HRD repair

[h8mtv]

Hi guys, I have been working on my Fender Hot Rod Deluxe. Whenever I turn up the reverb even slightly I get a terrible noise. I have done lots of stompbox builds but never much in the way of amp work. When I opened it up there were definately issues with the two 470r wire wound resistors so I got new 7w ones and floated them off the chassis as I have read to do. There is some scorching on the board in the area of the 470r resistors and the 1000uf caps. I also replaced the caps, but no improvement. Any advice?

[Andy Le Blanc]

If the traces on the pcb are ok, you should be fine.

fender got bad resistors and the show up in the darndest places.

the verb is SS, unplug the tank, see if the noise is still there...
ground the input, see if the noise is still there....
basically see if the recovery works, it might be a bad tank too...

[xtian]

Looks familiar!

https://tubeamparchive.com/viewtopic.php?t=12472

Before I made my repair to those two 470ohm resistors, the reverb and the channel switching did not work at all. Afterward, all fixed.

Is your channel switching working? All other features?

[selloutrr]

Clean the burn mark it can be conductive.
then follow the other steps mentioned you should be good.

[KellyBass]

Yeah! What is with Fender's resistors? Awful!...

[xtian]

I just read a review of the new Fender HRD III in Guitarist magazine, October 2010. No mention of the burnout resistors! Plenty of raves, though.

[Drumslinger]

it would be interesting to know what changes they made with the HRD III.

[Andy Le Blanc]

hot dropping resistors appear in those super 60 things in the power rail.
seems to have become a design trend. See them as the cause for a lot of
repairs that go across my bench, sometimes they burn, melt, stink, or
literally fall right off the pcb. In HRD it r78 and r79 +\- 16 v supply,
but they also show up in some of the SS fender things too.
I've had rashes of them, might as well replace as many of the plate resistors
in the pre amp as you can, fender got a flaky pile of those too.

[xtian]

According to the Guitarist article,

The two principal electronic changes...'tightened' overdrive: "More attack and less slop on the lower strings,"...Secondly, the volume and treble controls have a new taper.

The most common mods for the HRD, after replacing resistors R78,79, are to address the taper of the two volume controls, and to improve the distortion on the lead channel.

The volume controls aren't bothering me, but today I'm going to change C23 to allow more bass into the drive channel, play with C11 to address fizz in the high-gain distortion sounds, and add a jumper to the Mid tone control to allow the tone controls to completely shunt the signal to ground when zeroed (to address the fact that there's still a lot of bass in the signal when the Bass control is zeroed).

These mods should all be easy to do without removing the circuit board. I'll let you know how it goes.

[xtian]

OK, forget what I said about the mods being easy because of access to the traces. When you pop open the panel, you see the component side of the board.

Anyway, I went ahead with the three mods I mentioned.

Adding a jumper on the Mid tone control is dead simple, and an excellent mod. I have greater control over the tone now, and can dial the bass back further than before, a mod requested my many people for the HRD.

I added in parallel a 4n7 cap with C23, allowing more bass thru from the first gain stage to the Lead channel. Excellent! This has really made the Lead channel more useful. Before, it sounded like poor 80s solid state distortion. Now it's much warmer and tube sounding. However, if I dime the Bass control now in Lead mode, I can get some lo-fi, flubby, chunky distortion. Jack White would approve, but Fender's engineers would never let it out of the factory like this. Call it a bug or a feature--I like having the option on tap!

Finally, I added a 500pF mica cap in parallel with C11, which should soften the high frequency fizz in the Lead channel. I didn't do enough before/after testing, nor at high enough volumes to notice the difference.

Last valuable mod will have to wait for a new part: replace the Lead channel's Master pot with an audio taper. The stock, linear pot is really poor for adjusting bedroom volume levels.

[xtian]

This web site http://www.amprepairparts.com/pots.htm has the exact Fender pots I need for this mod. However, I have to choose 250K audio to replace the existing 100K linear.

I'm not sure how the voltage get's divided in a circuit like this. Is it OK to go up to 250K? Or should I get a mini pot from another manufacturer at 50 or 100K? See schematic below.

[h8mtv]

Wow, thank you all for the great info!

What should I clean the board with? Contact cleaner and elbow grease do not seem to be doing it.

[selloutrr]

cleaning the board...

You wont be able to get rid of the burn mark that's forever!... The elbow grease and cleaner, you could have also used 90 proof alcohol, Is to clean up any traces that could have been conductive. If you have any doubt set your volt meter to continuity and touch the board and then probe around the eyelets / burn mark to make sure it's not making contact.

[h8mtv]

If the traces on the pcb are ok, you should be fine.

fender got bad resistors and the show up in the darndest places.

the verb is SS, unplug the tank, see if the noise is still there...
ground the input, see if the noise is still there....
basically see if the recovery works, it might be a bad tank too...

When I unplug the tank the noise goes away. Are you talking tank input to ground or amp input?

[Andy Le Blanc]

OK... give the input (verb return) a pop test or touch to the chassis to ground,
should be just like the guitar patch when theres no guitar, buzszszs buzszs.

You should be able to control the vol on the reverb knob etc... its OK.

If you pulled the input to the tank and the noise is gone there's a good chance its the tank.

I don't see issues with SS verb tank drivers, the tank is usually the culprit.
That doesn't mean that its not the verb driver, you can look at the signal out
with an O-scope, or if you have another amp you can feed the signal to it
and give it a quick listen, there are impedance issues doing this but its
quick and will do if you don't have another tank to swap.

Verb tanks are prone to fuzz and lint and broken wires and crud getting into
the electromechanical drive thing-a-ma-bob, it might just be a simple
broken wire or bad ground, any how, if the recovery is fine..
pull the whole tank out of the amp and bag (if there is one) and give it a
good look. You should be able to read the coils with a meter if you cant
then there's something up there. Matching the tank input impedance to
the driver is important when it comes to a replacement.

You can build your tank from the remains of others in a pinch too, fun fun.