In short, it was counterfeit parts. But read the whole thread if you want to understand the how and why and where.
CarZ I believe it was the VAS transistor that failed and maybe took out some other components. Fred had changed out just about every part on the low side of the amplifier and still no go. In the end it turned out to be a counterfeit part in one of the current sources/sinks that was causing the final problem. Once the original part was put back in the amplifier worked. This was a big lesson for all involved.
The original VAS transistor was under rated for Vce breakdown and was thought to have failed because of this.
Once a power amp has failed catastrophically, the cause is just a guess because the damage tends to spread out from the original fault.
David.
I'm not sure if this is clear but the original in use failure was due to a shorted output.
The design should, hopefully survive a shorted output just with a fuse blowing but
the early units were shipped with lower rated driver transistors that are a weak link
and there is a factory authorized upgraded driver. My unit has the upgraded drivers
from the factory and has simply blown fuses when the output was accidentally shorted.
Once we were able to test the outputs we found at least one bad output device,
probably due to a failed driver.
It is important to treat a failed amp like a crime scene, try to make as few changes
when you do your tests. I suggested a way to remove a few transistors and test
the output devices for leakage, the transistors were removed, but some new
counterfeits were also put in with far to low breakdown voltage. Once that part
was identified testing of the output stage went smoothly.
My guess is that with the shorted output a driver failed which then led to an output
and perhaps the VAS to also fail. The design is slightly marginal under short
circuit conditions and even warm temps.
The design should, hopefully survive a shorted output just with a fuse blowing but
the early units were shipped with lower rated driver transistors that are a weak link
and there is a factory authorized upgraded driver. My unit has the upgraded drivers
from the factory and has simply blown fuses when the output was accidentally shorted.
Once we were able to test the outputs we found at least one bad output device,
probably due to a failed driver.
It is important to treat a failed amp like a crime scene, try to make as few changes
when you do your tests. I suggested a way to remove a few transistors and test
the output devices for leakage, the transistors were removed, but some new
counterfeits were also put in with far to low breakdown voltage. Once that part
was identified testing of the output stage went smoothly.
My guess is that with the shorted output a driver failed which then led to an output
and perhaps the VAS to also fail. The design is slightly marginal under short
circuit conditions and even warm temps.
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I see a 2016 post here so I don't feel so bad following up. Great read. Watching the NBA Playoffs and reading this at the same time. What a great thread. I know the feeling the OP had when he finished (well sorta). I have a Pioneer Elite FD 111 Plasma TV that went out on me. Instead of paying ($600-800+) to have it fixed I went in myself. Got some info from youtube of all places. Pulled out my old plug-in-the-wall soldering iron out the closet (which after reading this thread is something I need to replace ASAP) and my trusty Fluke from an old job that we NEVER used. Order a 69 cent part (order 3 just in case) and fixed my own TV!!! It's been a few years and it's still going strong.
I also have an Adcom GFA 555 that I bought (original owner) back in 89, or so, during my stint in the Army and really wanted to bring it back in to service. It's been sitting in a closet for quite a few years now though (well over 10). Only work I've done on it over the years is replace the fuses from time to time when they've blown. I wouldn't dare do the work the OP did. But I'm concerned if I could just plug it in and turn it on. Seems like according to this thread there may be some surprises waiting for me if I do.
Congrats to the OP for getting it to work!
I also have an Adcom GFA 555 that I bought (original owner) back in 89, or so, during my stint in the Army and really wanted to bring it back in to service. It's been sitting in a closet for quite a few years now though (well over 10). Only work I've done on it over the years is replace the fuses from time to time when they've blown. I wouldn't dare do the work the OP did. But I'm concerned if I could just plug it in and turn it on. Seems like according to this thread there may be some surprises waiting for me if I do.
Congrats to the OP for getting it to work!
Does anyone have a master list of parts to update? It was promised (in post 658 of this thread), but Fred appears to have been dark since April 2016, so I don't know when we might hear back from him.
Also, what transistors are available now which could be used in place of the originals (due to their rarity/expense/difficulty to acquire), and what (if anything) needs to be modified in the supporting circiutry to make the new parts work?
Also, what transistors are available now which could be used in place of the originals (due to their rarity/expense/difficulty to acquire), and what (if anything) needs to be modified in the supporting circiutry to make the new parts work?
Late reply... I don't know what happened to Fred, but here's a list of the parts I supply with my GFA-555 board kits. Sorry for the formatting, it's pasted from a spreadsheet.
In order of assembly, low-profile components first
1 BFA-555 MK1 Circuit Board
4 R101, R201, R105, R205 Resistor, Metal Film, 0.1% 1K Vishay/Dale RN55E1001BB14 0.1% precision resistors on the input stage so that gain is precisely matched between channels.
