Weird Blackface Pro Reverb Tremolo Issue

[Charlie Wilson]

Hello, I have a Pro Reverb on my bench has a rather frustrating tremolo issue. It works fine when the chassis is out of the cabinet but as I slide the chassis into the cabinet the tremolo gets uneven and then just stops. I have checked the shielding screen in the cabinet for strands that may be hanging down and touching the chassis but there are none. I have worked on a lot of Fender amps and this is a first. Any ideas?
CW

[pompeiisneaks]

That's super weird, almost the opposite problem of roach issues. Usually if the roach get's light in, it messes it up, but in this case it sounds like exterior light may help but when it's inside the cabinet the LDR has too little light?

May be a red herring, but maybe the LDR side is becoming too insensitive and it needs extra light to make it work? Unless I'm talking about the wrong kind of trem unit, and it doesn't use a roach, I know some fender's dont.

Otherwise I have no clue

I definitely want to hear the solution.

~Phil

[Charlie Wilson]

Interesting Phil. I did not even consider the light or lack of as the issue. I was thinking maybe the speaker magnets were interacting somehow. Anyway, I can try a different roach and see if that solves the problem. I just hate replacing vintage ones unless I have to. The new ones don't sound the same.
CW

[pompeiisneaks]

yeah well basically the LDR may be failing in an odd way and it's become less sensitive, but I'm just thinking in my head what could cause that behavior. Yeah I remember watching a video with someone trying to find an LDR that seemed appropriate in a repair of a trem roach and they had to try several, but they built the roach themselves. Maybe someone else has practical experience and has seen the fault before and will know for sure.

~Phil

[sluckey]

Can you reproduce the symptoms by placing a piece of cardboard over the chassis while out of the chassis?

[Charlie Wilson]

Sluckey, I guess that would have been the easy way to do it. I wrapped it with electrical tape and it behaved the same way. Weird.
CW

[sluckey]

OK, one more little test. Can you reproduce the symptoms just by turning off all the room lights while out of the chassis? If so, you may want to go ahead and get a new roach. BTW, I've never seen an LDR act like this, but the amp does seem to be light sensitive.

[Charlie Wilson]

Yes, I turn the lights out and the tremolo stops. This is one of the oddest things I've seen in an amp. I just noticed that the neon blub actually stops flashing in the dark.
CW

[Reeltarded]

Yes, I turn the lights out and the tremolo stops. This is one of the oddest things I've seen in an amp. I just noticed that the neon blub actually stops flashing in the dark.
CW

Abbot and Costello are about to make an appearance. I just know it.

[sluckey]

I'm thinking more likely Svengoolie!

[martin manning]

Neon lamps are photo sensitive. Aparrently the room light is putting the strike voltage just out of reach.

[sluckey]

Neon lamps are photo sensitive.

I never knew that.

Since the bulb seems to be the faulty part of the roach, it's an easy fix if you want to keep the original LDR. Just remove the roach from the board and carefully cut the shrink tube away. Lay the new NE-2 in place and put new shrink tube on the assembly. Cost about a quarter and 15 minutes time. You can test the NE-2 in circuit before rebuilding the roach.

[pompeiisneaks]

Such freaking cool stuff! thanks, my 'theory' was right, but for the wrong part of the component. I was guessing light was the factor, but I also never knew that neon bulbs are light sensitive, so as it went bad it started being more sensitive to the light I guess?

Nice find!

~Phil

[Reeltarded]

Neon lamps are photo sensitive. Aparrently the room light is putting the strike voltage just out of reach.

Third base.

[Charlie Wilson]

Well new Ne2 bulb and I'm back in business. Thanks Sluckey.
CW