Hello all, I too have an ADCOM GFA-555II with a blown 2SC2912 in the left channel. I read here that 2SC3503 are a suitable replacement but can't find them. I also am thinking it would be wise to use the comps to the 3503 , the 2SA1380 but can't find those either. So are there new newer replacements for the 2SC2912 and 2SA1210? thanks
You can use KSC3503/2SC3503 & KSA1381/2SC1382. Always replace both BJT's and be careful when removing them, the pads can be very easily lifted from heat and age.
http://www.bdenterprisesinc.com/search.php?search_query=2sc3503
Give Bud a call over at B&D , if he's out he will direct you to a viable replacement.
http://www.bdenterprisesinc.com/search.php?search_query=2sc3503
Give Bud a call over at B&D , if he's out he will direct you to a viable replacement.
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Thanks for the info. Yea I noticed the board around some of the components looks a little brittle. Still intact but has been exposed to heat for awhile. Thanks for your help.
Try using a good sharp diagonal cutter and cut the device on the part side. Then use a quality solder-wick to remove the solder then carefully remove the lead. I would say a good 90% of the repairs I do have rework that has been performed incorrectly buy someone leading to more problems. It drives me crazy seeing leads bent over to form a replacement trace or pad. Anything worth doing is worth doing right the first time.
Help!!! 2 Adcom GFA 555 II with issues
Hi All,
I've got two 555 II amps with issues that i thought maybe you could help me with. I don't have much experience with these amps but am familiar with amplifiers in general and have done some repair in the past. My problem is my (old faithful) tektronix 475 oscilloscope. It's power supply decided to die around the same time my amps did so I'm at a loss troubleshooting withe just a hold meeter and a lot of guess work.
Here are the symptoms:
Left channel of Amp #1 has the dreaded -80v dc output. No blown fuses and right channel is fine. I disconnected the speaker within seconds of powering up. it survived so I've just got the amp to deal with. Sense i don't have a scope, I've been using the right channel as a reference for voltage measurements. I wondered if the amp had serious damage to the output stage but it seems to be functional. I pulled the board jumper for the temp breaker to see if it would shut down the output stage, and it did. The temp LED came on and the output setteled out a 0 volts so I believe the power stage is functional. with the jumper out i went a head and measured the current in the breaker circuit. it's about 5.5 ma so I believe thats about right as well (reading the service manual). The voltage at the emitter of Q101 & Q102 combo is about -1.5, the base of Q101 is close to 0, the base of Q102 is -3.5v so i'm pretty sure its off with Q101 carrying all the current (about 2ma). I would expect the collector of Q101 to be pulling as much current as possible but it remains at or close to the positive rail. It kind of points to Q106 being shorted or Q107 being open (or maybe Q101 open?). Any input here would be appreciated. I also need a good resource for replacement transistors. and the current correct parts. I've read a lot of different sources in this thread. Whats be current best replacement parts (transistors and where should I buy them from?
On to Amplifier #2:
I've not looked as far into this amp. Its symptoms are:
Left channel is fine, right channel distortion LED stays let and 0 volts on the output, no blown fuses. I know it behaves the same as the previous amp if I pull the jumper for the temp breaker. The Temp LED comes on and the distortion LED goes out. still with 0 volts on the output.
Again, any help or guidance would be appreciated.
Thanks
Hi All,
I've got two 555 II amps with issues that i thought maybe you could help me with. I don't have much experience with these amps but am familiar with amplifiers in general and have done some repair in the past. My problem is my (old faithful) tektronix 475 oscilloscope. It's power supply decided to die around the same time my amps did so I'm at a loss troubleshooting withe just a hold meeter and a lot of guess work.
Here are the symptoms:
Left channel of Amp #1 has the dreaded -80v dc output. No blown fuses and right channel is fine. I disconnected the speaker within seconds of powering up. it survived so I've just got the amp to deal with. Sense i don't have a scope, I've been using the right channel as a reference for voltage measurements. I wondered if the amp had serious damage to the output stage but it seems to be functional. I pulled the board jumper for the temp breaker to see if it would shut down the output stage, and it did. The temp LED came on and the output setteled out a 0 volts so I believe the power stage is functional. with the jumper out i went a head and measured the current in the breaker circuit. it's about 5.5 ma so I believe thats about right as well (reading the service manual). The voltage at the emitter of Q101 & Q102 combo is about -1.5, the base of Q101 is close to 0, the base of Q102 is -3.5v so i'm pretty sure its off with Q101 carrying all the current (about 2ma). I would expect the collector of Q101 to be pulling as much current as possible but it remains at or close to the positive rail. It kind of points to Q106 being shorted or Q107 being open (or maybe Q101 open?). Any input here would be appreciated. I also need a good resource for replacement transistors. and the current correct parts. I've read a lot of different sources in this thread. Whats be current best replacement parts (transistors and where should I buy them from?
On to Amplifier #2:
I've not looked as far into this amp. Its symptoms are:
Left channel is fine, right channel distortion LED stays let and 0 volts on the output, no blown fuses. I know it behaves the same as the previous amp if I pull the jumper for the temp breaker. The Temp LED comes on and the distortion LED goes out. still with 0 volts on the output.
Again, any help or guidance would be appreciated.
Thanks
I quick update. After looking over the schematics I concluded the bad component had to be Q107 (2SA1210). I replaced it and Q108 with a KSA1381E and KSC3503D from Mouser (just over $0.50 each). They worked perfectly. I was able to adjust the bias without problems and am very happy. Tomorrow I'll take on the second 555 II (distortion LED lit) and see if it's as easy to get going as the first. Thanks to the people on this forum for all the trouble shooting tips and the info on replacement parts and where to buy them.
Updat to update:
What a great productive weekend. The second amp is fixed. I ended up replacing the same two transistors after a little snooping around. Powered it up, warmed it up and again adjusted the bias. No problem to get to 10mv. Put the top on and they are both back in service. I purchased a few extra parts as I'm sure my other 5 555ii amps will soon be in need of repair.
Thanks again to this forum. You've reignited my love for trobleshooting this kind electronics. I look forward to the next challenge.
What a great productive weekend. The second amp is fixed. I ended up replacing the same two transistors after a little snooping around. Powered it up, warmed it up and again adjusted the bias. No problem to get to 10mv. Put the top on and they are both back in service. I purchased a few extra parts as I'm sure my other 5 555ii amps will soon be in need of repair.
Thanks again to this forum. You've reignited my love for trobleshooting this kind electronics. I look forward to the next challenge.
Joining the 555II Blown Channel fun...
Here I have a 555II blowing both rail fuses on the right channel. All 6 transistors on this pos rail test out of spec or have a section shorted. The information posted above will really help me find the replacements.
Question about the NPN outputs transistors, what is your recommendation, is it still the On Semi MJ15022G?
Here I have a 555II blowing both rail fuses on the right channel. All 6 transistors on this pos rail test out of spec or have a section shorted. The information posted above will really help me find the replacements.
Question about the NPN outputs transistors, what is your recommendation, is it still the On Semi MJ15022G?
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555
The On Semi is fine. Or I have some original Adcom marked devices you can PM me if you would like them.
The On Semi is fine. Or I have some original Adcom marked devices you can PM me if you would like them.
Thanks, I decided to get them from Mouser since I had them in the cart already. 38 dollars to re-transistorize half a channel.
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