Pye Cambridge

Posted on February 14, 2021 Updated on September 26, 2022

Something a little different this time…a dashmount Pye Cambridge transceiver for renovation.
I wanted to renovate one of these because I had a bootmount Pye Cambridge in the early 1970’s that was originally AM and converted to FM on 144Mhz. I later built a front panel for the bootmount along with a simple digital readout that showed the crystal channel numbers. In the 1980’s I had a dashmount Cambridge that I used in my shack, I also converted this to FM. I love the all valve transmitter with the QQV03-10 outputs. My first 2m transmitter used one of these.

I spotted this on ebay after looking for a long time for one of these old PMR transceivers. They are quite rare and when they do appear they are usually expensive. I got this one cheap, it is a bit of a mess, but it is an FM version and originally low band.

This transceiver had been converted to 144Mhz and a VCO added on receive. Inside is in good condition and the modifications seem to have been done by someone who was very knowledgeable. One thing that I could not understand is that the oscillator and phase modulator valve  (ECC81) has been changed to a 6BH6 valve and phase modulation applied to the second valve stage. I will investigate this further. The RF stages have had their transistors replaced for more sensitive devices.
When I powered it up it actually worked. The VCO appears to be working, but the tuning pot is very dirty and needs replacing, also the speaker is distorted and needs replacing if possible. The VCO is quite clever, it uses four receive crystals spaced out in frequency across the band. Each crystal is tuned +/- 100Khz approx, this covers most of the upper band of 2m. The FM discriminator circuit needs alignment.

On transmit, it puts out around 6w, tuning the coils and trimmers gave an output of almost 9w. I was using a dynamic microphone for the audio and found that the although the phase modulator is working fine, the audio is very low coming out of the audio board. Could be because I am using a dynamic microphone instead of the original Pye microphone?

Upper side of the transceiver with the VCO at the top right and crystal bank centre bottom. All the transmit crystals are FT243 type, all originally on the same frequency, but someone has moved the fundamental frequencies by ‘penciling’ . This is where you unscrew and take apart the FT243 crystal and make a pencil mark on the crystal surface. This can move the frequency to the one you want with a lot of patience! (I remember doing this myself back in the 70’s)

The underside of the transceiver, a 10 position switch had been added for the crystals.

So what needs to be done?  Well first, replacement of the electrolytics in the transmit audio stage and some modifications to improve the output. (the audio clipper / limiter needs bypassing)
The FM discriminator was aligned and I also removed a low pass filter in the audio output line to improve the receive audio quality.  The VCO potentiometer was replaced with a nice 10 turn pot and this works a treat making the tuning very smooth and noise free. Replacing the speaker proved to be quite difficult, I didn’t want to remove the front panel as I once had problems with this in the past, so I added a 3.5mm Jack socket on the side to plug in an external speaker.
Cosmetically, the Cambridge needs a bit of TLC. First, it has no finger plate, so I made a new one from aluminium. The extra crystal holder, trimmer and small switch on the front were removed as they are not necessary. I also removed the added microphone socket and wired a dynamic microphone straight in. It also needs set of knobs, preferably the original blue / green Pye types. An appeal on Facebook brought a donation from another amateur, so I now have a full set.
I cleaned front panel first with soap and water, then with acetone to remove the writing on the front. Next, I mixed some Humbrol paint to get the colour as close as I could to cover up the scratches on the plastic surround. The case was cleaned with Brasso which brought back the colour and gave a nice finish. Finally, I printed out the legends for the finger plate onto decal paper.

So here it is…a fully renovated and working Pye Cambridge complete with VCO on receive and crystal controlled on transmit.  I managed to work some local stations with it on 144Mhz, not bad considering that this little beauty was made in 1967 the Summer of Love!