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Thread: Damn near started a fire in my Hot Rod Deluxe

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    Damn near started a fire in my Hot Rod Deluxe

    Oh man. Was at Frys (remember them) and saw the still cary parts and thought I could buy the two power resisters that Fromel changes as part of the kit. Mine have blackened the PCB. Picked up a pair 5w4.7r and soldered them in with a little standoff as they are supposed to be. Fired it up, 10 seconds in, smoking. Turned it off.

    Two diodes got so hot they de-soldered themselves, one of the power resistors did the same, ruining the pad in the process. Nice work genius.

    So, the original are 5w470r. Replaced with 5w4.7r. I’m guessing I just let way to much current pass through? I’ve got some work to to put it back correctly but this is my “learn by mistake” project.

    I’ve got the diodes back in, ran out of time to reinstall the 5w470r’s that I removed.

    How big of a dumbass move was that?

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Oldmactech View Post
    How big of a dumbass move was that?



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    Enjoy. Every. Sandwich.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    If the diodes got so hot that the leads melted solder, say 190C, then the maximum junction temperature was likely exceeded. Therefore replace the diodes. Much easier than removing the whole board in two weeks time as the diodes failed.

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    Also eye up the caps across those zeners for any bulging.

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    If there is a lot of heat damage to your board, you might want to reinforce the traces in the LV supply. Using a small solder helper toolkit you can pick up for around $4-6, you can scrap away the solder mask from the copper traces pretty easily. - Like this:
    (keep in mind this board isn't all that bad, I was just using it as an illustration)
    Click image for larger version. 

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    Then I tin the traces by flowing solder along the exposed copper. I like to use a small amount of liquid flux over the traces. This really helps them take the flow of solder very quickly.

    Click image for larger version. 

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    I've done this on more blues/deville fender amps than I can count and it goes a long way in keeping those components and circuit board working reliably going forward.

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    Thanks, I’ll look into that.
    I have an exact, plenty of flux and good solder. Not sure what wire to use but I have different sizes. I assumed braided is correct.

    Like to replace the zener diodes but can’t find them on the schematic. They function currently but likely headed for failure.

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    Lifetime Member Enzo's Avatar
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    Can't find on schematic? Look in power supply. Can you find the two 470 ohm dropping resistors R78,79? They are directly connected to those resistors. CR13,14.

    If all you need is the number: 1N5353

    The 5w resistors have like an inch or so of wire lead. We usually slip them through the hole and trim off the excess. I prefer to keep the wires. I do as Soul did, cleaning the coating off the traces back from the holes. I then insert the resistors, then bend the excess wire down along the bared traces. I then run solder the length of the wire. SO it looks like what SOul did, but with wire running through the solder too. That extra wire adds to the heat sinking of the solder and trace. Also you can mount the resistors up off the board a little to increase air cooling.

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    Thanks. Finally found them on the schematic.

    Oredered a pair and ZenerDiodes from Digikey.

    I will try the bend wire to meet the trace. Like the concept. Need to look and see how many traces need work.

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    How best to proceed?

    I reinstalled the two power resistors and the two diodes that popped off due to heat. Ran temporary jumpers until my replacement parts arrive. I’ve spending time getting more educated on schematics so I’m operating at a C- grade level for that.

    I could use some symptom troubleshooting advice:


    When I turn on the power (standby) all tubes light- oddly the drive channel yellow light lights on standby, not supposed to. Switch off standby, 60 cycle hum, loud at first, dissipates after 1-2 seconds to light hiss. Guitar sounds surprisingly good, loud, seems stuck in drive mode. Tone controls function. Nothing else. Drive, master, reverb, presence not functioning. Going over the schematic with such widespread outing maybe a relay or more than 1 got toasted?

    Thanks for any help.

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    Do you have the proper +/- 16V at those zeners?

