Don't let someone do this to your amp...

[icefloe01]

I haven't been here in a while but when I saw this bias pot mod in my friends '93 Fender Blues DeVille, I just had to post it.

To start it all off, apparently whoever did the work was just too lazy to take the boards out and do the job properly.

Cathode of CR19 lifted with resistor placed in series and wire attached to anode to run off to the bias pot.


You can't see it here but R50, R57, and R58 have been replaced with jumpers and this guy floating in the air is the replacement of R51. Notice how thoughtful they were to cover the connection with a big wad of RTV...


Oh yeah, apparently the ribbon cable went bad...


And finally, the bias pot installation! I guess this gives new meaning to the term "solder pot".


It is now my privilege to figure out why the output quit working. The preamp out works at least...

[icefloe01]

Replaced the 6L6's and she's back in working order. At the very least though I'd like to get a ribbon cable and replace that orange wire crap...

[Chuck H]

Wow!?! That's shameful. Hope the traces and/or holes for the ribbon aren't buggered too badly. I would suspect why the old power tubes failed. If there is an intermittant short in that Alpha pot (as often happens to them when they're overheated) the new tubes may be on borrowed time. Along with the ribbon cable I might at least, also, install a cermet pot for the bias adjust... Just to get that badly abused and untrustworthy pot out of a circuit that could cause tube death. And in that amp PT or OT death too. Pretty much the most expensive stuff under that amps hood is relying on that pot to do it's job.

[Raybob]

Thanks for posting this. I needed a good laugh today.

[Mandopicker]

Even as ugly as the modifications appear to be, perhaps there is some logic to the removal of the ribbon cable. The wires being seperated can help to avoid any parasitic oscilations that might result from otherwise being so closely bundled together.

That said, I do agree that the whole operation could have been done more professionally and with the proper parts.

[icefloe01]

Even as ugly as the modifications appear to be, perhaps there is some logic to the removal of the ribbon cable. The wires being seperated can help to avoid any parasitic oscilations that might result from otherwise being so closely bundled together.

That said, I do agree that the whole operation could have been done more professionally and with the proper parts.

I don't think that was an issue with these amps though. The guy who owns the amp now I've known since 1974 so I know how to "read" him pretty well. I asked him if he had the amp worked on and he said he hadn't but the guy he bought it from had it modded. When I told him whoever did it had no business with their hands in this stuff, the way he chuckled told me that the guy he bought it from was probably the one that did the work. My wife happens to know the guy that we think did it. She went to high school with him and said he's a know-it-all d**k that thinks he can do anything. He also happens to be in our Facebook friends. If I was of a more acidic nature, I would consider posting the pics on Facebook with a little diatribe on how I could do a better job with a hammer, chisel, nail, and blowtorch. But, I'm a bit old for all that drama crap.

[jrfrond]

That's fugly work! If one of my techs cranked this out and I found out about it, they'd have to rework it on their dime.

[icefloe01]

I'm considering completely undoing the mod but it'll be a bit of a trick as the less than amateur person that attempted it destroyed a few traces in the process. It'll take a good bit of careful working to make it look presentable again.

[Jazz P Bass]

That is what a competent tech is all about.

[Enzo]

I really doubt the ribbon was replaced for parasitics. That work suggests teh tech didn;t know what a parasitic was, let alone how to cure it. I'd be wagering that by movong the boards enough, he snapped off a wire or two on the ribbon, and replaced it with wires.

[icefloe01]

Got a ribbon cable to finish this out. Yeah I know, it took long enough. I called Fender and the rep went above and beyond to help me. Turns out there happened to be about 80 cables left in their Mexico plant. Problem was since it wasn't set up as a sellable item, there was no way he could get the computer system to issue a shipment on it. So he looked up the old engineering drawings and gave me the specs. I found an 8 conductor 8" for 27 cents at DigiKey.

New ribbon:


Caps and diodes restored:


No more solder pool:


Resistor leads to repair traces:


Overall:

[JoeM]

Nice job fixing up the earlier mess.

[Prairie Dawg]

And you of course will repeat that Digikey part number pretty please??

[icefloe01]

Thank you! One of these days when I have time, I'm going to redraw the schematic. I have two different versions, both of which are very poor scans and both have the same error in the B+ standby switching section of the schematic. They both have a board layout page showing where the components are located (this was how I realized I had two different schematics). This particular amp actually had to have been one of the earliest releases as R94 is tacked on the copper side of the PCB because whoever did the layout forgot to include it on the board, the second schematic corrected that. Since I don't have access to a PCB based on the second schematic I don't know if they corrected the layout in regards to the quick-disconnect tabs on the board in the B+ supply. In my friends amp the PCB layout for the tabs does not match the schematic. The schematic shows the red leads on the transformer winding for the B+ supply connecting to CP14 and CP15. CP14 is supposed to be directly connected to CP21 which goes to one side of the standby switch with the other side connecting to CP21 which is supposed to feed the bridge rectifier circuit. That's not how the board is laid out and they didn't realize it until after the boards were made. On the board CP14 connects directly to the bridge rectifier and CP21 and CP22 were placed after the rectifier. Luckily there was a workaround on the board they were able to use. So they replaced CP21 and 22 with a jumper, one side of the winding still went to CP15, the other side was moved from CP14 to CP8 (which is tied directly to CP9 and nothing else), and the standby switch is connected to CP9 and CP14. So at some point I'd like to redraw the schematic to both reflect this correction and make the doggone thing more legible! I'm odd like that.

[icefloe01]

And you of course will repeat that Digikey part number pretty please??

Nope, because I just realized I totally lied. I'd been dealing with DigiKey for some other things and got myself confused. I will however let you know the Mouser part number I ordered: 538-28100-0335

28100-0335 Molex Flat Cable