Blown part in Fender Blues Deluxe Reissue

[titanxt]

I took the back off of my FBDR to see how to get at the speaker and noticed that this electronic piece had some damage. What is it? What does it do? How do I get it repaired? So far, my amp seems to work OK even though it could be causing some issues. I do get a hum now and then that is fixed by tapping the top of the amp.

Also, I have a small piece of the shielding tape on the side of the amp that is missing - about the size of a quarter. Will that cause issues?





Thanks!

[jrfrond]

It's the bottom filter cap in the screen grid supply for the power tubes. It's 100uF/350V. I'd replace both of them (they are right next to each other).

[titanxt]

How hard is that to replace? I assume you have to take the oard out because the part is soldered in. Why did that piece fry?

Side topic, what is it in these amps that can zap you to death? What should I stay away from?

[titanxt]

Will playing through the amp cause any futher damage? I will not get to a repair center until next month and have a couple of very low volume uses planned.

Thanks!

[trem]

Your amp is done 'till the new caps are in. Bad caps that short to ground are not a good thing. How old is your amp? I would consider replacing all the filter caps and biasing the amp while you are in there.
Good Luck

[icefloe01]

Originally Posted by jrfrond

It's the bottom filter cap in the screen grid supply for the power tubes. It's 100uF/350V. I'd replace both of them (they are right next to each other).

In the original Blues Deluxe schematic C41 and C42 are indeed 100uF 350v caps (so are C34 and C35) but this is supposed to be a reissue. I can't find a schematic on the reissue. My issue is that while the callout on the silk for that is C42, it's awfully small for a 350V cap. And the cap next to it is not C41. I wonder how much the circuit and part designations changed form the original to the reissue. The size of those (and location with respect to the pcb layout in the original schematic) seem to be more in line with 100uF 100V parts in the +/- 16VDC supply. Or am I way off base...?

Eric.

[Enzo]

icefloe is right. C42 is a 100uf/100v cap filtering the supply that is dropped down to the 16v zeners. If it blew up, I'd replace also the 1N4006 D12 rectifier that serves it.

[ctcpete]

Originally Posted by titanxt

How hard is that to replace? I assume you have to take the oard out because the part is soldered in. Why did that piece fry?

Side topic, what is it in these amps that can zap you to death? What should I stay away from?

What can zap (KILL) you are the capacitors, especially the larger value ones like the one fried on your amp. If you don't know how to discharge them, don't mess with it

[darrellcheng]

Originally Posted by ctcpete

What can zap (KILL) you are the capacitors, especially the larger value ones like the one fried on your amp. If you don't know how to discharge them, don't mess with it

But if you really want to know how to do this stuff properly and safely and you feel up to the diy by all means ask away

To discharge caps unplug the amp and take an alligator clip connected to the ground (chassis) and take insulated needlenoses and grab the other end of the alligator and discharge all the large caps. Do some reading up on safety tips http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#safety and servicing tube amps, a quick google search will give u loads of info. If you just want the problem fixed then by all means bring it to a shop

[Euthymia]

Originally Posted by titanxt

How hard is that to replace? I assume you have to take the board out because the part is soldered in. Why did that piece fry?

It's not that difficult to replace once the PCB is out of the amp. Getting that PCB out of the amp is a job for a pro or someone with pro-level skills.

The cap probably blew due to over-voltage. Enzo told you to replace the zener; the zener might have opened as well. Might have taken a spike from the wall, or maybe the zener failed and took the cap with it. The cap may also have been defective from the start and just finally blew up.

[jrfrond]

Sorry. I was using an original BD schematic.

In any case, an open cap needs replacement.

I agree that this can be a tedious job for someone who is not familiar with how to squeeze the pot shafts past the chassis lip on new Fender amps. You might want to enlist the help of a knowledgeable repair person. Also, there is a good chance that the amp is still under warranty if it is, in fact, a BDRI.

BTW- Electrolytic cap sizes are shrinking everyday. Size is not necessarily a good indicator of capacity and/or rating.

[titanxt]

Thank you for all of the information. I have contacted a local tech due to knowing nothing about this stuff. Would the amp still play even with the blown capacitor? The reason why I ask is because it never stopped working. I found the problem when I opened the back look at the speaker...

[Enzo]

ACtually I suggested replacing the rectifier that serves that cap, not the zener. If the zener downstream shorted, it wouldn;t hurt the cap. But if the rectifier were leaky or ssufferend from the bad cap, it might turn up as a problem later.

