Fender Super Bassman Automatic Bias issues

[nevetslab]

I had dreaded the day that one of these amps would eventually hit the bench with a problem. Not a simple, easy product to set open on a test stand. Granted, the main power amp chassis is on a light-weight T-shaped chassis. Power tube PCB has all of it's parts on the opposite side, while the driver/input stage board, with the power supply components are facing you.

The issue I found was the Automatic Bias display was reading steady...all ORANGE, with one GRN LED, nothing flashing. During power-up, the display is cycling thru all six tubes, whether Standby Sw is engaged or not. When it settled with all but one LED ORG, indicating either multiple tube failure OR no tubes installed, I pulled the tubes, pulled the chassis, checked all the tube circuit components on the power tube PCB, having no service documents...just common sense & experience on the large amps like this. Not having found anything, I put it back together, and tried one pair of tubes at a time, and ended up with ALL GRN LED's....steady, then it would flash, then one set of tubes would adjust, go steady GRN, then flash, the other set would adjust, return to steady, then flash, and the pattern would repeat. Power consumption now was about 250W....typical of biasing with around 25-26mA ea.

I plugged my bass into the Power Amp input jack, and saw while playing, I got steady ALL GRN LED indication.....which did agree with the notes in their owner's manual on the Automatic Bias. I checked with the rental dept to see how much use the amp has been seeing, and found a lot of use. I brought back a fresh matched sextet of KT-88's, just to see if I got a different display reading. Repeated flashing, while All GRN, is one of their readings indicating the tubes are weak, needing replacement. I wasn't so sure. After installing the new tubes, pressed the bias buttons to invoke Factory Bias Setting, after it had gone thru it's routine, I got the same flashing GRN & cycling bias adjustment behavior as before, so it doesn't appear to be tube issues. The phone rang, interrupting progress. I left it turned on, out of Standby, disconnected the input. When I got back, It was flashing all GRN, power consumption now 150W....typical of NO BIAS....just heater. Switching in and out of S?B made no difference. Powering Down, letting it sit a while, powering back up never restored operation. I put the 6550's back in, same marked order, still no bias.

Pulled the chassis back out, removed the tubes, and extracted the small CPU-based bias board for inspection. I didn't see anything obvious, as far as faulty soldering joints on either side. Checked all the semi's, got nominal readings on the NE5532 op amps matching stand-alone thru-hole parts of the same type. I plugged the board back in. Getting the 3-cond multiplexed display cable plugged back in was a 10 min ordeal, it not being user-friendly to re-connect with no service loop.

After re-connecting, all I've ever seen since then is solid ORG on the display, after it cycled thru the 6 tubes, all GRN. Didn't matter if tubes were installed or not. no replacement ribbon cable on hand, nor any replacement bias board assy on hand to see if that is the defective circuit. the 3 HT fuses are ok. So, for the time being, another boat anchor occupying floor space that looks nice....but certainly not service friendly.

I haven't yet gutted the case, cleared a lot of bench space in order to place the two free-standing power & output XFMR's, chassis, and power amp chassis, positioned somehow to prevent damage as well as harm to myself while probing. I'd rather work on SVT-CL's or VR's than this thing.

I searched thru all the forum threads, and only found documentation on the automatic bias circuit in a 7-page set of schematics for a Super Sonic 100, using the same PCB....though I can't say for sure the stuffing is identical (disregarding this application is for 6 tubes, not 4 tubes).

Does anyone have maintenance experience on this Super Bassman amp, with regards to the Automatic Bias board? I only found a little dialog about it, but without any conclusions or solutions.

I've attached the schematics for the Super Sonic 100, which has documentation on this automatic bias board, and similar circuitry for the power tube stage for clues to how it's all hooked in.

Super-Sonic_100_2011_schematic_Rev-B.pdf

[Jazz P Bass]

So how do you know that U1 is not the problem?

Without a working board to swap I guess you can't tell.

F###ing computers in a tube amp.

