Blues Deluxe channel Limbo

[pabothecat]

I've owned a Fender blues deluxe since 1995 (tweed covered model not the hotrod) and have always had channel switching problems.

While playing the amp in either the clean or dirty channel, the channel light will dim to about half the normal brightness (or light if on clean channel) and no sound will come out of the amp. Over the years, I have had it in the shop on three occasions to get fixed and it would work fine for a few weeks.

I have pulled the circuit board and looked it over for bad solder joints and PCB damage, but haven't found anything.

I am thinking about setting the original PCB aside and wiring in a turret board (may a 57 bassman or deluxe type thing).

I'll loose channel switching, but the damn thing has never worked and has been sitting in the corner for 2 years.

I have a good chassis, speaker, enclosure and transformer i would just need to add the circuit.

What do you guys think? Should I try to get the original pcb up to snuff or is it time to cut my losses and start anew?

Although I've never built a tube amp from scratch, i have made a few successful modifications over the years. I have proto-typed solidstate circuits and micro-controller circuits, so I'm not a complete noob.

I'm not a purist who thinks all PCB boards are bad. I just think the one I have is bad.

Any help or thoughts are appreciated

Steve

[Two Tone]

http://studentweb.eku.edu/justin_holton/faq.html

[Enzo]

Check the low voltage supplies. There are two 5 watt 470 ohm resistors along the rear edge of the main board, more or less centered - R85,86. Next to them are two 5 watt 16v zeners - CR22,23.

The resistors like to get too hot, and the solder connection can fail.. Also, due to thermal stress, the traces they solder to can lift from the board and fracture. Once repaired, you might glob some silicone around them to keep them from vibrating or moving. If you have to run wires from the resistors over to a more secure copper trace, do it.

The channel switching and other stuff run off the +/-16v rails. If they fail, you will get poor operation of those circuits.

DO NOT replace the 470s with 390s as some suggest. This will increase the dissipation there. Not desirable.

It is also possible the switching circuitry itself has an issue. For some reason that circuit baffles some tube amp techs with its lack of tube-ness.

I don't believe in a bad circuit board. I believe in a circuit board with a problem.

[R.G.]

Good advice, Enzo.

I might add to it that I have a personal belief that resistor-zener power supplies are inherently evil.

OK, just unreliable. The Blues Deluxe would be a lot better replacing those resistors and zeners with three terminal regulators, such as the 7815 and 7915.

[pabothecat]

Thanks for all the replys so far.

I checked the 2 470 ohm 5 W resistors and that is definitly the problem.

I thought about replacing the 5 watt versions with 10 watt versions, but i think the leads will get just as hot as they are now (they still have to disipate a little more than 2 watts by my calcs).


I have checked into linear voltage regulators i.e. lm317 and lm337 (adjustable positive and negative regulators) and i'm not sure this will work.

I am measuring 48 and -48 volts before the power resistors with a 3 volt ripple. I think this is over the max operating voltage of the regulators.
I think i found a high voltage lm317 that will work, but no luck on the lm337, all have a max Vin of -40 volts (or min to be more accurate).
I'm not sure if this is absolute voltage or the Vin - Vout

I also thought of putting some kind of headsink on the resistors, but i've never seen a heat sink for resisters.

By my calculations, the circuit is only producing about 68 mA. current draw should not be an issue. It seems like this is making a lot of heat for such a low current draw. I would prefer to find a linear voltage regulator solution than to having a small size frying pan in the back of my amp! Seriously, these things get hotter than the tubes. My right index finger has the resistor part# branded into it.

I agree that this resistor zener network is a very poor design.

I'm going to keep researching regulators and let everyone know what i find, maybe do a rightup if it works well enough.

I will gladly accept anyone's advice!!!!

Thanks,

Steve

[52 Bill]

Steve:
The early Blues Deluxes and Devilles suffered from the low voltage power supply channel switching problems that you are experiencing. There are 2-3 Fender Tech notes that offer the Fender revisions for the problems.

TN94-7: For Blues Deluxes serial numbers prior to T-36620, replace R85 & R86 (330 ohm 5 watt) with two 470 ohm 5 watt Wire Wound Footed resistors (Fender P/N 041256). They specifically require "footed" resistors (the ones with the little bumps on one side) that will keep the body about 1/8" away from the pc board.

TN95-5: Check for value of R71 in channel switching circuit. Early amps have a 10K ohm 1/4 watt. If it is, it should be changed to a 100k ohms 1/4 watt.
The lower value, left too high a voltage on the relay coil when it was supposed to be off, and some relays would "float" between channels.

Other areas to check in general on these amps are the input jacks, the FX loop jacks, and the impedance switching contact on the speaker jacks (if they have them).

