Hi Guys
I would have thought I already posted this, but it appears not.
This is an STC radiophone VHF FM set operating on split frequency for repeater use.
It’s a valve/ solid state hybrid. Valve audio preamp for the mic, and valve RF final valve.
It’s the final valve that was gassed and had to be replaced.
The original mic is also a carbon element the same as STC elements in old telephony.

25 Watts RF output is wound down to my permitted 10 Watts by simply
lowering the DC supply voltage to the entire unit... love analogue electronics!

https://www.youtube.com/watch?v=B3z9Vo3qmzE

Cheers, Brek.

Hi Brek

In the late 60s early 70s these were the radio to have for both VHF lo band and VHF Hi band. Great performance and very reliable. Very popular with taxis and couriers. TV service co. I worked for at the time had hybrid AWAs. They were rubbish compared with the STCs. Their all solid state MR25s were better.

The mic is actually not carbon but a 200 ohm dynamic earphone! They worked very well as mics. Used in almost all 2way radios at the time.

A better way to set the power is to adjust the tank coupling coil position. Not a good idea to under-run valve heaters.

What is the model no. of your unit. It looks a little older than the STC 151s which a customer had when I was servicing two-ways in the late 80s/early 90s. They were still in daily use in trucks, just a little hard of hearing compared with then current radios.

In the 1976ish, working for a mining company in NW Aust, we threw about 2 dozen of these in a hole and buried them.
Worth digging 'em up?

Sorry I haven’t dropped in here for a while. Yes dig them up and send one to me!
I never did get the transmit audio sounding good. Maybe the previous owner’s experiment to put a carbon mic in it.
He also had to have replaced the industry crystal with the amateur repeater crystal.
It definitely was a phone mic whether original to the radio or not, unless they look exactly the same.
A little single transistor PCB was inside the mic as well. That part was probably original being a circle cutout to match the handset moulding.

The model I’ll check, although I can be heard, it’s no good as a transmitter at the moment. Well strictly speaking, the FM carrier is good.
I did use the coil at the PA valve to tune it with a Jpole antenna, and was afraid to detune it.
The Jpole (VHF) isn’t up at the moment though.

"25 Watts RF output is wound down to my permitted 10 Watts by simply
lowering the DC supply voltage to the entire unit... love analogue electronics!"

What supply voltage are you running at? By running the entire unit under voltage I can imagine several reasons why you are sounding distorted when transmitting. Much better to just run the final power amp at a reduced voltage or adjust the coupling on the output as Ian mentioned earlier.

Haha, Yes, I don’t think the digital synthesis boards would like that much.
It was not far. This is going, back, but might have only been down to around 10 Volts.
I did try the full power though many times, both here and at the club.

It was able to happen though. I had reports of moments where I "suddenly went hi fi and dropped out again”,
but I did the mic cabling at least. It was ok up to the radio enclosure.

Based upon my own experience with early AWA Carphone transceivers, I'd be taking a long hard look at the coupling capacitors in the mic preamp. Simple enough to replace them if there's any suspicion at all. Amazing how many intermittent audio quality faults are caused by electrolytics with internal open circuits and/or loss of electrolyte and reduced capacitance. They should generally be low voltage units, so replacements will be readily available and should cost you very little {:o)

Plus of course, if your microphone really is a carbon unit ex telephone, it's about 75% likely to be causing poor audio quality faults. Rough test. Next time you have the unit set up, give the mic a few very firm sideways taps on a hard surface, to see if it changes the audio quality at all. If it does, it's time to replace it with a dynamic insert. A CB radio mic would be ideal. NOTE A carbon mic will have been set up with a standing DC bias across it - they have to have this to operate. The same bias will cause much pain for a dynamic mic, so you will need to trace back where the bias DC is sourced and disconnect it. Or instead, you could just wire a 1 or 2 microfarad poly cap in series with the insert to block the DC component.

That radio is similar in appearance to the radios that were in the Aussie cop drama Homicide and made it's appearance from about 1966 to about 1968 or 1969 when they were replaced with the Vinten MTR27 (there were 2 versions of that set, the MTR26 covering the VHF Low band and the MTR27 covering the VHF High band and they were all transistorised) made by Plessey which was used till the end of the series in 1975.

A screen cap from an episode of Homicide where this rig is used in Car 99:- https://au.pinterest.com/pin/553379872943435590/

If it has the PCB on the back of it, that mic MUST be an earphone insert, the carbon mic will not work properly.

The PCB was to allow the use of the magnetic earphone in place of the unreliable and scratchy carbon mic.

The earphone should read around 200 ohms - stable and consistent - if it is really an earphone. If you apply light air pressure to the orifice (by "kissing" it" - sounds awful but it works - if it's a carbon mic its resistance will change dramatically. So you will know.

I can recall how these units used to sound on transmit - clear and punchy audio.

Mic SHOULD be an earphone insert from a period (60's) telephone. The STC and AWA ones were interchangeable.

Sorry for multiple posts on this issue but the memory is coming back slowly!

We also had mic issues with our early AWA carphones. The preamp board in the mics would start to fail in hot summer weather, when the vehicle was parked in the sun during the day for an hour. Mic would burn your hand when you picked it up!

We replaced the two PNP germanium transistors with BC177s - modern equivalent BC557 - and never had any more trouble.

Sounds like that could also be an issue with your STC.

How I knew what these radios sounded like? We had a courier company (Braggs) on the same low-band simplex channel. Some of their vehicles had STCs (car 11 as I recall was one), most were AWAs. You could always pick the STCs, they punched through the noise so much better!