I have my father's GFA-545 that he thinks got smoked by a power surge many years ago. He took it to a shop and they said don't bother. But he doesn't remember if they diagnosed anything.
My troushooting:
-getting 81 VAC from transformer which seems high. No load, output fuses removed.
-getting 112 volts DC out of rectifier measured from top of cap to ground rail. (54 expected)
-left channel diode test across empty fuse holder output side beeps (short?). Right channel does not.
-left channel blows 4a agc fuse immediately. Did not try right after seeing voltage readings.
-nothing looks crispy
I have fond memories of this things pure sound so would be tickled if I can fix it. Do you guys think it's worth the time? Anyone familiar with these amps know if my readings are nonsense? Any other tests I can perform with multimeter?I'm especially curious on the power supply readings. I don't know much about electrical engineering but am eager to learn.
Thanks for reading,
Nick
My troushooting:
-getting 81 VAC from transformer which seems high. No load, output fuses removed.
-getting 112 volts DC out of rectifier measured from top of cap to ground rail. (54 expected)
- rectifiers test out fine
- L channel diodes d609 and d611 beep at me both ways with diode test
-left channel diode test across empty fuse holder output side beeps (short?). Right channel does not.
-left channel blows 4a agc fuse immediately. Did not try right after seeing voltage readings.
-nothing looks crispy
I have fond memories of this things pure sound so would be tickled if I can fix it. Do you guys think it's worth the time? Anyone familiar with these amps know if my readings are nonsense? Any other tests I can perform with multimeter?I'm especially curious on the power supply readings. I don't know much about electrical engineering but am eager to learn.
Thanks for reading,
Nick
Hi,
I had that amp and it's a very decent amplifier and worthy of fixing if you can do it on the cheap.
Did you find a schematic (Hifiengine) for it as it's probably going to be of value in order for you to proceed in finding the fault since it might have extensive component damage.
https://www.hifiengine.com/manual_library/adcom/gfa-545.shtml
I had that amp and it's a very decent amplifier and worthy of fixing if you can do it on the cheap.
Did you find a schematic (Hifiengine) for it as it's probably going to be of value in order for you to proceed in finding the fault since it might have extensive component damage.
https://www.hifiengine.com/manual_library/adcom/gfa-545.shtml
Thanks. I have printed out the service manual and it has come in handy. That's why I was concerned with the output voltage from the transformer. Seems almost double expected.
You need to be absolutely certain that the output from the transformer is incorrect because everything hinges on that result. If you have more than 55 to 60 volts DC across any of those four large caps then you have big problems somewhere.
Running a 110/120 volt AC configured unit on 230 volts for example would give way higher than expected voltages.
If you really have over 100 volts across any of those four caps then please switch off immediately as they could explode.
Do a check on your meters calibration to be sure.
The amps looks to be conventional bjt (ordinary transistor) so not sure where you get the mosfet and checking between drain and source from. 2SA and 2SC transistor devices are PNP and NPN types respectively.
Running a 110/120 volt AC configured unit on 230 volts for example would give way higher than expected voltages.
If you really have over 100 volts across any of those four caps then please switch off immediately as they could explode.
Do a check on your meters calibration to be sure.
The amps looks to be conventional bjt (ordinary transistor) so not sure where you get the mosfet and checking between drain and source from. 2SA and 2SC transistor devices are PNP and NPN types respectively.
Without looking the schematics look like you have this voltage between Positive and Negative rail. You may be should take the measure between Ground and Positive or Negative Rail.
Thanks. I retested making sure to take positive to ground rail. I disconnected caps from rectifier for safety. Seems to be a happy voltage. I must have been measuring + to - last time. Second pic is AC across the transformer outputs.
Regarding transistors, I read that they were mosfets somewhere. They say C3855 and A1491 on them. My test was following procedure without comprehension from YouTube. Risky I know but gotta learn it somehow.
Finally, I checked voltage from the wall to confirm multimeter wasn't throwing me off. Measured nice clean 115.
