Bogner alchemist resistors

nunonaos · March 23, 2019, 10:17:51 AM

Hello,

Can anybody help me out
What is this brown thing covering the resistors? Could it be on purpose when assembling the amp or are the resistors starting to fail?
I've mesured them from the bottom of the board and they are ok, on the top of the board i don't get any readings.

I'm looking at these because my bogner alchemist is changing channels on it's own.
I've looked at a schematic and noticed that these resistors are in the path that gives 3.3v, witch is the voltage for the microcontroler that changes the cannels.

bluebunny · March 23, 2019, 12:39:03 PM

Hmmm... Looks more like "lunch" than "failed".

If you can measure the correct resistance from one end of that string to the other, then they're OK. BTW, they're not providing the 3.3V - that'll be the regulator a little further downstream (one of those hairy great three-pin things with metal tabs). Check the voltages on each of those pins when you worked out which one it is. Expect to find 3.3V on one of them.

And welcome.

nunonaos · March 23, 2019, 12:49:35 PM

Quote from: bluebunny on March 23, 2019, 12:39:03 PM
Hmmm... Looks more like "lunch" than "failed". If you can measure the correct resistance from one end of that string to the other, then they're OK. BTW, they're not providing the 3.3V - that'll be the regulator a little further downstream (one of those hairy great three-pin things with metal tabs). Check the voltages on each of those pins when you worked out which one it is. Expect to find 3.3V on one of them.

And welcome.

What do you mean "lunch"?
Yes I can measure the correct resistance but only on the bottom of the board. On the top where that brown thing is it doesn't give any reading (maybe because the solder is covered).

I know the 3.3v comes from the regulator, but if the resistors fail I think that the regulator will not work correctly.

bluebunny · March 23, 2019, 01:07:28 PM

Quote from: nunonaos on March 23, 2019, 12:49:35 PM
What do you mean "lunch"?

Looks like someone tipped beer/jam/curry over it.

Resistors don't leak, so the brown goo is probably not important.

Quote
I know the 3.3v comes from the regulator, but if the resistors fail I think that the regulator will not work correctly.

True, but confirm the regulator voltages. If you have any voltage at all on either of the outside legs, then the resistors are OK.

PRR · March 23, 2019, 04:46:15 PM

Agree with the bunny. I've never seen stuff like that come out of a resistor (I have not seen it all). I have seen that when people eat lunch over a PC keyboard, and even under a piano keyboard. So why not a pedal?

I do not know the specific pedal. If it is quite sealed against food/drink sloppage, then I'm clueless.

EDIT- ah, this is an amp head. Closed wood box. Vents in back but that seems like an awkward way to inject crumbs. But I suppose I have seen some strange accidents in the truck. Stop short, amp tumbles into a case of Cola, cans burst, brown fizz everywhere?? Or: it is Mud Season here, and one slip-step can fling mud funny places...

nunonaos · March 23, 2019, 05:07:42 PM

It's a combo amp.
A can guarantee that it's not something that was dropped on it. That part has lots of components and none of the other has that brown stuff.. only those two resistors... It would be very very strange...

bluebunny · March 23, 2019, 05:47:48 PM

For anyone who wants to take a look, here's the whole service manual (including schematics and board layouts). This power board is page 41. Layouts and artwork are on pages 11, 12, 25 and 26. And here's a big picture of the whole guts: our board is top-left, parallel with the front panel (you can see those regulator tabs peeking out).

Rob Strand · March 23, 2019, 05:52:21 PM

QuoteA can guarantee that it's not something that was dropped on it. That part has lots of components and none of the other has that brown stuff.. only those two resistors... It would be very very strange...

It could be something like hot melt glue or contact adhesive (I'm not speculating why that would there!). Then the resistors got hot and it all melted. The weird thing is I can't see any use of adhesives on the board in other places.

The other thing to notice is *both those* resistors are different so it is unlikely to have come out of the resistor.

My vote goes to the the fact it's a combo and some glue used on the enclosure (eg gluing the tolex) has accidentally spewed out and dropped onto the board.

mth5044 · March 23, 2019, 05:52:50 PM

What's that weird little lead tacked on to the top metal tab of H3 (most obvious in second pic) The regulator north of the left resistor in the last pic looks like it has rust or some brown on it too.

nunonaos · March 23, 2019, 05:57:32 PM

Quote from: mth5044 on March 23, 2019, 05:52:50 PM
What's that weird little lead tacked on to the top metal tab of H3 (most obvious in second pic) The regulator north of the left resistor in the last pic looks like it has rust or some brown on it too.

I forgot to mention that.
The regulator has a metal tab on it's top. It's like some resistor lead got loose and soldered to it...
The regulator also has a little bit of that brown stuff.

Let's not forget that the problem of the amp is that it changes channels automatically and the digital board is powered by 3.3v

Rob Strand · March 23, 2019, 06:19:18 PM

QuoteI forgot to mention that.
The regulator has a metal tab on it's top. It's like some resistor lead got loose and soldered to it...
The regulator also has a little bit of that brown stuff.

It's has to be some glue that's leaked down on the board. The glue there has no purpose.

At first I though the clear blob on the left was silicon. Is it actually clear hot melt?

QuoteLet's not forget that the problem of the amp is that it changes channels automatically and the digital board is powered by 3.3v

Possible a bad solder joint or bad connection on a connector. If you can work out what signals control the selection you could look around those areas, perhaps pull any relevant connectors off and on give them a clean contact.

nunonaos · March 24, 2019, 04:59:32 AM

The blob is hot glue.
Some connectors have it. I even had to remove some to get that board out to measure the resistors.
I'll try to measure the 3.3v but it's hard with the board in place (I can't reach the bottom of the board). But I'll try to measure it in another place. And I'll try to check if it's a problem with a connector

duck_arse · March 24, 2019, 10:06:23 AM

my vote on the brown gunk is "contact cement" - or whatever they call it on their production lines. it has that sticky trail draped over the resistor like I get every time I use it. and it turns brown with heat/age in everything I've ever seen it. I'd guess the resistors are glued for mutual support.

the stick on the regulator tab is odd. in this photo, the reg marked ?80? [maybe 7805 or 9] has the thinnest tab I've ever seen on a TO220 package - a topic that came up recently about bad regulators .....

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Bogner alchemist resistors