Killed my Princeton treble

[MacGwyn]

Been trying some tone mods on a blackface Princeton Reverb '67 and somehow managed to render the treble control inoperative.  I've put everything back to original now... checked resistors and capacitors for proper values and continuity-checked the entire tone circuit to pots and pre-tube one.  Substituted new preamp tubes and installed new 250k treble pot.  Still, volume works normally, bass works normally, but treble has no effect at all.  The good news is, it sounds great!.  Bad news, I want my treble control back in operation.  Any thoughts on what to try next? 

[sluckey]

Double check your wiring. It's still not wired properly.

[MacGwyn]

Sluckey, It looks right according to the AA764 layout notice.  Looking from rear, treb center lug connects to vol right lug... treb right lug connected by white wire to 250p cap on board... and treb left lug connected to mid lug on bass pot.  See anything that needs changing?

[HotBluePlates]

Sluckey, It looks right according to the AA764 layout notice. 

Hoffman's Law: "If it was wired right, it would be working now."

It worked, then you tinkered, now it doesn't work. You re-wired "correctly" but it still doesn't work. Therefore something you changed is still not back to stock. We don't say it to be jerks, but to reinforce that you have to suspect everything you altered in the tone circuit area when you tried mods.

Looking from rear, treb center lug connects to vol right lug... treb right lug connected by white wire to 250p cap on board... and treb left lug connected to mid lug on bass pot.  See anything that needs changing?

All that is as the layout shows, but we can't see your amp. Did you change any wiring or parts locations on the circuit board? Did you move a wire that runs from tube socket to the board? Did you triple-check you wires and really have them routed as you described (and not a simple error)?

Another possibility: Let's say all wires and parts are in correct locations. Did you have a cold solder joint at the 250pF cap? A break in the solid-core wire between 250pF cap and Treble pot? Cold solder joint at the pots?

[sluckey]

It looks right according to the AA764 layout notice.  Looking from rear, treb center lug connects to vol right lug... treb right lug connected by white wire to 250p cap on board... and treb left lug connected to mid lug on bass pot.  See anything that needs changing?

I'll assume you mean AA1164 layout? The treble pot is wired IAW the AA1164 layout. But there's more to the tone circuit than just the pot. Something is still not wired properly. Or you have a failed component, broken wire, bad solder joint, etc.

[MacGwyn]

The amp is labeled AA764.  I have schematics for both that one and the AA1164, and the tone circuits appear to be the same.  Could not find a layout plan for 764... so I am using the 1164 layout from Fender.  I've pretty much eliminated cold joints and broken wires by using VOM continuity checks and by individually "doubling" each connection with alligator jumpers... still no action from the treble control.  I know it's either my bad or a bad component - so I'm thankful for any and all helpful suggestions!

[sluckey]

Tell us about the tone mods you were trying. Post some hi rez pics of the pots and circuit board. Maybe we can spot something.

[MacGwyn]

Tried several different values for the three tone caps and the associated 100k resistor.  The 'mid' resistor on the bass pot is a bit higher than the standard 6800 - but that worked fine before the treble control went dead.  At one point, I replaced that mid resistor (6800) with a pot to vary the resistance value to ground, but that has now been removed.  Tried to upload photos but got a "file too large" message.

[HotBluePlates]

Then use a program on your computer to resize the pictures. Windows Picture Manager works on a windows machine, and is typically installed.

[Fresh_Start]

Have you checked the operation of the treble pot with a DMM?  IOW one clip on the wiper and the other on either end of the pot. Does resistance change when you rotate pot?

It's also easy to fry those small caps. Is there any DC voltage on the treble pot end of the treble cap?

Respectfully,

Chip

[sluckey]

The 'mid' resistor on the bass pot is a bit higher than the standard 6800

What size is it?

[Willabe]

Here's a link for how to resize a photo to post from our host Doug.

http://el34world.com/Forum/index.php?topic=464.0


                    Brad     

[eleventeen]

With power off, clamp your ohmmeter leads on the treble control 1-2 and 2-3 and be sure that rotating the pot changes resistance between either end and the wiper. Also check end-to-end resistance, 1-3. If you had the amp in a car and took a sharp corner and the amp fell over, I have seen the result of that be a pot that seemed to be OK but the impact cracked the horseshoe of resistance material. OR...the impact tweaked the wiper assembly in such a way that it no longer makes contact with the resistive horseshoe. Either of those faults would cause the treble control to not work and gets my vote for the villain here.

Unless the pot is damaged, it *has* to be the 250 pf in the tone network, doesn't it? You could tack-solder any reasonable value cap in there say 100pf-.02 uf just to see if the action of the treble control actually did/does anything. Pretty rare for those to go out.


[size=78%]And by the way, just to rule out the completely ridiculous....the setscrew in the knob is tight against the pot shaft and is actually turning it, right? [/size]
[/size]
[/size][size=78%]Small font brought to you courtesy of "the cloud". [/size]

[MacGwyn]

The currently installed mid resistor on the bass pot is just over 11k.  I understand that 15k+ is acceptable, in order to decrease the mid-cut.