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    There might be other issues to look at involving the IC's on the board. If stuck in drive mode then there is a 4560 IC that would be a suspect. In fact, the other IC's would be suspect since they could have been damaged as well and that may help indicate why the reverb is not working. Do as G1 says and post your measurements of the -/+16v supplies. At this point post the version of the Hot Rod Deluxe schematic that you are referencing so everyone is on the same page.

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    The schematic is available at thetubestore.com I believe it’s REV A 1996.

    I measured the Zeners and neither is +/-16.

    C41 is -13.1
    C40 is 3.1.

    I measured these on DC Voltage 20 scale on standard DMM.

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    Last edited by Oldmactech; 03-05-2020 at 11:12 PM.

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    Supporting Member The Dude's Avatar
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    Schematic Here

    I'm presuming you mean CR13 and CR14- C is the designation for a capacitor. Is that a typo, or do you actually have 3.1V on CR13 and not 13.1 like its counterpart? If so, you likely have a near short on that rail. What are your voltages on the other side of those 470 ohm resistors- the side right off of the rectifier diodes?

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    Yes thanks.

    I’m reading 46.8 at R78 -46.8 at R79. R78 is -1.28 R79 out is .11. Nowhere near +/- 16v.

    Cr 13 reads .10, CR14 reads -1.28 now.

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    You had checked these zeners and they measured ok, yes?
    Are any of the IC's feeling real hot?

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    No, sorry, not sure how to check the zeners. I just received replacements by mail and will install new ones as soon as the 470 resisters I ordered arrive.

    How do I check the zener diodes? Sorry for what must be a beginner question.

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    Supporting Member The Dude's Avatar
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    Thanks. And I carefully prodded
    the IC’s with a finger and didn’t feel any heat. I know the 470r resistors are hot but that is likely normal.

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    Supporting Member The Dude's Avatar
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    The resistors are gonna get real hot if they're dropping 45 volts. You shouldn't run it more than long enough for testing that way. You need to find the shorts/near shorts on that supply first. My guess is that the zeners are shorted, but it could be any number of things.

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    Thanks. If I have time today I will install the new Zeners and see if that improves it.

    The Zeners I got from Digikey have the specs/part number but are 2/3 the physical size. Is that a concern?

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    Was just about to install new Zeners when a heard a Loud boom and the power went out. Went outside, neighbors tree feel across the power line even took out the telephone tree- found parts of the transformer up near my house in pieces.

    Gonna take the rest of the day off.....

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    Don't forget the joker g1's Avatar
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    Wow.
    Click image for larger version. 

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    As far as the zener size, the specs I found for new one is approx. 3.5mm diameter x 9mm length, so yes, quite smaller than older type, but still spec at 5W so no problem.

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    Lifetime Member Enzo's Avatar
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    Make sure to install them the proper direction. Zeners "zene" in reverse direction. Forward they are just diodes. So in correctly, a 15v zener has 15v across it. If in backwards, it is just a diode and you have about 0.7v across it.

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    Reality check: the Diodes have a silver stripe (collar) on one end. The PSB has an outline that has an arrow on one end. Safe to assume the collar goes in the direction of the arrow? I guess I should use the correct terminology- cathode side or anode side.

    Btw. I tested the old Diodes with my DMM and the readings were way off, tested the new ones and both were something like 580 ea. Not at home so that may not be an accurate number.

    Prolly get the 5w 470k resistors in the mail tomorrow so I will put this all in on Monday if we have power.

    Thanks to all for the patient guidance. I’m gaining a least a small bit of knowledge.

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    Lifetime Member Enzo's Avatar
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    The line end is the cathode, the arrow is the anode. The arrow points at the line

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by Oldmactech View Post
    Reality check: the Diodes have a silver stripe (collar) on one end. The PSB has an outline that has an arrow on one end. Safe to assume the collar goes in the direction of the arrow? I guess I should use the correct terminology- cathode side or anode side.

    Btw. I tested the old Diodes with my DMM and the readings were way off, tested the new ones and both were something like 580 ea. Not at home so that may not be an accurate number.

