. Now i measured from the fuse to the chassis. On the left channel there is a voltage of 30 V when i turn on the amp which goes down to about 2 V after a minute or so
Perhaps this is the result of the operation of the protection of the power supply (malfunction or overload).
Possible malfunction of the corresponding amplifier channel
(usually output transistors or drivers).
Possible malfunction of the corresponding amplifier channel
(usually output transistors or drivers).
Hi,
Thanks for posting the diagram.
Yeah, i hope I can narrow it down to which components i have to replace. Is it possible to conclude that the fault is in the side that has no voltage since the relay engages?
Thanks for posting the diagram.
Yeah, i hope I can narrow it down to which components i have to replace. Is it possible to conclude that the fault is in the side that has no voltage since the relay engages?
I measured between the negative side of an AC outlet on the back and 304 and 344. I measured 0 ACvoltage
Looks like it's in protect. But let's make sure, ACV on 303 and 304? If those voltages are good we can get it out of protect by removing the speaker fuses, F2 and F3. The protect circuit senses the DCV AFTER the fuse so removing the fuses will eliminate any DCV getting to the protection circuit. Now measure 343 and 344 again, ACV?
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Hi!
303 and 304 both have a ACV of 28,3V
I removed the 4A L and R fuses and measured 28,3 AcV on 343 and 344
303 and 304 both have a ACV of 28,3V
I removed the 4A L and R fuses and measured 28,3 AcV on 343 and 344
OK, power supply and protection are working. What are the DCV on 341 and 342? Now reinstall one fuse and measure the ACV again.
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341 measured the first time 97V but went down to 89V
342 measured 0 V
I screwed in the R fuse and the points
303,304,343,344 all measured 29V
342 measured 0 V
I screwed in the R fuse and the points
303,304,343,344 all measured 29V