POWERHOUSE BLUES

[gottcent]

In my continuing effort to find out why my first generation TMCC set won't work, I've discovered two things that may be of interest. Both involve the crystals installed in the CAB-1 and the PowerHouse unit.

First, the crystal in the CAB-1 is labelled 27.255 MHz. On the PowerHouse, although the plastic cover that holds the crystal is also labelled 27.255 MHz, the actual crystal inside is labelled 26.800. Shouldn't the Cab-1 and the PowerHouse be "tuned to the same frequency"?

Also, when I put an ohmmeter across the electrodes on the crystal in the CAB-1, I get zero resistance (the needle swings all the way to 0). When I do the same on the PowerHouse's crystal, the ohmmeter needle doesn't move.

Am I right in assuming I should have a PowerHouse crystal labelled for the same frequency as the one in my CAB-1, and a crystal that shows zero resistance? If so, does anyone know if these crystals are still available, and where I might find one?

Thanks.

John Gottcent

[lionelsoni]

Note that the difference in those numbers is 455 kHz, which is a standard intermediate frequency used by superheterodyne receivers.  (10.7 MHz and 45 MHz are also standard IF frequencies).  A superheterodyne receiver uses a local oscillator offset from the radio frequency to heterodyne (mix) the radio frequency to a lower, intermediate frequency for amplification before the modulation is detected.  So a receiver designed to receive 27.255 MHz would use a 26.8 MHz crystal.

Crystals normally appear to be open circuits at DC.  If you measured a less than infinite resistance, you were probably measuring the circuits connected to the crystal, not the crystal itself.  This is a risky practice, since an ohmmeter invariably passes current through the device whose resistance is being measured.  When that device is an active semiconductor circuit, that current can damage it.  To measure the resistance safely, disconnect the component from the circuit first.

[Buckeye Riveter]

I thought I had heard this discussion before and I have:

http://cs.trains.com/ctt/f/95/p/186906/2044430.aspx#2044430

[gottcent]

Thanks, guys. I did measure each crystal's resistance with the crystal safely removed from any circuitry. If I understand you correctly,the crystal in my CAB-1, which is measuring zero resistance, may be defective. Re: the previous conversation about this which you remembered--I will look through that and see if anything there helps me out.

Thanks again.

John