"The Twin"-Reverb problem

[Teleman55]

I have a 1994-The Twin,after the amp warms up for ten minutes or so the reverb cuts out,but the amp still works otherwise.When it first came to me it would cut out when you banged on the amp,I found a loose connection at the input jack and thought all was well.I have tried replacing tubes,and searched the board with a magnifiying glass for bad joints,I checked the cables and tank the first time around.It must be something heat related because it wont happen if I have the chassis out of the cab unless I place a board over the open chassis-Help!

[Mark Black]

Those amps are particularly troublesome in terms of cracked solders and even cracked traces. They also don't put up with too many movements of the PCB's before the interconnecting wires start to break.

If it is a heat-related problem your best friends might be a heat gun (or a hair dryer) and some freeze spray. Use the heat gun (carefully) to get the failure to happen with the chassis opened up and then start nailing things one by one in the reverb circuit with the freeze spray to find which component is the troublemaker.

If you don't have any freeze spray an "air duster" can might do in a pinch. I haven't tried for this application but have noticed if you hold them upside down you get something similar to a freeze spray effect.

Have you tried poking/prodding/tapping on components with a chopstick or similar non-conductive tool? That might reveal if there is a not-too-obvious solder crack or broken (but barely touching) component lead.

Most of my memories of working on those are not fond ones...

Good luck!

[Enzo]

You can also generally heat the chassis by covering it with a couple layers of towel.

OK, so the reverb quits. Isolate the problem. Is it the drive or the return?

When it is in the failure mode, kick the amp. DO you hear the SPANG of the reverb springs flapping around? If so, the return is working. Of pull the cable from the OUTPUT jack on the pan and touch the tip of the plug with a finger. Loud hum? Then it works. No loud hum? then the return is not working.

Likewise the drive end. Pull the cable from the INPUT jack on the reverb pan and scope it for signal. Or set your meter to AC volts and see what comes out in the way of signal. Or connect the cable to another amp to listen to what is there, or connect it to a small speaker even.

Once you know which half of the reverb is bad, it will be easier to track down the failure.

[Teleman55]

Enzo-Always good to hear from you.I covered the chassis with a board and let it get good and hot ( set on High power).I whacked it a couple of times and could not induce the problem.When I removed the board the reverb cut out.The only items in contact with the board were the ribbon cables.I isolated which one and determined it was the multipin connector on the main board-Ugh-Resoldered those and a couple other suspect looking solder joints and all set.I find a lot of newer amps suffer from weak solder joints.If the heat don't get them sudden impact will.Thanks for the help, I was getting frustrated and had to walk away from it for a while,it cuts down on the tendancy to over think things.-Kevin

[Teleman55]

Hi Mark-Thanks for the input,some really good tips I will have to file for further use.Yeah I agree that these amps are a pain.When I know I have to pull one of the boards I get the willies,but after having done a few I know its not as bad as I make it out to be. If I pay attention to what leads bind things up,cut enough wire ties and determine the proper angle of removal it goes ok.It does pay to be stubborn and Obsessive.