E-635: Philips BX-925A

E-635, BX-925A; developed and produced by Philips Gloeilampenfabrieken, NL-Eindhoven.

There is some evidence that the Philips BX-925A has not only been evaluated by the Swiss PTT (post telegraph telephone service) but also saw use with the Swiss Army in small numbers.

E-635: Philips BX-925A

Technical Data

Power Supply


  • 490 x 255 x 415 mm, 32 kg



The communications receiver BX-925A is a bulky communications receiver made by Philips, in fact it is one of few commercial receivers ever made by Philips and it seems to me, it was not a great success, as not many of these receivers have been ordered and appear on the second hand market.

The 16 valve receiver can powered from 110 - 245 V mains, the set in a heavy steel cabinet with carrying handles has a weight of 32 kg, it's dimensions are 49 x 25,5 x 41,5 cm.

On the front panel, you find the sector type main frequency dial in the the left upper half, in the middle the fine tuning dial and at the right the signal strength meter.
The main tuning knob is located in the middle of the front panel, the band segment switch at the left and the I.F. bandwidth selector at the right.
Leftmost, you find the switch for the internal crystal calibrator and the rotary controls for tone and antenna tuning, at the right hand the AGC switch, the controls for the noise limiter and the headphones jack.
In the bottom row of controls, you find the RG gain control, the BFO switch and BFO pitch control, the crystal filter control, a receive - transmit switch and the volume control AF gain.

To operate the receiver, select the correct mains voltage at the rear face of the radio and connect to mains, connect a long wire antenna to socket A1 or a dipole antenne to A1/A2.

The receive/transmit switch is only in use on connection with a transmitter, it should be in the REC position, use the volume control AF gain to turn the receiver on.

To listen to Radio Deutsche Welle on 6075 kHz, select band range 3, the respective frequency dial is now visible in the frequency dial window. Use the main tuning control and tune from the 6 MHz mark slightly up until you hear the typical german newsreader from Cologn shortly after the hour - nowadays, shortwave broadcasting stations do not broadcast longer interval signals to identify the station any more. You might write down the numbers of the small logging scale on the main tuning dial and the number on the mechanically coupled 0 - 100 fine tuning dial, once you have found the station, to make it easier to find the station again on later occasions. This procedure using logging dials and a logbook were replaced by electronic memories some years later. With the crystal calibrator active, you find a frequency mark every 500 kHz to make determining a frequency of an unknown station or tuning a station on a known channel easier.

The RF gain should be turned up fully clockwise, the AGC should be set on 2 (fast) or 3 (slow), the bandwidth to BROAD (to listen to a strong signal without adjacent channel interference), for AM reception, the BFO has to be turned OFF.
With the beat frequency oscillator (BFO) switched on, you can receive morse code (CW) signals and single sideband transmission e.g. in the amateur bands around 80 or 40 meters, use the BFO control to regulate the pitch of the morse code signals.

Normally, you tune the receiver with the main tuning knob slightly pulled out. In the position „press in“, the BX-925A features motor driven tuning, the receiver is tuned up or down when you turn the tuning knob to the right or to the left.

The receiver can be used in crystal controlled fixed channel operation. Insert a matching frequency crystal (reception frequency + 735 kHz in ranges 1-3 or reception frequency - 735 kHz in ranges 4-6 respectively), after you have switched to fixed frequency operation with a screwdriver, an „X“ instead of a „N“ for normal operation will be visible in the tiny indicator window.

Technical Principle

After two HF preamplifier stages (two EF93), the RF signal is fed to the mixer (EK90), with the oscillator signal from V4 (EF93) to generate an intermediate frequency of 735 resp. 740 kHz. After two IF amplifier stages (EF93), the signal is demodulated, here acts the AVC and the noise limiter, which is realized by two antiparallel diode sections of an EAA91 or EB91. The audio signal is preamplified in an EF93 and fed to the EL90 in the final audio stage.
For CW reception, the signal of the BFO (EF93) is mixed to the intermediate frequency, another EF93 is used in the calibrator, which provides calibration signals every 500 kHz on the dial.

Valve setup

V1, V2 (EF93 or 6BA6, HF preamplifier); V3 (EK90 or 6BE6, mixing stage); V4 (EF93 or 6BA6, oscillator); V5, V6 (two EF93 or 6BA6, 1st and 2nd IF amplifier stage); V7 (EB91 or EAA91 or also 6AL5, demodulator); V8 (EB91 or comparison types , AVC]]; V11 (EB91 or comparison types, noise limiter); V9 (EF93 or 6BA6, AF preamplifier), V10 (EL90 or 6AQ5, audio final stage); V14 (EF93 or 6BA6 , BFO), V15 (EF93 or 6BA6, calibrator);
V12 (5Y3GT, plate voltage rectifier, V13 (0D3 or 150C1K, stabilizer), V16 (EL90 or 6AQ5, voltage stabilisator)


Field use


Additional information

en/e-635.txt · Zuletzt geändert: 2019/04/07 09:29 von mb