KA-3500 cleanup

[rickl]

I hate to call this thread a resto after reading hopjohn's excellent 3500 restore https://audiokarma.org/forums/index.php?threads/the-ka-3500-restoration.840144/

This will be more of a clean up.

Here is my latest find. Went to pick up a KR-5030 but it was sold after my 1 hour drive. Checked out his other stuff and found this 3500.

There was a tag that (the guy quickly pulled off) said "right channel out". Got it cheap and brought it home.

Brought it up on DBT (60w bulb) and passed. Bulb glowed briefly and went out.

Without output protection, I don't know if the amp works and just need cleaning. Once I get it cleaned up, I'll measure the DC offset and see if rail voltage is present.

Might be a nice little amp.

 

[rickl]

With a DBT, I measured DC offset.
Left -18.1mV
Right -60.2mV

Injected a sine wave and both channels are working.

Left channel has a little more gain. Adjusted volume to 34Vpp on left, right has 32Vpp. Might not be enough to notice.

I haven't checked bias but it would be nice to get the right channel DC offset lower. Need to read up on Hopjohn's resto.

 

[rickl]

Replaced all e-caps in the signal path with film or audio e-caps. Updated a few PS caps and muting circuit per hopjohn's recommendation. Replaced Qe2, Qe4 and got the right channel DC offset lower. Let it warm up and set bias to 40mv for each channel.

It sounds pretty good. All controls are quiet. Some wear on the face plate by the volume control so my guess this had been used a lot. Kind of beat up but cleaned up nicely. I always love the feel of a freshly waxed face plate and knobs.

 

[hopjohn]

Nice work. I'm listening to my KA-3500 right now. It punches above its weight no doubt about it.