It's up to you if you do not want to match parts or use orginal. You asking for sugestion from experanced and proven repairs. I never said the FC parts are largly sub-par just not orginal or meet all spec's of the orginal. I cut and pasted the data sheet to make it easy for you to compair. I have rebuilt and repaired countless number of them. I have also had to re-do the so called "budget repair" on these a number of times just because someone did not want to be bothered with getting the orginal parts. If you are pleased with what you have selected then as they say "go for it". I would be interested to see you final results verfied by a good distorshion anlyzer. The bottom line numbers will tell the whole story in the end but then again, low THD/DISTN and other factors may not be what your looking for.
I have access to an Rohde & Schwarz UPV and an HP 3561A at work, so I could do an analysis on the 555 later, if I can get my hands on a good signal generator. It wouldn't be conclusive as there would be no "before" test, but if it would put the matter to rest...
I didn't say anything about not matching the diff pair. I plan to match them as close as I can with my Heathkit IT-18. Shouldn't be hard as Fairchild seems to have tighter groupings than Toshiba did. I plan to buy a good amount anyway to have on hand for future projects.
I didn't say anything about not matching the diff pair. I plan to match them as close as I can with my Heathkit IT-18. Shouldn't be hard as Fairchild seems to have tighter groupings than Toshiba did. I plan to buy a good amount anyway to have on hand for future projects.
Have you seen Chris's (Anatek) transistor matching jig? This is what I use to match my diff pairs, works for matching the darlingtons in the 565/585's also. I rough them out on the IT-3120 then look at breakdown on the curve tracer then on to the "jig" for close matching. It sure helps get the DC offset down s close as posible.You may have acess to just the same equipmet in you lab.
This is my first triple nickle repair. I noticed there were two extra boards sandwiched in-between the outputs on the heat-sink but couldn't find any documentation at all on them and I was missing the negative on one channel and traced it back to one of them. I removed it and saw that it was a regulator board ( 80v to 62v ) and after a good look I found that one of the trim pot's was open on one end. My question is this. Why is it so hard to find a schematic for this apparent factory upgrade? All I have is the original service manual and it's not in there. I sent pictures to all of the repair shops that ADCOM recommended online (10 or 15) yet not one came back with any idea what those boards were for or who put them in the amp!? I saw that there is a thread for the 62 volt regulator board somewhere here so somebody knows something??? This amp has -10 volts at the bias pot in one channel and there is no change at all in adjustment looking across one of the emitter resistors? The schematic uses +/- 80 volts and all of the voltages are referenced to that and not 62 volts. I'm tempted to just crank up the trimmers and bypass the regulator boards but I know they are there for a reason. Was the 80 volts too much for these amps or what? Is this a mod or a factory update? Does anyone have a good schematic for this amplifier with some voltages on it? Thanking you in advance! Alley :-(
Back in the '80s, maybe '90s, there was an Audio Amateur magazine article that added regulators to the 555. I'll look for it when I get home. If all else fails you just return the amp to stock. Old Colony Sound Labs used to carry the boards and possibly the kit but OCSL was bought by Elektor I think and no more Audio Amateur PCBs.
Craig
Craig
Happy! Happy! Joy! Joy! ( It would be way too cool if you would send me a scan of the article! ) I just did a search and came up with zilch! Oh yeah, there was one guy I found on this site who claimed that he has tried them and that they aren't regulating very well at all. Here's the thread: http://www.diyaudio.com/forums/solid-state/106957-ryan-regulators-not-regulating.html At least the "Mystery Boards" have been identified, at last!!! Thanx Guy's!!!!!!!!!!!!!!!!!!!! ;-)
Hi guys, I need some help fixing my old '86 GFA-555 Mk.1.
It was bought with a pretty high DC offset of around -70mV on one channel and after recapping the unit (the PSU caps were fine so these could stay), changing the bias pots and readjusting the bias current, the offset somehow raised to -110mV.
Measured the 2SC2240 transistors in the differential input and got hFE of 150 and 250 so that could explain the offset.
However, at some point when I was working on the amp the thin red wire going to the (probably) NFB circuit snapped right at the PCB, I didn't notice it at first so when I turned the amp on I was greeted by +82V at the loudspeaker terminals and a faint trace of smoke from the main PCB, yet the none of the fuses went off and I directly switched off the amplifier.
I've spent all day removing and measuring every part on that channel, including the OP devices and couldn't find any broken parts at all.
The other channel was disconnected at that time.
I'm not an expert in electronics so if someone could explain what could've happened with the circuit when the cable snapped off, it would be greatly appreciated. Could it be oscillation/lack of balance and trying to amplify DC?
It's marked as "O" on the schematic.
It was bought with a pretty high DC offset of around -70mV on one channel and after recapping the unit (the PSU caps were fine so these could stay), changing the bias pots and readjusting the bias current, the offset somehow raised to -110mV.
Measured the 2SC2240 transistors in the differential input and got hFE of 150 and 250 so that could explain the offset.
However, at some point when I was working on the amp the thin red wire going to the (probably) NFB circuit snapped right at the PCB, I didn't notice it at first so when I turned the amp on I was greeted by +82V at the loudspeaker terminals and a faint trace of smoke from the main PCB, yet the none of the fuses went off and I directly switched off the amplifier.
I've spent all day removing and measuring every part on that channel, including the OP devices and couldn't find any broken parts at all.
The other channel was disconnected at that time.
I'm not an expert in electronics so if someone could explain what could've happened with the circuit when the cable snapped off, it would be greatly appreciated. Could it be oscillation/lack of balance and trying to amplify DC?
It's marked as "O" on the schematic.
An externally hosted image should be here but it was not working when we last tested it.
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