Aargh.

[nevetslab]

As luck would have it, the other Super Bassman in our inventory is out on rental. Waiting for a reply from Fender on obtaining a replacement bias PCB assy. I'd have pulled the other one apart, and swapped the boards if it was on hand. Could be the ribbon cable, but also, without another one, can't tell either. Reseating multiple times at both ends didn't change the outcome.

[Jazz P Bass]

How hard would it be to bypass the circuit & make it 'normal'?

[nevetslab]

Interesting question, considering how the packaging is done on this amp. Cutting the bias module loose and patching the power amp stage's grids back together so there's a pair of manual bias pots....probably not that difficult. You'd have to put the bias pots, and sense points on the rear panel, or it would drive anyone nuts. Mechanically, this isn't powered-up-servicable amp in the normal sense.

I haven't seen the rest of their circuits on this amp, with regards to how they control Standby. We don't have any of the Auto-biased Super Sonic 100's here, to see if that is done in a similar fashion. At least that amp is built in the traditional fashion, that can sit in a service cradle with most of what you need accessible. Getting user control over Standby again would be one of the keys to eliminating the Auto-Bias scheme.

As I've not read about any major complaints on this amp in our forum nor elsewhere......where I've looked so far, I'm not sure the time investment would pay for itself. But to have some cute clever CPU module disable your amp for reasons yet unknown, I'd be furious if this was my bread-and-butter amp.

Some photos of the 'main' power amp chassis. Transformers are mounted to the wooden case floor, plug into the T-shaped chassis, as does the overhead preamp...connects to the front end of the 'T'

[Jazz P Bass]

Holy Moly.

I think Leo just did a flip.

Makes you wonder about this new batch of engineers.
Vo-Tech Night School grads maybe?

[52 Bill]

Thankfully I haven't had one of these cross my bench. I have fixed two of the SuperSonic auto bias units. Both were bad/broken solder on the ribbon cable connectors. I think that board is not the same as the one shown here.

I have assumed that the reason these new amps are more and more difficult to repair, is because the people designing them do not have to fix them. The circuit is drawn up on a computer, and then the computer lays out the circuit board to fit the space that another computer has specified so that the chassis can be made to fit into a case that another computer has specified to fit into a cardboard box that another computer has specified to fit onto a pallet that another computer has specified to fit into the back of a semi-trailer that another computer...

[Jazz P Bass]

I don't know why but that post reminded me of Lee Iacocca when Chrysler was on there knees.

In looking at transportation costs, it was noticed that the charges where not similar.

"Find out why", Lee commanded.

It was the fact that when shipped by rail, sometimes two, sometimes three autos could be placed on the rail car.

So Lee countered with the following command: "Make our cars that three will always fit".

For better or worse (The K car: yuck) the transportation savings helped in bringing Chrysler out of the hole.

[nevetslab]

Not the Auto-Bias board, but stinkin tiny ribbon cable!

Had to go into waiting mode to see if a new Auto-Bias board would cure the problem. It didn't.....same issues. At that point, I fumbled about inside to extract the ribbon cable from the power tube PCB. After ringing it out, I had less than 20 connections, so...having chopped and re-terminated ribbons before (larger ones on 0.10" centers), I figured I'd try the same. Not a chance. When those connectors get to be 2mm centers, the contact material is a one-time crimp. And, couldn't find that size raw parts to make some cables, so....kicked myself for not ordering some ribbons along with the Auto-bias board. Had to place an order for new ribbons from Fender.

The replacement ribbon cables arrived today. Finger-crossing time. I first rang it out, and all 20 connections were intact. Ah...a chance. Installed it...somehow, that wasn't difficult getting it back into the bottom side of the tube board this time.

Powered it back up, having left the hardware off. It went thru it's usual scanning routine, then began slowly flashing all GRN....that had promise. One half of the GRN LED's began flashing, as they do when its' adjusting bias. AC Power Analyzer was also showing 250W Mains consumption, a VERY good sign, as it was sitting at 140W when it wasn't running. It stopped flashing, remained stable, all GRN LED's lit, without going back thru scanning and cycling, as it did when it first came to me.

