CH-25 and 26 SSB Transceiver


SPECIFICATIONS

Modes: SSB (Upper/Lower), AM, CW
Frequency Range: 3 MHz to 22 MHz
Power Output:  100 watts SSB PEP; 60 watts phone
Frequency Control: Six crystal controlled positions.
Power Input: 12 VDC confirmed; possibly an AC power supply is available.
Marconi Type: 189-575
Dimensions: 11 x 7 x 13 deep
Weight: 13 kg ( 28.6 lbs)
Comment: Used in Canada's North in 1960's and 70's.

Some comments on the CH-25 are provided by Jack van Oostveen, PA3GYM, The Netherlands.

"It was a hybrid transceiver. The receiver is solid state as well as the local oscillators and balanced SSB mixers in the transmitter section. The transmitter driver and PA stages use tubes - two 6883B's  with 12 volt filaments in the PA , one 6CL6 as a driver tube and a 7360 dual beam mixer tube.The HT supply is 750 volts, bias approximately  - 66 volts and the transmitter LT line is 300 volts. A 9.6 VDC power supply is used for the receiver circuitry and that portion of the transmitter which is solid state. During transmit, the 12 VDC  input was converted to 750 VDC by the "trill converter unit" which produced a very audible noise.

When my set was purchased, it came with one crystal for 5386.5 kHz for USB. The other five crystal positions and coil receptacles were empty. Since the set lacked a bandswitch, it was necessary to plug in the appropriate Rx/Tx coil set for the desired band of operation and made sure it matched up with the crystal frequency. When crystals were ordered for the CM-25 set they had be either 455 kHz above or below the desired operating frequency. A maximum of six different operating frequencies could be "programmed" that way.

For amateur radio operation on the 80 meter band, I altered the RX input coils, and TX coils, added a variable preselector and driver selector and a new adjustment for the PA stage. Instead of operating with crystals, I made a home brew VFO. For SSB operation (both USB and LSB) I used two crystals (453,5 kHz and) 456,5 kHz) which resulted in a  1.5 kc  wide passband while still giving good audio. SSB filtering is accomplished by a Collins 455 kHz mechanical filter.

In AM mode, the passband is 6 kHz band wide. For CW operation there was a side tone oscillator running at  800 Hz. My home brew VFO covers the band between 3.600 kHz to 3.800 kHz and has a digital readout display, thuis providing the actual RX / TX frequency".

One technician made this remark about the CH-25. "My time at Bell Canada was spent in the field servicing the Marconi CH25 HF-SSB transceiver (as in Completely Hopeless Model 25) which went through endless finals when they weren't blowing out the cathode resistors.  But they used Collins mechanical filters so they had some redeeming value".

marconi/ch25.jpg
The CH-25 in a museum mockup of a Northern Canadian radio station. (Photo by Harry Foster) 
Two CH-25's that were on sale on E-bay had the following crystals installed: 3171, 4532, 4571,4573, 4825, 4922, 5313, 5814 and  9462 KHz .

PHOTO SECTION
ch25_front_pa3gym.jpg
This CH-25 is owned by Jack van Oostveen, PA3GYM
ch25_under_chassis.jpg
Under chassis view.
ch25_rf_compartment.jpg
R.F. compartment showing the two 6883B output tubes.
The solid state portion of the transmitter with the Collins SSB filter under the shaft.
ch25_sockets.jpg
The sockets for the crystals and coils can be seen in the top right portion of the photo. 
ch25_trill_unit.jpg
The Trill Unit which converts 12 VDC to 750 VDC. This CM-25  was likely made in 1971 as attested by the date code on the power transistors.
All photos in this table by Jack van Oostveen, PA3GYM

 
ch25_xtal_coil_location.jpg
Cover plate with TX and RX tube/coil channel info.
ch25_nameplate.jpg
CH-25 nameplate.
All photos in this table by Jack van Oostveen, PA3GYM

 
ch25_chassis.jpg
Overall chassis view including a generous layer of dust. (E-bay photo).

 
ch25_005.jpg
ch25_001.jpg
Other views of a CH-25 example. 
All photos in this table via E-bay.

CH-26

The CH-26 was the 12 channel version of the CH-25. It used the same circuits and channel kits. The radio was wider, almost double the width but same faceplate style
 



Contributors and Credits:

1) http://www.civilization.ca/hist/phase2/mod11e.html
2) Jack van Oostveen, PA3GYM <jaap.oostveen(at)hccnet.nl>
3) Donald LeFevre KD5RCG <Donaldb946(at)aol.com>
4) http://mailman.listserve.com/archives/html/collins/2006-07/msg00146.html
5) Walter IsaacsonVA7ANI [wi1940@telus.net]
 

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May 10/10