[GreenKeys] Model 33 Q1 transistor replacement

Tue Jul 22 11:19:13 EDT 2025

One correction.  In the schematic I noted that the Q1 base voltage during
SPACEing would be  -4.7V.  I don't think that's right.  I think that  -1
volt is a more correct value for the voltage at Q1 base during SPACEing.

Paul

On Tue, Jul 22, 2025 at 11:01 AM Paul Newland, ad7i <ad7i at ad7i.net> wrote:

> Oh, gosh.  That schematic reminds me of the good old days at AT&T.  I used
> to have mild arguments with drafting as to how a schematic should be shown
> on paper.  Their approach was often that the schematic was just a graphical
> netlist and the goal was to use as few pages as possible.  My position was
> that a schematic told a story, in graphic form, a story about how a circuit
> worked, so that the reader could easily understand its operation.
>
> Anyway, here's a redrawn version of the schematic with some commentary
> below that.  Please let me know if you spot any transcription bugs in the
> schematic.
>
> [image: image.png]
>
>
> A couple of things to note here.  First, I don't see any reference to
> earth ground in the original schematic.  To me, that means that this
> circuit "floats" with respect to earth ground, and the relationship with
> ground is determined by the applied 20mA (or 60mA) current loop (one side
> of which is likely grounded directly or through some finite resistance).
> When I call out a voltage on the schematic or text, that voltage is with
> respect to the Zero volt bus.
>
> Second, the selector current is BIG, about 0.5 A.  And when Q2 is
> conducting,  Q2 is operating in its linear range, so it will get HOT.
>
> Let's look at what happens in the MARKing condition.  In the marking
> condition the 20 mA (or 60 mA) of positive current is shoved toward the
> base of Q1.  In this condition the loop current flows through CR5 and the
> R3 (0.8 ohms) as well as through R1 into the diode network.   Also in this
> condition the voltage at the base of Q1 is clamped to about +1.1V, and the
> BE junction of Q1 is cut off, so Q1 is out of the picture in the MARKing
> state.  The base of Q2 is held at -5.4 volts by the diode network and the
> voltage the the emitter of Q2 is held at 4.7 volts (due the diode voltage
> at the base, less the diode drop across the Q2 BE junction).  So with a
> constant voltage at the emitter of Q2, and a constant resistance between
> the emitter and the 0V bus, Q2 forms a constant current driver.  If Q2 has
> a decent gain value, then Ic = Ie = 4.7V / (0.8 +1.5 + 8) = 456 mA.  So
> that's the marking condition.
>
> In the case of the spacing condition, the 20 mA loop current is disabled
> and Q1 is biased on via R1 and the diode network.  This causes Q1 to turn
> on and current is sent from the collector of Q1 toward R8, which tends to
> rob current from the diode network, sending the voltage at the base of Q2
> toward zero volts.  It won't get to zero but it will be close (maybe -1
> volts).  With lower voltage at the base of Q2 (and 0.7 to 0.5V less than at
> Q2 emitter), that will reduce the selector magnet current significantly
> (but I don't think it will go to zero).  In a working circuit the actual
> MARKing and SPACEing selector current can be determined by measuring the
> voltage across R7 and dividing by 14.
>
> Paul, ad7i
>
>
>
> On Mon, Jul 21, 2025 at 5:04 PM RODNEY HOGG <radio_man at wbsnet.org> wrote:
>
>> Paul,
>> Do you have a type number or p/n for the transistor.  I have 1000s of
>> transistors, civilian and military types, be glad to help you out.
>> Rod
>> K0EQH
>>
>> ------------------------------
>> *From: *"Erik Bruchez" <erik at bruchez.org>
>> *To: *"Paul Newland, ad7i" <ad7i at ad7i.net>
>> *Cc: *"Greenkeys" <greenkeys at mailman.qth.net>
>> *Sent: *Monday, July 21, 2025 1:58:36 PM
>> *Subject: *Re: [GreenKeys] Model 33 Q1 transistor replacement
>>
>> Paul,
>>
>>> It's probably not all that difficult to find a working replacement, but
>>> in a different form factor.   Can you show a larger portion of the
>>> schematic?  I'd like to see if this transistor is operating as an on/off
>>> switch, or if it's trying to work some magic to regulate the voltage on the
>>> magnet for fast pull in and then low current idle.
>>>
>>
>> I attach the whole thing. There are explanations in the schematics.
>>
>> I measure about 0 V on the base of Q1, but also about 0 V on the base of
>> Q2. My understanding (which could be wrong) is that when one is on, the
>> other one should be off, and vice versa, which means that 0 V on Q1 should
>> lead to about 4.7 V on the base of Q2. In any case, I am not getting any
>> voltage to the selector magnet through Q2 (the large transistor on the heat
>> sink, separate from the board), which is the original problem I am having.
>>
>>
>>> Some basic DC measurements might give you some insight into the
>>> health of the transistor without removing it from the circuit board.
>>>
>>
>> This is what is leading me to Q1: it seems that there is continuity
>> between emitter and collector, which I don't think there should be! I have
>> another instance of the board where this is not the case. (However, that
>> second board also doesn't seem to work, but there might be something else
>> wrong with it.) I could desolder the transistor on that one and transplant
>> it to see if that gets my first board working, but I was trying to avoid
>> doing that immediately.
>>
>>
>>> I'd be curious to know the voltage from base to emitter when the
>>> transistor is "ON" (or supposed to be on).   Also (can't see that part of
>>> the schematic) but if there's a resistor in series with the base I'd be
>>> curious to know the voltage across the resistor (and the resistance of the
>>> resistor), and then use ohms law to determine the base current.  Same thing
>>> for the selector magnet current, if that's possible.
>>>
>>
>> -Erik
>>
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