In our shop, we see 1-2 of these a week for various problems, but the low voltage supply problem seems to only effect early versions of the amps like yours. The revisions seem to have solved the problem. I've seen people fit 10 watt resistors in there and once, I had one where someone had bolted down to the chassis 2- 470 watt Dale resistors (in the metal cases) and run wires back to the circuit board. I don't know who or how the earlier repairs were done on your amp, but I would recommend doing the Fender fix and only redesign if it fails to hold up.

Bill

[pabothecat]

Thanks for the help Bill.

I checked R71 and it is 100 Kohms.

The 2 resistors are exactly as you described; 470 ohms and have feet on the side facing the board. silicone or epoxy is under and between the two resistors.

I guess I'm going to try and re-solder the power resistors and see if it holds up.

If it fails again, i'm going to modify the power section with lm317 type regulators. hopefully it will hold up long enough to complete my voltage regulator research!!

All advice has been excellent so far, and i will gladly accept more!!

Steve

[Enzo]

Why bother with the adjustable regulators? Just use 7815/7915. DOn't worry about the 1 volt.

For the record, the 10 watters will not get the wires as hot. They are larger and have greater thermal mass and will run cooler, thus less heat conducting out the wires.

If you want to get real anal, mount a couple of those aluminum Dale resistors on the side of the chassis and run wires to the pc board. The 50 watters are only a couple inchesd long and come complete with a couple little mounting hole tabs. They are $3-4 each.

[pabothecat]

the reason I was going the adjustable regulator route was they handle more voltage input and total voltage drop. I was looking at 7818/7918 for +/- 18volts and letting the zeners take care of the other 2 volts, but with 48 volts in and 18 volts out, a drop of 30 volts, no fixed regulator could handle this kind of drop (at least not with the simple type of circuit i was going to build).

I wasn't sure about the heat for a 5 watt vs a 10 watt, so if my amp fails again, i'll probably try a 10 watter next.
Thanks for setting me straight Enzo!!!

For the record, I did re-solder the power resistors.
I've played on it for about 30 minutes and have had no problem so far.
Only time will tell!!

Thanks for everyones input!! I may not have it fixed this time, but at least I know what to do now! Knowing IS half the battle!

Steve

[Enzo]

With the zeners, the droppers have a set 32v across them. Your 7815 has a max in of 35v, so lets make that 25 or 30. Now all we have to drop is 18-23v. Do that with a resistor. That resistor doesnt work nearly as hard.

[pabothecat]

well, I didn't think of that!! voltage regulator combined with a drop resistor would work great!!

What do you think of using 7818/7918 (18 volt regulators) with a drop resistor and letting the 16 Volt zeners reduce the ripple and regulate to 16 volts? I think this would be a dead easy and reliable circuit.

I am a little concerned about dropping to 15 volt with 7815/1915 because 16 volts is used as a reference voltage on the channel switching circuit on R62 (may be 1 or 2 other places as well). I'm not sure if this would be a big deal or not. I haven't analyzed the circuit that much, just noticed it was there.

I've played for another hour or so through the amp and so far, the solder touch up on the 5 watt resistors is still holding up. I'm guessing this will hold up for a couple weeks, then fail again. (just my past experience)

Thanks,

Steve

[Enzo]

I think it is unecessarily complex.

The 16v is not a reference, it is simply a rail. The references are all simple diode drops. The circuit is comparators. Outputs are high or low, +15 or -15. The volt won't matter.

If you wanted to be real anal about it, you could elevate the ground terminal of the 78xx with a diode drop or two. That would also elevate the 15v output the same amount. I wouldn't bother.

There should be darn little ripple coming out the 78xx, and good practice has a small cap there anyway. Like a 10uf and a .1uf.

REducing the zeners to 1 watt would help. Remember the circuit current does not flow through the zener.

[eeijcon]

Hi,

I seem to have the same problem as Steve. 1995 Tweed Blues Deluxe that goes silent (if playing the clean channel the channel light comes on). It either goes away intermittently or sometimes seems to responds to a gentle thump. Seems to become less of a problem the longer the amp is on/more it's heated up but yuou never know when it's going to happen.

I've checked R71 value and it's rated at 10k ohms but only reads 6.2k (?). Should I replace with 100k? The 2 R85 & R86 resistors are 470 ohm big boxy things (no, I don't know much about these things). Perhaps I should try re-soldering?

Any advice would be much approeciated.

BRs,
John.

[Enzo]

John, we will be happy to help, but please start a new thread for your amp. We call this "highjacking" a thread.

[Steve A.]

...
I don't believe in a bad circuit board. I believe in a circuit board with a problem.

What about the original Epiphone Valve Junior circuit boards???

Steve Ahola