Probably a stupid HR Deville question

[bradicusmaximus]

Background - a buddy of mine brings his amp to me (HR Deville) that ran into some problems while he was on the road. When he told me about it, he was halfway across the country and the channels stopped switching. He said he could get it to work by jiggling the channel LED.

Simple enough - it sounded pretty much like a loose connection somewhere. But then he decided he wanted to try fixing it himself, since he was on the road and liked the amp and seemed to have packed a soldering iron with him. After a few minutes, a mild shock, and some sparks, he decided not to continue on that path - along with some stern encouragement from the band leader. However, the amp stopped working completely after that.

Flash forward and the amp is on my bench. After going through the logical steps of first checking the solder joints and board connections and doing a lot of touch up work, I got the amp to work again. Add in a cap job and some rejiggering of some power resistors to get them off the board for cooling and the beast seems to live pretty well.

However, the channel switching problem still is a problem. For the sake of simplicity, I replaced the LED completely as the old one was a bit hammered from all the jiggling that was done. But, I am way outside my comfort zone on this as I really don't deal with solid state any more than some very, very simple stuff.

This appears to be an almost 20 year old amp. It seems to be a first revision from 1996 or thereabouts. I'm hoping that over the almost two decades of this model line being out, someone here might have a suggestion or two that will put me in the right direction.

Any thoughts or insights would be most appreciated.

Thanks!

[xtian]

The green/red, three legged LED is an important part of the switching circuit. Did you replace the LED with the correct part, not just any old LED?

(Yes, I know the amp shows red and YELLOW indicator lights, but that's because it's turning on both the red and green elements together, and that creates yellow.)

[bradicusmaximus]

The green/red, three legged LED is an important part of the switching circuit. Did you replace the LED with the correct part, not just any old LED?

(Yes, I know the amp shows red and YELLOW indicator lights, but that's because it's turning on both the red and green elements together, and that creates yellow.)

I actually did trace down a new proper LED according to the part number and popped it in. From the research I did, I found out that the LED actually is functional in the switching circuit so I was careful about replacement. Not to mention, since it was for someone else, I wanted to make sure it's done right.

[Leo_Gnardo]

The pedal jack is likely to be knackered in a 20 y/o amp that's seen a lot of road use. Plus the pedal switches, cable, & plug. Push switch too on the dashboard. Whadda hassle.

[bradicusmaximus]

The pedal jack is likely to be knackered in a 20 y/o amp that's seen a lot of road use. Plus the pedal switches, cable, & plug. Push switch too on the dashboard. Whadda hassle.

This has definitely been quite the challenge for me. It an amp that's been on tour with major acts for a bunch of years and has seen its share of road fixes. Amazingly, it's held up quite well. It's just those new-fangled transistor do hickeys that get me all flummoxed. lol

[Leo_Gnardo]

The pedal jack is likely to be knackered in a 20 y/o amp that's seen a lot of road use. Plus the pedal switches, cable, & plug. Push switch too on the dashboard. Whadda hassle.

This has definitely been quite the challenge for me. It an amp that's been on tour with major acts for a bunch of years and has seen its share of road fixes. Amazingly, it's held up quite well. It's just those new-fangled transistor do hickeys that get me all flummoxed. lol

Couldn't have said it better meself.

Nonetheless, it's the s.o.s. that goes wrong, gotta attend to plugs jacks cables switches. I have confidence you'll restore the ol' road dog.

[bradicusmaximus]

The pedal jack is likely to be knackered in a 20 y/o amp that's seen a lot of road use. Plus the pedal switches, cable, & plug. Push switch too on the dashboard. Whadda hassle.

This has definitely been quite the challenge for me. It an amp that's been on tour with major acts for a bunch of years and has seen its share of road fixes. Amazingly, it's held up quite well. It's just those new-fangled transistor do hickeys that get me all flummoxed. lol

Couldn't have said it better meself.

Nonetheless, it's the s.o.s. that goes wrong, gotta attend to plugs jacks cables switches. I have confidence you'll restore the ol' road dog.

I hope so. Luckily, the owner is gone for another 12 days on tour out west, so I have some more time to trace this thing down and get it figured out.

[bradicusmaximus]

After a bunch of futzing around with this amp, I've made some headway. But I have a head scratcher that has me closer to my comfort zone but still kind still has me hating taking on this project.