[MacGwyn]

With amp turned on, there is zero voltage on the treble pot-side of the 250p capacitor.  Opposite side of the cap has 150+ volts.
In resistance, the treble pot ranges:
Left lug to right lug, 205k to 252k
Left lug to middle, 1 ohm to 205k and
Right lug to middle, 1 ohm to 252k.
I did not remove the pot's connections for the test, so they were all in place.
This is a new 250k pot, by the way.  Still have the original which was replaced yesterday during trouble-shooting.

[MacGwyn]

Pots picture

[MacGwyn]

Amp board pic.

[MacGwyn]

I've also tried replacing the 250p cap - no response at the treble control.  Sound is nice, crisp and balanced... bass tone works fine... treble is unchanged no matter what position that pot is in.

[HotBluePlates]

The wire from the middle lug of the Treble pot to the Volume pot looks... uhhh... cut in half? You picture shows what sure appears to be 2 non-continuous pieces of wire for that connection.

EDIT: Added circle to picture to show the break I'm talking about.

[eleventeen]

Very weird. (You already knew that)


Sure seems like you have a broken or an intermittent wire somewhere or a bad solder joint. I know you say you have checked for these possibilities. 


I guess the next thing I would do would be to lift one end of the .1 and the .047 uf tone forming caps. You could do the same by disconnecting the blue and brown wires from the bass control; that might be the easiest physical means. Or, you could heat up the solder joints and try to lift up (only) one end of those caps on the parts board, on the front-panel side of those caps. If you do this, there is no path from V1 to the volume control to the grid of V2 other than through that 250 pf and the treble control. With signal source on the amp input, a "listening amp" or signal tracer or preferably an oscilloscope...somewhere, the signal is disappearing between the plate resistor of V1 and the wiper of the volume control. There are about 5 or 6 soldered connections along that path and at one of them or between two of them, the signal goes bye-bye. And that's about it.

[eleventeen]

I saw that, too, HBP, and yes it looks that way, but I'd have to guess it's an illusion and the wire is recurved on itself. Unless the OP is missing something very much "in your face" obvious. 


Cut in half or not...why/how is that wire so long?

[MacGwyn]

The wire in question does look cut, but it is only folded.  It was originally tucked up under the lip of the chassis (thus the length)... and will go back there when this problem is cleared.  I'll try lifting the other caps as suggested tomorrow and report the results.

[eleventeen]

If you have no other test gear, you should be able to tack-solder one side of a cap (any size, typically a .01 / 400) to the 2nd half-tube's grid and a 10-12" length of wire to the other side of the cap as a "probe". Solder one lead of the cap directly to the tube socket, pin 7, do not rely on any other piece of wire. The "probe" end of the cap just hangs in the air. In effect, you are going to use the remainder of the amp as your signal tracer.  With a signal generator or minimally a radio or CD player playing into the amp input, you should be able to safely follow a signal all the way back to (and including) the plate of V1 with its 180 volts and either get or not get a signal. This would be good to do with the .1 and .047 caps lifted.


*IF* you touch an obvious high-voltage point with your "probe" you'll want to discharge the cap by touching it to ground/the chassis before the next measurement.


You're gonna tell us what the problem was when it's all over, right?

[HotBluePlates]

The wire is solid-core. A fold that severe could break it.

Regardless, the problem has to be simple: wrong/broken wire, cold solder joint. Anything you messed with for modification would be suspect.

I strongly believe that when you find and fix the problem, you will see that it is something you assumed to be properly connected, but wasn't.

[MacGwyn]

I disconnected the .1 and .047 tone caps and could hear that the treble control is working to some extent.  With vol set at 3, treble control sent a "tinny" signal to the output from a strong signal at 10, to almost no volume at 0 treb.  However, when i reconnected the mid and low tone caps, the treble sweep was again, not discernible.  Seems that the mid and bass are overwhelming the treble control, somehow.  I'm thinking now of replacing the bass pot with a new 250k.
In the mean time, I did discover a one of the original caps was pretty badly out of spec, just "up river" from the tone stack.  I replaced a 500p with a like-value new one... but that didn't change the treble control problem.
I'm a little scared to try the test-probe suggestion.  If you could be a little more specific on what components should be tested - and what it is I'm looking for when they're "probed" - I'd like to try it.

[HotBluePlates]

Do you have another 250pF cap handy? Maybe the original ceramic cap? If so, swap it in.

I think it would be highly unlikely there is any defect in the pots themselves (cold solder joint or poor wire connection would be much more likely).

[eleventeen]

[with the probe/signal tracer idea] you are not testing "components" per se, you are testing "nodes". You'd be checking for signal presence at the series of soldered connections starting let's say from the hot side of the 250 pf (which is the plate of v1 and you could probably eliminate that one since that same point is adequately feeding the bass ctrl) >> the cold side of the 250 pf > the top of the treb control > the wiper of the treb control > the wire going to V1 pin 7, then V1, pin 7 itself. You are looking for that place where the signal disappears. Once you find that, then you assign blame.