5 R102, R202, R106, R206, R223 Resistor, Metal Film, 0.1% 22K1 Vishay/Dale RN55C2212BRE6 or YR1B22K1CC 0.1% precision resistors on the input stage so that gain is precisely matched between channels.
4 R103, R203, R108, R208 Resistor, Metal Film, 1% 33R2 Vishay/Dale RN55D33R2FB14
4 R117, R118, R217, R218 Resistor, Metal Film, 1% 68R1 Vishay/Dale RN55D68R1FRE6 Original value is 68.2R which is not available.
2 R121, R221 Resistor, Metal Film, 1% 100R Vishay/Dale RN55D1000FRE6
2 R116, R216 Resistor, Metal Film, 1% 130R Vishay/Dale RN55D1300FB14
2 R114, R214 Resistor, Metal Film, 1% 332R Vishay/Dale RN55D3320FRE6
4 R109, R209, R115, R215 Resistor, Metal Film, 1% 475R Vishay/Dale RN55D4750FRE6
2 R111, R211 Resistor, Metal Film, 1% 681R Vishay/Dale RN55D6810FRE6 Original value is 682R which is not available.
2 R107, R207 Resistor, Metal Film, 1% 825R Vishay/Dale RN55D8250FB14
2 R124, R224 Resistor, Metal Film, 1% 1K Vishay/Dale RN55D1001FRE6
2 R112, R212 Resistor, Metal Film, 1% 2K74 Vishay/Dale RN55D2741FB14
4 R119, R120, R219, R220 Resistor, Metal Film, 1% 4K75 Vishay/Dale RN55D4751FRE6
2 R113, R213 Resistor, Metal Film, 1% 22K1 Vishay/Dale CMF5522K100FHEB
2 R110, R210 Resistor, Metal Film, 1% 47K5 Vishay/Dale RN55D4752FB14
2 R122, R222 Resistor, Metal Oxide 5R1 KOA Speer MOSX3CT521R5R1J
8 D103, D104, D105, D106, D203, D204, D205, D206 1N4148 Vishay/Dale 1N4148
2 P101, P201 Trim Potentiometer Bourns 3362P-1-202LF
2 Pair Q101, Q102, Q201, Q202 TO-92 Transistor On Semi KSC1845FTA Matched Pairs
4 Q110, Q210, Q103, Q203 TO-92 Transistor On Semi KSC1845FTA Unmatched for non-current-mirror functions
6 Q108, Q208, Q105, Q205, Q109, Q209 TO-92 Transistor On Semi KSA992FBU
4 Q107, Q207, Q106, Q206 TO-126 Transistor On Semi KSA1381ESTU
2 Q104, Q204 TO-126 Transistor On Semi KSC3503DS
2 Q12 TO-220 Driver Transistor, NPN On Semi MJE15032G Drivers with improved breakdown voltage headroom. Install on output modules.
2 Q11 TO-220 Driver Transistor, PNP On Semi MJE15033G
4 TO-126 Heatsinks Assmann (Yes, really) V7237B For Q104, Q107, Q204 and Q207. No heatsink needed on Q106 and Q206
4 Machine screw with integrated lock washer, M3, 8mm
4 Nut, M3 with integrated lock washer, M3
2 C102, C202 Mica Capacitor 20pF Cornell Dubilier CD15ED200JO3
2 C104, C204 Mica Capacitor 39pF Cornell Dubilier CD15ED390JO3
2 C101, C201 Mica Capacitor 300pF Cornell Dubilier CDV16FF301JO3F
2 C107, C207 Polypropylene Capacitor 1uF Kemet F461BK105K160C Original silver-mica capacitor is awesome, so you might want to consider re-using it.
2 C105, C205 Polyester Capacitor 4.7uF WIMA MKS2B044701K00KSC9 Original cap is electrolytic. Wima MKS2 is an ordinary polyester cap, but of a usably compact size, and far superior to any electrolytic.
6 C151, C251, C152, C252, C106, C206 Polypropylene Capacitor 0.1uF Kemet R71MF31004030K
2 C103, C203 Electrolytic Capacitor 47uF Nichicon UKZ2A470MHM Audio type for feedback to ground. Some people have their own preference, such as Elna Silmic II.
4 C153, C154, C253, C254 Electrolytic Capacitor 47uF Panasonic EEU-EE2C470 Power supply bypass
1 S1 Toggle Switch NKK B22AV or A22AV
1 Power Switch Carling RGSCC901-R-B-B-E
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