YMI5150 Early '68 12xxx build journal

[Strat78]

Awesome mounting of the board bias pot. How did you get that bottom leg to come off at a right angle to meet the turret? Also, you have a cap on the volume pot, I forgot to do that, what does it do? I nearly dimmed mine today for the first time, wow, what a beautiful woody tone!

[vh junkie]

here's a few shot. first documenting "time of birth" and the others are of the bias pot. I thought it might be suspect for high bias voltage but it appears correct. went ahead and powered up for a second and no change. reading is -121 VDC.

Where are you reading this voltage?

What reading are you getting at the ends of the two 220k resistors(on the board) where they connect to the wires going to the power tubes(via 5.6k resistors)? Can you vary this voltage with the board pot? Generally the instructions say to set this to the most negative voltage you can(i.e. -70vdc is better than -30vdc).

[vh junkie]

Awesome mounting of the board bias pot. How did you get that bottom leg to come off at a right angle to meet the turret?

He has a different style pot where the center lug comes out each side, instead of the bottom.

Also, you have a cap on the volume pot, I forgot to do that, what does it do?

It allows more treble to pass as you turn down the volume. Don't need if you always gonna be dimed (using a PPIMV or attenuator/load), otherwise might want to add it in there.

[YMI5150?]

pin 5, V5. also measure about -127vdc just off the board pot and no change when adjusting the pot. i do get a change in resistance when adjusting the pot so its good. this is gonna be tough to troubleshoot since I'm at work when most of you are on the boards...

[YMI5150?]

i wonder about the bias pot wiring. it matches the layout but it seems like the bias legs are bridged (if that's possible). the value is about double and the pot has no effect on either leg.

[YMI5150?]

I get -125vdc at pin 5 on v4-v7 with no effect from pot. nice and cool...

looking forward to figuring this one out. so close...

[YMI5150?]

just started measuring resistor values. the 47k AB resistor on the presence leg is measuring 4.7k. looks like they selected orange when they should have selected red for the third stripe. not sure if this would be the problem but needs to be swapped out.

[vanhalen5150]

just started measuring resistor values. the 47k AB resistor on the presence leg is measuring 4.7k. looks like they selected orange when they should have selected red for the third stripe. not sure if this would be the problem but needs to be swapped out.

Same thing happened to me. I'm not sure if you can measure across it while its on the board or not. You may have to lift one end. Mine was definately orange. There is a pic in my thread. I left it in.

[vh junkie]

I get -125vdc at pin 5 on v4-v7 with no effect from pot. nice and cool...
looking forward to figuring this one out. so close...

Everything looks right in the pics. Is there a ground connected to the positive side of the bias caps and therefore the center and right leg of the bias pot.

Are the 3 bias resistors the correct values? Looks like they should be 27k, 25k, and 47k? In mine looks like I have a 56k instead of the 47k.

I would expect the voltage to be a larger negative vdc (-100) without the tubes in and drop into a lower value(-50vdc) once the tubes are in. It just bugs me that the pot doesn't seem to do anything.

just started measuring resistor values. the 47k AB resistor on the presence leg is measuring 4.7k. looks like they selected orange when they should have selected red for the third stripe. not sure if this would be the problem but needs to be swapped out.

Keep in mind, you generally can't measure the values of resistors without lifting one end of the circuit, because any other resistance in parallel will lower the value being read. In the is case(with the tubes out), what you are seeing may be correct... lift one end and check it again... it definitely should not be that small...

[YMI5150?]

@ work now so I can't get any additional data until tonight. it would be nice to get an action plan together during work.

the bias pot's three legs are all continuous. the top right leg (see pic) varies with the pot setting. the right bias cap turret is not grounded. aside from the pot, that leg terminates to nothing but the turret. looks like that's the issue. the struggle here is the references are not consistent. according to the attached schematic, it should see ground on that leg. advise on where to ground (if it matters).

[vh junkie]

One of the two turrets that hold the bias pot to the board needs to be grounded... it doesn't matter which one...

[YMI5150?]

does it matter where you ground it? can i ground to buss between presence and bass?

[vh junkie]

does it matter where you ground it? can i ground to buss between presence and bass?

That would work... but there may be a better place either at the power board or on a chassis lug. I don't have any layouts that show this.

EDIT: The NightOwl instruction have it going to buss between presence and bass.
WHOOOAAA: Is your buss wire behind the pots connected to the chassis anywhere? I don't see it in the NightOwl instructions either???

Looking at vanhalen5150's thread: on strat78's build I can see the a black wire from the pot ground buss going under the power board... I think it is going to the chassis lug on the F1 chassis mounted cap. I think this is the best way to do it. You could also hook to the power board on the ground side(black wire) lugs of one of the (leftish) smaller screen caps.

I don't see this wire on vanhalen5150's build either???

[YMI5150?]

call me an addict. i took the afternoon off to fix this sucker.

i emailed Fido. he says it should go to buss between bass and mid. just did it and am about to fire it up. that's also where my dual 33uf cap is grounded.

the buss wire does not go directly to the chassis but does via all the pot housings.

i'll be checking in frequently. hang tight. and thanks for rallying the others over to this thread. cheers

BTW, my bias resistor is 47k like Nightowls, not 68k. please advise if I should change.

[YMI5150?]

we're good to move on. got -39.79 vdc as the highest negative voltage at pin 5 of v4 - v7.