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
Attachments
The inconsistencies on the diagram labelling have led me astray and I now believe its only the rail voltages on the power transistors that are altered this is backed up by there being 4A fuses in the 24.9V feeds . Likewise the labelling on board P701 ( J704 ) would appear to be correct and not as my above post . Also it is the power across the output transistors that would need to be reduced when working class A on class B heat sinks .
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So Ive done some more measuring and voltages on rails seems good on both sides. shifts as per scheme with class A.
Continued measurements around the bias adjustments pots. Lots of small voltages on scheme all checks out. So no abnormalities
I cant seem to quite wrap my head around what controls the bias voltage, so Im a little stumped
I popped the bottom lid off and had a look at PCB 701. Could not see any bad solder joints, but there was some dark spots clearly from heat directly under the areas around Q701, Q702, Q703, Q704
Continued measurements around the bias adjustments pots. Lots of small voltages on scheme all checks out. So no abnormalities
I cant seem to quite wrap my head around what controls the bias voltage, so Im a little stumped
I popped the bottom lid off and had a look at PCB 701. Could not see any bad solder joints, but there was some dark spots clearly from heat directly under the areas around Q701, Q702, Q703, Q704
Found the MKII Spescial Edition manual and there seems to be some hybrid-bastard-combination stuff going on with my amp. There are some parts corresponding to the SE-manual/schematics, but mostly the ordenary MKII checks out. Probably not uncommon for mass produced electronics to have overlap and mix/match of parts....
I'll try the SE bias settings and give the amp a go
I'll try the SE bias settings and give the amp a go
All that stood out for me about the SE was it didn't have tone controls , but it wasn't a comprehensive search through the schematics . The SE schematics have the errors corrected at least the ones i identified.
The AB biasing is the same and you ended up with the correct A bias 90mV/250mA
Perhaps it was a modification to the MKII that didn't get updated on your handbook , assuming it came with the amp . What is the YK...… number on your P701 board is it different to YK188J191-0 .
The AB biasing is the same and you ended up with the correct A bias 90mV/250mA
Perhaps it was a modification to the MKII that didn't get updated on your handbook , assuming it came with the amp . What is the YK...… number on your P701 board is it different to YK188J191-0 .
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I did a bias adjustment according to the SE manual, and hooked up the amp last night. It played nicely for several hours but I could not turn the volume very high due to sleeping baby in the next room. Did not try class A but I will
If all works I am a little confused, as I am sure these bias values are what I landed on in the first place - ref. my first post.
Anyway, I will pull out the amp and check the P701 pcb number, and have a closer look at the glass fuse. I'll report back as soon as possible
PS; the handbook I'm referring to came from the internet, as I have no papers left from when it was new
By the way - thank you for taking the time helping me out! Much appreciated
If all works I am a little confused, as I am sure these bias values are what I landed on in the first place - ref. my first post.
Anyway, I will pull out the amp and check the P701 pcb number, and have a closer look at the glass fuse. I'll report back as soon as possible
PS; the handbook I'm referring to came from the internet, as I have no papers left from when it was new
By the way - thank you for taking the time helping me out! Much appreciated
P701 board is marked YK188J191-3
Tried playing music at moderate level in class A. Worked fine for about 20min then protection mode. Comes back to life when turning down volume. Tried switching to AB immediately but still tripped protect mode when volume is turned slightly up. Fuse from before is burned off one third in the middle
Tried playing music at moderate level in class A. Worked fine for about 20min then protection mode. Comes back to life when turning down volume. Tried switching to AB immediately but still tripped protect mode when volume is turned slightly up. Fuse from before is burned off one third in the middle
That's the board number on the "se" handbook schematics I have which gave R749 as 47K .
Interesting that it lasted 20 mins . From cold , in AB mode , try turning the amp volume up and see if it prematurely goes into protection .
I have had that kind of fuse blow when the fuse was just too low a rating and the inrush current at switch on progressively aged the fuse , however on the "se" they still quote a 3.15A rating .
Interesting that it lasted 20 mins . From cold , in AB mode , try turning the amp volume up and see if it prematurely goes into protection .
I have had that kind of fuse blow when the fuse was just too low a rating and the inrush current at switch on progressively aged the fuse , however on the "se" they still quote a 3.15A rating .
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I'm not sure about the production years for the different versions, but serial number on mine starts with 0096. Maybe they made modifications to the latest mkII's witch followed into the SE. I don't know. Pretty confident that this amp has never been opened, repaired or modified
Fuse might have blown due to age. It's worth mentioning that I did not have 3,15A so i tossed in a 4A. Maybe not the best idea....
I'll test AB like you mentioned and report back. Wonder what the protect mode actually is tripped by. Read about one guy here that had dc leak to the speaker terminals with tripped PM
Fuse might have blown due to age. It's worth mentioning that I did not have 3,15A so i tossed in a 4A. Maybe not the best idea....
I'll test AB like you mentioned and report back. Wonder what the protect mode actually is tripped by. Read about one guy here that had dc leak to the speaker terminals with tripped PM
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