Help debugging strange behavior in rangemaster clone build: battery gets loaded

[ItsGiusto]

I'm revisiting an old rangemaster clone build (positive ground) of mine that's acting up.

Here, you can see that when I turn the pedal on, the LED lights up bright for just a split second, before dimming significantly:
https://www.youtube.com/shorts/zbXI19StF98

Here you can see that when I measure the voltage across the battery terminals, the voltage will be up around 9v, but as soon as I plug in the input cable (which connects the positive battery terminal to the circuit), the voltage across the battery drops to something like 2.5v.
https://www.youtube.com/shorts/rbhz37nh8zg

Here you can see that when I plug in a 9v dc adapter (which also disconnects the battery from the circuit), the LED is at full brightness. When I measure the voltage from positive to negative in the circuit at this point, the voltage is at full 9.4ish volts.
https://www.youtube.com/shorts/9PH5M5_LFdQ


So, it seems to me that something in the circuit is loading down the battery. But the DC adapter has enough juice that it doesn't get loaded as much. So when the battery is on its own, it'll have good voltage. But when it's actually connected to the circuit, it gets loaded down to 2.5v. But this doesn't happen for the DC adapter.
Or maybe just the battery is old? Not sure.

Does anyone have any ideas of how I can trace this problem down? Why might this be happening?

[ItsGiusto]

Nvm, you know, I think maybe just my battery was dying!

[Lino22]

Measure the resistance between the battery clips.

[GibsonGM]

^  Yes. That may tell you if there's something close to a short. It SHOULD ramp up from low R up to much higher over (some) time.  The power supply filter cap will look like a dead short initially, very low R, but will charge via the meter and R should rise, changing rapidly.   If it remains low ohms, you have some kind of short, 'low ohms bridge', or failing component somewhere.

Make sure you have no battery installed (of course), and plug something into the input just to complete the circuit  

[ItsGiusto]

^  Yes. That may tell you if there's something close to a short. It SHOULD ramp up from low R up to much higher over (some) time.  The power supply filter cap will look like a dead short initially, very low R, but will charge via the meter and R should rise, changing rapidly.   If it remains low ohms, you have some kind of short, 'low ohms bridge', or failing component somewhere.

Make sure you have no battery installed (of course), and plug something into the input just to complete the circuit

Thanks! I just did this on your suggestion. I wasn't sure which clip I should attach which probe lead to. When I did it one way, it eventually settled on 25k, and when I did it the opposite way, it was around 50k, though it started dropping gradually. I didn't stick with it for too long, maybe one minute, but it dropped to about 49.5k, and was continuing to drop.

Does this sound like it's supposed to be? I mean, there are two set of voltage dividers that run from positive to negative across the circuit in parallel, so I feel like something in this range does make sense. One set of voltage dividers is the 470k resistor + 68k resistor, and then the other is the 10k pot + the transistor (which could measure as ohmic, not sure) + the 3k9 resistor.

[antonis]

Without powering the pedal, measure resistance between reverse polarity protection diode cathode / filrer cap (+) leg /470k resistor node and GND..
(don't bother about transistor 'cause it will be in cut-off region..)

[Lino22]

25k means 0.3 mA. That is just a normal value. Try a different battery.
An old 9V battery that has been down to 7V unloaded can be pressed down to 1-2V by that resistance, but not a new battery that still shows 9V unloaded.

[ItsGiusto]

25k means 0.3 mA. That is just a normal value. Try a different battery.
An old 9V battery that has been down to 7V unloaded can be pressed down to 1-2V by that resistance, but not a new battery that still shows 9V unloaded.

Hmm, that's strange then. I mean the battery is old. But it's definitely reading 9v while unloaded.
FWIW, though, I tried a new battery and it's working fine, no significant voltage drop when loaded, and the LED is bright. Though I am worried that maybe there's some drain, and it'll lose charge fast. But we'll see.

[Lino22]

Measure the current draw. Leave one battery clip on and put the probes on the other clip and the free battery pole. Set your DMM to DC current mode and about 20mA current scale. Typical RM current draw is 0.5mA + LED if you have any.

[Lino22]

[ItsGiusto]

Measure the current draw. Leave one battery clip on and put the probes on the other clip and the free battery pole. Set your DMM to DC current mode and about 20mA current scale. Typical RM current draw is 0.5mA + LED if you have any.

It draws 0.2 mA when in bypass and 1.6 mA when not in bypass.
It does have an LED, btw.

[Lino22]

Then it does just what is supposed to. Have fun.

[anotherjim]

There's a bunch of little cells connected in series in a 9v battery. Quite often multicell batteries fail when all the cells are perfectly good, but there's a bad contact between at least two of them. So that will read good off load.

[antonis]

From electrical analysis point of view, what Jim said above could be expessed as " Bad contact -> High resistance -> Raised total internal resistance -> Lowered output"
(due to voltage dividing effect on the internal resistance / load resistance node..)

[pinkjimiphoton]

a bad ge transistor can leak enough to kill the battery quickly sometimes. maybe the q's bad? sometimes they grow whiskers inside and ya get a dead short in a previously working circuit.