Hot Rod Deluxe channel LED

[Doug]

I've got a hot rod deluxe where the clean channel LED indicator (off) flickers to yellow or red slightly, depending on the drive channel setting. It only does this at higher volumes and intermittently. I can get it to flicker by tapping on the board near the LED but I can't find any bad solder joints. The channel doesn't switch from clean to drive when the LED flickers, just like to fix before I lose a switching relay or maybe some other problem down the road
Thanks
Doug

[Don Moose]

Since it's just the LED, and not the switching, then going by this schematic, I'd look carefully at:

R86, R92 - blown, bad solder, broken traces
C51 - shorted
Q3 - starting to fail short, conductive crud between Collector and Emitter - a flake of solder that _sometimes_ makes contact (look at the back of the board, too)

You're looking for a reason Q3 would start to conduct - it's probably not U3A, as you're not actually GETTING the More Drive, just the light that reports it.

Hope this helps!

[Mark Black]

Also it could be as simple as a cracked solder right at the LED, which has 3 legs as I remember.

If you go to the trouble of getting the PCB's out of the amp you should take the time to re-touch all the pots, switches, jacks, LED's, large and/or heat-producing components, tube sockets, as well as any other solder-starved-looking joints you see along the way.

Good luck!

[Dave Curtis, dB AudioTech]

Hi Don,

The fact that you can make it happen by tapping generally indicates a bad solder joint. Reflow all joints in the vicinity; it's quicker than trying to find a bad joint. I've fixed a few by just resoldering the LED (it's one of those "stressed connections). Just use enough heat to flow a bit of new solder on the joints (try to get the heat on the component lead rather than the board).

While I've got the board out, I add solder to all the "stressed" connections on it such as pots, jacks, any large components, etc.

All it would take is a little touch-up after the board is wave-soldered at the facory and we wouldn't have such silly problems. Some OEMs are better than others, but it's been going downhill...

(stepping off the soap-box)

-DC

p.s. Keep me/us posted.

[Doug]

I checked some voltages at test points 31 32 33 etc... when in the drive channel (13V?) they're solid, they're sometimes stable for the clean channel (16v) then they swing up and down, not enough to switch the channel, just a dim led or a flicker. It sometimes stays dim, stuck there, in clean. channel switch not working at all, I have to short the chip between legs 6 and 7 to reset the relay. Also when I first power up the amp, it is stuck in drive for several minutes before it will allow me to switch to clean. Is this a cap problem. I believe the switching is drive channel grounded, clean channel otherwise. Is a cap failing until it charges up? Thanks guys. I'm going to check out all your suggestions.

[Enzo]

There are two power resistors R78,79 - 470 ohm 5 w - right next to the bias adjust. Pull the board and look at their solder. Are they trying to unsolder themselves? At least redo the solder. I'd suggest new ones, and sand the green coating off the traces they solder to for a half an inch or so. Run the resistor leads through the holes, and then lay them down flat on the traces for a ways. Now solder all along the length of that wire-on-trace at each end. This will be more reliable and spread the resistor heat out more.

Of course if those are fine, it must be something else.

[Doug]

Thanks Enzo,
Tapping on or moving slightly those power resistors was the only consistent way of reproducing the problem. I re-soldered and made good connections, now I've got more of a problem somehow. It's stuck constantly on clean while the led is dimmly lit. The voltages at the chip which were close but unstable before are now not half what they should be. I'll keep looking,
thanks.
Doug

[Don Moose]

Keep looking at those resistors - it's quite possible you now have a broken trace near those pads.

Enzo's suggestion about reinforcing the traces with the replacement resistors' leads would have prevented that. You could use an inch of wire (solid, AWG 22 or 24 if handy) soldered to the spot where the resistor lead comes through the board and along the trace that you've prepped per Enzo's directions.

Do traces leave those pads on the TOP of the board, too - reinforce those as well.

Hope this helps!

[Doug]

Found a broken trace around that bad solder joint, someone must've been in there before. There had been some work around there. Everything's perfect now, until it happens again I suppose.
Thanks for all your help.
Doug