I am trying to repair a vintage Dixco Model 797 tachometer from the 1960s. It is a one transistor system and the transistor is not switching off. From the circuit, it is clear that the transistor is a PNP transistor. It is marked GE AJX1C1224 and I have been unable to find any information on it.

Using my diode tester across the E-B junction, the tester says the forward voltage is 0.18V and that is about what I see on the scope too, which makes me suspect that it is a germanium transistor. The circuit uses both silicon and germanium diodes in it.

I am hoping someone has some specs or knows where I can get specs for this transistor.

For completeness, here are the schematics I have captured:



Hope someone has something in their archives...

Thanks,

Mark

 

PNP germanium transistors are still available as guitar pedal fanatics still swear by them. Maybe try a small signal transistor and see if it works? Cutting out R6 for 6 cylinder operation suggests that changing R6 will facilitate calibration.

 

https://worldradiohistory.com/ should have data books with info on old germanium transistors. But the markings on the transistor could be unique to that buyer. It wasn't unusual for volume purchasers of semiconductors to have them marked with their in-house part numbers.

 

Here are the specifications of 2N1224. I don't know if it is the same as the one you seek.

PNP, Ge
Vce -40V
Ic -10mA
Pc 120mW
hfe 20 @ -1.5mA Ic
fhfb 30MHz

 

Test the hfe of the transistor and consider the schematic when selecting a new transistor.

nteinc.com turned up nothing on your part number.

 

As it's just an emitter follower, it's not going to be too critical. Try anything you can find that's PNP Germanium.

 

Oh, it's a techometer? I would use an MJE2955T, but a photo of your transistor would be nice.

 

Oh, it's a techometer? I would use an MJE2955T, but a photo of your transistor would be nice.

No that's ridiculous. Firstly it is silicon, so it will have a 0.6V offset instead of 0.15V and secondly, it is far too big a transistor for the job.

 

In the circuit shown I would expect a silicon transistor to function satisfactorily, albeit perhaps with a bit of re-calibration of the meter reading. Are you merely interested in getting the tacho working, or is this a near-to-original vintage restoration project?

 

I assumed that it was a near-to-original restoration. Otherwise I‘d just throw away the entire circuit and replace it by an LM2917.

 

As it's just an emitter follower

I don't see how, since the emitter is tied to Vbatt? Methinks the transistor is being used simply as a switch.
I'm also puzzled as to why C3 has a charge path (via D3 or D4), but no apparent discharge path?
Is this tacho for a +ve-ground vehicle?

 

I don't see how, since the emitter is tied to Vbatt? Methinks the transistor is being used simply as a switch.
I'm also puzzled as to why C3 has a charge path (via D3 or D4), but no apparent discharge path?
Is this tacho for a +ve-ground vehicle?

You‘re correct - I don’t know how it’s meant to work as it appears to have no ground connection, except through the points.

 

Thanks for all of the replies. I can't test the hfe of the transistor because the E-C junction appears to be a short circuit.

I looked at the 2N1224, but saw that it was an RF transistor, so I figured that was not it. I think bassbindevil is right, Dixco had custom marked transistors, maybe extended temperature specs for automotive apps?

The only ground is via the points. The transistor turns on when the points close which charges C1 and C2. When the points open, the transistor turns off and the caps jump up to about 16V (charge pump) and then discharge to 12V through R4 and the network to the right which puts current through the meter.

The function of D2, D3 and C3 are not clear to me. D2 appears to clamp the collector voltage to about 12.64V, but that's all that is obvious to me. If anyone has any ideas as to why they would put those components in, I am all ears.

The intent of the project is to learn how to repair these old tachometers so that people can keep their classic cars working correctly. You would be surprised how many people want to keep everything as original as possible in their cars, even down to the workings of the tachometers.

I use NTE158s to repair Gen 1 Smiths tachometers and I use NTE176s to repair Gen 2 tachometers, so I will give them a try and report back.

Thanks again guys.

Mark

 

Both the NTE158 and 176 behaved exactly the same as the original GE transistor, not switching off. It looks like I will finally need to learn how to use the Digilent AD2 transistor tester that I bought a few months ago...

 

The only ground is via the points.

Is the meter housing grounded, or insulated from ground?

 

Is the meter housing grounded, or insulated from ground?

The meter housing is insulated from ground by a ceramic carrier.

 

Hmmmmm let me seeee; let me take a guess,

The purpose of D2, D3 is to clamp the voltage seen by C3 as the difference of the forward voltage drops between the germanium and silicon diodes. That way C3 is charged to more or less constant voltage, such that the pumped charge Q= C * V remains constant.
That way the average current that the meter will read is only proportional to the repetition frequency.
Very clever!

 

EDIT; what did you mean by “the transistor is not switching off” ?
How did you determine that the transistor was conducting?

 

EDIT; what did you mean by “the transistor is not switching off” ?
How did you determine that the transistor was conducting?

I assumed it wasn't switching off because the Base voltage was going to 12V when the signal was at 12V and the collector was staying flat at 12V instead of going above 12v as seen to the left of R4. However, you are on the right track. I removed the GE transistor from the circuit and tested it and the transistor works:



I also tested the the NTE 176 and the curves are very close. I am pretty sure the NTE 176 will be a good substitute for the GE unit. I haven't figured out how to control the axis scales on the AD2 curve tracer yet. Sorry:



It appears something else is what actually broke.

 

Hmmmmm let me seeee; let me take a guess,

The purpose of D2, D3 is to clamp the voltage seen by C3 as the difference of the forward voltage drops between the germanium and silicon diodes. That way C3 is charged to more or less constant voltage, such that the pumped charge Q= C * V remains constant.
That way the average current that the meter will read is only proportional to the repetition frequency.
Very clever!

So that's how they make it a one-shot! That makes sense. Thanks for that!