I know this has been discussed many times, the amp is blowing dc rail fuses. I've gone as far as I can with my diagnosis,all of which I have learned here, mostly by reading and rereading cogeniac and anatechs posts, #203 ,207etc. I own a variac and a fluke volt meter. So far ,after jumping the soft start resistor I can hear the relay clicking in. The amp and the variac buzz at about 15 volts, dead short somewhere. When the rail fuses are removed the amp powers up to 110 volts, no magic smoke. If I put either rail fuse in the amp powers up ok,as long as iI put only one fuse in at a time, doesn't matter which side.. fuses in power lines to rails disconnected (post 186) powers on to 110 volts.points 3,4 on output board disconnected , everythig else connected amp powers on to 110 volts.could both sides be biasing at once?. The factory leaky board caps were replaced 15 years ago wit nichicons. I'm hoping the board is bad, as I'll buy a new one from chris hoppe. I don't know where to go from here. When the amp failed it didn't take out my speaker,nor did it pop when turned on or off.
The fact you can power up one rail at a time, suggests to me that the output sections are OK and not shorted. The amp won't attempt to bias with either rail off.
There's so many things that can go wrong with the input board, that the easiest way to tell if your input board is bad, is to just make sure the output sections are all good. They are easy to test; every component can be checked in circuit, and if everything tests OK, you can be nearly certain the input board is the issue. Here's copy-pasta from my documentation for the BFA-565 board.
Yes, it's possible for a board to go DC years after it's been fixed. The electrolyte is soaked into the board and it slowly dissolves further into the fiberglass. Eventually it reaches the high-impedance parts of the circuit, the input section is right next to the bad caps. 15 years does seem an awfully long time for it to take though.
You might also want to try running your input board without the output sections connected. You can just disconnect points 3 and 4, the drive signals, and tie two 1K resistors into a Y that goes back to the feedback... Procedure is in the documentation for the BFA-565 and EBFA-565 boards. If the board behaves normally without the output sections connected, then you know it's the outputs.
Compare your board's readings to the annotated schematic attached.
There's so many things that can go wrong with the input board, that the easiest way to tell if your input board is bad, is to just make sure the output sections are all good. They are easy to test; every component can be checked in circuit, and if everything tests OK, you can be nearly certain the input board is the issue. Here's copy-pasta from my documentation for the BFA-565 board.
Yes, it's possible for a board to go DC years after it's been fixed. The electrolyte is soaked into the board and it slowly dissolves further into the fiberglass. Eventually it reaches the high-impedance parts of the circuit, the input section is right next to the bad caps. 15 years does seem an awfully long time for it to take though.
You might also want to try running your input board without the output sections connected. You can just disconnect points 3 and 4, the drive signals, and tie two 1K resistors into a Y that goes back to the feedback... Procedure is in the documentation for the BFA-565 and EBFA-565 boards. If the board behaves normally without the output sections connected, then you know it's the outputs.
Compare your board's readings to the annotated schematic attached.
Thanks for your quick reply Phloodpants. I had to deal with a broken water line in the yard right after I posted, but was able to repair it. The schematic will come in handy. I will check both the output modules before a new board is installed but am hoping to check the control board first as it will help my diagnostic skills.
Hey Analogrider if you get to the outputs and they need to be replaced reach out to me. I have hundreds of NOS 193/194s from the same lot codes. They have been sitting around in the new packing for at least 15 years. Free of charge. I would just like to see some get used. BTW owned one of the first Gen 555s and ran it on some Acoustat 2+2s for about 8 years. So yes I have had to do my share of replacing outputs. The one thing I finally did was attach muffin fans to each heatsink and problems went away. Let me know
Hi, the amp now turns on and does not blow output fuses. I took the control board out and reinstalled it more carefully this time. The dc on the speaker terminals is 80 mv with the input shorted. Played music through it It is distorted. I need to check the output transistors but don't know how.
Yeah, 80mV is bad on a GFA-565. There is a DC servo that should be taking it down to less than 5mV.
That doesn't sound like bad outputs, but it's a good idea to check.
This text is from my document "GFA-565 and GFA-585 Circuit board replacement notes". Google that to find the document, I suggest you read through the whole thing, it's relevant even without installing a new board.
Often there are blown resistors on the output modules. Usually, the output transistors are fine. Here’s how to test everything.
If you find that you just have one or two blown transistors, it is recommended that you replace the whole set of ten, as they need to match. I recommend On Semi MJ21193/94, but there are many good substitutes. If you order from a reputable source like Mouser or Digikey, you will probably receive a whole set with the same batch code. Manufacturing tolerances are so good these days that a set from the same batch code will likely be decent matches.
That doesn't sound like bad outputs, but it's a good idea to check.
This text is from my document "GFA-565 and GFA-585 Circuit board replacement notes". Google that to find the document, I suggest you read through the whole thing, it's relevant even without installing a new board.
Testing the output modules:
Please do not skip this step! You must be certain your output modules are good before applying power to the amp. This GFA-565 is direct-coupled from one stage to the next. If you try to power up the amp with blown transistors on the output modules, the “Domino Effect” may ripple backwards through your amp, blowing up everything all the way back to the input stage. It’s a big job to repair the damage.Often there are blown resistors on the output modules. Usually, the output transistors are fine. Here’s how to test everything.
- Lift one leg of R201, R202, R301 and R302. Test these resistors and replace with suitable metal-oxide types if they are bad.
- Test all the other resistors on the board, including the 10R base stoppers and emitter resistors. (Hint, most meters do not read the emitter resistors accurately, due to their low impedance. If they read a little high, they are probably actually fine. An LCR meter works better.
- Lift one leg of the reverse-protection diodes D201 and D301.
- Using your meter’s diode check, you should see the usual, approximate 0.5-0.6V drops from B-E and B-C on all transistors. Q202, Q203, Q302 and Q303 will show 0.6V from C-E in one polarity only. All the output transistors should read open-circuit from C-E in either polarity. If you read 0V anywhere, you have a shorted transistor.
- Check C202 for a short circuit. (Rare but it happens.)
- Test your bias comp transistors in the usual manner.
If you find that you just have one or two blown transistors, it is recommended that you replace the whole set of ten, as they need to match. I recommend On Semi MJ21193/94, but there are many good substitutes. If you order from a reputable source like Mouser or Digikey, you will probably receive a whole set with the same batch code. Manufacturing tolerances are so good these days that a set from the same batch code will likely be decent matches.
Hi phloodpants, Do I need to lift one leg of all resistors on the output modules or just r201,202 and r301,302 ?. Where are the base stoppers?. Is pin 6 on the control board where I check dc servo voltage? As you can see I'm learning. In some ways my Fisher x 1000 tube amp was simpler to understand. Thank you for your patience, Kevin
Hi,all. I am now testing the output transistors The readings I'm getting don't make sense to me. As I have never tested a transistor before I'm wondering if I am doing something wrong. Black probe on collector, red on base or emitter multimeter on diode check, .54 and .55.red on collector black on base/emitter reads open. E to b reads .018. all the transistors on one bank read like this. I checked a few on my working 565 . The few I checked read the same. Am I doing something wrong. ? .Also put a fresh battery in the fluke meter. Kevin