I let it idle for a while, before picking up the bass and plugging into it. Remained stable. Re-adjusted the bias setting range, lowering it, then raising it, and it re-adjusted and set itself up accordingly, with seeing accompanying change in AC Mains power draw at each setting, so this is back to normal.

Plugged in, hammered away on the strings for a bit, and not a whimper with the Auto-Bias circuit....remained solid and stable. Finally!!

What everybody needs....a computer in their bass amp, using the smallest IDC ribbon cable one can find!...that when it flakes out, you're dead in the water.

[johnk_10]

Had to place an order for new ribbons from Fender.

The replacement ribbon cables arrived today. Finger-crossing time. I first rang it out, and all 20 connections were intact. Ah...a chance. Installed it...somehow, that wasn't difficult getting it back into the bottom side of the tube board this time.

Powered it back up, having left the hardware off. It went thru it's usual scanning routine, then began slowly flashing all GRN....that had promise. One half of the GRN LED's began flashing, as they do when its' adjusting bias. AC Power Analyzer was also showing 250W Mains consumption, a VERY good sign, as it was sitting at 140W when it wasn't running. It stopped flashing, remained stable, all GRN LED's lit, without going back thru scanning and cycling, as it did when it first came to me.

I let it idle for a while, before picking up the bass and plugging into it. Remained stable. Re-adjusted the bias setting range, lowering it, then raising it, and it re-adjusted and set itself up accordingly, with seeing accompanying change in AC Mains power draw at each setting, so this is back to normal.

Plugged in, hammered away on the strings for a bit, and not a whimper with the Auto-Bias circuit....remained solid and stable. Finally!!

What everybody needs....a computer in their bass amp, using the smallest IDC ribbon cable one can find!...that when it flakes out, you're dead in the water.

glad to here that you got it sorted! do you have a part number for that ribbon? i have a SB300 that i'm working on that has its auto bias acting up, and i've tested just about everything else already.

[nevetslab]

Ah..........ask a question like that 3 years later. Let's see if I have that info in my service notes from that repair. I'm at home, and I'm missing the service notes from 6/8-6/12/15 when I began the repairs on the amp in this thread. I do have the Fender P/N on the replacement Auto-bias PCB assy 0081737000. I'm sure I would have made note of the Ribbon cable, since that was a separate item I ordered, with it coming in on 7/8/15. I'll check on that missing service note file when I get into the shop tomorrow AM

[johnk_10]

that would be awesome. thanks so much! i really appreciate it.
i saw that this was a 3 year old thread, but i figured that it was worth a try.

[nevetslab]

I found my notes from the following week. I see I had gone thru what you probably also found....the pitch size of the ribbon cable and the IDC connectors didn't seem to be stocked by anyone...nothing in that size was even listed, when I was making a search for them. There is a Fender Artist Relations office here on our studio lot, and my notes indicated I took the bad cable over to our guitar dept manager and instructed him to go over to Fender and have them order the replacement ribbon cable. He got the part ordered, and I'm sure it came in one of the Fender wall-hook bags with their cardboard ID cover, which would have had the part number. For some reason, I never thought to copy the P/N into my records so I'd have it for the next time I ran into that problem.

So, you'll have to contact Fender on that one. Sorry.

Steven

[johnk_10]

okay. thanks for looking into it anyway.

[johnk_10]

i got the part number for the 20 pin ribbon cable (008143600) from Fender customer service but they said that i have to purchase it from an authorized service center. i called a bunch of them and the ones that could get it said that they have to install it and charge me their bench fees and the others said that they can't even find it and order me one. i was wondering if this one on ebay would be the same part (or at least fit) because it sure looks like it:
https://www.ebay.com/itm/5Pcs-20-Pin...91866#shpCntId