Fender Hot Rod Deluxe 2001 Troubleshooting Low Volume, Distorted Clean Channel

[FenderHRDlx]

Hi, thanks for reading my post.

I had my Fender Hot Rod Deluxe amp (2001 on my PCB) start making some crackling popping sounds (almost like a bad patch cable--def. not the cable, this has been ruled out quickly), I figured it was tubes (have owned the amp for about 15 years, play it low volume, relatively no gigging) so I swapped out all V1-V5 (preamps with EHX 12AX7, tubes are now TAD 6L6GC-STR, matched pair).

Tubes were not my problem apparently. I plug into the pre-amp in and still the same low volume distorted clean sound. The other channels work, i.e., they add more distortion, reverb circuit seems to work.

I've checked the plate load resistors R57 reads 81.2kOhm (spec 82kOhm), R58 reads 98.1kohm (spec 100 kOhm). Voltage drop across R57 is 398V to 257V; R58 goes 398V to 249V.

Screen resistors R61 is at 467ohms (spec 470 Ohms) same with R62, 467Ohms (spec 470 Ohms).

I've learned the resistors at R78/79 are prone to overheating, but now they read 483 Ohms/459 Ohms respectively. Voltage on the high side of R78 is 16.7V and R79, high side, is -16.7V. Low side for both is 48.7V.

Tubes V5 and V4 read at pin 3 show voltages of 436 V. Since they're above 400Volts I don't think its the output transformer giving me an issue.

V3 reads 258V at pin 1 and 250V at pin 6.

The IC caps aren't leaking as far as I can tell, but they do all read 0 volts at the high/negative side, is this normal? C31 reads 439V low/positive side, C33, 437V, C53 400V and C36 is 351V.

Visually inspecting I don't see any burned/charred components, the amp is plugged into the left jack (looking into the back of the amp). I cleaned the contacts there as well, no help.

Any clue on what to check further? Thanks!

[glebert]

Does the problem come on right away or after the amp has warmed up for a minute or two? Never a bad idea to plug a guitar into the power amp input and see if you get a clean output tone. It won't be super loud, but should be clean. If that is clean you can rule out the PI and power tubes.

[vintagekiki]

plug a guitar into the power amp input and see if you get a clean output tone

[vintagekiki]

https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Hot-Rod-Deluxe-Schematic.pdf

https://www.manualslib.com/manual/239251/Fender-Hot-Rod-Deluxe.html

1)
Fender Hot Rod Deluxe rev A, B, C service manual
https://vdocuments.mx/download/hot-rod-deluxe-service-manual-rod-deluxepdfhot-rod-deluxe-service-manual-contents

[FenderHRDlx]

@Glebert: The amp has the issue when first turning on, and after idling. If I plug into the pre-amp in I still have the distortion. I've tried across multiple guitars, cables, this is an amp issue for sure. I don't think I can rule out the PI or the power tubes.

vintagekiki, thanks for the schematic links. I had been working on one dated 1996, even though I know my PCB is 2001, so not sure if major changes (aside from component venders) occurred from 95/96 til 2001.

Thanks to you both for the quick replies!

[vintagekiki]

...

Distorted clean sound at low volume may be due to low idle current. Check the voltage on TP30 (60mV) and if necessary adjust it with bias adj (25k)

Since the amplifier has worked many years, transient phenomena (crackling popping sounds) can occur due to cold solder on the components in the filtration circuit.
At the power supply check the solder condition from C31 to C36, and if necessary a resolder cold solder point.

Test Fender HRD
During all tests, the speaker must be connected to the Fender HRD.
Check Fender HRD to see if it works as a preamplifier.
Connect the preamp out jack J3 to the input any external guitar amp which serve as slave.
If the Fender HRD preamp works on external guitar amplifier, then the fault is in the communication between preamp and power amp, or the power amp of the Fender HRD is faulty.
To test Fender HRD power amp plug the guitar into the power amp in jack J4.

[FenderHRDlx]

vintagekiki I’ll put it on the bench tomorrow and report back. I’m grateful! Thanks for these suggestions

[Enzo]

the amp is plugged into the left jack (looking into the back of the amp)

What?? DO you mean speaker? There are two speaker jacks, one has the extra shorting contact and the other does not..MAKE SURE you plug the speaker into the jack with the cutout contact.

[FenderHRDlx]

Enzo correct, I wasn’t clear. I’m aware of the shorting jack. My point was it’s jacked correctly.

[FenderHRDlx]

So I've confirmed this is properly biased to 60mV at test point (TP) 30 (i.e. 30mA to each of the balanced power tubes V4/V5). Input is in main speaker jack (J7 on schematic). All the solder connections seem solid.

