I have retired and wanted to spend time listening to a loved audio system from earlier years. I am now 75. My background is an Electrical Engineer from many moons ago and have practiced medicine for decades. I have an ADCOM GFA 5500 bought in the 90's with one channel not working and some distortion in the other. I would love to have advice how to go about making the diagnosis for what looks like the need for electro-surgery or get psychotherapy to accept the equipment's mortality. Thanks for listening!
Hi All, My initial step in GFA 5500 diagnostics was to check the power supply. Both the working and not working left side had symmetric voltages but not what the circuit diagram specified. The 73 volt taps had 75 volts and the 60 volt taps has 75 volts going to the source on the MOSFET. Is this indicative of a capacitor failure even though the rt side of the amp works?
If the power supply is common to both channels, it should not be a problem.
The DC voltages are likely to be high because of one channel not working, and also the AC line is higher than it used to be.
Check the resistance of the high power emitter resistors (with power off), in case some are open.
The DC voltages are likely to be high because of one channel not working, and also the AC line is higher than it used to be.
Check the resistance of the high power emitter resistors (with power off), in case some are open.
Hi All, Checked all components and finally decided to check bias of adcom gfa5500 with left channel not working. Bias was 20 and spec is about 50 mv. I adjusted it up and all started to work. The question now is why did the bias drift down so low over the years and now I notice the heat sinks get very warm after running for a while at a low volume. Do mosfets require high current speakers to dissipate the energy. I was using Martin Logan sl3 before. I was just using 4 ohm test speakers certainly not matched to the amp. Thanks for the help in advance. Barry S
You've got around 30W per channel on the heat sink from the bias current alone, so it's not surprising if it's warm.
Maybe it was never set right, or your AC line is higher now.
Maybe it was never set right, or your AC line is higher now.
Hi Rayma, How do you calculate that there is about 30 watts going to the heat sink per channel. Amp is now working but how do I decide if any of the caps power or amp need replacement. Also if the bias drifts down again do you think I need to replace the Zener diodes. This is definitely a learning experience for me, I love it. BarryS
For each channel, there are 5 pairs of outputs, each biased at 0.05A, and the rail DC voltage is +/- 60V.
So 5 x 0.05A x 120V = 30W per channel.
This is a complex circuit, and hopefully someone who has worked on one of these will chime in.
So 5 x 0.05A x 120V = 30W per channel.
This is a complex circuit, and hopefully someone who has worked on one of these will chime in.
Thanks for reply, makes sense. If the bias drifts down slowly is that likely the Zener diode leaking below its threshold or would you look elsewhere? Also, how do know if any caps need replacement or if there is any minimal distortion. While I have it open should fix anything else that will improve sound and eliminate any distortion. Thanks Barry S
Hard to say what the reason is. Most likely just gunk in the pots.
I bought a very used Stasis years ago. One side was close to set the house on fire, the other side was cold as ice.
Changed the pots and readjusted bias. It's been spot on ever since.
I bought a very used Stasis years ago. One side was close to set the house on fire, the other side was cold as ice.
Changed the pots and readjusted bias. It's been spot on ever since.
Small thermal drift is ok, but large drift would indicate a problem.
Many would replace the expensive large main supply capacitors and pots.
Look for bad solder joints, bulging capacitors, and check resistor values.
Look for dried out thermal paste and loose bolts on the output devices.
Maybe someone with direct experience with these will chime in.
Many would replace the expensive large main supply capacitors and pots.
Look for bad solder joints, bulging capacitors, and check resistor values.
Look for dried out thermal paste and loose bolts on the output devices.
Maybe someone with direct experience with these will chime in.
Hi Everyone, My ADCOM GFA 5500 was working well but the bias has been drifting down until one channel stops working. What do you think I need to check to diagnose the issue ? All caps and cmos and z diodes look ok on inspection. As always help is appreciated BarryS