Hello All,
I just repaired an ADCOM GFA-5500 that according to the original owner was damaged due to a power surge, he said back in the 90's he sent it to a Service Center just to be told that it wasn't repairable, for sure it was since I just fixed it and I'm just a DYIer, well log story short is that according to the non accurate service manual and schematic that is all over the net the power supply is supposed to provide DC voltages of +73- and +60- but this one is putting out +89- and +78- instead, dangerously close to max out the capacitors, in the manual it says that all measures were taken with a 115VAC input and since I'm getting 125VAC out of the public mains I expect some higher values but not that high.
According to a person in Argentina some of these were manufactured with out of spec transformers and he said he has asked ADCOM and they said it was "fine" and suggested to adjust the bias to 10mv instead of 50mv...
Is anybody familiar with this issue?
Thanks!
I just repaired an ADCOM GFA-5500 that according to the original owner was damaged due to a power surge, he said back in the 90's he sent it to a Service Center just to be told that it wasn't repairable, for sure it was since I just fixed it and I'm just a DYIer, well log story short is that according to the non accurate service manual and schematic that is all over the net the power supply is supposed to provide DC voltages of +73- and +60- but this one is putting out +89- and +78- instead, dangerously close to max out the capacitors, in the manual it says that all measures were taken with a 115VAC input and since I'm getting 125VAC out of the public mains I expect some higher values but not that high.
According to a person in Argentina some of these were manufactured with out of spec transformers and he said he has asked ADCOM and they said it was "fine" and suggested to adjust the bias to 10mv instead of 50mv...
Is anybody familiar with this issue?
Thanks!
So far I have not been able to see a schematic for it many have been removed from the manuals on websites but the general opinion is that the capacitors aren't up to scratch ( dry out etc etc ) including the ones in the signal path/frequency response causing loss of low frequency drive .
I would like to see a schematic before commenting on possible fault areas.
What is the AC output to the rectifier ?
I would like to see a schematic before commenting on possible fault areas.
What is the AC output to the rectifier ?
Thanks for chiming in, attached is the Schematic I have and the only one I have found so far.
The AC out of the Transformer and o the Rectifier is +60- VAC, then it puts +55- VDC out from the rectifier to the filtering area. Now there are 2 pairs for each of those.
Thanks!
If the output from ( I take it ) a bridge rectifier is connected to a filter capacitor which is faulty then the AC component is NOT filtered away and is added to the DC component to produce a combined voltage which would account for the higher voltage measured .
Check for an AC component in the OUTPUT from the filter capacitor(s) , as I don't know if the power supply is a simple straight supply directly connected to the power amp voltage rails ( I take it ? ) I hope you can tell me ?
Check for an AC component in the OUTPUT from the filter capacitor(s) , as I don't know if the power supply is a simple straight supply directly connected to the power amp voltage rails ( I take it ? ) I hope you can tell me ?
I have measured every component in the power supply and no AC voltage was found other than where it should, such as bridge rectifiers and rectifier diodes.
I have also measured another GFA-5500 that I have used for over a year and, well, it also has the same higher voltages, and I have used that one for long hours and its in a non air conditioned environment and yes, it gets HOT, but never had an issue with it, I'm back to thinking this is either an error in the schematics or a very common production flaw.
If you have measured another GFA-5500 and it has the same higher voltages and -quote "you have never had an issue with it " then that leaves one sure check to be made .
What is the WORKING voltages of the main smoothing capacitors relating to the power supply .
I don't mean peak or anything like that I mean WORKING voltages ?
If they are within 10 volts or a bit more then there is a design fault in your amps or somebody is cutting corners , you should have at least 20 volts to spare, you cannot argue with actuality of components fitted by the manufacturer .
In my amplifiers I build 55V +/- power supplies with at least 80 Volts (each rail ) main capacitors or if its a small class A running at rails of say 25 Volts then I use 50 volt capacitors .
What is the WORKING voltages of the main smoothing capacitors relating to the power supply .
I don't mean peak or anything like that I mean WORKING voltages ?
If they are within 10 volts or a bit more then there is a design fault in your amps or somebody is cutting corners , you should have at least 20 volts to spare, you cannot argue with actuality of components fitted by the manufacturer .
In my amplifiers I build 55V +/- power supplies with at least 80 Volts (each rail ) main capacitors or if its a small class A running at rails of say 25 Volts then I use 50 volt capacitors .
Totally agree with you, that's why I mentioned at the begging that they are dangerously close to be maxed out, that's because the working voltages for all electrolytic capacitors is 100V and 250V for the mylar capacitors.
So the theoretical design was all right with the 73+/- VDC but the amplifiers are at 90+/- VDC, a bit too close to my liking...
Ok , this is a novice question but is regarding the same subject matter. I'm in Panama and want a class A amp to drive my 95db speakers. I have a gfa 5500 sitting around not being used and am considering modding it with single ended class A boards . Using the 120v power supply in 240v configuration to half the rail voltage. There are 73 and 60 vdc rectified voltages . Can I use one of these ? They should be halved with the transformer configuration change . Assuming I have boards requiring 36vdc , either voltage could fall in range .
How screwed up is this idea in function terms , I know some will protest modding a good amp but It's not being used and selling gear here is difficult at best.
How screwed up is this idea in function terms , I know some will protest modding a good amp but It's not being used and selling gear here is difficult at best.
Last edited:
Low voltage PS caps in Adcom amps.
It seems that Adcom (amps) use PS caps ~ too close to rated operating voltages ~, 63 volt in both 545 amps. I own, have heavily modified and completely recapped several 545v1 and 545v2 with 80wvdc PS caps, they are actually listenable / more relaxed sounding.
I think that is a portion of why I was never particularly fond of them... I cannot (relax and) listen to stock Adcom [500 series] amps in my system.
Adding to that (in the US, at least) where I live the AC line voltage is higher and higher, taking (all) vintage gear that I own [modify] and use, "at risk" from the lower voltages present *in the day* that they were designed, built and marketed. I have started adding thermistors [for start up] and to slightly reduce the in-line AC voltages applied to them for longevity.
It seems that Adcom (amps) use PS caps ~ too close to rated operating voltages ~, 63 volt in both 545 amps. I own, have heavily modified and completely recapped several 545v1 and 545v2 with 80wvdc PS caps, they are actually listenable / more relaxed sounding.
I think that is a portion of why I was never particularly fond of them... I cannot (relax and) listen to stock Adcom [500 series] amps in my system.
Adding to that (in the US, at least) where I live the AC line voltage is higher and higher, taking (all) vintage gear that I own [modify] and use, "at risk" from the lower voltages present *in the day* that they were designed, built and marketed. I have started adding thermistors [for start up] and to slightly reduce the in-line AC voltages applied to them for longevity.
Last edited:
yes so the main ps caps are 100v , I'm thinking running the amp on 120volts but using the 240volt transformer taps would halve voltages throughout the psu. That seems to also be what Nelson suggested in another thread regarding this amp . Bringing rectified voltage down to 30-36 vdc.
I'll share my experience with my 5503. High rail voltages were brought down to within 10% of spec by replacing resistors on the power supply board. Amp runs cool with 60mV bias. Next step - look in to ultra fast diodes for the 73v rectifier circuit.... Or better yet, make 3 mono amps out of this thing. Where does one source transformers with different secondary voltages ?