Bought a defective GFA-555 II off EBAY. Right Channel was good, Left channel was dead and the instan. Distortion LED was on.
I replaced Q107 & Q108 and the left channel came back to life.
I replaced all the elec caps.
I biased both channels and they came in right at 1mV.
Output DC offset is less than 1 mV on each channel.
When I tested the amp in bridged mode, 1 kHz signal into an 8 ohm load, when the output was at about 150 watts, it went into oscillation and blew a fuse.
It's done that twice now.
When I replaced the fuse and test again in Stereo everything is fine.
Any ideas what I should check/look at? I'm guessing a cap somewhere but not sure.
Thanks for the help,
Gary
I replaced Q107 & Q108 and the left channel came back to life.
I replaced all the elec caps.
I biased both channels and they came in right at 1mV.
Output DC offset is less than 1 mV on each channel.
When I tested the amp in bridged mode, 1 kHz signal into an 8 ohm load, when the output was at about 150 watts, it went into oscillation and blew a fuse.
It's done that twice now.
When I replaced the fuse and test again in Stereo everything is fine.
Any ideas what I should check/look at? I'm guessing a cap somewhere but not sure.
Thanks for the help,
Gary
Were you testing the amp with a signal generator with a grounded plug? If so, you might have sent a current through the patch cable from the signal generator. The GFA-555 II has a 100R buffer resistor between each channel's signal ground and amp ground. Check resistance between your amp's RCA shields and speaker ground. You might have popped those resistors. It's a metal-oxide type if you need to replace it. (Metal oxide for its high frequency performance.)
Finally
I finally got back to this project this past week!
Yeah it's been a while, a few pieces of test equipment got in the way of repairing the amp.
It wasn't as bad as I thought. Once I took the amp apart it was obvious.
The resistors in the zobel network on the output terminals burned up. Resistors
R901 (left channel) & R951 (right channel) were pretty well toasted.
Both resistors are 10 ohm 2 watt.
R901 tested at 17k, this channel had the worst oscillation.
R951 tested at 350 ohms, this channel mostly worked.
I replaced both resistors and also C901 & C951 (0.1 uF). See pic.
The amp is back to it's normal self. I'm listening to it now.
The right channel does have a little higher THD+Noise number than the left channel. Not sure what's up with that. Anyone have any ideas? The right channel was about .025% at 1KHz and 200 Watts @8 ohms. The left was .003% same conditions.
Thanks everyone for the help and comments.
Gary
I finally got back to this project this past week!
Yeah it's been a while, a few pieces of test equipment got in the way of repairing the amp.
It wasn't as bad as I thought. Once I took the amp apart it was obvious.
The resistors in the zobel network on the output terminals burned up. Resistors
R901 (left channel) & R951 (right channel) were pretty well toasted.
Both resistors are 10 ohm 2 watt.
R901 tested at 17k, this channel had the worst oscillation.
R951 tested at 350 ohms, this channel mostly worked.
I replaced both resistors and also C901 & C951 (0.1 uF). See pic.
The amp is back to it's normal self. I'm listening to it now.
The right channel does have a little higher THD+Noise number than the left channel. Not sure what's up with that. Anyone have any ideas? The right channel was about .025% at 1KHz and 200 Watts @8 ohms. The left was .003% same conditions.
Thanks everyone for the help and comments.
Gary
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Yeah, I've had a board for the 555 MKII for a while now!
ADCOM GFA-555 MK2 Input Board | Hoppe's Brain
Your measurement of 0.025% THD+N at 200W, 8-ohm sounds about right to me. Spec is <0.09%.
0.003% seems too low. Might that be a measurement error?
ADCOM GFA-555 MK2 Input Board | Hoppe's Brain
Your measurement of 0.025% THD+N at 200W, 8-ohm sounds about right to me. Spec is <0.09%.
0.003% seems too low. Might that be a measurement error?
Not sure how I missed it on your website, thought I had covered it all. Thanks
.003, most likely a memory error or fat fingers!
Just got another broke 555 MkII from Ebay, it's a basket case. New input board might be the path of least resistance. Plus I just want to try one of the boards and hear how it sounds.
Indeed! The damage to the input board can be pretty extensive, especially if someone else has tried to fix it!
I originally designed the boards to save me time in refurbishing amps. I can build a new and better board in the same time it takes to fix and modify an old one.
Just make sure everything on your output modules is good and it should be a slam-dunk.
I originally designed the boards to save me time in refurbishing amps. I can build a new and better board in the same time it takes to fix and modify an old one.
Just make sure everything on your output modules is good and it should be a slam-dunk.
I see that sort of thing sometimes. The most common reason is a bad wire connection(corroded wire crimps) on the internal speaker wires to the output terminals. Try testing distortion before the wires at the board to see if it's better. Also test distortion on the input directly on the board to see if the problem is an input connection. Most of the time it is something simple.
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