Problem witjh Mojotone 5E8-A - Treble pot acting as Volume control

[nickb]

You removed the 220K and left the treble in place. You need to lift the 220k at the wiper end of the treble pot and also the wire that goes to the treble pot wiper and join them together, leaving the treble wiper open circuit. This allows you to test just the bass path.

[Steve A.]

There are only 9 components between V3 and V4 which is the entire passive circuitry for the tone controls so I would recommend confirming all connections and all of the values of the resistors, pots and capacitors with a good DMM. (I have seen kits with some of the parts off by a factor of 10 or 100.) Before removing any components for testing I would recommend checking for resistance to ground at each eyelet, comparing it to what you would expect looking at the schematic, with each pot full on, full off and right in the middle. It seems to me that something is draining the signal to ground with some of the settings for the controls.

[nadles]

You removed the 220K and left the treble in place. You need to lift the 220k at the wiper end of the treble pot and also the wire that goes to the treble pot wiper and join them together, leaving the treble wiper open circuit. This allows you to test just the bass path.

OK thanks. I misunderstood. I'll give this a try.

[nadles]

There are only 9 components between V3 and V4 which is the entire passive circuitry for the tone controls so I would recommend confirming all connections and all of the values of the resistors, pots and capacitors with a good DMM. (I have seen kits with some of the parts off by a factor of 10 or 100.) Before removing any components for testing I would recommend checking for resistance to ground at each eyelet, comparing it to what you would expect looking at the schematic, with each pot full on, full off and right in the middle. It seems to me that something is draining the signal to ground with some of the settings for the controls.

Thanks Steve. This is my project for the weekend .

As well as draining the signal to ground, its also draining the filter caps as soon as the amp is switched off.

[Jazz P Bass]

Are we positive that what you are seeing is NOT the way it is supposed to be?
Tone controls can often affect each other.
Also, Fender has in the past, used tone circuits that literally removed that band of frequencies. (ie: full cut).
There is your 'volume loss'.

[Les s of Paul]

Turning the volume pots doesn't affect the resistance readings on the treble pot legs.

Uhm... considering only one leg is connected to ground why is the pot not changing the reading?^^

interesting, seems like the signal is being forced to ground, yet no immediate wiring problems seem to be the cause.
Does the volume pot itself have a significant effect on the volume? or has the treble pot made it essentially useless? i'm probably barking up the wrong tree, but might the problem rest with a bad volume ground?
i would consider using your tape deck again, and tracking the signal through the path with the AC mV setting on the meter, you might find where the signal is vanishing to.
Doesent appear to be a pot problem, although that's what i thought until it turned out my volume pot was partially disconnected on one lug on the inside.

I don't pretend to be an expert, but by asking stupid and sometimes meaningless questions an epiphany can result.
please don't take offence, i'm trying to help, and learn at the same time.

[loudthud]

I suggest that you ohm out all the wires that connect on the back side of the eyelet board. Either between eyelets or to tube socket pins or pots etc.

[Steve A.]

The picture on the Mojotone site shows the eyelet board but no blank plate to go behind it to insulate it from chassis. They apparently included spacers to raise it up from the chassis but it is possible that a solder drip or too-long component lead is shorting out to the chassis. (I suspect that the Fender practice of screwing the eyelet board (with a blank back plate) directly to the chassis kept the noise level down on those old amps.)

Steve

EDIT When I run across a problem like that (tube voltages OK, unusual loss of signal) one of the things I do is check the DC resistance to ground from every eyelet, copper pad and terminal within the area of the circuit that seems to be the problem, using printouts of the schematic and layout drawing to compute the expected readings. Looking at the schematic you should be able to tell if any of the readings are "out of whack" (to use the technical term.)

One formula to remember: to find the total resistance of two resistors in parallel add together the reciprocals of the values- and then find the reciprocal of that sum. I use the Windows calculator all of the time since it has the 1/X button. To figure out what value resistor you need to add to an existing resistor to end up with the desired resistance you SUBTRACT the reciprocal of the existing resistor from the reciprocal of the desired resistance- and then take the reciprocal of that. BTW that formula also works with capacitors wired up in series.

WOW! I just noticed that most if not all of the coupling and tone caps are polypropylene orange drops. I usually use polyester caps unless there is part of the circuit that needs to be brighter. The Orange Drops have finally been discontinued and I expect the remaining stock to keep going up in price. In most cases I think that Mallory 150's work as well as the polyester orange drops. (For BF/SF tone stacks I like putting two big Orange Drops next to each other- I have a hunch that there is some parasitic action going on between them- but that would depend on how you orient the outer and inner terminals.)

[nadles]

Are we positive that what you are seeing is NOT the way it is supposed to be?
Tone controls can often affect each other.
Also, Fender has in the past, used tone circuits that literally removed that band of frequencies. (ie: full cut).
There is your 'volume loss'.

I don't think its how it should be.

