H+K Rotosphere troubleshooting

[surfdaworld]

hey y'all!
just wondered if any of you have had any experience troubleshooting an H+K Rotosphere?

a friend asked me to take a look at his--it works fine, but the 60 Hz hum is unacceptably loud.
I already tried swapping the tube out, to no avail.

any ideas?

[Rodgre]

The first question I would have is whether they used a different adapter than the one it comes with (12VAC).

Other than that, have you tried a new tube in it?

Roger

[jonathan perez]

I already tried swapping the tube out, to no avail.

[alfafalfa]

I already tried swapping the tube out, to no avail.

What exactly is the tube ? Is it a 12ax7 or a 12au7 ??
I have one and tried to open it but couldn't get it done >>

How does this thing open ?

Alf

[surfdaworld]

off the top of my head I believe its a 12AU7--but don't quote me on that 

the case opens by unscrewing the 2 end plates, releasing all the 1/4 in. connectors on the back, and (carefully) sliding the whole shebang out the end of the aluminum enclosure.  be careful of the ground wire and lug that connect the top and bottom boards, as it tends to catch on various pieces along the way.

my new scope just arrived--as soon as the probes get here I'll be spending a while poking around in this box to try it out 

[surfdaworld]

I know this thread is pretty old, but after finally getting this thing going, I thought I'd pass along what I found in case someone else could benefit.....

after mucking around without schematics, I was able to trace out 3 separate power conditioning stages in the circuit--the culprit turned out to be the step-up transformer stage that powers the tube.   The two 15uF filter caps had started go out, so the power to the tube wasn't being filtered adequately--and as close as everything is packed in there, several of the op amps were picking up the hum as well.

Incidentally, I found out after the fact that the owner of the box had already taken it to the pro audio repair shop here in town, where he was told it was hopeless, and there wasn't anything they could do with it      I could see from the marks on the board that they had checked out the first two filtering sections, but apparently hadn't even touched the 3rd one