Hearthrob issues

[ParlorCitySound]

Hey,

Just built a hearthob trem and it doesn't quite work yet.

I think I've narrow the issue down a good bit, I'm just a bit lost as to what is going on.


These are John Lyon's voltages for his Mark m hearthrob
Q1
C 0v
B fluctuates
E 0v

Q2
C 2.9v
B 2.2v
E 1.6v

Q3
C 9.6v
B 1.2v
E .6v

Q4
C fluctuates
B .6v
E 0v

Mine are very similar, including the Q4 collector fluctuating; however, I can't seem to get Q1's base to fluctuate. It stays at a constant .83v


The schematic I used:


I also pulled Q1 per another debug and got clean signal.
mods: I was missing a fourth 2N5088 so I substituted Q2 for a 2N3904

Any help is greatly appreciated.
Cheers,
-Kevin

[Gurner]

I'm not familiar with the cct, but if Q4 collector is flutuating, yet Q1's base aint, then seeing the simple amount of components in that part of the chain, I'd say the most likely candidate is Q1 itself - try lifting Q1's base leg out of circuit & see if you get fluctuatons at the evacated Q1 base pad. If you do, then take Q1 out and test it properly

[ParlorCitySound]

with the base of Q1 lifted, the pad is getting full voltage (8.36v). It continues to fluctuate until the right hand side of C8.

[Gurner]

that doesn't seem right ....R7 & P1 are acting as a simple divider (for the 9v)....the max DC you should have at their junction is about 1v (depending where P1 is set).....what happens when you adjust P1 to both extremeties?

[ParlorCitySound]

I was curious about the voltage there as well, turns out, 1 color band on a resistor makes a big difference 330 is not 330k.
I switched it for the correct value (voltage is now .6v but P1 isn't changing anything) and everything went funky.

new transistor voltages:

Q3
E - 7.6v
B - 7.6v
C - 8.2

Q4
E - 0v
B - 7.4v
C - 8.2v

[Gurner]

Putting those transistor voltages to one side for a moment, you need to sort that R7/P1 junction DC voltage out - so next, I'd have gone for detaching R14 & C8 from the R7/P1 junction- , you should have about 1.18V with P1 set at its highest point (as measured at their junction)

[ParlorCitySound]

I disconnected R1 and C8, the voltage is a constant .35v, irregardless of pot location. This is a bit of an odd setup, the R7 connection and the P1 connection are on opposite sides of the Q1 base. Also, I didn't have a 25k pot, so I used a 50K pot with a 47K resistor between lugs 1 and 3 (to make a 25k pot).

[Gurner]

Oops, earlier I missed the extra 15k resistor (& worked on your p1 being 50K), but nevertheless, what you've now got is this arrangemnt...



clearly adjusting the pot should give you a varying DC voltage (albeit small 'range') with the max being about 1V with P1 at its max value  (as measured at the junction of P1 & R7).

You need to nail that aspect before you'll get anywhere as it suggests something definitely amiss with the Q1 base setup arrangement.

[ParlorCitySound]

I've got it a bit sorted out. The pot shifts the voltage from .3v to about .55v. I'm not sure as to why its half of what it should be.

[Gurner]

I'm way too lazy/sloppy with a calculator, so when faced with such an issue I normally quickly punch the numbers into Excel...



you should be getting between 0.39V & 0.97V with that resistor divider network.

Assuming your supply voltage *is* a good solid 9V, then having just eliminated two issues relating to that junction.....you've still got a problem that needs sorting. Once you've got your reading varying between the two voltage I've calculated above - connect your circuit back together.

Edit: I'm doing this all too quickly - ignore the typo in that jpg - I can't be arsed to redo it!!