Hot Rod DeVille- gain switching problem- no yellow mode only red

[BStringThumper]

Hi,
Looking for some help please with a Fender Hot Rod Deville 2x12. It says 1996 on the main circuit board.

The channel switch works, but when it switches to the dirty channel it automatically goes to the "more drive" red mode. Actuating the drive switch makes no difference. It will not engage to the "drive" yellow mode. I have both +/- 16vdc on the opamps.
I have replaced Q1,2,3,4. Also replaced U3 opamp. I have diode tested the LED and can see both Red & Green glow dimly. I reflowed the solder joints on the 330 ohm 5W resistors just for good measure and touched up any that looked even remotely marginal.
Anyone seen this before? Thanks for any help/advice.



TP31 Spec Actual
Normal 1.39 1.4
Drive 1.08 1.15
More 1.08 1.15

TP32
Ch. Out .536 .482
Ch. In 9.69 9.63

TP33
Normal 16 15.54
Drive -13.0 -13.11
More -13.5 -13.11

TP34
Normal -12.5 -16.5
Drive -4.90 -16.19
More -4.14 -16.9

TP35
More Drive Out -.52 -.480
More Drive In -9.92 -9.50

TP36
Normal -15.5 15.8
Drive -15.4 15.8
More 16 15.8

[Enzo]

So go fix whatever is going on with TP34.

What actual type transistors went into Q3, Q4? ANy chance one is backwards?

Does CR22 have proper zener voltage across?

[BStringThumper]

Hi Enzo, thank you for the response.

Q2 is 2N4401
Q3 is 2N4403
Orientation is correct with the silkscreen flat side.

CR22 is 0V cathode side, -4.97V anode side.

I just rechecked CR17, another 5.1V ZD that I had checked off as good on my schematic, now I'm getting -15.89V reading from ground to either side of the ZD or 0V reading across the ZD itself. It tested fine under a simple diode test

[g1]

Is R101 and it's connections ok?
At Q4 you have -16V from CR17 at the emitter and TP33 voltages at the base?

[BStringThumper]

Hi g1,

Thanks for the suggestion-

R101 tested good at 27.1K ohms.

Q4 emitter voltage= -15.81
Q4 base voltage= -15.3
TP33 voltage= -13

I substituted CR17 ZD with a new 5.0V (instead of the 5.1V, all I had at hand) they both tested good under diode test, no change in voltages.

[nevetslab]

I'm in the midst of updating the rental inventory of Hot Rod series amps (@ CenterStaging, LLC Burbank, CA), and one of them wasn't getting the Drive Mode...it would change directly to More Drive mode. The voltages you're reading at TP34 was very similar to what I was reading, and after changing out U3, all was back to normal again. I just used my last NJM4560 in that repair, and was wondering just how much difference there is between the 4560 Dual Op Amp and a 4558/1558/LM248. The 4560 has a bit more current drive than the others, as well as being a bit faster/higher BW. I haven't yet tried any 4558 types in that circuit (U3) to see if it would work in a pinch while waiting for the 4560's to arrive.

Anyway, U3 looks like the culprit in your case.

[BStringThumper]

Hi nevetslab,

Thanks for the info.

I did replace the U3 4560. I pulled out a BA4560, I also soldered in a DIP socket. I have a NJM4560 in stock. I'll swap it out to try it. Thanks.

[g1]

These things seem pretty picky about the U3 & Q4 combination. Lowell had pretty much the same issue here:
http://music-electronics-forum.com/t45878/#post478991

[BStringThumper]

Hi g1,

Thanks for the link. Very similar problem, but I can't get the Yellow light to come on, it goes right into Red mode. I did swap U3 to a NJM4560 as nevetslab recommended, no change. I even put in another new BA4560, no change. Please take a look at my TP34 & TP36 voltages. In another article Enzo recommended jumping Q3 E&C, that activated the Green/Yellow light. So the LED is good. Thanks Enzo. So I'm back to the Q3 emitter & base voltage TP34 & 36. I've been reluctant to pull the board up again since this amp has been blown up and repaired badly before, flimsy board, but I need to pull some components out of circuit for testing. Starting with CR19. Any other thoughts?? Thanks, everyone.

[nevetslab]

I came across another Hot Rod DeVille today that also had no Drive mode. The LED's change as normal, and all the Test Points were correct, but still only went from Normal to More Drive. Both of the JFET's Q1 & Q2 that switch out the 100k resistors in series with the cathode bypass caps are shorted. Take a look and see if you see near0 ohms across those 100k's R23 & R24.

[BStringThumper]

Hi nevetslab,

I pulled the board and checked R23, R24 out of circuit. They are 100K. OH!! Q1 & Q2 were replaced with PF5102 transistors, not the J111. The supplier had them listed as the same characteristics as J111. I looked at the data sheet and they don't look very similar to me, but I'm not sure which parameters are the most important to match. Has anyone used these before in place of J111?

[nevetslab]

You'd check them in circuit in this case. each of the resistors has the JFET acr9ss them, and would normally read around 100k until the switches are turned on. I'll be replacing the J111's on the amp I have on teh bench at teh moment, and will confirm this approach, since I found the 100k's shorted out by the JFET's, which would account for why I don't' get the Driver mode, when all the rest of the control logic's voltages are nominal. I wouldn't expect the 100ks to have failed. I've never tried the switching FET's you've selected.

To be continued.....

Back from the trenches. Having just swapped out Q1 & Q2 JFET's in this Hot Rod Deluxe 112, whose Drive mode wasn't working, .that cured it. AND, you CAN place an ohmmeter across the 100 k resistors and observe the FET switching (or NOT switching) under power. The cathode bypass caps C8 & C9 prevent DC Voltage on the 100k resistors in this case,

[Enzo]

And though they may be laid out the same, there is no guarantee the pins on the J11 are in the same order as on the PF5102.

[BStringThumper]

Hi nevetslab,

Thank you for the update.

I measured the 100K resistors before I pulled them, they were only reading 107 ohm & 110 ohm, respectively, in circuit, not 100K. Out of circuit they were 100K. Powered up there was 0V flowing, reading from both sides of the resistor. I wish I would have checked for the FET switching before I took it apart.
I really want to learn and troubleshoot the problem before just throwing parts at it. Enzo has brought into question the PF5102 transistor, I'm going to order the correct J111 transistors and associated parts to have at hand. Thank you all for the help. Any additional thoughts/advise is appreciated. Have a great day.

[nevetslab]

After you replaced U3, are you still getting the same voltages as you stated initially? TP 34 behaves as though CR22 is open, and no longer a zener, since you get around -16V in all three switch modes. You've already replaced the two FET switches and Q3 & Q4 with no change, as well as CR17.

Also, are you just using the front panel switches for this. We had recently a HR series amp that was using a different foot pedal and not the HR Series pedal, which gave incorrect results.