I didn't spot the web adress in disguised form. Here it is for those interested:

valveradio dot net website article, or enter "valveradio", "adding FM to AM" words into a search engine

Lots of other good valve stuff especially for those with deeper practised knowledge of valve technology (not me).

Re Ian's question in Post 6, Is the radio reflexed?

I can't see any obvious reflexing connection to the 6AR7GT IF valve in the N78 version.

I don't see any such connection in the 6AQ5 version either (also 6AR7GT IF); however it is not so clear as it has the added complication of feedback from the secondary of the output transformer back to the 6AQ5. Would feedback and reflexing be combined? Sounds like an invitation to instability.

I am not so good at picking up feedback and reflexing connections though, so I will submit the circuits to Brad for inclusion in this Post. Won't be able to do that until later today.

Adding this mod to these radios raises problems with their different output valves and circuits, despite the mods elegance of operation, so am also considering bluetooth.

AWA 429MA Service Information
AWA 429MA Circuit Diagram

There is no reflexing in this circuit

The website is valveradio.net

https://valveradio.net/radio/adding-fm-reception-to-an-am-valve-radio.html

Thanks for the tip re no reflexing Dave.

I put the website in disguised form because that how it is given by the vendor in the ebay listing.

Should have done this earlier, I just downloaded the manual for the 429MA from Kevin Chant's site.

Yes, no reflexing, a very simple radio.

The 6AR7 was designed by AWV especially for reflex designs, so it's unexpected to see it used that way.
A 4 valve radio was never a very impressive performer without reflexing and with a relatively low gm output bottle.Unless you do away with the AGC.

OK thanks Ian, I didn't think so either but sometimes these things are done in a subtle way so I wasn't sure.

Re the valveradio website; while he goes into a lot of detail, things are explained very well. I can't say electronics are in my bones, but I can usually nut things out. I just wish I had the depth of understanding and experience of the Ians, Marcs and Freds!

Probably the best way to turn it into a respectable performer, both as a radio and as an amp for the FM adaptor, is to add a transistor audio stage after the volume control. It needs to have a high input impedance and a voltage gain of about 10 times.

Easy to do that with an npn and series voltage feedback to keep the input impedance high. Happy to send you a circuit, 1 transistor, 2 caps, 3 resistors would do it.

Ok thanks Ian I think that that is the way to go as I do want to keep AM to receive Sydney stations, especially those with music, and ABC Radio National which is not available locally on FM. Don't want to miss out on The Science Show, What The Duck and Dr Norman Swan's Health Report. I will email you shortly.

Documents uploaded to Post 17.

I decided to have a look at my radio, which is the Hotpoint version, can't quite make out the model number, L?4ME, but it's the same as the 429MA.

The first thing I did was to measure the G2 voltage, which was 110 volts. This exceeds the maximum specification for both the 6BE6 and the 6AR7GT. Generally, the G2 voltage should not exceed 1/3 of the anode voltage, unless the maker says otherwise. In this case, with 219 volts B+, it was way over, and the 6BE6 showed its displeasure by getting hot. I changed the 10k resistor to 18k, giving 91 volts on the screen. The B+ rose to 225 volts. It still drifts noticeably during the first minute, but seems to settle down after that. I'll leave it running for a while and see what happens.

For a 4-valve set with no reflex, it works pretty well. I could hear the Sydney ABC stations with just the short wire stub. Holding the wire of course brought up the volume considerably. My house station produces lots of volume, just to the point of distortion when the volume is turned up fully.

A last examination found a wax paper capacitor hiding behind the transformer, so I swapped it out. The schematic show the tone control on the N78 anode, but in this radio it's connected to the grid circuit instead. Works well enough as is.

There's an IF trap on the antenna circuit. Not sure why I disconnected it, but I assume it was dragging down the sensitivity.

So, if your 429MA's are working, you'll have some pretty good radios.

After over an hour it's still ok, so I guess the drift problem has been fixed.


BRAD: the system time is wrong on the forum. It says 7:34 here, but the forum says 7:00 .

The time should be good now. I have a feeling that the motherboard battery on the server may be on its way out. As a precaution, next time I do Windows updates on it, I'll do a shutdown and replace it.

So your idea of this defect given in your Post 2 shows in your radio Robbert. So thats the first thing that I will check when they are back in my hands.

Mine would also pick up Sydney on a few metres of wire from north of Wollongong, about the same distance that you are at Hill Top. So mine performing OK compared to yours which is encouraging.

Will also sub in NOS 6BE6s as Ian suggests, if the voltages are OK and fading persists.

Do beware of atmospherics. Pretty much everything here will pick up Sydney& Adelaide at night, however 2AY North side of Albury, whilst fine during the day; At around 5pm it will start fading in and out.

Some often forget the oscillator; It can lose strength and bring down the modulation within the tube. That is also a normal thing when tubes like 6A8 are used for SW and start to fall over at about 25 metres.

Due to drifting of coils with age and the recommendations of many manufacturers, after changing RF parts and with any sign of "deafness": Recalibrate it, if you have the gear. Here that usually involves a Signal Generator, Frequency Counter and Oscilloscope, coupled to an attenuating box that couples them all together.

Yes all bets are off at night, even 630 Newsradio can be difficult to listen to due to fading and interference from other stations on 630. The appearance on 90.9 has fixed that. 702 Sydney also fades.

Will look at alignment as these were out of my hands before I built the micromite gear.