Hi,
I bought a used Adcom GTP-500 (not the II) knowing the tuner was out and thought it would be a simple trimmer cap repair. Unfortunately there was more to it!
My problem with my tuner appears to go beyond the trimmer cap issue these tuners suffer from as I have 1 amp fuses blowing on one of the 3 transformer secondaries. The first winding serves the tuner section, the second provides AC to the display, while the third (where the 2 blowing fuses live) provides 22.5 volts either side of the center tap and drives the tone controls and the rest.
Kinda stumped!
Any ideas welcome as I visually look for singed components and start looking for shorts. Diodes seem to be most likely, but isolating parts of the circuit seem difficult the way the thing is built. Does this seem like a good application for the series lightbulb?
Again, any ideas would be welcome!
Mitch
I bought a used Adcom GTP-500 (not the II) knowing the tuner was out and thought it would be a simple trimmer cap repair. Unfortunately there was more to it!
My problem with my tuner appears to go beyond the trimmer cap issue these tuners suffer from as I have 1 amp fuses blowing on one of the 3 transformer secondaries. The first winding serves the tuner section, the second provides AC to the display, while the third (where the 2 blowing fuses live) provides 22.5 volts either side of the center tap and drives the tone controls and the rest.
Kinda stumped!
Any ideas welcome as I visually look for singed components and start looking for shorts. Diodes seem to be most likely, but isolating parts of the circuit seem difficult the way the thing is built. Does this seem like a good application for the series lightbulb?
Again, any ideas would be welcome!
Mitch
fuse blowing in the 22+22 AC area can mean a problem in the power supply and can be any of the parts related there ...you need to trace it part by part
can be shorted capacitor (rear) shorted diode most likely , and shorted transistor (possible but not fuse blowing damage )
Diodes to be checked are around Q 906 ,905,916,917 Just before them
Kind regards
Sakis
can be shorted capacitor (rear) shorted diode most likely , and shorted transistor (possible but not fuse blowing damage )
Diodes to be checked are around Q 906 ,905,916,917 Just before them
Kind regards
Sakis
Hello Sakis,
Thanks for the reply.
Should the unit checked for voltage when nit is powered on with a dim light bulb tester or should it be powered off and traced with an ohm meter looking for shorts to ground?
Appreciate your help,
Thanks,
Mitch
Thanks for the reply.
Should the unit checked for voltage when nit is powered on with a dim light bulb tester or should it be powered off and traced with an ohm meter looking for shorts to ground?
Appreciate your help,
Thanks,
Mitch
no ...Just a DVM to verify that diodes /transistors or zener have no short ACROSS them NOT to ground
Sorry to say that but if you don't have what it takes to proceed please seek expert help .
If we try to perform a ""tele-repair" it will not work since we speak different language in electronics even though my intention and probably other forum members is to help you out ....
Kind regards
Sakis
Sorry to say that but if you don't have what it takes to proceed please seek expert help .
If we try to perform a ""tele-repair" it will not work since we speak different language in electronics even though my intention and probably other forum members is to help you out ....
Kind regards
Sakis
Thanks Sakis.
I will work at isolating the various power supplies downstream of the secondary fused by F991 which is opening and find the responsible component or components.
You help and advice are appreciated and if the problem proves more that my abilities allow, I will seek local help.
Thanks again,
Mitch
I will work at isolating the various power supplies downstream of the secondary fused by F991 which is opening and find the responsible component or components.
You help and advice are appreciated and if the problem proves more that my abilities allow, I will seek local help.
Thanks again,
Mitch
In case this may be useful to anyone working on an Adcom GTP-500, here is an update.
Repair Update:
I finished testing all the electrolytic caps in the power supply section and found 4 that were open (C907, C908, C924 and C925) and 5 that had a high ESR value (C910, C918,C919, C920 and C923). Also found a shorted diode (D923). Since I was in there, I decided to replace all the 220uF 10 volt, 16 volt, 25 volt and 50 volt caps and the dead diode (performing when initially tested more like a 7 ohm resistor!), and the tuner/pre-amp is now up and running just fine. All power supplies look stable and show no ripple and no more blown fuses!
Another classic saved from the landfill!
Thanks for the pointers, Sakis.
I guess my success suggests that I am learning the 'language'! "Tele-repair" may not seem viable, but "tele-tutoring" is a great aid.
Mitch
Repair Update:
I finished testing all the electrolytic caps in the power supply section and found 4 that were open (C907, C908, C924 and C925) and 5 that had a high ESR value (C910, C918,C919, C920 and C923). Also found a shorted diode (D923). Since I was in there, I decided to replace all the 220uF 10 volt, 16 volt, 25 volt and 50 volt caps and the dead diode (performing when initially tested more like a 7 ohm resistor!), and the tuner/pre-amp is now up and running just fine. All power supplies look stable and show no ripple and no more blown fuses!
Another classic saved from the landfill!
Thanks for the pointers, Sakis.
I guess my success suggests that I am learning the 'language'! "Tele-repair" may not seem viable, but "tele-tutoring" is a great aid.
Mitch
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