Fried Fender Bassman 100t

JJJr

Bought a Bassman 100t on second hand, played it 3 times then it started blowing fuses, after a month it had a small electrical fire inside taking part of the mains pcb board with it.

Ive spoke to fender, they dont make the pcb boards anymore, my amp guy says he doesnt think he can do anything.

In a situation like this is there anything that can be done? I dont want to bin a £1000 amp but it seems unlikely it can be fixed, with the rest of the amp intact any recommendations on what to do with it? Repurpose it maybe?

RiftAmps

Do you have a pic of the damage? It's probably not worth attempting a repair but a handwired conversion is possible however that'll cost you somewhere in the region of a replacement.

ICBM

Dear god… that’s ridiculously complex for a valve bass amp.

Do you have any pics of the damage?

JJJr

Thanks pal, yeah its the mains pcb board, Fender are checking if any of their parts are compatible but likely not, photo below. Cant believe im gonna have to bin a £1000 amp because of a little £20 plastic Fender doesnt make anymore.

https://i.postimg.cc/9F1t6w11/IMG-1503.jpg

ecc83

If we assume the fire was on the power output PCB and the pre amp sections are still ok then that is the sort of "project" I would have taken on 20 years ago.

Stripped out the valve power section and fitted a 300W class D amp.

I fixed a couple of transistor amps years ago by fitting a Maplin "150W" MOSFET power amp kit. Still have one I use as a bas amp.

But as Mr Rift says, hell of a lot of work and for a tech 'in business' not viable.

Shame.

Dave.

ICBM

JJJr said:

Thanks pal, yeah its the mains pcb board, Fender are checking if any of their parts are compatible but likely not, photo below. Cant believe im gonna have to bin a £1000 amp because of a little £20 plastic Fender doesnt make anymore.

https://i.postimg.cc/9F1t6w11/IMG-1503.jpg

That’s nasty, but probably repairable. The first thing to do is clean off all the soot and remove the burned components - it’s possibly just that big resistor (which should not have been mounted tight to the board like that), but if the ones nearby are too contamined with carbon they might need to be replaced as well. With the schematic it should be possible to work out what goes where and whether any broken traces need to be bridged. It may be necessary to move some of it to a separate board, which you could do with stripboard mounted above the PCB. It’s probably many hours of faffing around, but it should be feasible and definitely economical given the value of the amp. You just need to find a tech willing to invest the time…

I have to say I wouldn’t want to own it even if it’s been successfully repaired though - it just looks like another similar disaster waiting to happen somewhere else.

ICBM

Comparing the two pics it looks like that resistor has actually been replaced once already, and the tech used a larger/higher-rated one, to try to stop it happening again - given the level of destruction I think that was optimistic though. The question is why it’s being overloaded like that…

It’s interesting that there are two regulators, but only one is fitted with a heatsink, despite both having the outline for one printed on the board. It could be that there’s a conflict with R280, but if so I would cut the corner of the heatsink away rather than not fit it at all. I would definitely check that regulator, it looks more blistered than I would expect just from the soot from the resistor.

JJJr

ICBM said:

Comparing the two pics it looks like that resistor has actually been replaced once already, and the tech used a larger/higher-rated one, to try to stop it happening again - given the level of destruction I think that was optimistic though. The question is why it’s being overloaded like that…

It’s interesting that there are two regulators, but only one is fitted with a heatsink, despite both having the outline for one printed on the board. It could be that there’s a conflict with R280, but if so I would cut the corner of the heatsink away rather than not fit it at all. I would definitely check that regulator, it looks more blistered than I would expect just from the soot from the resistor.

Thanks for this, gives me some hope, Fender are seeing if any of their current boards are compatible but im not holding out. Im almost certain the issue was caused by the layout or a faulty issue with the amp over anything a tech has done in the past, he mentioned something about a faulty digital bias circuit but not sure if it was in reference to this amp or another Bassman. 

It worked fine for a week or so then just started popping fuses, then finally caught fire, nothing on the interior was messed with since I bought it.

Habanero

JJJr said:

ICBM said:

Comparing the two pics it looks like that resistor has actually been replaced once already, and the tech used a larger/higher-rated one, to try to stop it happening again - given the level of destruction I think that was optimistic though. The question is why it’s being overloaded like that…

It’s interesting that there are two regulators, but only one is fitted with a heatsink, despite both having the outline for one printed on the board. It could be that there’s a conflict with R280, but if so I would cut the corner of the heatsink away rather than not fit it at all. I would definitely check that regulator, it looks more blistered than I would expect just from the soot from the resistor.

Thanks for this, gives me some hope, Fender are seeing if any of their current boards are compatible but im not holding out. Im almost certain the issue was caused by the layout or a faulty issue with the amp over anything a tech has done in the past, he mentioned something about a faulty digital bias circuit but not sure if it was in reference to this amp or another Bassman. 

It worked fine for a week or so then just started popping fuses, then finally caught fire, nothing on the interior was messed with since I bought it.

Given the warning on the PCB, I wonder if someone had replaced a fuse with the wrong value before you got it.

ICBM

Given how quickly it died, I do suspect it might have been repaired and then immediately sold by the previous owner - which doesn’t mean he deliberately ripped you off, he may have been confident in the repair… just unfortunately wrong.

randella

That is one complicated device. From those interior shots Rift posted, it looks more like a modeller than a tube bass amp.

I hope you get it mended somehow, seems a crying shame if it dies.

