Fender Blues Deluxe Reissue bias doesn't match schematic, has intermittent jump in bias voltage

[MichaelNuzum]

Hi there-

I have a Blues Deluxe Reissue from 2017 that has a strange problem in the negative bias circuit. I searched the forums and found a thread from 2012 where someone seemed to have a similar problem, but their's was constant not intermittent, and it wasn't totally clear if they resolved the issue. (https://music-electronics-forum.com/...es+deluxe+bias) Other than that I can find little-to-no info about this anywhere on the web.

My problem is much like the one in the above referenced thread. I too see around -105V at pin 5 of the power tubes, but only intermittently. Always at start up, never cuts out or changes if it successfully turns on.

I also have a negative bias circuit that does not match the most recent available schematic (2014, REV A, page 2 (https://www.thetubestore.com/lib/the...atic-Rev-A.pdf). I have contacted Fender in an attempt to see if they have a "REV B" or some circuit update, and the customer service rep told me he didn't have access to anything newer than REV A.)

Specifically, the negative bias supply has C37 and D10, which should not be in the Deluxe. Also, some values are changed. C37 and C43 are 47uf/160v (should be 100uf/100v), R76 is 82K (1.5K in Deluxe, 10k in Deville), and R77 is 68K (should be 100K). R83 is not installed.

It has the proper power transformer for the Deluxe with the two brown wires, no center tap.

So, that finally brings me to my questions:

1) does anyone know anything about this circuit change? Is this the new normal? If so, is there any sort of schematic/docs regarding it?

2) anyone have any idea why this newer circuit might fail, permanently like the above referenced thread, or intermittently like the one I am looking at?

3) anyone have any idea why this circuit might be better than the one on the schematic? And,

4) is there any reason not to put it back to the wiring on the schematic? It seems to double, then divide the voltage. Any reason to just not double it?


Thanks in advance for any insight anyone is able to offer.

[The Dude]

...... I too see around -105V at pin 5 of the power tubes, but only intermittently.......

First, -105 is way to much negative voltage. Second, what does that statement mean? -105 is intermittent, then it settles to what? Or, bias is completely lost? Or, other? Can you better explain the intermittent part? What happens to bias? The fact that something is intermittent makes me think connection or solder joint problems.

[g1]

Don't rewire it to match a different schematic.
Can you post some pictures of the board, including the bias circuit area and any board number markings?

[MichaelNuzum]

Yep, -105 is way too much, but it is what is generated at the junction of D15 and C43. Then dropped through R76 down towards the more normal range of -55-ish. It does that in the DeVille bias supply, but isn't supposed to for the Deluxe according to the schematic.

The intermittent refers to, let's say every fourth or fifth time it is turned on, the power tubes see the full -105 at pin 5, so they never conduct, no sound. Turn it off. Next time, turns on just fine, sees around -45-ish at pin 5 like it should.

Bias is never lost, it just is too much negative bias voltage so no current at bias probe point.

I too suspected solder, but ran it several times and probed with chopstick on all components in the negative bias section, and never saw the voltage waver from physical manipulation.

[MichaelNuzum]

g1- I don't have a good camera with me tonight. I will post pics tomorrow. From what I can tell, everything looks stock, original soldering, etc.

[The Dude]

I'm wondering if you're losing circuit ground. Check ground connections. What does the filament winding AC do? Is it staying constant across P15 and P16?

[Enzo]

I would suspect that whatever configuration you have, aside from the glitch, the amp works fine, the bias is fine. SO the thing to do is repair the amp, not redesign it or modify it to some other circuit.

Most manufacturers state that circuits can be changed at any time without notice.

I doubt there is a problem specifically with the bias circuit, more likely connections. If the voltage leaps to -100, then it sounds to me like the voltage divider is losing its ground connection.

[DFuller]

I would agree, it sounds to me like you're losing ground reference somewhere.

[g1]

Sounds like this unit matches the Hot Rod, not the Blues.
Here is Rev.B which shows the items you mentioned:

[Enzo]

DeVille/Deluze series - hot rod and blues - developed over time. You could follow them as they worked to consolidate the models. Using the same board for several models, just install different parts into them, or like the schematic, wire jumpers for one model but not the other etc.

[MichaelNuzum]

Well, you guys are right, it seems to be an intermittent loss of ground!

I pulled the board and was horrified by the condition of some of the solder joints. I have found several questionable joints around the bias circuit, the bias pot and R77, specifically. I removed the crappy looking solder with chip-quick and solder braid, then laid in fresh clean solder. Before, when it would see excessive negative voltage, I could trace voltage all the way to the bias pot, but nothing at R77. Now I see the proper negative voltage on the leg of R77 that ties to the bias pot.

So now I have good negative bias voltage, but I have too low voltage on the low voltage (+-16v) section. The power dropper resistors (R78,R79) are seeing +-48 volts at input, but putting out something like +-2.6v at the output end.

The weird thing is, all the components in the low voltage supply tie into the same ground as R77. All of them show good continuity to ground. It had fine low voltage power before I reflowed the solder joints, so it is something I have done! Kills me because I have been very careful with the ribbon cables, and I have pretty good solder skills, so I wasn't expecting to make anything worse. Man, this amp hates me!

I will keep plugging away. Obviously I will re-check any joints that I hit, hopefully that's all it is. I will update when I figure it out.

[Enzo]

I of course have not seen it, but think of this:

Voltages were OK before, but then you restored a ground, and now that allows some other defective part to conduct current to ground, dragging down the supplies. Maybe?

[MichaelNuzum]

Enzo, I totally hear what you are saying, and will check out along those lines, thanks.

The only rub I have with that logically is, I didn't restore a ground that was never there, I restored a ground that was usually there, and when it was there, everything worked fine. All I did (hopefully) was take a solder joint that was working 90% of the time and make it 100% (as far as the bias ground reference). When it was working that 90% of the time, so was everything else. Channel switching and everything that runs off the low voltage worked.

I have to believe that as a result of my re-soldering, I messed something up. No solder blobs or connected traces or anything like that. As I mentioned I have decent soldering skills. Nothing sloppy or messy, but something went sideways.

I checked all the joints I re-soldered and they all look very clean. Everything leading in and out from them has conductivity where it should. But the low voltage section is whacked.

It is being fed proper voltage to test points 42 and 43. When I measure across R78 and R79 they both say they are dropping 46-ish volts. Then after the dropping resistors, I only see 2.5-ish volts. Weird part is, both diodes (D13,D14) and caps (C40, C41) show good conductivity to ground (measured around circuit board and at chassis).

I'm going to give it a little more time tonight. Hopefully something will reveal itself.

[g1]

What does your meter say on diode check across those zeners?
There's not a lot running off those low voltage rails. 3 IC's and a couple other items. Easy enough to de-solder the IC supply pins (4 & 8) and see if the voltages come back up.

[MichaelNuzum]

g1- I read .671 forward, .753 reverse, and .641 forward, .788 reverse on diode mode. Under power they drop 2.1 and 2.5 volts. They both have good ground continuity. I haven't pulled them to test out of circuit. Should I?