    Prolly get the 5w 470k resistors in the mail tomorrow so I will put this all in on Monday if we have power.

    Thanks to all for the patient guidance. I’m gaining a least a small bit of knowledge.

    470K ??? 470 ohm, right?

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    Quote Originally Posted by nickb View Post
    470K ??? 470 ohm, right?

    Yes, 470R is what they are. Poor typing skills.

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    Installed the two 470r resistors (had to jump one connection on each), installed two Zeners (had to jumper them also.)

    Fired it up and the yellow channel light was still lit,no reverb, no channel switching. Got late so I gave up for the night.

    Readings at the resistors is troubling. R79 -49.6 -17.9. R78 55.8 55.8. I obviously missed something, maybe jumper failed although I checked continuity. Possible the resistor failed out of the box? No smoke, actually played GTR through it for a minute-not bad.

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    Look for a broken connection coming out of R78 (zener side).

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    Rebuilt the traces on the 470r resistors and the two Zener Diodes.

    R78 and R79 are now +/-48 and +/-16 in spec.

    Some symptoms remain, sone changed.

    Yellow overdrive light still lights even before standby switched.
    Stuck on drive channel, master and drive pots now function not previously. No channel switching.
    Normal volume now no longer functions. .
    No reverb.

    Guitar sounds good if you like the drive channel. Input one is louder than input two.

    Going back to the schematic to look for clues.

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by Oldmactech View Post
    Rebuilt the traces on the 470r resistors and the two Zener Diodes.

    R78 and R79 are now +/-48 and +/-16 in spec.

    Some symptoms remain, sone changed.

    Yellow overdrive light still lights even before standby switched.
    Stuck on drive channel, master and drive pots now function not previously. No channel switching.
    Normal volume now no longer functions. .
    No reverb.

    Guitar sounds good if you like the drive channel. Input one is louder than input two.

    Going back to the schematic to look for clues.
    Well, you're getting close. I think you may have issues with not only U3 (4560D), which controls the channel switching, but also U2 which drives the Reverb tank and amplifies the return form the tank, and if it's bad, U1 probably is also, since you've had issues with the bipolar 16V supplies. There's a very good Test Point chart on the schematic to aid in troubleshooting the channel switching (U3A & U3B). On U1 and U2, their outputs at pins 1 & 7 should be at/near 0VDC, when good. Of course, a broken lead in the reverb tank will also be a cause for no Reverb....a very common problem, and simple fix, as long as the tiny reverb xfmrs DCR readings are nominal (about 78 ohms on the input, 220 ohms on the output of the two xfmrs that connect to the RCA connectors).

    If your three IC's are all good, then the clues will be found in the Test Point chart for the switch settings using both the front panel buttons as well as the foot switch, if you have one.

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    No footswitch......dammit.

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by Oldmactech View Post
    No footswitch......dammit.
    No matter......those can be tracked down used, and simple to repair.

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    Measured the iCs

    Pin 1 Pin 7

    U1 15.69 15.69
    U2 -17.1 -17.1
    U3 -8.6 -8.3

    These are VDC.

    I measured several of the test points still trying to make sense of the readings.

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    Supporting Member nevetslab's Avatar
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    replace IC1, IC2, IC3

    Quote Originally Posted by Oldmactech View Post
    Measured the iCs

    Pin 1 Pin 7

    U1 15.69 15.69
    U2 -17.1 -17.1
    U3 -8.6 -8.3

    These are VDC.

    I measured several of the test points still trying to make sense of the readings.
    IC 1 & IC2 (T072) are both bad....replace. IC3.....Pin 1 is TP33, and Pin 7 is TP36. Neither of your readings are correct as shown on the Footswitch TP chart on the schematic, so that IC also is bad...replace with 4560D. TL072 will NOT work for U3.

    NJM4560_E[1].pdf
    Hot_Rod_Deluxe_Schematic.pdf

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