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K100 : PC900 Module 8 Volt Regulation Problem [message #21837] Wed, 12 November 2014 10:58 Go to next message
Shoreline Electronics is currently offline  Shoreline Electronics
Messages: 3
Registered: November 2014
Location: Clinton, CT
Junior Member
Hi Folks !

Got a head scratcher here.

Kustom K100 5 Head

When amp is first powered on, it works great.

After a few minutes both the regulated +8 & -8 Volt supplies slowly drop go down to about 2V & -2V

When this happens... 39.5 & -39.5 rails are perfect.

Pulled all the tantalum electrolytics and tested out of circuit...they are fine.

First thing I did was to disconnect these feeds from the rest of the amp so it's now isolated to just the PC900 module. Same results

Also tried freeze spray on the circuit in case it was a thermal problem...no change

Replaced both Q911 & Q915 with 2N3055's

Not wanting to spend all day on this, I started replacing other transistors in the circuit and check all resistors. I've replaced all transistors except the 2N3638's....those will be here today as I ran out of them

I did replace Q900 and checked the precision resistors in the biasing circuit for it also as these supplies feed that also.

My feeling is it has to be something in common to both supplies to do this.....but I haven't cracked the code yet!

Anyone out there ever had this issue before?

Thanks !


++++++++++++++++++++++++++++++++++++++++++++
Jeff Stielau
Shoreline Electronics Repair
344 East Main Street
Clinton, CT 06413
(860)-399-1861
fixmyamplifier@aol.com
www.shoreelxandsound.com
++++++++++++++++++++++++++++++++++++++++++++

[Updated on: Wed, 12 November 2014 11:05]

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Re: K100 : PC900 Module 8 Volt Regulation Problem [message #21840 is a reply to message #21837] Wed, 12 November 2014 13:18 Go to previous messageGo to next message
chicagobill
Messages: 1957
Registered: April 2003
Senior Member
Welcome to the place.

The 8 volt regulators are pretty straight forward. You need to fix the positive side first, as the negative side just follows the positive side.

There are voltages on the schematic, how do your voltages compare? If it starts out at 8 volts compare the readings when it's cold to the readings when it fails.
Re: K100 : PC900 Module 8 Volt Regulation Problem [message #21841 is a reply to message #21840] Wed, 12 November 2014 13:22 Go to previous messageGo to next message
Shoreline Electronics is currently offline  Shoreline Electronics
Messages: 3
Registered: November 2014
Location: Clinton, CT
Junior Member
Will do ! Thanks !

++++++++++++++++++++++++++++++++++++++++++++
Jeff Stielau
Shoreline Electronics Repair
344 East Main Street
Clinton, CT 06413
(860)-399-1861
fixmyamplifier@aol.com
www.shoreelxandsound.com
++++++++++++++++++++++++++++++++++++++++++++
Re: K100 : PC900 Module 8 Volt Regulation Problem [message #21842 is a reply to message #21837] Thu, 13 November 2014 06:16 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4299
Registered: June 2004
Location: NY
Senior Member
Welcome to the place also!
If nothing else than by process of elimination you will have it fixed when you recive the transistors you ordered,lol!
Re: K100 : PC900 Module 8 Volt Regulation Problem [message #22687 is a reply to message #21837] Sat, 21 March 2015 13:31 Go to previous messageGo to next message
Shoreline Electronics is currently offline  Shoreline Electronics
Messages: 3
Registered: November 2014
Location: Clinton, CT
Junior Member
Finally got back to this one : Replacing all of the 2N3638's with 2N4403's ( ECG159 equivs) fixed the problem.

One thing I see now is that our 8V supplies are more around 10V now. They dip a half a volt or so when you push the amp hard. Should be good to go !


++++++++++++++++++++++++++++++++++++++++++++
Jeff Stielau
Shoreline Electronics Repair
344 East Main Street
Clinton, CT 06413
(860)-399-1861
fixmyamplifier@aol.com
www.shoreelxandsound.com
++++++++++++++++++++++++++++++++++++++++++++
Re: K100 : PC900 Module 8 Volt Regulation Problem [message #22688 is a reply to message #21837] Sat, 21 March 2015 14:56 Go to previous message
stevem is currently offline  stevem
Messages: 4299
Registered: June 2004
Location: NY
Senior Member
Yup, your good!
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