Thanks all.
Regarding transistors, I read that they were mosfets somewhere. They say C3855 and A1491 on them. My test was following procedure without comprehension from YouTube. Risky I know but gotta learn it somehow.
Finally, I checked voltage from the wall to confirm multimeter wasn't throwing me off. Measured nice clean 115.
Thanks all.
Don't use 2 hands to probe with the power on, voltage >24 across the heart can stop it. Use an alligator clip lead on the analog ground. Don't wear rings or jewelry on hands wrists or neck, 1v @ 30 amps through a ring can burn your flesh to charcoal. Wear safety glasses power on or soldering, parts can explode and solder splashes. Don't probe the AC power area. Other than that hard to hurt yourself.
When you measure across both rails as in 2nd picture post 6 you get double rail voltage. Measure from one side to the joined terminals (analog ground) and then from the other side to the joined terminals. Probably +- 40 v.
I have a klein meter from HD and it is **** for measuring resistances & capacitances. Everything is ----. Won't show any current flow on resistance check on electrolytic caps. Okay for 480 3 phase power I guess. My sears Craftsman ~1995 was stolen and I haven't found any 2020 up meter yet that tests with 2 v source on resistance scale. I also bought a tenma from newark, the new one diode scale doesn't work either. They gave me my money back.
The heat sinks are nice & big, main thing wrong with this amp is they drive output transistors with TO92 transistors. Works a while but not very long at full power. For 1 watt use in home that is okay, but don't turn up at parties. Drivers should be TO126 at minimum(mje340/350) , and TO220s (MJE15028/29) work fine in my AX6. Heat sinks on drivers makes them loud play resistant, but cost $1 each in labor so most consumer amps leave them out. Can't make the sale if your product costs $5 more than the next brand.
Usual suspect on class AB amps is output transistors bad which destroys drivers etc. However this may have drivers bad first.
Failing double diode test is first screen for bad transistors. 400 to 600 mv forwards, 9999 or ---- backwards. Sometimes they can get only damaged and require a open base current test at 12 v or more to break down. 12 v goes c to e through current scale of dvm series a 47 k resistor or so to avoid blowing fuse in the meter. Current =12/47000 your transistor is leaking.
If you're in US I'd replace outputs with mjl1303/4281 . Drivers if you can find tall to92 they are more watt insensitive, but mpsw05/56 have been discontinued. Those were 125 degC/w instead of 200 degc/w of normal mpsa06/56.
Light bulb limiter box in AC feed can reduce damage as you're testing to see if you're done yet. Meltdowns can proceed from the O.T.'s through the base line back to the input stage in some cases.
When you measure across both rails as in 2nd picture post 6 you get double rail voltage. Measure from one side to the joined terminals (analog ground) and then from the other side to the joined terminals. Probably +- 40 v.
I have a klein meter from HD and it is **** for measuring resistances & capacitances. Everything is ----. Won't show any current flow on resistance check on electrolytic caps. Okay for 480 3 phase power I guess. My sears Craftsman ~1995 was stolen and I haven't found any 2020 up meter yet that tests with 2 v source on resistance scale. I also bought a tenma from newark, the new one diode scale doesn't work either. They gave me my money back.
The heat sinks are nice & big, main thing wrong with this amp is they drive output transistors with TO92 transistors. Works a while but not very long at full power. For 1 watt use in home that is okay, but don't turn up at parties. Drivers should be TO126 at minimum(mje340/350) , and TO220s (MJE15028/29) work fine in my AX6. Heat sinks on drivers makes them loud play resistant, but cost $1 each in labor so most consumer amps leave them out. Can't make the sale if your product costs $5 more than the next brand.
Usual suspect on class AB amps is output transistors bad which destroys drivers etc. However this may have drivers bad first.
Failing double diode test is first screen for bad transistors. 400 to 600 mv forwards, 9999 or ---- backwards. Sometimes they can get only damaged and require a open base current test at 12 v or more to break down. 12 v goes c to e through current scale of dvm series a 47 k resistor or so to avoid blowing fuse in the meter. Current =12/47000 your transistor is leaking.
