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Old 19th June 2017, 05:30 PM   #41
DPH is offline DPH  United States
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Originally Posted by Andersonix View Post
In my attempt to corner the market on used 5200's, I now have a newer updated GFA-5200 to play with (date sticker says 3-97). It has separate windings for the input stage, so input rails are +/-54V while output stage rails are ~42V. (Maybe it can swing the full output rails?) Then it also has separate PS (bridge rectifiers and caps) for each channel, so a useful advantage over the original.
It appears to have been assembled very quickly, and the input signal cable is actually RESTING on one of the rectifier bridges, so clearly cable routing is not one bit important to these hacks. Nor is capacitance: It has only 4 puny 6800uF caps, so it needs some help getting the PS ripple down from an unacceptable ~50mV.
Now, "protection" is something they obsessed over: It has more transistors watching for faults (10) than for making sound (9). I will be referring to the schematic for the 5300 (attached) which is very close to the 5200.
It is my intention to replace the cover with a perforated aluminum cover which will be attached to the existing heatsink for maximum passive heat rejection without the use of a fan.
Stay tuned!
While not being anywhere near as facile with circuits (and this drawing does make it a little more confusing), any thoughts of trying to split C102 into a two-pole/TMC compensation to push more LF loop gain around the output transistors?
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Old 19th June 2017, 06:00 PM   #42
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Nope, no such thoughts whatsoever!

I just throw tons of caps at the PS and crank up the output stage bias.
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Old 19th June 2017, 06:12 PM   #43
DPH is offline DPH  United States
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You could do worse!

(Maybe snubber the rectifier/transformer secondaries, but that's the end of anything "fancy")
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Old 19th June 2017, 06:21 PM   #44
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For years I sat on a borrowed oscope hoping to get around to the snubber cap and resistor, but could never bring myself to disconnect the amp from the system. I should find someone with a Quasimodo bellringer setup:
Simple, no-math transformer snubber using Quasimodo test-jig
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Old 19th June 2017, 06:30 PM   #45
DPH is offline DPH  United States
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Mark is right in your neighborhood!
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Old 19th June 2017, 06:37 PM   #46
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"Hi Mark, you don't know me, but I'm a DIY-er who's too lazy to make my own, so can I come over and use yours? <click> Hello? Hello...?"
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Old 19th June 2017, 07:36 PM   #47
srinath is offline srinath  United States
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Default Exact opposite

Quote:
Originally Posted by Andersonix View Post
Srinath, on yours the ps is common to both channels. One fuse is for the positive rail, and the other is the negative.
Are you saying yours has visibly blown output transistors on both L and R channels?
I'm seeing minty beautiful board and transistors with nothing more than a light darkening in the areas of the resistors. Overall, nothing alarming at all.
Thanks.
Srinath.
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Old 19th June 2017, 07:46 PM   #48
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Originally Posted by srinath View Post
Awesome, I will get my transistor tester and get on it sometime after some of these already cracked open (snip)
OK, I read it as if your transistors were cracked open...
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Old 19th June 2017, 11:57 PM   #49
srinath is offline srinath  United States
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Default sorry

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Originally Posted by Andersonix View Post
OK, I read it as if your transistors were cracked open...
No, I meant I will get to this amp after I get the "other already cracked open amps" worked on.
As in - I have a lot of projects in various states of progress toward repaired status.
Let me know what you think please. I suspect power supply side cos both fuses are blowing.

Thanks.
Srinath.
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Old 20th June 2017, 08:37 AM   #50
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Originally Posted by Chamberman View Post
The fuses that blow inside of the amp are the rail fuses, they are after the main filtering caps and will not blow when one of the main filtering caps fails. The fuse on the rear panel will blow when a main filtering cap goes bad.

The fact that oversized 5A rail fuses were installed means that whatever was failing is almost certainly dead now. I would expect to have to replace the output mosfets in the affected channel.
Srinath, I agree with Chamberman.
(Although it's not necessary for the rear panel main fuse to blow with a bad PS cap; a bad cap will just make the ACV really high and the amp buzz in the output.)

You could satisfy yourself that the PS is fine by leaving out the rail fuses and measuring the PS DC voltage and AC voltage (ripple). If AC V is around 50mVAC or lower, then the PS is as designed. If the ripple is much higher then it will need new PS smoothing caps.
You could then load test the PS by loading it with a suitable power resistor, the value of which I will leave for you to calculate.

ps. It's likely somebody put in 5A rail fuses instead of the original 4A because that's all that Radio Shack had available, but that's a red herring.
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