New amp GFA-555 broken

ben62670

Well I have been dabbling in the repair field for a while, and have acquired a bit of knowledge, and parts. So....
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=290207076535&ssPageName=STRK:MEWN:IT&ih=019



Second pic shows sign of excessive heat.
Third pic shows mild stupidity with unfused extension cord/power switch.

ben62670

I started from the beginning, and disconnected the rectifiers. So far I replaced the bridge rectifiers(less than $10). I had some new ones laying around so I threw them in there. 81.5 volts all around after I connected to the supply caps. Inserted the rail fuses next, and no smoke. Took a voltage reading from the speaker terminals, and 78vdc from the left channel. Started chasing shorted output transistors, and one bank of outputs had .116 ohms on all four(Ben scratches head). I figured that the input board may have an issue so I started to disconnect the driver outputs from the input board when I noticed that the output driver transistor was touching the thermal coupling transistor. I separated them, and fired it back up. No pegged output:) The DC offset is a little high at around 42mv, but that is acceptable, and I have not yet set the bias. I wish I had a heavy duty woofer down here. Most of my stuff is in Georgia:(

Edit
Pegged again 78vdc at speaker terminial:eek:

agfrost

Nice fix, Ben.

Did Britney hand-deliver it, naked? (Would this be a good thing if she did??)

Jay

PS-I'll need to talk to you about the possibilty of ordering some more ICs, once I figure out exactly what I need.

ben62670

agfrost wrote: »

Nice fix, Ben.

Did Britney hand-deliver it, naked? (Would this be a good thing if she did??)

Jay

PS-I'll need to talk to you about the possibilty of ordering some more ICs, once I figure out exactly what I need.

I jumped the gun. Its misbehaving again. Shoot me a PM when you are ready.
Ben

agfrost

Kick it.

(Yeah, I'm not much of an electronics tech)

Jay

danger boy

that amps looks like it's seen better days.. props to you Ben if you can get her up and running again.

ben62670

Well I disconnected the input board from the outputs. Still 78 VDC at speaker terminal. Either a driver is pegged, or an output transistor is shorted. I'll get it. Done for the night though. No fuse, and no sleep equals burnt power cords, and skin + smoke, and sometimes a little juice to "the repairman".

ben62670

Well the amp is done now. New 14/3 power cord. Stock was a 16/2. I added a ground, and upped the Ga. I replaced the little transistors 970's, and 2240's. DC offset gone! Bias set properly 50mv when hot. Also replaced the output drivers. Not the main ones, but the ones that drive the mains. All I had in stock were MJE15032, and MJE15033. These drivers handle slightly less currant, but I am taking it easy till I get the MJE15030's, and MJE15031's. The 32, and 33 have slightly more gain, but are not up to snuff for max power output. I am driving the snot out of my Ipod thingy speakers, and they take it well. Adcom's kinda amaze me. They seem like smart amps that just control speakers well. I have driven my big custom SDA's with one, and then I hook it up to small speakers, and it doesn't blow them yet it cranks. The sound is so sweet. It sounds a bit better than my other 555. I don't know why but it does. The shop that worked on it last cut the power cable, and threw out the fuse holder. I guess they though it was beyond repair:D I am torn whether or not to sell it or keep it. I want to get rid of the silly ground bar, and upgrade it to point to point wiring. I have another GFA-555 transformer, and was thinking of going nuts with this one. I was thinking of paralleling the 2 transformers to double the va, and doubling the output trannies. Also upping the caps to 68,000 mfd x 4. Stock is 15,000 x 4. I would like to improve the dampening factor, and headroom. This might require making the GFA-565 input boards. It would be very similar to the GFA-585, but stronger. I would need a larger case, and have to do a little milling to make heat sinks, or water cool it. We'll see.

Refefer

Cool hobby. Wish I knew where to begin, electrically speaking, to do what you've been doing.

ben62670

Refefer wrote: »

Cool hobby. Wish I knew where to begin, electrically speaking, to do what you've been doing.

I started with basic knowledge of electronics, and did a lot of online reading. It cost way more to do my first couple repairs than if I sent them to a shop, but the experience was worth it. I was one of those kids that tore apart everything.

Ben