Using the preamp out to slave another amp (J3) indicates that the tone is still low volume/distorted in the slave amp. Again, power amp in (J4 on schematic) also provides low volume distortion.

When assessing TP29 (green wire on J6) I'm not getting any voltage. If I'm reading TP 27 and 28 correctly--by touching the solder joints--(just upstream of TP29), I'm reading 433V on both, schematic indicates this point should be 66.5V at each (!??).

As always, I'm grateful for any insight.

[Enzo]

DC voltages are in square boxes. The circles are for AC voltage WHEN THE TEST CONDITIONS ARE MET.

Look om tjhe power supply drawing. You get 430v on the plates, that supply is 430v, all OK. Of course there is zero DC volts on the speaker jack.

Your terms are confusing. High side and low side generally refers to the voltage It doesn't refer to which way the cap sits in the chassis

[FenderHRDlx]

Thanks Enzo , already I'm learning. I had reset the amp to the "test conditions" (i.e. knobs at locations, drive settings etc., as per schematic). Sorry for mixing term, was just trying to lend clarity, but see I was confusing the issue.

Seems like it's the power amp, but I can't spot any of the well document issues for this amp.

Can anyone recommend a repair tech in the Washington DC area?

[vintagekiki]

Can anyone recommend a repair tech in the Washington DC area?

https://www.tdpri.com/threads/vintage-amp-repair-dc-area.904334/
Vintage amp repair - DC area

https://www.thegearpage.net/board/index.php?threads/amp-techs-in-dc.751385/
Amp Tech's in DC

https://support.fender.com/hc/en-us/articles/214344223-Where-can-I-take-my-instrument-and-or-amplifier-to-be-repaired-
Where can I take my instrument and/or amplifier to be repaired?

1)
https://support.fender.com/hc/en-us/requests/new
Fender Contact Support

[FenderHRDlx]

Many thanks!!

[nevetslab]

If you still want to continue pursuing the troubleshooting and repair on this, you'll need to first take a close look at all of the solder joints on the tube PCB. Bright light & magnification is recommended so you can clearly find bad solder joints on any/all of the tube terminal connections, as well as those on the screen resistors, ribbon cable connections, etc. My avatar, showing surgical headlight and loupes is what I use at the bench, and it CLEARLY REVEALS SOLDER JOINT FRACTURES! These are common ailments on the Hot Rod Series. Once you're beyond those on that pair of PCB's, then you need to drop the main PCB. Getting it free from the control panel is a bit of a pain. I normally clip off the tie wraps of the transformer wires coming out below the bottom of the Main PCB. I also remove, one by one, the Power Xfmr Secondary wires, labeling each per their PCB connection number so you can return them when reassembly. Also remove the AC primary wiring, as all of those inhibit dropping the main board down. After you've removed all of the control panel hardware, and removed the screws/washers from the PCB, along with the GRN Grounding Wire to the chassis, now ou can begin pulling the main PCB down enough to get the ends of the pots to clear the chassis flange. THAT's what is a PITA, but it will come down.

It's on this PCB where I regularily find solder joint fractures on the control terminals of the panel pots, as well as fractures on the 5W power dropping resistors ahead of the 16V/5W zener diodes. Those leads oxidize, as well as overheat. I often have to desolder the leads, scrape away the surface of the leads to get to fresh metal, then re-solder them so there's good connection to the solder pads. The solder pads/PCB quality is generally poor, and easily fail or break away from the foil traces during repairing solder joints, so be patient and careful. I often find fractures on the power supply filter caps. On some generations of the PCB's, Fender reduced the solder mask diameter well below the diameter of the actual solder pad, so there's very small plated pad left to solder the leads onto. I will usually scrape away the solder mask and plate those solder pads so there's the full size of the pad to get the solder to wick up onto the leads.

While you have the main PCB dropped down, take time to tightened up all of the round standoffs, as they tend to be loose. Pulling on each with pliers as you twist to tighten is all that's needed....other side has screw & lock washer, so it tightens back up just fine.

Input jack connections, as well as those for the Power Amp Input/Preamp Output jack, as well as the Foot Switch Jacks often have fractured solder joints. Get all those connections sound throughout this main PCB along with the tube PCB's.....I suspect your problems will be gone.

[glebert]

Using the preamp out to slave another amp (J3) indicates that the tone is still low volume/distorted in the slave amp. Again, power amp in (J4 on schematic) also provides low volume distortion.

This seems pretty unusual to have the same problem in both power amp and preamp. The voltage for the power tubes seems good, what about the voltages for the preamp and PI tubes (V1-V3)? I would try "chopsticking" around on the board (tapping components gently with a wooden stick) while having a test signal applied to see if something makes the volume jump (up or down).