Internal fuse blowing, CD650, help understanding circuit

[dotrs]

Hi guys,

I'm trying to bring my old Laney CD650 mixer/amp back to life. Cant remember the cause of the problem as has been stored in the loft for a couple of years out of action. Always been looked after well, and never spilled/dropped anything on it. Can't see any burnt out components, but my electronics skills are not the best, so would like to understand the circuit better before I start testing things.

The problem is an internal fuse keeps blowing. With fresh fuse, the unit works as normal for approx 30 seconds before the (timelag) fuse blows again. The fuse is in the power stage, and it is a 2 channel amp - always the same channel with the problem so could possibly get some help from the working side?



Here is a link to the service manual:

http://www.eserviceinfo.com/download.php?fileid=48004



the affected fuse is 500mA Timelag fuse labelled F4 (top right of page 11; overall layout page 3)

If anyone could offer any help as to why this fuse keeps overloading, and how to repair the promlem, I would be very grateful.

Many thanks in advance,

Adam

 

[Resqueline]

The problem must be with BR2, a 1.5A 200V bridge rectifier having developed a partial short (or C19, or the wiring beween F4 & BR2, but that's rare).
Whatever it is it should get hot if it's the culprit (unless it's C19 or the wiring suddenly shorting out). Are the fuses real timelag/delay or just ordinary slow (T) btw.?
But that circuit (F3, F4, BR2, etc.) is common to both (all) channels however, how come you say only one channel is affected..?

Edit: It could in fact also be a break in F3 (or in the tracks to & from, or in BR2). This will leave F4 to do the full duty (for both), and the amp will go dead when this fails.

 

[shrtrnd]

Watch out for your large electrolytic capacitors near that bridge too.
I don't know how old the unit is, or where you stored it, but the electrolytics seem
to go more often than anything else in power circuits (causing damage to other components
like the bridge rectifier).

 

[olfogie1]

fuse blowing

The previous reply's are real good and to the point. My tinkering skills with this short of stuff may help to possible. As for a condenser if it is an electrolytic they will usually bulge out at the top when they go bad thats what the lines on top are for to help prevent them for acting like a miniature bomb when they explode. You definitely have a short as that is what the fuse protects or a sudden voltage spike like from lightning. As I was not able to download the diagram with the rectifier I try to measure each section to see if it shows a short which would continuity in both directions. You might have to isolate each section though as sometimes other components will effect the reading you might get. As I am no expert here with solid state circuits this may or may not be of any help.