Input problem

[EFK]

I'm sure it's something stupid or perhaps staring me in the face but for the life of me I cannot figure this out. This amp has two sets of inputs, one set per channel. The treble input for each channel will not work - if I plug in, silence. The normal inputs work fine. If I stick a plug into either normal jack, then the respective treble input will work if I plug into that at the same time. The normal jacks are switched, the treble jacks are not. All grounds seem to check out. ???

[PepeSilvia]

Check continuity across your switched leads when you don't have a plug in the normal jack. You might need to clean or replace the jack.

[Albert Kreuzer]

Maybe the switching contacts and input contacts on the normal jacks are reversed? Not all jacks have the same layout.

[Euthymia]

What kind of amp is this? That input scheme is kind of unusual.

[Enzo]

Don't plug anything in. measure resustance across that 470k resistor. SInce the normal jack puts the 22k in parallel with it, I'd expect 20k or so. However, if you measure a short to ground - that is the 470k reads zero ohms - then you have miswired the normal jack.

Also, when you use the normal jack, there is a 470k load on the signal. But when you use the bright jack, then there is only that 20k load, that may be too low for your pickup. Plugging something into the normal jack lifts the contact and thus the 22k is out of circuit, Now that way even the bright jack has the 470k load.

[EFK]

This is one of my many Supros. I have a bit of a problem - I can't stop accumulating them. Thanks for the suggestions guys. I haven't touched this one yet - all I've done so far is change out the filter caps. The circuit is entirely untouched. I have seen this input scheme on many of them, or variations on it, but the value of the 22K resistors varies - some as high as 100K. Continuity all seems to check out on the normal jacks - Enzo I really appreciate the advice and will check that out tomorrow morning. As a side question, what is the purpose of loading down the input in this manner? To keep the volume down? 470K seems like a lot of loading.

[Enzo]

470k is a lot? Seems like a pretty high resistance to me. The 470k is going to have very little affect on the signal level. That 22k is doing all the loading down.

[EFK]

WEll it gets weirder. I measured across the 470K w/ nothing plugged in and got a O ohm reading. I took a look at another one of my Supros which appears to be wired exactly the same way and double checked everything, and the jacks appear to be wired correctly. The other amp has 100K in place of the 22K, however. On a whim, I removed one of the 22K resistors and alligator clipped in a 100K resistor - treble inputs now work. ????? I tried clipping in a 47K, got nothing. Can someone explain this? Apparently for this to work, there needs to be a particular resistor value, or a minimum value, in place. I'm at a loss, because the jacks definitely are wired correctly - at least, wired as per every other switched jack I've ever seen or used.

[52 Bill]

Is it possible that the bright input works, but is so attenuated by the 22K resistor, that you can't hear it without turning up the volume?

[EFK]

52 Bill - if it's possible, I don;t understand it. I understand how a typical Marshall 4-holer works; in that scenario, the lower the resistance (68K vs. 34K vs. 22K etc) the more signal that is getting to the first stage. I'm assuming that this works the same way but apparently not; how is a lower value here (22K) attenuating signal whereupon a 100K is not?

[52 Bill]

The 22K resistor is grounded through the normally closed input jack switch, creating a simple voltage divider, sending most of the signal to ground. Increasing the value to 100K reduces the amount of signal that is being sent to ground.

[EFK]

So it reduces the signal from the treble input sent to ground? What about the normal input? It seems that increasing the resistor allows more signal through the treble input, but maybe reduces the signal which has to pass through it from the normal input?

[EFK]

Now it's odd-er yet. I replaced the 22K w/ 100K on the 1st channel and both inputs work perfectly. However, even w/ the 100K on the second channel, the treble input still won't work. Thinking possibly the switched (normal) jack was funky, I replaced that with another. No difference - treble input still will not work unless I push open the switched jack. I have repeatedly tested for continuity, checked the treble cap with LCR meter.... what the heck is going on here?

[Enzo]

When you checked the jack wiring, did you just compare the solder terminal positions? Not all jacks have the terminals in the same positions, so if there are two different brands of jack involved, there could be some confusion.

Remember what I said in post #5? If you measure across the 470k and get zero ohms, your jack is miswired.

Look at your schematic. The 470k sits there with a 22k next to it, and the end of the 22k away from the 470k is the end that should be grounded, NOT the end connected to the 470k. The 22k is grounded through that cutout contact.

Answer me this, is the 22k just soldered from one jack terminal to the other? Look closely at the construction of the jack. Look at the layers and where the terminals come out for each thing. Look at the schematic. See the point where the 470k and the 22k are joined? Note that that point is NOT a terminal on the jack. If your 470k is soldered to the jack, that doesn;t sound right.

[EFK]

Enzo I'll double check on all that shortly. The jacks are old switchcraft non-isolated switch jacks, three terminals. I've triple checked them - they are wired correctly. The two ground lugs are bussed together on all of them. None of the components are on the jacks, they are all on tag strips. The two input schemes (each channel) are mirror images of each other, save that the channel two input does not have the 2.2K resistor into the grid of the triode. Don't know if that makes a difference but I wouldn't think so. Here's a rough sketch: