Ian,
Yes, I also cut my amateur teeth on various VHF PMR radios, with sometimes quite fiddly adjustments to get the modulation right, and adding to the power output, with an increase in PA current on speech peaks, rather then a reduction.
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Not something I have really pondered since, especially as the effect at the received end is not noticeable usually.
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With regard to PA failures on the 931 family, on the two radios I've replaced PA's on, they were operating into an antenna via the roller coaster internal ATU, on 80m, into quite short antennas and at a guess the PA's were exposed to a higher RF voltage when tuning (or the antenna changing impedance), due to the quite high Q. ?
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By comparison, the same PA devices when used in a PRC-320, seem a lot more rugged, although the internal ATU on the 320 is lower Q, with ferrite pot-core inductors that are lossier overall. ?
A PRC-320 with an ALC/APC fault can produce around 100-110W of RF, although it obviously gets very warm, quickly, so the devices seem to have a degree of head room.
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I'm glad that your 931 is now producing full power again, however it modulates!
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73
Al
G8LIT
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------ Original Message ------
From: Ian@...
To: [email protected]
Sent: Monday, March 24th 2025, 12:23
Subject: Re: [RacalRadio] TRA-931
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Many thanks to Alan, G8LIT
For his enlightening reply ... Downwards Modulation.
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Wow! It must be at least 45 years since I last heard that term. Being first licenced as GM8LWR back in 1976, when most of us operated ex-PMR AM mobiles, we were very familiar with Downwards Modulation, characterised by the ammeters on our PSUs dipping, rather than peaking on speech peaks. I suppose that since then, we have been accustomed to AM and SSB being the product of the same Balanced Mixer.
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The 931 is an interesting beast in that the TX side actually bears a close resemblance to the MA79 Drive Unit. The 1.4MHz in each case is the 14th harmonic of a 100KHz signal. Both use a balanced mixer ... and even the following filters are configured in a similar way, with the AM (or DSB, in the MA79) filter being somewhat similar. Given that the MA79 generates AM and DSB from a very simple balanced modulator, I do find it odd that in the 931, the modulator is deliberately unbalanced in order to generate AM in this form. I understand it, I just don't know that I understand the reasoning.
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Anyway ... everything is working fine. Press the PTT in AM mode and you get 20W of carrier ... apply an appropriate audio signal, the ammeter on the PSU drops from 3.5A to 2.5A and the power drops to 8W.?
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Thanks again,
Ian, GM0UHC
www.chavfreezone.me.uk?
