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Hartke A70 repair

R

Riot666bomb

Guest
Apr 30, 2016
3
0
35
#1
I've got an A70 that was KIA by faulty electrical wiring in a friend's practice space. I've since gotten a much better amp (Ashdown Klystron 500) but would still like to get this pup running again.
The electrician that eventually fixed the electrical issue said he tested the outlet it was plugged in to and it was jumping between 60v and 120v.
It may have been two or more issues that eventually put the amp down; leading up to the incident it seemed the limiter would be triggered and I would lose 99% of volume output. (this was even on a "safe" circuit)
After replacing the fuse and a couple resistors, the amp will power on with the limiter still stuck on, no sound coming out until the power is switched off and it will produce a soft "bweeep" on powering down.
 
#2
Ummm, what kind of knowledge do you have with analog electronics? What test equipment do you have? When you power the amp up, does the cone move and stay in that position?
 
#3
I don't have much experience in circuit board repair, but I didn't make it worse so there's that. ^_^
When I power it on, the speaker will give a normal "thud" and slightly flex
I have a cheap multi meter.
 
#4
If the speaker moves and stays in the new position, you have a major power amp failure. You do not want to leave the amp on in this condition as it will likely destroy the speaker eventually.

Without understanding the schematic (the power amp is deceptively simple), you might be able to narrow the fault down to the preamp or the power amp. With no speaker attached, can you measure the DC voltage at the output (speaker jack)? This will help with how I would suggest going about a repair.

This is actually a fairly sophisticated little amp from the preamp side of things. There are some clever implementations that are well ahead of it's power level expectations (IMO)
 

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