With the tone on 0 the sound is muffled and the volume is very low. Turn up the treble and the volume and brightness increase significantly, but with vol on 7 and treble on 7 its only as loud as Deluxe Reverb on 3.5. I would expect the volume to be closer to my JTM45 and at 7 that's painful to be near. with the tone stack removed from the circuit there was a dramatic increase in volume and its is now on a par with the JTM.

[nadles]

Uhm... considering only one leg is connected to ground why is the pot not changing the reading?^^

interesting, seems like the signal is being forced to ground, yet no immediate wiring problems seem to be the cause.
Does the volume pot itself have a significant effect on the volume? or has the treble pot made it essentially useless? i'm probably barking up the wrong tree, but might the problem rest with a bad volume ground?
i would consider using your tape deck again, and tracking the signal through the path with the AC mV setting on the meter, you might find where the signal is vanishing to.
Doesent appear to be a pot problem, although that's what i thought until it turned out my volume pot was partially disconnected on one lug on the inside.

I don't pretend to be an expert, but by asking stupid and sometimes meaningless questions an epiphany can result.
please don't take offence, i'm trying to help, and learn at the same time.

Thanks. The volume pots do work, but the treble pot has a greater affect on the overall volume. This problem affects both channels. This weekend I'll plan to check all components and trace the signal as you suggest.

[nadles]

I suggest that you ohm out all the wires that connect on the back side of the eyelet board. Either between eyelets or to tube socket pins or pots etc.

Will do. I've checked for continuity, but not resistance.

[nadles]

The picture on the Mojotone site shows the eyelet board but no blank plate to go behind it to insulate it from chassis. They apparently included spacers to raise it up from the chassis but it is possible that a solder drip or too-long component lead is shorting out to the chassis. (I suspect that the Fender practice of screwing the eyelet board (with a blank back plate) directly to the chassis kept the noise level down on those old amps.)

Steve

EDIT When I run across a problem like that (tube voltages OK, unusual loss of signal) one of the things I do is check the DC resistance to ground from every eyelet, copper pad and terminal within the area of the circuit that seems to be the problem, using printouts of the schematic and layout drawing to compute the expected readings. Looking at the schematic you should be able to tell if any of the readings are "out of whack" (to use the technical term.)

One formula to remember: to find the total resistance of two resistors in parallel add together the reciprocals of the values- and then find the reciprocal of that sum. I use the Windows calculator all of the time since it has the 1/X button. To figure out what value resistor you need to add to an existing resistor to end up with the desired resistance you SUBTRACT the reciprocal of the existing resistor from the reciprocal of the desired resistance- and then take the reciprocal of that. BTW that formula also works with capacitors wired up in series.

WOW! I just noticed that most if not all of the coupling and tone caps are polypropylene orange drops. I usually use polyester caps unless there is part of the circuit that needs to be brighter. The Orange Drops have finally been discontinued and I expect the remaining stock to keep going up in price. In most cases I think that Mallory 150's work as well as the polyester orange drops. (For BF/SF tone stacks I like putting two big Orange Drops next to each other- I have a hunch that there is some parasitic action going on between them- but that would depend on how you orient the outer and inner terminals.)

Thanks Steve. There is a blank board between the eyelet board and the chassis.

I'll check all DC resistances to look for anomalies, but probably won't be able to do this till the weekend. Any specific "out of whack" value i should expect if a is component sending the signal to ground (high or low or just wrong )

The orange drops come standard with the kit, in past builds I've used Mallory 150s.

[Steve A.]

Here is a link to the instruction manual so everybody can see some pictures. It shows a whole shitload of wires on the back side of the fiberboard. As suggested previously you should check the wiring there. I think that you should do that before you start ohm-ing everything out (which can get very tedious.)

http://mojotone.com/manuals/Tweed%20...05499&gc=clear

[nadles]

Trouble shooting's begun in earnest. As I work through the suggestions above I'll post the results.

Here is a link to the instruction manual so everybody can see some pictures. It shows a whole shitload of wires on the back side of the fiberboard. As suggested previously you should check the wiring there. I think that you should do that before you start ohm-ing everything out (which can get very tedious.)

http://mojotone.com/manuals/Tweed%20...05499&gc=clear

I've checked continuity on all back of the board wiring, including connections to pot lugs and valve sockets, and all check out OK.
I'll post a photo i took of the back of board wiring before the board was installed.

You removed the 220K and left the treble in place. You need to lift the 220k at the wiper end of the treble pot and also the wire that goes to the treble pot wiper and join them together, leaving the treble wiper open circuit. This allows you to test just the bass path.

I've tested the the bass path - lifted the wire from the wiper of the treble pot and connected it directly to the 220k connected to the wiper of the bass pot. Results as follows:

Bass pot 0-2: - No output. With vol control on full very faint sound barely discernible.
Bass pot 3-9 - Volume comes on at 3 and increase as bass pot turned up to 9.
Bass pot 9-12: highs roll off no real change in volume.

Next step will be to check the values of the components in the tone circuit before moving on.

[nadles]

Back of the board


Front of the board