Fender Rumble 100 - faulty Power Supply Stage.

[Noodle]

Hi.

I have a second-user Rumble 100 in immaculate condition. It recently went 'pop', and a cursory look inside shows that one of the small snubber caps (C356-C359) has blown, a not-uncommon occurrence in these amps according to various reports on Google. This seemingly indicates a larger problem, which is beyond my abilities to investigate.

My options are to sell it on as 'spares or repair' to someone with the necessary skills to fix it, get it repaired professionally - at an unknown cost, but likely to be close to, or exceed, the amp's value - or to try and effect an 'alternative' repair.

For 'alternative', I was wondering how daft it would be to consider installing a suitable third-party switching supply as a separate unit inside the amp case, and tying its output to the two large smoothing caps C360 & C350 in order to supply the boards? (The existing PSU components would be electrically isolated).

Is this nuts? Looking at the schematic, it appears straight-forward... Page 5: https://www.fmicassets.com/Damroot/Original/10002/Rumble 100 V3 Schematics-Diagrams.pdf

Thanks for any thoughts

 

[Nigel Goodwin]

Assuming it's one of the four capacitors C356-C359 that's failed? (I wouldn't call them snubbers), they are just to help protect the bridge rectifier from possible mains spikes, then it shouldn't cause any damage elsewhere.

If it is one of those?, then just remove it, and the amp should work fine - they have no effect on the working of the amplifier, and often aren't fitted anyway. But if removing it cures the fault, you should replace it.

I would consider 'snubbers' as C351 and C361 - these are high voltage, high frequency, capacitors, usually small blue discs, and VERY prone to failure. Mostly easily identified by obvious splitting, burn marks, or having exploded. I used to keep as wide a range of possible of suitable replacement capacitors, because they are quite difficult to source, so I stocked every value I could get (but still only in single figures).

IF (and it's a big IF!!) you're lucky, then just replacing the capacitor (and mains fuse) might cure it - but often it takes the SMPSU with it, and requires a fair number of components replacing, such as Q302, U303 etc, etc, etc.

But you should check Q302, and see if that's blown - and double check which capacitor you think has failed - and perhaps post a picture of that part of the board?.

Edit:

Just had a quick google, and found pictures of the affected capacitors across the bridge rectifier - and that they are surface mount ones blown on the bottom of the board - not surprising for tiny SM capacitors, stupid idea using those.

I would expect removing the capacitors (all four of them) and replacing the fuse will cure the amp - unless you've been really unlucky?. If that cures it, I would suggest replacing them with decent sized through-hole capacitors, rated for such use (you could solder all four across the bridge rectifier pins on the bottom of the board).

 

[Noodle]

Many thanks, Nigel, for such a comprehensive reply.
Yes, it's the wee surface-mounted caps, and they were described as 'snubbers' by one of the repair agents I contacted. They added that, one of these blowing shouldn't stop the rest of the PSU from working - they are mostly interference-suppresion - but my amp is completely 'dead', so I guess there are more issues.
Here's the fellow:

https://imgur.com/ZTDGWbY

I'd be happy to replace Q302, but would baulk at U303. I'll also visually check out C351 and C361.
No fuse has blown - the board is receiving mains power.
Thanks!

 

[Nigel Goodwin]

Remove the four little capacitors, clean the board, and check and see if there's been any damage - it could have a track, if the fuse hasn't gone.

Assuming you have a multimeter?, and are OK working on live mains circuits?, then power it up and check for high DC voltage across the large capacitors - from positive of C360, to negative of C350 - it should be about 340V DC. If not, then there's very little it can be - it's got to be to the left of C350/C360 on the schematic - if the fuse hasn't blown, and a PCB track hasn't blown, then TH302 is about the only possibility.

You could check with a multimeter on low ohms, from either side of the mains plug, to the two centre pins on the bridge rectifier in your picture above, live should go to one, and neutral to the other.

 

[Noodle]

Thanks again for such concise info.
Yes, I have a decent digi meter, and know how to be safe on 'live' circuits.
I'll report back - it may be a few days before I have the chance, tho'.

 

[rjenkinsgb]

my amp is completely 'dead', so I guess there are more issues.

I would not be surprised if the bridge rec has failed; it looks as if there was an arc between an AC pin and the + out.
That would effectively short some diodes across the supply.

 

[Nigel Goodwin]

I would not be surprised if the bridge rec has failed; it looks as if there was an arc between an AC pin and the + out.
That would effectively short some diodes across the supply.

Possibly, though bridge rectifiers are usually pretty tough - but in either case, it's a trivial fault to locate.

 

[ZipZapOuch]

I think the vapors of the capacitor explosion very likely caused an arc across the Bridge diode terminals. There is too much "spatter" in the bridge just to be fall-out from the capacitor. I'd say it is likely blown or a good first place to look if the unit is dead.

That bridge is easily removed if you cut the four leads first, then remove the pins one at a time (suck the solder away with some solder wick to release each pin). A replacement is only about 1£ . Good luck.

I would not be surprised if the bridge rec has failed; it looks as if there was an arc between an AC pin and the + out.
That would effectively short some diodes across the supply.

 

[Noodle]

Thanks all.

I'm certainly happy to replace the rectifier - as you say, it's one of the easier tasks!

Looking at the PCB, I'd not baulk at tackling Q302 and U303 - I really don't want to get to the stage where I mess things up for the next (more skilled) person.

So, the rect and the cap - and we'll see

 

[Nigel Goodwin]

Thanks all.

I'm certainly happy to replace the rectifier - as you say, it's one of the easier tasks!

Looking at the PCB, I'd not baulk at tackling Q302 and U303 - I really don't want to get to the stage where I mess things up for the next (more skilled) person.

So, the rect and the cap - and we'll see

Just remove the four little capacitors, clean the board (with iso-propol alcohol and a tooth brush), then simply check the bridge rectifier in-circuit, see if it reads short between any of the legs.

IF the SMPSU has self-destructed (which is unlikely) then changing Q302 and U303 might not be all that's required, and you could cause more trouble if you replaced them and powered it up again. I used to make up 'repair kits' for popular supplies, and if it had gone 'bang' I simply replaced all items in the kit - I had probably six different versions.

But as I said, check the bridge, and check the connections from mains plug to bridge.