[icefloe01]

Originally Posted by titanxt

Would the amp still play even with the blown capacitor? The reason why I ask is because it never stopped working. I found the problem when I opened the back look at the speaker...

Yes. It's a filter on one of the power supply lines for the op amps. So it would still work but you would have increased hum and the possibility of it (or the rectifier feeding it that Enzo mentioned) taking something else out down the line.

Eric

[merlot]

Newby, here, and new owner of a Blues Deluxe. I'm very interested in this thread because I'll probably be experiencing similar problems (although I love the sound of this amp!) I downloaded the service manual from the above link so that I can replace the output tubes and adjust the bias when needed (I can't leave anything alone). I cannot find TP47 on any of the schematics to check the voltage. I've traced from the pot (R82) and still cannot find TP47.
Any help greatly appreciated.
Bob

[R.G.]

The low voltage supply for +/-15V in the Blues Deluxe and also the Hot Rod Deluxe is a disaster waiting to happen. It is extremely common for these amps to fail in either the zeners, the resistors feeding the zeners, the filter caps before/after the zeners, and/or the rectifiers feeding the whole mess.

Perhaps the most common failure is that the resistors feeding the zeners get so hot that they either melt the solder off their leads and unsolder themselves on the spot. This can happen because of an overheated and shorted zener, or any overload on the circuits they feed. This is probably also the worst failure, because the resistors can also char the PCB material under them so that the PCB is permanently damaged in that spot.

The best thing to do is to substitute a pair of three-terminal regulators for the resistor/zener combination. This permanently fixes the problems by converting it into a current- and thermally-limited regulated supply. This is, unfortunately, a job for the professional repair tech.

I can only guess that the Fender designer or, more likely, the MBA in charge of making money on that design, went with saving $0.20 on parts by going to a less-reliable design. The reasoning goes like this "if we put in the cheaper parts, we save the parts cost on every amp. Out of that money, we have to pay for the few amps that fail within the warranty period. If the number that fail in the warranty period is lower than the parts savings, then I get my next performance bonus. And I'll probably be moved to some other area before the amps start to faily in the field and give us a bad name ... kewl..."

[merlot]

Wow! If I'm calculating correctly, they pushed it to the limit with those resistors. At 48 volts and 470 ohms, that's a power rating of 4.9 watts--way too close for comfort on a 5w 10% resistor. I can see one mod I'll be doing very soon. I imagine it's that close with the zeners, too.

[jrfrond]

Originally Posted by merlot

Wow! If I'm calculating correctly, they pushed it to the limit with those resistors.

Pushing to the limit is not exactly a new concept in guitar amps!

[newsong]

I have a '96 Blues Deluxe and boy have had problems with those 470ohm resistors. I've resoldered them twice and now the board is charred with no sound. I tossed the sandblock 470's and decided to try mounting some aluminum housed 500 ohm Dale resistors off the pcb and mounted to the steel chassis behind the board, wired in with 20 ga teflon silver wire, but did not replace the zeners.
When I fired it up there was a real bad hum, although I now have sound.
do you think if I replace the zeners, It will take care of this?

[melvin]

Originally Posted by jrfrond

Sorry. I was using an original BD schematic.

I agree that this can be a tedious job for someone who is not familiar with how to squeeze the pot shafts past the chassis lip on new Fender amps.

Is there a better way than taking the tube pcb out to make room for the squeezing bit? I really hate that whole process... Ampeg had it right when they had a removable window on the other side...

[darrellcheng]

Yea, I've managed to squeeze the board out by unscrewing the control panel stuff, and then managing to flex the ribbon cables so that i can get the board out without undoing all the wires.

[Enzo]

The ribbons are not what hangs me up, I always have to clip some tie wraps to free the wires to the transformer.

[Gtr_tech]

Take some voltage readings and find out. If one/both of the 15v rails are off then you have other problems.

[michael peck]

my hot rod deville had same prob, 10 pin wire straps rattled loose next to solder, after about 5 yrs of use, i over heated board fixing it and amp works great now but has nasty hum, ok for loud gigs but not in rec studio

[newsong]

Just an update: I changed the Zeners, which I don't think were bad. What was causing the hum was an open in my #1 input jack. Once that was retouched, everything seems to work fine and those outboard dropping resistors should remedy the cold solderjoints on the board. Thanks for all the help.