Consider: Let's say we are talking about the wire that connects V1 plate to the 250pf. Under normal conditions, we assume that each and every point along that wire (assume it was a bare wire) and the solder joint on the 250 pf and the lead going into the 250 pf and pin 1 of V1 and the solder joint at pin 1, V1 are all exactly the same electrical point...and that would be true.  If for some reason we wished to connect a wire to that point, we could connect to any point along the wire, either solder joint, the lead going into the 250 pf and we would not be concerned that some electrons would be headed one way and some the other. I'm giving a silly overdetailed explanation.


But the fact of the matter is that your amp is acting like one of those elements is losing the signal. Yes, it is strange. What we'd like to find out is, where does the signal disappear?

[MacGwyn]

Just got a new 250p cap in from Hoffman this morning... no difference after installed.

[sluckey]

I'd like for you to put a 6.8K resistor on the bass pot just like the original. Does that help?

[MacGwyn]

Yes - put a 6.8k on the bass pot... again, got it in this morning from Hoffman.  No change in treble control.

[eleventeen]

We need an MP3 of some scary suspenseful Alfred Hitchcock music for times like this.

[mresistor]

Maybe the problem isn't in the tone stack...   maybe something is happening like a "raw" control turned up.
(just thinking out loud)

[sluckey]

maybe something is happening like a "raw" control turned up.

That's why I wanted to see a 6.8K put back on the bass pot.

[mresistor]

curious - what is the plate voltage on the first triode stage?

[MacGwyn]

I get 140v on pin 1 and 150v on pin 6 of the first preamp tube.

[mresistor]

Sorry I see in post #14 you said 150v on the first triode plate. When you continuity checked the tone stack did you use the diode setting on the meter or did you use the ohm function in auto mode? I ask because if there is a break in the white wire going from 250pf to treble pot and there is significant resistance it could be your problem. Bad solder joint could do it too. Clip on to the pot end of the cap and the pot terminal metal on the other end and check ohms reading. (I know there is good chance it is OK but there has got to be an answer to this problem) See if treble works with this jumper in place.

[MacGwyn]

MResistor is the winner!  The jumper between treble lug and 250p cap brought the treble control back to life!  Could have sworn I had already tried that.
Problem now is, it's backwards... max treble at 1, minimum at 10 (on the treble knob).  Does that mean that on the treble pot, I should keep the wiper connected to volume... and swap the left and right lug connections?  (As currently configured, left lug goes to the bass pot and right lug to the 250p cap, as per the Fender layout sheet).

[mresistor]

Bravo!  and yes on the wire swap.  Replacing that white wire might be a little tricky as it goes under the board. I'd loosen the board mounts and pull up a little on it, then unwrap the white wire from the others and then, after wicking all the solder from the cap/wire junction, try pushing the wire up a little through the hole, enough to solder the end of a new wire to it and then carefully pull the new wire through. It's really tricky and you need some finesse.
Maybe someone else will chime in with a better idea of how to replace it.

[HotBluePlates]

MResistor is the winner!  The jumper between treble lug and 250p cap brought the treble control back to life!  Could have sworn I had already tried that.

Reply #3: "Another possibility: Let's say all wires and parts are in correct locations. ... A break in the solid-core wire between 250pF cap and Treble pot?"

If you're troubleshooting an amp, the most dangerous thing is to assume, especially that you checked something or completed some procedure correctly. The only safe approach is to assume everything is bad, until a conclusive test shows that it is good.

[mresistor]

MResistor is the winner! 

Naw, I'm not the winner, HPB did point out the possible problem long before I did. I just couldn't believe the problem could be something else (along with everyone else at this point) and figured that we should take another look at the obvious. Sometimes people looking for help on the forums don't have a lot of experience with the actual hands on troubleshooting steps and some of the nuances in the physical application of the procedures to trouble shoot. And sometimes we all make mistakes and overlook clues that are staring us in the face. (pointing the finger at myself here, LOL)
 I'm just gratified that the problem is resolved and the amp will be restored to normal operation; as that is the ultimate aim of this forum.

[MacGwyn]

Well said... thanks to everyone for your interest and help. 

[eleventeen]

Plus, it should be said, that 50-year old electronics can have faults we would never imagine in say, a 10-year old device.

[super&plexi]

This 50 yr old's faults are usually attributable to my acting like a 10 yr old..... or two. Glad you worked it out.


ran into similar problem. was melted jacket coating my wire connection. even with jewelers loupe I couldn't see it

[Indyryder02]

I have one in the shop, clone.. had same problem.. oddly it shows good continuity from cap to treble pot lug but doesn’t work without the jumper. And it’s also backwards.. from the fender layout..  same as this one was described..
not sure why on either of these!
Batch it these pots assembled backwards? Is that possible? Haha