Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: halfsmith on June 26, 2020, 12:55:37 pm

Title: "94 Twin" Ribbon Cable causing channels to switch and blow up caps
Post by: halfsmith on June 26, 2020, 12:55:37 pm: Hello,
I'm trying to repair a "94 Twin Amp" that has been stuck on Ch2 since a cross-country move.

Found two obviously blown caps on the board (C47, C48) and I replaced them and the two Zener Diodes next to them (CR12, CR13). BAM! fixed.

Then when I was pressing the Ribbon Cable back into position to reassemble it crackles and switches back to Ch2 and sticks there.
The board flexes a bit when I press down on the Ribbon Cable port and the Channels go back to working (can switch channels and all works)>

My first guess is the Ribbon Cable port (not sure what to call the plug end that's soldered onto the board) has lost its connection due to board flexing... I'm about to flip it over and resolder it.

I haven't been able to find any replacement cables. It's a 9PIN, soldered directly to the front board (where the volume pots are) and plugs into the port on the "middle" board.

Anyone have advice?

much thanks
Title: Re: "94 Twin" Ribbon Cable causing channels to switch and blow up caps
Post by: sluckey on June 26, 2020, 02:51:03 pm: Quote
My first guess is the Ribbon Cable port (not sure what to call the plug end that's soldered onto the board) has lost its connection due to board flexing... I'm about to flip it over and resolder it.
Look closely for a broken trace on the board too. And whenever plugging those ribbon cables into the header, try to get a finger under the board to prevent/minimize board flexing.
Title: Re: "94 Twin" Ribbon Cable causing channels to switch and blow up caps
Post by: halfsmith on June 26, 2020, 05:32:28 pm: Thanks Sluckey!

I actually just flipped the board over and touched a hot iron to all the leads and reseated them, then (as you suggested) supported the board as I attached the Ribbon. Seems to have done the trick!

I noticed the board flexed when I attached the ribbon the first time, and when it was intermittent would work while pressed, so I guessed the contacts may have cracked loose. She's humming again!

Thanks to Hoffman website and all the schematics/parts lists, and this forum.

Take care!
Title: Re: "94 Twin" Ribbon Cable causing channels to switch and blow up caps
Post by: neddyboy on June 27, 2020, 08:33:46 pm: You may want to replace the ribbons entirely with 9 strands of wire. I had the '90s ribbon cables in my Blues Deluxe and they gave me fits!