Onerr Tungsten Drivemaster TO-3 burnt resistor replacement

[Iffy Jiffy]

Hi everyone, I got a TO-3 Onerr Drivemaster pedal at a garage sale and it has a burnt resistor which I cannot identify. I removed it and would like to replace it and see where I go from there as there may be other components that got damaged. This resistor was directly connected to the external power jack and D1. I removed and tested the diode and it appears normal. I can only suspect that an incorrect ac adapter was used that caused the resistor to burn. Only the outermost color bands on the resistor were visible, the red and the gold 5% tolerance band. The band next to red was burnt and did appear to be brown but that may have been caused by the heat  so I cannot be sure. Searches online for a schematic or a photo of the internals were unsuccessful. This brand was made in Brazil and does not appear to be operating any longer. Is there any way for me to determine what would be a suitable resistor or is there a standard value resistor connected to the power source used in most pedals. The pedal can be powered with a 9 volt battery or by an external power source. The resistor gave no reading when tested from each end as well as from either end to the center of the body where I sanded to make contact. Hopefully someone can offer some suggestions regarding this problem.

[PRR]

I would try 100 Ohms.

That does not match "red" but is probably safe. If there is a large voltage drop, the original might be 2? Ohms; if a small drop the original may be 2?0 Ohms. 22, 27, not much difference.

[Rob Strand]

You might want to look at the tracing of the Onerr Tungsten Overdrive TO-1 on "the other" forum.   

The TO-1 schematic shows a transistor regulator + zener.   The zener resistor R37 is 470 ohm and the zener is D9 (from what I can see incorrectly shown as a 1N4007 in the schematic.)   If you can see a transistor near the diode then your board might also use the zener regulator.

There's some traces of other Onerr pedals on "the other" forum and they have a different power circuit.  A couple have a reverse diode across the power rail  and no series protection resistor.   That company might not be consistent with the power arrangement across all pedals.

All I'm saying is make sure what you are dealing with.   If you can confirm it's just a series protection resistor, then given it has been severely cooked it's likely the resistor is 22 ohm or 27 ohm.    Often the value for such a resistor is fairly arbitrary and you might find the pedal will work with a wide range of values.   If you measure the voltage drop across the resistor it's usually a good idea to keep the voltage drop below 1V - of course that doesn't mean the manufacturers stuck to that!


EDIT:
Look here.  It states the pedal consumes 30mA.   I find that hard to believe but suppose it does then the voltage drop across a 22 ohm would be 30mA *22 ohm = 0.66V and the voltage drop across a 27 ohm resistor would be 30mA * 27 ohm = 0.81V.   So if the 30mA is correct it would explain the low resistor value (and the fact it was badly burnt after the fault).

https://www.ninjasom.com.br/pedal-para-guitarra-tungsten-drivemaster-to-3---onerr/p

[Iffy Jiffy]

 I had a look at the schematic for the TO-1  but my understanding is somewhat limited.  From what I can see there are three diodes. D1 close by the power input jack is a 1N4007, the other two diodes  D2 and D3 are both 1N4148. There are two K30AGR transistors in close proximity to D2 and D3. The other two transistor on the board are both BC547B which are general purpose NPN types. The resistor  R35 that burnt is larger in size (looks like 1/4 watt )  than the others on the board which are smaller.  The electrolytic caps were removed to test before I noticed the damaged resistor.

[Rob Strand]

The resistor  R35 that burnt is larger in size (looks like 1/4 watt )  than the others on the board which are smaller.

To me it looks like R35 just goes in series with the power rail and D1 is across the power rail for reverse protection.   The simplest scenario.  Extremely unlikely to be anything other than that.

As for the value it's not possible to deduce the precise value.    If it were mine I'd probably use a 22 ohm then check the voltage across it is 0.1V to 1V.

[Iffy Jiffy]

Thanks for your suggestion. I will see if I have that value resistor or something very close in resistance. Will let you know how it goes.

[Iffy Jiffy]

I had to wait on the arrival of the 22ohm resistors. I installed it and everything works fine. Fortunately there were no other components that got damaged. When I checked the voltage at each end of the resistor, it was 4.45v and 4.10v respectively.  Not sure if I took the measurements the correct way. Thanks to Rob strand and PRR, without your assistance I would not have gotten this up and running.

[Marcos - Munky]

When I checked the voltage at each end of the resistor, it was 4.45v and 4.10v respectively.  Not sure if I took the measurements the correct way.

You probably measured the voltages in a wrong way, since you'd expect close to 9V. Anyways, if it's working as it should, problem solved!

[Rob Strand]

I had to wait on the arrival of the 22ohm resistors. I installed it and everything works fine. Fortunately there were no other components that got damaged. When I checked the voltage at each end of the resistor, it was 4.45v and 4.10v respectively.  Not sure if I took the measurements the correct way. Thanks to Rob strand and PRR, without your assistance I would not have gotten this up and running.

The way I read that is the drop across the resistor is 4.45V - 4.10V = 0.35V.  Which looks in the right ball-park.    The current drawn by the pedal is 0.35V / 22 ohm = 16mA.    Not the 30mA in the specs but certainly a believable figure.

As far as the value goes I'm pretty sure it's about right.  It's unlikely to be 220 ohm as that would make the voltage across the resistor 3.5V, which would be unusually high.   Splitting between 22 ohm and 27 ohm is simply not possible without having another pedal to see the real value - at this level it's almost an arbitrary choice from a design perspective.

Anyway, all looks good.