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

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

    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

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    Lifetime Member Enzo's Avatar
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    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?

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    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

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    Last edited by BStringThumper; 02-22-2018 at 01:31 AM.

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    don't forget the joker g1's Avatar
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    Is R101 and it's connections ok?
    At Q4 you have -16V from CR17 at the emitter and TP33 voltages at the base?

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    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.

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    Senior Member nevetslab's Avatar
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    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.

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    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.

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    don't forget the joker g1's Avatar
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    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

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    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.

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    Senior Member nevetslab's Avatar
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    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.

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    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?

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    Senior Member nevetslab's Avatar
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    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,

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    Last edited by nevetslab; 02-23-2018 at 08:19 PM.
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    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.

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    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.

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    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.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    TP34 is stuck at -16V yet CR22 has the correct -5.1V on it's anode . This tells as C22 is OK and so is R93. That leaves just one resistor R94 between the correct voltage and the incorrect at TP34. Also U3 has been replaced so that should not be it (it's prudent never to assume - I've been caught out my bad new parts before). Turn off and allow the +/-16V rails to go to close to zero then measure the resistance between pins 3 and 4 of U3. I'm expecting a short. If there is, look for a solder bridge between those pins.

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    Hi, Everyone-

    Thanks nevetslab & nickb for the additional ideas. I checked U3 pin 3 to pin 4 and there is no short. I tried another new BA4560 & NJM4560, I got the same -16 at TP 34. I tested R93, R94, both are good at 10k ohms.

    CR17 was removed, it tested good, but I put in a new 5.0V zener just to make sure. CR22 is testing good, R101, R96, R93, R94, R23, R24, are all good. Q4 has been tested.

    I'm going to replace Q1,2, with the correct J111. Any other thoughts?

    Thank you for taking the time to read this stuff and your very appreciated advice.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by BStringThumper View Post
    Hi, Everyone-

    Thanks nevetslab & nickb for the additional ideas. I checked U3 pin 3 to pin 4 and there is no short.
    Darn!

    I tried another new BA4560 & NJM4560, I got the same -16 at TP 34. I tested R93, R94, both are good at 10k ohms.

    CR17 was removed, it tested good, but I put in a new 5.0V zener just to make sure. CR22 is testing good, R101, R96, R93, R94, R23, R24, are all good. Q4 has been tested.

    I'm going to replace Q1,2, with the correct J111. Any other thoughts?

    Thank you for taking the time to read this stuff and your very appreciated advice.

    I think replacing Q2 and Q2 is pointless as until you find out why U3 pin 3 is at -16V it will never work. What resistance did you measure between pin 3 and 4?

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    Senior Member nevetslab's Avatar
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    I looked up the FET you had used, and it's the same pin-out, and Fairchild even stated 'See J111 for characteristics' on its' data sheet. So, it can't be the FET's.
    The voltages at TP32 & TP35 are ok. You are NOT getting the 0.5V difference between Drive and More Drive...it remains the same. So, that voltage controling the base of Q5 is puzzling. That's about the time I curse at it. I can't see how C45 would be involved.

    And, you're NOT getting the YELLOW LED to light. If ou have another Bi-color 3-leaded LED, I'd change that out. It has been stated before (maybe not in this thrad) that if it's bad, the circuit won't fully work. I've had the Drive NOT work, while the RED, YELLOW and No LED light status, with U3 or the JFET"s being the culptrits.

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    Lifetime Member Enzo's Avatar
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    If you get red but no yellow, that means the green side is not lighting. You should be able to test the green side by shorting over to the red drive.

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    Hi,

    Thank you all for the additional suggestions. Nickb, your suggestion to check for a short between pins 3 and 4 on U3 was excellent advise, there was in fact a
    solder bridge between those pins. When I responded saying there was no short it would have been nice if I tested the correct pins?? DOH! I was looking at the solder side when I tested what I thought were pins 3 and 4. I wish I would have seen the tiny little solder string/bridge on the 3 and 4 pins. Enzo, before removing the board I did short Q3 and the green light does illuminate to make the yellow light. Nevetslab, thanks for taking the time to compare/confirm those JFETS. I wonder what data sheet I was looking at then?? Thanks so much for the help, sorry I missed something so simple as that solder bridge. I hope to have time to reassemble the amp by Monday and report back. Thanks!!

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    SOLVED FIXED DONE.

    The amp works perfectly now. My sloppy solder job on the those two little pins sure was a "make work program" for myself.

    I learned a lot from my mistake. Another thing I learned is that I couldn't have done it without the skilled advice of everyone on this thread.
    Thank you! I hope someday I can help someone else out with a problem.

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