Fender Hot Rod Deluxe : instability when in More Drive mode

[stokes]

Problem is, many time only those crap IC caps will fit on the board. Spragues won't fit and F&T probably won't either. Stupid Fender.....

F&T's fit okay.Only problem is you have to cut the old caps out and leave the leads connected to the board and then twist the leads of the new cap to the leads on the board,as removing the board to get to the underside requires removing too many leads.Changing the bias caps on these amps requires removing the entire board,as these caps are not axial leads.

[g1]

Well putting back the cover wasn't necessary to stop the oscillations....
The audio ground does connect to the chassis, and thus to the earth when present. How this has an influence on the oscillations... I don't know

So you tried it in the ungrounded outlet with the back cover on and it still had the oscillation?

[ricothetroll]

I didn't try it again since those tests, as I've ordered F&T caps to replace those crappy IC, I left the amp open. But the first test I did was with the back cover, and there was oscillations.
It's strange though, I played on a HRD during a few years and in a lot of places, and never had this oscillations problem. Now my HRD is gutted and the original circuit is replaced by a Vox AC15 circuit (only the original OT and the box left)

[stokes]

I've seen it too many amps from Fender.Even in amps that seemed to be okay after moving the ribbon cable,which by the way is a fix described in Fender tech notices,if the caps were changed the ribbon cables position didnt matter anymore.I really think Fender got a bad batch of those IC's.In about a 2 year period I saw about a dozen amps with the same problem.All of them were Hot Rods,Deluxes and Devilles,Blues Jr's and a coupke reissues.One that belonged to a friend seemed to be okay after repositioning the ribbon cable,we changed the caps anyway and the ribbon cable could be in any position and no problem.

[ricothetroll]

I'll measure the IC caps after replacing them, to see how wrong those were.
BTW I'll also replace the 5W/470R +/-16V voltage drop resistors by 10W offboard ones. She should then be fine for years, excepted with the 6L6GC...

[stokes]

I'll measure the IC caps after replacing them, to see how wrong those were.
BTW I'll also replace the 5W/470R +/-16V voltage drop resistors by 10W offboard ones. She should then be fine for years, excepted with the 6L6GC...

Dont expect to see anything "wrong" when measuring the IC's.I'm not an engineer,so I cant explain why the caps fail to properly decouple the stage,but the failure occurs when high voltage is present,and in this case,when the extra gain of the drive channel is applied.I have checked some caps for leakage and at lower voltages the cap showed no leakage,but did show it when working voltage was applied.

[Alex R]

actually the boards do come out ok like this. Have the amp upright on the bench and take off the rear panel. Free the pots and jacks from the control panel and take off the ground wire (green) that screws to the cab at the side. Cut the cable ties on the wires from the transformers etc, take out the board retaining screws, then carefully pull down the board till the pot shafts clear the lip of the panel then hinge the board towards you. You can work on it whilst it is down like that, the cable joints don't seem to crack, prob because there are so many of them. I've done it countless times with no problem.

[ricothetroll]

Hi,
I switched the PSU caps for F&T. BTW those fir the board perfectly as they are only slightly bigger in size as the IC.
Now the oscillation problem is gone, with or without earthing the amp !
You were right, those crap IC caps were the problem....
Thank all of you guys for helping me with this one !
Best regards
Eric

[christarak]

Thank you Jazz P. You suggested this 4 years ago, but you saved me from wasting any more hours chasing my arse around the switching circuit.