Fender Hot Rod DeLuxe Channel Switch Problem

[CJ]

this amp has a rep for doing channel switching by itself,

there is some information online that points to supply rails moving due to damaged traces on some 470 ohm dropping resistors that feed the +/- 16 volt zeners,

so we mounted  the 470 ohm resistors on the chassis with some heat sink grease underneath, ran the leads to some good traces which were scraped with a razor to expose the copper, got rid of the messed up pads, now they run cool but the problem seems to be there still,

does this circuit look prone to tripping states due to being unstable, if  so, how do we tweak it to be more stable?

do we really need 1 meg feedback resistors, thinking maybe if we reduce those resistor to maybe 100K it will not act like a sensitive comparator so much,

pwr supply is shown on bottom right, Fender put out a blurb on reducing the pwr resistors from 470 to 330 but some folks say this is not a fix, puts excess current thru the 5 watt zeners, 

seems like a lot of parts to get 3 way switching, eh?
the relays (K1,K2) switch the signal thru different gain vac tube channels,
looking at a possible problem with loose contacts on the pc mount foot sw jack also,


thanks for any help,

related info>

http://aztechmusic.com/213/guitar-amp-channel-switching-problem-and-how-to-repair-it/

relay current goes thru the  tri colored LED>

http://music-electronics-forum.com/t25935/

schemo for the whole enchilada>

http://support.fender.com/schematics/guitar_amplifiers/Hot_Rod_Deluxe_Schematic.pdf

 

[joaquins]

I'd try changing CR27 for a 5V zenner or something like this to increase the threshold for switching, if that's the comparator which is causing the problem.

It definitely looks over complicated for 3 way switching... and if it's unstable and a known issue... come on fender guys, you can do it better!

JS

 

[Andy Peters]

I have one of these amps. I haven't noticed the channel select being twitchy, but then again, I play through it once a year for ten minutes.

Why oh why do they use an op-amp instead of a comparator? It all looks way too complicated.

 

[CJ]

well, don't rack your brains out over that switching kludge, turns out to be the cheezball push button sw, so we are wiring in some micro toggles for drive and more drive,

the switch, number one component to fail in all of electronics, always go for the sw first, 

 

[PRR]

> Why oh why do they use an op-amp instead of a comparator?

What is the difference?

Oh, I know.... but speed is not needed, TTL-level output is not needed, and most important: the op-amp is in the parts-bin and no comparator is. To get a comparator someone would have to pitch a business-case for buying another part-bin, specifying specs, approve vendors, add a line to the inventory database, tell the human/robot to fill the bin, keep it filled, but never over-filled, which means keeping track of expected and actual demand, and when that demand stops.

> definitely looks over complicated

I've seen worse.

It does cleverly get multiple functions on ONE conductor, by applying AC and letting the remote switch do different things on opposite half-cycles. It also supplies power to the LEDs in the remote switch. The input protection could perhaps be a few cents cheaper, but once you have PCBs and parts-bins, adding parts to a product costs very-very little. Perhaps less over the run of production than you pay the Designer to design-out parts yet prove the simpler plan will NOT give trouble down the road (in users' hands, or when vendors do minor changes to parts).

Amps similar to this (+/-15V from power resistors) do, *some of them*, tend to have problems with these resistors cooking. Apparently the Designer was too stressed to really think about the stress on these resistors and the PCB-stuff when boxed-up with a hot power amp and played on hot stages or out in the sun. (Or maybe it has been calibrated to fail long after warranty expires, so you have a reason to buy a new amp.)

Failed switch. Who wudda thunk??

 

[CJ]

a big circuit board in the same box as a 12 inch speaker is gonna get some mechanical stuff going on, so the push button switch would go off and on while the amp was played,

also had an 82K resistor go to 120K in the phase inverter which caused a mismatched signal being fed to the power tubes, reverb pan was swapped for a non working model, B+ for the inverter was shorting to the pwr tube pd board mounting screw, wrong opamp was stuffed in the TL072 spot, other than that, a piece of cake,  :