ICBM

randella said:

That is one complicated device. From those interior shots Rift posted, it looks more like a modeller than a tube bass amp.

I hope you get it mended somehow, seems a crying shame if it dies.

Likewise, although to me it looks like a cautionary tale about making things far more complex than they need to be, and possibly about mixing valve-amp voltages and densely-packed PCBs. PCB itself is not the problem, it’s perfectly possible to build extremely reliable amps, even valve ones, that way - but I would not mount that size of resistor, which can clearly receive many times its rated power when something else fails, tightly to the board like that - it should be on stand-off posts, which will minimise the risk of board damage. The other question is why whatever else failed did - the burned resistor is the result, not the cause.

On the other hand I’m just an (ex) repair tech who cares about total reliability - I didn’t want repaired amps coming back - not a designer working for a large manufacturer where everything is costed down to the minimum that will get the job done.

ecc83

ICBM said:

randella said:

That is one complicated device. From those interior shots Rift posted, it looks more like a modeller than a tube bass amp.

I hope you get it mended somehow, seems a crying shame if it dies.

Likewise, although to me it looks like a cautionary tale about making things far more complex than they need to be, and possibly about mixing valve-amp voltages and densely-packed PCBs. PCB itself is not the problem, it’s perfectly possible to build extremely reliable amps, even valve ones, that way - but I would not mount that size of resistor, which can clearly receive many times its rated power when something else fails, tightly to the board like that - it should be on stand-off posts, which will minimise the risk of board damage. The other question is why whatever else failed did - the burned resistor is the result, not the cause.

On the other hand I’m just an (ex) repair tech who cares about total reliability - I didn’t want repaired amps coming back - not a designer working for a large manufacturer where everything is costed down to the minimum that will get the job done.

The shame of it is IC that these things were learned decades ago. The early PCB CTVs were prone to cooking (Thorn convergence board was notorious in the trade!) but they learned and as you say, stood power resistors off the board, sometimes with ceramic beads, sometimes running the wires down ~1mm brass tubes that acted as both stand offs and heatsinking.

There were no more tightfisted bstd bean counters  than in the rental TV industry but they did do this.

Dave.

ICBM

For what it's worth, this is an original Fender Bassman 100 chassis...



It's not the neatest thing ever made, and a lot of it is definitely built down to cost - eg the carbon-comp screen resistors rather than wirewounds - but there's really not a single bit of circuitry it doesn't need, and things like those resistors aside (and which are easily upgradeable if they do fail) it will be repairable more or less forever, if it even goes wrong at all.

Progress?

This is one of the reasons I recently bought its little brother, a '73 Bassman 50.

ICBM

Looking at it on a bigger screen I've also just spotted that the red cap second above the resistor in the pic - C239 - has also failed, it has a noticeable bulge and crack in it. I can't tell whether it goes to the burned resistor to since the traces are on the underside of the board. Fixing something like this first go so it doesn't end up being a rinse-and-repeat process requires thoroughly checking all the components even remotely connected to the damaged area. Even then it's possible there's a fault you can't see - Marshall JCM2000 boards had a very nasty problem where they sometimes arced through the thickness of the board itself, due to poor design... traces at about 500V apart directly on opposite sides of the board.

Keefy

JJJr said:

Bought a Bassman 100t on second hand, played it 3 times then it started blowing fuses, after a month it had a small electrical fire inside taking part of the mains pcb board with it.

Ive spoke to fender, they dont make the pcb boards anymore, my amp guy says he doesnt think he can do anything.

In a situation like this is there anything that can be done? I dont want to bin a £1000 amp but it seems unlikely it can be fixed, with the rest of the amp intact any recommendations on what to do with it? Repurpose it maybe?

Definitely don’t ‘bin’ it. If you are unable to arrange a repair, there will be plenty of people who could make good use of the valves, transformers and other hardware.

Modulus_Amps

What Keefy said, There are loads of guys that would buy that for the parts and then build a new amp inside.

You could also reach our to some of the UK builders to see if any of them would build something decent for you using the parts. That won't be very cheap though.

JJJr

These are all good ideas, im considering seeing if someone can build something with the internals, the price is scary as though as these run £800 second hand so ..

ICBM

Given that it’s the power supply board which has fried, which is the simplest board in the amp, it *might* be possible to build a hand wired tag strip/stripboard replacement - possibly simplified further if there’s stuff which isn’t that critical and could be left out - although it’s still a lot of work. You would probably need the Fender schematic, although reverse-engineering it isn’t impossible - but time-consuming, unfortunately.

Failing that, a completely hand-wired amp based on the old Bassman circuit, which you could probably add the mid sweep etc to, would be possible, but most likely nearly as expensive as buying a new amp. On the other hand it would probably then be better than a new one of these…

Mojoampguy

That looks very much like the amp I have in for repair at the moment. I'll sort it and get back to you all.

Mojoampguy

BASSMAN 100T FIXED. As I said the pictured amp looks like the one I had in for repair. owner was told "not economical repair". He brought it to me and I've fixed it. I had to replace the following parts in this particular case.

M1, M2 - MosFets
D261 - 16v 5 Watt Zener Diode
D256, D257 - 16v 0.5W Zener Diodes
R283 - 22 ohm 2W resistor
D252 - 1N4006 diode
Mains Fuse F4Amp
Screen Fuse - F100mA

Several tracks were obliterated and needed to be rewired. 

Now its all back together and functioning as it should.  Hope you find this useful. And before you ask, No I don't know what caused it to fail in the first place. All valves are in good working order. Best of luck with yours.