If you're in US I'd replace outputs with mjl1303/4281 . Drivers if you can find tall to92 they are more watt insensitive, but mpsw05/56 have been discontinued. Those were 125 degC/w instead of 200 degc/w of normal mpsa06/56.
Light bulb limiter box in AC feed can reduce damage as you're testing to see if you're done yet. Meltdowns can proceed from the O.T.'s through the base line back to the input stage in some cases.
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Is that the 'Low Battery' symbol showing on the meter? If so then readings may not be accurate.
If any of the transistors on the heatsink read short circuit from collector to emitter then they have failed. Use either the diode test range or the '200' ohm range. Amp OFF for doing that.
If any of the transistors on the heatsink read short circuit from collector to emitter then they have failed. Use either the diode test range or the '200' ohm range. Amp OFF for doing that.
545 I & II are bipolar ouptuts. Later Adcom amps (5400/5500/5800 etc) were an entirely different design that employed Mosfet outputs.
Thanks all for advice and input on safety and amp. Half the output transistors on the right are shorted across collector and emitter. All of the output transistors on the left are shorted. Driver transistors too small to test without removing from case. They are probably shot too. Seems power supply is good at least.
I'll probably pull boards next and confirm testing with a little less cramped conditions. Then try and find replacement parts.
Think I have decent odds of fixing it or will this just lead me to the next issue?
I'll probably pull boards next and confirm testing with a little less cramped conditions. Then try and find replacement parts.
Think I have decent odds of fixing it or will this just lead me to the next issue?
For each channel, the three output transistors connected to the positive supply are in parallel.
Same for the negative supply. So only one bad transistor of the group of three could cause a short
reading across the CE of all three.
You won't know how bad it is until you test more parts.
Same for the negative supply. So only one bad transistor of the group of three could cause a short
reading across the CE of all three.
You won't know how bad it is until you test more parts.
If only some are failed, can I replace part of the array? I remember reading something about matching parts as much as reasonable.
If the transformer is okay, it is economically repairable by amateurs. Not at $80 an hour by pros in real estate that pay taxes.
You complained you can't touch the leads of the small transistors. One thing I really hate about the Klein meter is the plastic guards around the probes. Nib on end it too tiny to check TO-92 or IC legs. If you replace them with probes from panoma or parts-express, the banana plugs pop out of the jacks. The klein probes are even too short to connect with pamona grabbers, the ultimate tiny part sensor.
Replacing parallel output transistors, one usually buys more than you need. Check Vbe, mark them and don't use the outliers. All parallel output transistors need to be from the same batch, same production date.
Note when O.T.'s go, the emitter resistors often do and frequently the driver transistors.
You complained you can't touch the leads of the small transistors. One thing I really hate about the Klein meter is the plastic guards around the probes. Nib on end it too tiny to check TO-92 or IC legs. If you replace them with probes from panoma or parts-express, the banana plugs pop out of the jacks. The klein probes are even too short to connect with pamona grabbers, the ultimate tiny part sensor.
Replacing parallel output transistors, one usually buys more than you need. Check Vbe, mark them and don't use the outliers. All parallel output transistors need to be from the same batch, same production date.
Note when O.T.'s go, the emitter resistors often do and frequently the driver transistors.
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Also check the large white emitter resistors, some may be open or off value.
Learned this the hard way! this is what shorting my watch wristband to a live PCB did several years ago. Amp is fine, but a good reminder is permanently on my watch. Nice amp BTW.
Attachments
Providing you can get one good pair (a 2SC and a 2SA) per channel then you can get the amp working. The parallel pairs are for the high power high current capability but the amp will all work normally with just one pair.
Always use a DBT (dim bulb tester) when powering up for fault finding to limit current and damage.
Always use a DBT (dim bulb tester) when powering up for fault finding to limit current and damage.