Blown Fuse - what to check ?

[VBB]

I've got an HK 930 Twin Power receiver that I picked up a couple months ago. Been working great, then the other day while playing a cassette, I heard the speakers do a little pop and cut out. First one side, then a split second later the other side. The main 3A fuse blew (the L and R fuses are good). I inspected for a dead short and couldn't find anything, so I put in a new fuse and when I powered it up, I heard a hum (through the speakers) for a couple seconds, then the new fuse went.

Any idea what to check first?

 

[sKiZo]

First thing I'd check is the speaker wires. Might have gotten yanked about and shorted somewhere. Check both the receiver and speaker connections.

Also make sure you're using "slow blow" fuses.

 

[VBB]

I checked the speaker wires, but for fuses, I replaced with a standard AGC fuse. There's no marking like F for fast blow. Not sure which it is.

 

[sKiZo]

AGC just designates the size of the fuse. Standard fuses usually have a straight wire element inside, slow blows tend to be wound wire ...

 

[VBB]

Yeah, I knew AG was auto glass. The C must be the length. I'll try to find some slow blow fuses somewhere. Maybe RadioShack? Should the fuses in the R and L sockets also be SB, because none of the fuses in this receiver look to be?

Thanks for your quick reaponses!!

 

[sKiZo]

You might find almost anything in 50 year old gear - Gotta read the book. Just did here (it's available on HiFiEngine, but it was less than useful. It shows all fuses are 3a, but doesn't mention whether they're standard or slo blo, which would normally mean they're standard fuses.

** DO resist the urge to put a higher rated fuse in - doesn't help, and can really hurt.

I also see these little beasties are prone to blowing out bridge rectifiers, That said, the HK Twin Power completely isolates each channel (kind of a dual mono design really) - and I'd think that would also blow out at least one of the channel fuses.

Next step I suppose would be hooking up a dim bulb tester and looking for a dead short ...

 

[avionic]

Perhaps a DBT until you find the "root cause".

 

[VBB]

I'll get at it with a dim bulb tester. Thanks for the direction!

 

[Freddymac2]

I do not recommend a slow blow fuse, a fast blow can do enough damage. For real quick trouble shooting wrap the fuse with foil, and the bad component will go up in flames!!!!!!!

 

[VBB]

Any idea what wattage bulb I should use in a DBT?

 

[VBB]

Ok, I made a DBT last night. Works great, tested known working gear. When I test this HK930 the bulb shines bright! So, if I understand, this would indicate a dead short? Any suggestions on where to start looking?

 

[sKiZo]

Bridge rectifiers, or the power transistors are a good place to start ... those are both known culprits on these things. I'd think a bad transistor would also blow the corresponding channel fuse as well though.

 

[UncleBingo]

Check the voltage at the input to the rectifier (transformer output). Check for DC at the rectifier output.

 

[VBB]

I thought the corresponding fuse would blow too.

Thanks for the ideas. I appreciate you guys being patient with me. I really wanna learn how to work on my own equipment!

 

[triode17]

Yeah, I knew AG was auto glass. The C must be the length. I'll try to find some slow blow fuses somewhere. Maybe RadioShack? Should the fuses in the R and L sockets also be SB, because none of the fuses in this receiver look to be?

Thanks for your quick reaponses!!

Radio Shack? What's that? cry, cry ,cry

 

[triode17]

Yeah, I knew AG was auto glass. The C must be the length. I'll try to find some slow blow fuses somewhere. Maybe RadioShack? Should the fuses in the R and L sockets also be SB, because none of the fuses in this receiver look to be?

Thanks for your quick reaponses!!

The L&R fuses are fast blow. It sounds to me like you have some blown out power transistors. Check the heatsink parts with a diode checker on a DMM.
The channel that dropped out first should be the problem child. Once it blew, the fuse was next, then the other channel dropped because it de-energized.