Channel switching - Blues Deluxe

[lowell]

A customer is saying this amp intermittently switches channels on its own. He is NOT using a footswitch and is always playing through the clean channel when it happens. I've had the amp in before and could not get the problem to happen. I had it on all day and kept going back to it and playing it. As a possible solution I replaced the footswitch jack w/ a switchcraft metal shorting-type jack. The customer came back 2 days later saying it was still happening. I've now had the amp here for 2 days and cannot get this problem to happen. I've hit the amp with my fist, pushed on components with a chop stick and all seems fine.

I just tested TP30 and it reads 3.4VDC on clean setting and 0.6VDC on drive setting. CR7 and R63 test good. Any thoughts on this anyone?

[52 Bill]

There must be a hundred posts about this problem, all relating to the 16v supplies.

Check R85 and R86. The current version of the amp should have 470 ohm, 5 watt resistors in place. If not change them. Check the mounting pads of these resistors as well as around CR22 and CR23 for cold solder and loose leads.

[lowell]

52 Bill I checked the supplies. They seem fine and all voltages and test points seem good except the one I noted in the initial post. Does that 3v instead of the noted 6v mean something to someone? I tested all connected components and all seem fine which leads me to suspect U3.

[R.G.]

The +/- 16V supplies on those amps are a disaster. As a former power supply designer, I'd have fired the guy who let that design go out, especially if he was an MBA bean counter.

Zeners do NOT make for reliable supplies. On all the BDs I see (not many, I admit) I replace the resistor/zener with a pair of three terminal regulators and power resistors. The power resistors only wipe off enough voltage to keep the regulators cooler. The three terminals are thermally self protected and current limited, and therefore about 10 times as hard to kill and 100 times less temperamental than the zeners.

But that's just my opinion.

[mac1amps]

I can recall running into a similar situation with a few Fenders in the late 80's (I'm pretty sure they were the "red knob" Twins). After replacing various "suspect" components with no results, as a desperate "last ditch" effort (I was just about ready to start slamming my head into the wall from frustration), I went over virtually all of the P.C. board soldering (the usual "cold" solder joints from those "wonderful" wave soldering machines at the factories). And, what do you know? It worked like a charm after that, no more channel switching on it's own. Anyone who thinks that "cold" soldering means nothing in a guitar amp is sadly mistaken.

[lowell]

Thanks guys,
Will 15v regulators work here? Also can one type of regulator be used for both sides + and -?

[52 Bill]

Thanks guys,
Will 15v regulators work here? Also can one type of regulator be used for both sides + and -?

R.G. is right about the power supply design, but in my situation I would not alter the low voltage supplies of the amp unless the amp is out of warantee and the owner wants it done.

15 volts should work ok, but you can up the voltage by adding a diode to the regulators ground connection. The normal 78XX and 79XX three leg regulators are either for postive or negative voltages.

How old is this amp? There was an early version problem with one of these models, where there was a TSB regarding random switching problems. The fix was a resistor change, but I don't remember which model this covered. It could have been a Hot Rod version.

Have you checked the footswitch jack? If the normally closed contact is dirty it will cause the amp to switch randomly. Also in some of these amps, if the led indicator breaks loose from the board it will cause problems in the switching circuit.

[lowell]

great thanks! Good idea w/ the diode b/t regulator and ground! I read that the resistor change was to replace 330ohms w/ 470 ohm 5 watts. This amp already has the upgrade.

[lowell]

Should I rid of the 470ohm resistors? I put the voltage regulators in w/ the diodes in series w/ ground pin and voltages are off and the amp is putting out a very high pitch at low volume. Correction: The positive regulator is putting out the correct voltage but the negative is way off... will check further but need to know if should keep those dropping resistors in there.

[lowell]

ok these regulators (mc78m15) are for positive only. I'll order some others...

[Enzo]

7815 are for positive, the 7915 are for negative. And look up the data sheet, the two have different leg patterns.

The probem with the Fender low voltage supply is not the voltage, it is the resistors dropping voltage to the zeners. They get too hot.

If the negatove zener voltage is way off, then it is one of these most likely:
1. Failing zener
2. failing 470 ohm resistor or the solder path to it.
3. something in th circuit dragging it down. Bad op amp, shorted diode, etc.

[mikeboone]

Check the solder joint on the indicator LED itself. I went through this nightmare with my HR Deluxe until I noticed that a physical jolt to the amp chassis would cause it to switch.

[MarkusBass]

Lowell,

I think that it is important to understand how the channel switching works in this amp. The U3A opamp is configured as a comparator. It compares two voltages; the first one is voltage on CR9 diode (it should be 0.6V) and the other is the voltage available at TP30. If you check the schematic, it says "greater than +0.6V for Clear channel" and "less than +0.6V for Drive channel" (please note that this is +0.6V and not 6V). Quick simulation shows that the voltage at TP30 should be roughly +2.4V (on Clear channel) and 0V when the footswitch is shorted to ground (Drive channel) (or close to 0V if there is a LED in the footswitch).
I suggest that you measure these two voltages with DMM (and the difference between them). In my opinion the footswitch jack is not contacting (leads 1 and 2) due to some dirt in it. In this case CR6 diode gets disconnected and it switches the Drive channel. Just clean the jack.
Of course I assume that +16V is correct.

Mark

[lowell]

thanks for the tips on this guys. Markus I had already replaced the fs jack so it's got to be something else. Enzo shall I put 10w 470 in there? I guess I will measure the voltage drop across them, square that number and divide by 470 and see what the wattage dissipation is... then I'll put twice that rating in there. I DID notice those dropping resistors were ridiculously hot to the touch, but wasn't sure if this was normal or not.

[Enzo]

If you felt like it you could mount a couple of those aluminum finned ones of to the side on the chassis wall and run wires back. I find that instead of sticking the resistor wire leads straight through the hole and soldering, I bare the copper trace a ways away from the hole and lay the resistor lead along it for half an inch or so. This acts as a small heat sink and is more sturdy. Others have stood the resistor up off the board on little tubes around the wire leads. Little sections of brass tube can be soldered and also act as heat sinks. Even just raising the resistors off the board a little adds wire lead length for extra heat dissipation but also allows more air cooling. A glob of silicone keeps them from moving.


But why not consider RGs suggestion of mounting a 7815 and 7915 in there?