Author Topic: HELP! AS100 A575 blowing caps/rectifier diode  (Read 556 times)

impaladb

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HELP! AS100 A575 blowing caps/rectifier diode
« on: November 01, 2017, 06:21:02 PM »
Ok this one is kicking my butt. I cannot figure out why.

Got an A575-02 100 watt unit from 4/66 that I have recapped. Upon turn-on it starts to cook the rectifier diodes and the power supply filter caps.

I've checked all the new caps on the preamp board and there is nothing wrong. Resistors are all within spec.

With the chassis sitting upside down, preamp on the left and transformer on the right I am seeing continuity between the base and emitter of 4 of the output transistors. Those transistors are the 2 in the middle on the right, and the 2 outer ones on the left. I don't know if this is proper due to the push pull parallel circuit design or not. I've disconnected a couple of those transistors and tested for continuity and there is none, making me believe that the transistors are fine.

I've replaced all 4 rectifier diodes and the 1500mfd filter caps a second time and it's still happening.

I need the help of an expert!

impaladb

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Re: HELP! AS100 A575 blowing caps/rectifier diode
« Reply #1 on: November 01, 2017, 11:50:52 PM »
Just to update everyone I've recapped about 20 different Astrosonic chassis, but this is the first AS100 I have done.

TC Chris

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Re: HELP! AS100 A575 blowing caps/rectifier diode
« Reply #2 on: November 02, 2017, 06:29:49 PM »
I haven't worked on one of these, but I seem to recall that some of the sold state Magnavox devices had the polarity of the HV power supply, and therefore the orientation of the filter caps, reversed from the usual negative ground.  Any chance you inadvertently installed filter caps backward?

Chris Campbell

electra225

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Re: HELP! AS100 A575 blowing caps/rectifier diode
« Reply #3 on: November 04, 2017, 10:34:17 AM »
Do you have a schematic for this instrument?  Have you checked to see that everything is wired per the schematic?  Have you checked for an errant blob of solder somewhere?
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

AstroSonic100

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Re: HELP! AS100 A575 blowing caps/rectifier diode
« Reply #4 on: November 04, 2017, 12:20:45 PM »
I just uploaded the Sams photofact for the A575-01-00; -02-00; -03-00 amp for the Astro Sonic 100.  It's under the Service Manuals and Schematics - AMP in the Downloads section.


A word of caution regarding the polarity of electrolytics in the power supply. 


The schematic shows electrolytics C1, C2, and C5 with positive lead to ground and minus leads connected to resistors R34 and R33.  This is for the -24 and -32 volt source.  Also, electrolytic C4 has a positive ground and the negative lead goes to the negative leads in Diodes X17 and X18.


Electrolytic C3 has the negative lead going to ground and the positive lead going the positive leads on diodes X15 and X16.  This is for the 34 volt source.


I have the same amp and I had to really be careful when I was recapping the power supply to make sure I had the polarity correct.




Ray

darrenwgaransi

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Re: HELP! AS100 A575 blowing caps/rectifier diode
« Reply #5 on: February 14, 2018, 03:42:10 AM »
I would go back and double check the original filter cap to check whether or not the case was isolated from the chassis with a mica wafer. Also check the new cap to make sure it isn't defective. Magnavox was weird with the power supply positive ground setup.