[Boatanchors] OT: power supply troubleshooting

[W2HX]

Update: Ok guys, truth is, this is hard to fix.  Mr. Maples (and others) was quite right. I was looking at the pre-amp section. The power amp section is totally inaccessible and un-removable without taking out the saw, as far as I'm concerned. Perhaps it is removable, but I've given up!

So without further ado, I am offering a Polytone Minibrute II for the cost of shipping.  Dibs first goes to those who responded to my troubleshooting call (many done privately).  It is a 100W unit, dimensions are approx. 15.5"H x 14"W x 10.5"D  , weight is about 26lbs.

Contact me off list please. And again, I do sincerely appreciate all the help and advice (it's actually for a fender Rhodes keyboard).

Many thanks again 

 
73 Eugene W2HX
 

-----Original Message-----
From: boatanchors-bounces at mailman.qth.net [mailto:boatanchors-bounces at mailman.qth.net] On Behalf Of W2HX
Sent: Sunday, January 20, 2013 10:57 PM
To: boatanchors at mailman.qth.net
Subject: [Boatanchors] OT: power supply troubleshooting

Hi all, this is OT and solid state related (please delete me asap!) 

I have a guitar amp that is blowing fuses. The circuit seems to be AC input, fuse, transformer then to the power switch (interesting that the switch does not switch the AC Mains, but after the transformer).  When I measure the input to the switch I see 30VAC. But when I switch the switch to ON, the voltage drops to millivolts. I suspect this is because there is a short after the switch.  After the switch there are 4 discrete diodes forming a full wave bridge. Then two filter caps 470uF/35V electrolytics and then to a 7815 and 7915 regulators to create +/- 15V DC.  A fairly standard looking circuit.

First thing I did to troubleshoot this was measure the in-circuit ESR of the filter caps with an ESR tester. The ESR seems to read 0.1 ohm. I believe this is acceptable, no? but then again, the ESR would contribute to higher than normal ripple, but I don't know if it will indicate a capacitor short?  Do electrolytic caps fail short? I know that tants can.

So could the problem be the diodes? Do diodes fail short? Could 4 diodes in a full wave bridge fail in such a way as to create a short? 

If not, do I look at the regulators? 

Any general advice? 
Thanks

73 Eugene W2HX



______________________________________________________________
Boatanchors mailing list
Home: http://mailman.qth.net/mailman/listinfo/boatanchors
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Boatanchors at mailman.qth.net

List Administrator: Duane Fischer, W8DBF
** For Assistance: dfischer at usol.com **


This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html