I spent some time reading more about transistors and solid state electronics and realized that I was blindly overlooking something right in front of me. As I was measuring the voltage after the two main power resistors (330 ohm / 5w) in position R78 and R79, I was seeing right around +.8V and -.8V. I obviously spaced on the decimal point. Instead of the rest of the solid state components getting + / - 16V, they were getting 5% of the expected.

Obviously, that's pretty odd and would definitely cause problems.

So, in good trouble shooting fashion, I traced it back to a verified point. From the tap on the power transformer, I was seeing 26.8 VAC @117VAC from the wall. Low, but well within a 10% variation so close enough (in my book at least). Both rectifier diodes were getting that voltage so I measured on the other side and + / - 28V. That dropped lower than the 33.3V specification. Still, roughly ok - not perfect. However, on the other side of the resistors it was dropping to + / 1 0.8V.

This happened with both the original resistors and the replacements I put in. I triple checked the resistors and they are, in fact 330 ohms. The other replacements I made were the rectifier diodes. I haven't replaced the zeners or the caps.

Does anyone have a suggestion on where to go next on this? After my research, I'm out of ideas, which I think is more where the limits of my circuit knowledge is showing through. I would rather tackle things in a more reasoned way than simply swapping all the parts out. And, honestly, I really want to learn why this is happening.

Thanks!

I've included the relevant portion of the schematic marked up with what my measurements are and what's been replaced.

[xtian]

Something is shorted to ground, drawing the voltage way down. The two Zeners CR13/14 and caps C40/41 are the likely components to check, though something further downstream (not on the part of the schematic you show) could be drawing the current. There's probably no way to easily disconnect the +/- 16v rails from the rest of the circuit, huh?

[dkochan]

I am a rank amateur, but I will take a swing at where to go next.

Something is causing you to drop about twice as much voltage as you should be across those 330 resistors. I know you didn't touch the zeners or the caps, but are they bad?

This looks to be the PS for the TL072. Pre-amp outout buffer? Anyway, is that chip getting hot? Might be a clue... Oscillation or just unhealthiness in that area of the circuit?

Anyhow, that's where I'd look next. I wish I could be of more help.

Dave

Edit: Yes, what xtian said. Might be wishing for too much with a PCB amp. That is the cool thing about wired amps - you can break the circuit anywhere you please!

[bradicusmaximus]

Unfortunately, no. This whole 16v rail just snakes its way through the board from top to bottom, left to right.

I guess I'll pull the board and start tracing grounds. Is it possible something might have gotten too hot and leached over to the ground?

It's kind of goofy because it seems this to have happened all at once on him during a gig and it's not the usual suspects. Nothing really appears burned or otherwise obvious according to the usual problems with these models.

I've also included the full schematic if there's something obvious that I'm missing.

[bradicusmaximus]

I figured I would post a conclusion to this saga in case anyone was looking and wanted some finality to the story.

In the end, after A LOT of tracing and researching and learning on solid state circuits, essentially most of the important components of the channel switching, reverb, and more drive circuit were fried. This included both zener diodes for the 16v rail as well as one of the 22f / 25v caps before the channel switch and both U2 and U3 integrated circuits as well as the LED.

How did this happen?

After some prodding of the owner and the band leader who was in the hotel room on the tour, our intrepid guitar player decided to try and fix it himself. With soldering iron in hand and way too much voltage applied to the circuits, he almost electrocuted himself approximately five times in his efforts. And while it's impossible to figure out exactly how or what he did, he touched the wrong things at the wrong time and broke a lot of stuff in there.

From the original problem, it sounds like it was a common problem with these amps and the power resistors in R78 & R79 like 99% of the problems turn out to be. But, by noodling around, he made it much, much worse.

At the very least, it's off my bench now and the owner has standing orders to not touch things unless he knows what he's doing.

Another happy ending thanks to a bunch of people here that gave me some leads to explore. Thanks guys!

[xtian]

Great job!

And did you get to charge for all 943 hours of your time?

[dkochan]

Holy cow!

I was wondering how this one went. Thanks for posting the update! "A cautionary tale..."

Dave

[bradicusmaximus]

Great job!

And did you get to charge for all 943 hours of your time?

I wish! There's a reason I have a day job to pay the bills because I'm way, way, way too nice of a guy. lol