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Old 19th December 2004, 07:14 AM   #1
tman is offline tman  United States
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Default Please Help with ADCOM GFA-555 II

Hi everyone,

I'm new to this forum and to audio electronics. I just picked up a Adcom GFA-555 II that is in need of repair. I will try to explain the known problems best I can given my limited experience.

Problem: Left channel does not work. Blowing the negative fuse on the left channel. (the fuse that has a "-" above it. Also, when checking the transistors on the power board protected by that fuse my rusults are 19ohms on the top three and 0 ohms on the bottom transistor (labeled Toshiba 2SB554).

Also, a resistor located on the audio board (R114) is giving me a reading of roughly 100ohms. That resistor's mate for the right channel (the side that is working) R164 is reading at 300 ohms.

Just wondering if I'm in the ballpark with this or not as far as figuring out what problem may be causing the left channel to blow fuses.

I would appreciate any comments whatsoever. Thanks for your time,
tim

PS I can post pics if anyone is intersted is seeing the suspected components.
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Old 19th December 2004, 11:37 PM   #2
anatech is offline anatech  Canada
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Hi tman,
Welcome to the forum!
There are other threads on the forum with information on this model. It seems you will be replacing a matched set of outputs and drivers and possibly predrivers. Not a terrible job. Do it right and the unit will last for years to come. You will likely find some burned resistors.
So, take your time and don't buy the cheapest parts. Use On-Semi parts where possible - real ones. There is lots of help here, but do a search and read the previous posts.
-Chris
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Old 20th December 2004, 08:29 PM   #3
K-amps is offline K-amps  United States
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Tim,

I have the original Toshiba replacements for the 555/ 555II. I upgraded many 555's to MJ21195/96's so I have many Toshiba's I pulled out.

Send me a self addressed envelope, I will send you a 554.

Cheers!

K-amps.

PS: Will buy that 555II off you if you don't want to repair it... or even exchange with a working 555. ;-)
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Old 20th December 2004, 10:23 PM   #4
anatech is offline anatech  Canada
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MJ15022 / MJ15023 or MJ15024/MJ15025 will work fine for outputs as well. You must change all the outputs in the bad channel.
-Chris
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Old 20th December 2004, 10:36 PM   #5
K-amps is offline K-amps  United States
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Quote:
Originally posted by anatech
MJ15022 / MJ15023 or MJ15024/MJ15025 will work fine for outputs as well. You must change all the outputs in the bad channel.
-Chris

"Must"..... ok I will keep shut. ;-)
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Old 20th December 2004, 11:51 PM   #6
tman is offline tman  United States
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Thanks for the info everyone,

I would be more than glad to pay you for the part. Just let me know. By the way, I found this amp which looks brand new for $50 dollars. I hope I can repair it myself but I will keep you in mind if I can't. Part of the reason I'm trying this is to learn more about electronics. The more I read on this forum, the more I realize how much I need to learn. Oh well, guess that is part of the fun of a hoppy.

Thanks,
tim
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Old 21st December 2004, 02:03 AM   #7
anatech is offline anatech  Canada
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Hi K-amps,
Yup, must.
It really isn't worth losing more parts to one transistor that tested okay with a diode test. Over 25 yrs experience to prove this.
Remember, parts are way cheaper than speakers and a tech's time to do it again.
-Chris
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Old 21st December 2004, 02:36 AM   #8
K-amps is offline K-amps  United States
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Anatech,

I use a 100 watt bulb in series... yet to lose an amp on power on....

Happy Holidays.


Hey Tim,

I have roughly 20-30 pairs of original 555 output pairs from Toshiba, let me know how many you want. Email me with details.
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Old 21st December 2004, 11:56 PM   #9
tman is offline tman  United States
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Default Hey k-amp

Hey Kamp,

I could not retrieve your email.

If you see this message, please email me.

Thanks again,
Tim
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Old 22nd December 2004, 01:21 AM   #10
K-amps is offline K-amps  United States
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just emailed u my prodigy email
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