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inertian

Intermittent Reverb on HD-130 Reverb Head

Friend brought me his recently acquired HD-130 Reverb head, saying the Reverb sounds great - when it works. Apparently the Reverb cuts out and stays off at inopportune times. Then returns just as abruptly. It was not working when I got it on the bench.

It's an early unit with 12AX7 PI, all parts date from early to mid 1974. It is very dirty, but has a strong clean tone. Both channels work fine, Tremolo is good, Bright switch works well, Deep switch works but is subtle.

A quick inspection revealed the main filter caps were physically deteriorating, so I replaced them. Will do a complete cap job when I receive the other parts ordered.

Cleaned and tightened everything - pots, jacks, plugs, switches, tube sockets, IC sockets, board screws, etc. The Reverb tank and cables look/measure fine. Set the bias to .5 volts DC as per the directions,

Amps sound great both clean and dirty (Volume up, Master down). Reverb was working and sounded good - until it stopped working again. Still intermittent.

When I chopstick the board where the Reverb control wires connect (wire solder points), I sometimes hear the tank crash.

Moved some of the ICs around to see if that made any difference, Reverb has not failed since but I only checked it over an hour or so.

I will reflow some solder joints and check for bad or out-of-spec resistors in the Reverb circuit. Sugestions welcome.

mm210
mm210's picture

Those little wires INSIDE the

Those little wires INSIDE the tank are usually what I find when this happens. JMHO. Mike.

inertian

Intermittent Reverb Resolved!

I tried the HD-130 with several known good reverb tanks I had on hand, same issues with reverb working well, then not at all, So that eliminated the original tank as the culprit. Also tried different RCA cables, no difference.

I loosened the main board screws attaching it to the chassis to look for any issues underneath, Checked that the Reverb was working. On a hunch, I tightened the board screws back down and... the Reverb stopped working! Hmmm. Loosened the screws up again, blew some compressed air under the board, checked for any pinched wires, and retightened. Reverb was still working, so I ran the amp at high power for about five hours, checking now and again that the Reverb was working and the bias was stable. All seems ok now.

So I'm thinking there was some kind or short or grounding issue that was heat related, cleaning under the board and loosening/retightening its screws seems to have fixed the problem.

Returning the amp to its owner tonight, we'll see how it holds up.

mm210
mm210's picture

Great catch. Could have been

Great catch. Could have been a loose blob of solder or some other little piece of gremlin. Amp heats up, expands and something touches. Good one! Mike.

inertian

It's Back! (HD130 Reverb Issue)

And now the Reverb just buzzes, gets louder as you turn up the Reverb control. Tried another tank, cable and transistors in Reverb section of the circuit, no improvement. Some component in the circuit must've gone bad, or something is grounding out the tank. Suggestions welcome.

inertian

Cautiously optimistic that it's fixed...

Traced the reverb-ed signal with my audio probe last night, found it wasn't appearing at 220K R-17 or .1uf C-15, working back I found a lifted trace under the board for one end of .0033 cap C-12. Reinforced the trace and made sure it was making solid physical and electrical contact, and the reverb started working again. This might explain when the Reverb would suddenly work when the amp was on for a while, then not after being off (cooling down).

Fired it up again this morning, and it still works. If it's good again tonight, I'm calling this one done...

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