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]

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]

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]

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]

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]

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]

Where's the testpoint #47?

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