| The "Power Rock" was Collins answer to the MW5 at Harris.
Just as Collins was being taken over by Rockwell, the PDM transmission system was being perfected.
PDM - or PWM as the Harris version was called - used a 70 kHz switchmod arrangement to modulate the transmitter. Positive peaks were stunning ... the transmitter would effortlessly exceed 150%
As it developed at Collins, Rockwell, and finally Continental, the 838E-1 was a solid transmitter with a good sound and high modulation capability ... except for a certain Crystal Gale record!
(This picture is of the Continental edition - largely the same as the Rockwell/Collins version, except for the paint job and "new number: 316R ... the R standing for Rockwell.) |
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| Alan Alsobrook recalls: If that transmitter has low PDM audio drive it
gets no RF. A common cause of audio failures I've seen in that transmitter is
the collector resistors for the 2n6575's going DOWN in resistance. So when
checking them not only make sure they aren't open but make sure they haven't
dropped way LOW!! I guess the carbon is fusing together. Burt Weiner says: I had a problem with a Power Rock that would not output any RF. The Power Rock requires 28 VDC applied to the PDM card to enable or turn on the PDM signal. The 28 VDC is looped through several interlocks as well as external antenna switching contacts that may be part of your external equipment. The 28 VDC is applied through a 10 K resistor to the base of a 2N2222A on the PDM card. This is a comparatively high impedance input in that almost no current is drawn from this 28 VDC. Part of the path is through the plate contactor, a large 4PST device, as I recall, on the left side front at about knee level. Since there is so little current being drawn by PDM mute circuit there is no sealing current and no "contact cleaning" taking place. In my particular case, once I cleaned the contact on the contactor everything started to work again. I also placed a 5 K resistor across the 28 VDC at the input on the PDM card to draw a little "sealing current". The Power Rock is a great transmitter - when they are working. Be sure that your Driver Ic is about 2.5 - 3.0 Amps. This is reading more than just the driver Ic but it's a good indication of low level stuffs. The Filament rheostats are prone to failure (especially at the end of travel). Make sure the filament voltages are where they should be and that you can reliably adjust them. Another interlock issue that sometimes gets hidden is the air interlock. This is a pressure sensor on the bottom of the RF amplifier cabinet. Most of the problems that affect this contraption are dirt build up on the switch mechanism, not the contact but its the arm that gets glopped up. Make sure that the air flow input hole on the thing isn't clogged. I found a bug halfway in one one time. Spritzing the mechanism with a good cleaner and usually a very slight adjustment of the screw will usually remedy that problem. Be sure that the blades in the squirrel cage blower are not clogged causing the problem.
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