Fender automatic bias

[Wakculloch]

I have a Bassman 100t for repair. Duff 6L6 sorted, but now the auto bias is turning all the valves off. I know they're ok; they were ok yesterday, today all dead.
Has anyone had this before? Is there a way to reset the software? The only thread i could find mentioned known problem on the board, and he then ripped it all out. I've looked at this amp before and it behaved oddly, but has been going strong for a year.

[Jazz P Bass]

Never ran into a Bassman 100t.

Any chance that you could get Fender Support to send you a schematic?

I find it odd that the auto bias would be software driven.

[tedmich]

Holy Moly! U gotta reboot your Bassman! Strange days indeed good luck!

[g1]

How do you know the auto bias is turning the tubes off? What are the auto bias lights showing? Are the screen fuses good?
There is some info on the auto bias in the owners manual, bias reset is press and hold up and down arrows.
Owners manual: http://support.fender.com/manuals/ba...nual_rev-A.pdf

[Wakculloch]

Screens good at about 490V. Grids when turned off at -80V, no signal getting through at all. Two lights red, two orange.
Tried the reset, now only centre two valves off. I suspect tomorrow could be different!
Do you know what the trigger is for the bias setting?
You may laugh but this is the future and unfortunately it's here now. I can see end users thinking this is the end to all those pesky visits to the tech, no more charges for valve changes and re biasing. Ho, ho.
I think i may be in line for a long conversation with Fender tech support

[g1]

Have you tried some other known good tubes?

[Wakculloch]

I am sure they are all good. Just to make certain i shuffled the valves about and the fault lights do not follow the valves. Latest was green -52V, orange -65V, red -79V, green -49V.
Heading in the right direction but for no discernable reason.

[Wakculloch]

Latest on this thing; left it on standby for a few hours whilst doing other stuff, flicked it on, all lights green, bias voltages all around -50V.
The bias is triggered from the cathode resistors, via a micro, controlling a PWM signal modulating the bias supply. It would seem that if the micro becomes confused, the grids see full bias supply of -80V and are effectively shut off. This is guess work as fender have been silent on the subject.
Why it should take two days for it to get it's shit together is beyond me, but it would seem patience is what one needs to work on these amps.

[Jazz P Bass]

I would go in & resolder whatever has anything to with the bias circuit/ controller.

[g1]

These things have been around long enough that it is out of 5 yr. warranty?
Or you are doing a warranty repair and that's all the help you get from Fender?

[52 Bill]

Thankfully, I've only seen one of these amps. My recollection is that the controller chip is a third party unit, sorta like a dsp chip. Fender just buys them and installs them.

The one I did, had a problem with the connector cable not making all contacts.

[Wakculloch]

Yes, all the bias control is done on a double sided smd board so no chance of going over the solder joints.
I shall certainly have a play with the cable and make sure it's making good contact.
Fender warranty is only two years in Scotland.

[R.G.]

I designed an automated uC controlled bias circuit for the Workhorse amps 8-9 years ago. It did much of what is described there. We decided to remove it just before we did final release for manufacturing. The "wait 20 seconds for it to bias" stuff is very real. I found that I didn't like the drift on bias in anything under a minute in testing with real tubes.

We did the red/green light bias knobs with four selectable "heats" then too.

It will be interesting to see what Fender's patent that's pending is on.

[g1]

some info on ribbon cable replacement for Bassman 100T here:
https://music-electronics-forum.com/...l=1#post550819