Hello all
I hope I can ask for some guidance in my attempt to find and repair the fault of this amp. Quickly explained - This is probably not something everyone finds worth repairing, but this amp belonged to my late father so it has some sentimental value. My electronics skills are limited but not hopeless. That is to say, I have some education from earlier and have built some stuff over the years, but I lack the real experience acheved by messing with this stuff on a weekly basis
This is the situation:
- Amp was used by me for a few years then the speaker relays started acting
up. Stored it a few years and then pulled it out and replaced the relays last month.
- Got hold of the manual to check the biasing, witch was slightly off in class A/B, and far off in class A
- Adjusted bias in A/B to spec (50mA/18mV). Could not reach correct bias in class A (500mA/180mV). Adjusted to about 90mV to even both channels
- Amp vorked on low volume for a couple of hours then died. Glass fuse 3,15A on AC mains side was gone
- Replaced fuse and tried again only in class A/B, wich worked on low levels but amp triggered fault mode if volume was increased, but no blown fuse
- I have now done some measurements, mostly voltages and there is a few odd things going on
1. No DC on speaker terminals, witch is a good thing I presume
2. B+ and - seems good, if not a volt or so too high
3. Class A is stated to drop B+ to 24,9V witch is not happening
4. Class A switch circuit is working in terms of activating the opto-couplers
5. Some small voltages seems off at the input side
I'm wondering if the lower voltage of b+/- in class A stated on schematic is supposed to happen just by switching over to class A, and if so is this the circuitry pulling down the voltage, or is the feed lowered? The measurements are equal on both L and R
Schematics are power amp board only, but I have the rest if needed
I hope I can ask for some guidance in my attempt to find and repair the fault of this amp. Quickly explained - This is probably not something everyone finds worth repairing, but this amp belonged to my late father so it has some sentimental value. My electronics skills are limited but not hopeless. That is to say, I have some education from earlier and have built some stuff over the years, but I lack the real experience acheved by messing with this stuff on a weekly basis
This is the situation:
- Amp was used by me for a few years then the speaker relays started acting
up. Stored it a few years and then pulled it out and replaced the relays last month.
- Got hold of the manual to check the biasing, witch was slightly off in class A/B, and far off in class A
- Adjusted bias in A/B to spec (50mA/18mV). Could not reach correct bias in class A (500mA/180mV). Adjusted to about 90mV to even both channels
- Amp vorked on low volume for a couple of hours then died. Glass fuse 3,15A on AC mains side was gone
- Replaced fuse and tried again only in class A/B, wich worked on low levels but amp triggered fault mode if volume was increased, but no blown fuse
- I have now done some measurements, mostly voltages and there is a few odd things going on
1. No DC on speaker terminals, witch is a good thing I presume
2. B+ and - seems good, if not a volt or so too high
3. Class A is stated to drop B+ to 24,9V witch is not happening
4. Class A switch circuit is working in terms of activating the opto-couplers
5. Some small voltages seems off at the input side
I'm wondering if the lower voltage of b+/- in class A stated on schematic is supposed to happen just by switching over to class A, and if so is this the circuitry pulling down the voltage, or is the feed lowered? The measurements are equal on both L and R
Schematics are power amp board only, but I have the rest if needed