Forum problems

[okaman]

Dear Les,
  could you check Storypainter@hotmail.com
i bought positron on ebay 10 min. ago by credit card

[top204]

Many thanks for the donations my friends.

[charliecoutas]

Thank you Les for your patience and dedication.

Charlie

[top204]

Many thanks for your kind words Charlie and your 'most welcome' donation.

At the car boot sale this morning, I got a Dansette transistor radio from 1961 for £1. :-) It doesn't work yet, but it will when I get my workshop up and running. It will, most likely, need its electrolytic capacitors replacing. Beautiful objects, with older components in them that take me back to my boyhood in the 1970s when the neighbours used to give me their old electronic items when they were finished with them, so I could strip them for components or repair them, in our back garden shed.  :-)

The radio in the link is the one I got at the car boot sale:

https://www.radiomuseum.org/r/dansette_rt222rt_22.html

[John Lawton]

Probably teaching you to suck eggs....

Check the local oscillator is working using another MW radio nearby.
Check the audio stages with a mains hum injection or signal injector.
That should narrow down the problem(s) a bit.

Then the problem is what t listen too. There is Radio Caroline on 648kHz... only took them 50 years to get a licence.

John

[John Drew]

Those transistor types bring back memories Les, especially the OC44 and OC45.
I think my first transistor was an OC72.
John

[top204]

Same here John... Good memories of the old germanium transistors that dad had when I was a boy.

I only got into silicon transistors in the 1970s, because they were a bit too expensive for me, and all the old radios I got given or found used the old germanium types such as the OCxx types and the OAxx germanium diodes. These transistors and diodes now cost a bloody fortune. Even for second hand used ones! Utter crazyness!

The Dansette has OC44 and OC45 transistors in it as well as a few other germanium transistor, but earlier ones that are not the longish black glass types or metal types for the OC7x types that I remember as a boy. They are smaller squat types with coloured shrink wrap around them and look rather early. I can't wait to get into it. I tried a PSU on its battery terminal but there was no power draw at all, so it may be as simple as a wire broken or circuit board crack or dry joint or On/Off switch fault.

Then I will get into its inners and check with the signal injector John, and attach my home made wobbulator and make sure the IF is OK and aligned still. I made the wobbulator many, many, many years ago. Too many to be comfortable with actually. :-)

I will keep the rest of us nerds on this good forum updated. :-)

[John Lawton]

Most modern engineers have probably not heard of a wobbulator

AM radio service information usually said align the IF stages for maximum output but I think that is a mistake especially with transistor equipment that usually has two IF stages and aligning for peak output will give too narrow a bandwidth, giving a muffled audio characteristic. In my view, the ideal IF bandwith is somewhere between +/- 6 - 9kHz depending on how much adjacent channel interference you can tolerate. 

This is why valve radios usually had better fidelity, my SDR Uno radio is great as you can adjust the IF bandwidth easily to suit reception conditions.

John

[charliecoutas]

You're welcome Les. We have a computer at the museum that has of thousands a germanium transistors (mainly OC44): an Elliott 803B. It is looked after by a very enthusiastic chap called Peter Onion. If you are interested in 803's, Google Peter. He refuses to run the machine if the temperature is very far above 22 degrees (C), in case of damage. The old gal still runs well although an average instruction time of around 200uS is a bit slow these days.

Best to all
Charlie

Sorry I haven't been around but I've been immersed into a major project that has now been completed, successfully.

[John Drew]

Ahh, memories. At one stage a mate and I did experiments with light communications using low voltage pea lamps to produce a point light source and a large reflector from a vehicle to create a beam. The best modulation was achieved when the globe was barely visibly red, I think one reason was the inertia of the filament was least and so less distortion of the modulation, it's also possible that our detector was better toward the red end of the spectrum. A microphone and amp drove the pealamp (6V 40mA from memory).

Here's the crunch: the detector was a germanium transistor with the paint scraped off placed at the focus of a large diameter lens. It was followed by a high gain audio transistor amp probably using silicon transistors as by then we could access these from old computer boards at our local disposals store. If you think about it you can see why the transmitter had a reflector while the sensor used a lens.
Testing was done in a quiet longish street in Adelaide. We managed about 1km of range with voice modulation.

Other experiments with chopped light gave much greater range as we could use headlight globes.
Nowadays it is all so easy with high power LEDs there is no fun in it.

Kids don't do those experiments now, what a shame.
John

[SeanG_65]

The process is ongoing, but rest assured I WILL honour my offer.

We had to take a slightly different approach which entails setting up a bank in a foreign jurisdiction outside the grabbing hmrc and the criminals at the FCA.

The security checks are horrendous, but not surprising given over all there is some £90Bn involved overall.

[SeanG_65]

BTW, if you are into restorations / fixing stuff, look up Shango99 on YouTube, also Mr. Carlson's Lab

[top204]

I got the Dansette working. It was, as usual, a few electrolytic capacitors that had dried out, and a few dry joints stopping it powering up. It sounds OK after 60 years or so, however, MW is now, virtually, dead as a broadcasting frequency and most of what I heard on it was the noise from the LED lights in the workshop and around the house, and the computer monitor etc...

Now it is sitting on the rack and will not be looked at or used again, but it was fun fixing it. :-)

I still remember my dad showing me that an OC type transistor could be made into a phototransistor by scraping the black off its glass housing. The ones with the black scraped were more expensive and called OCP types. Just because someone had scraped a square off the housing's paint of a normal OC type?

I made a simple light coms unit as well John, as a boy back in the early 1970s. With a torch's reflector and bulb for the transmitter and an OC7x for the receiver, with its black paint scraped off, and a sine wave bias to give it more fidelity. Good memories. :-)

[John Lawton]

Hi Les,

good stuff on refurbishing the Dansette. You were going to work on a MW modulator unit - we had a bit of a debate on the merits or otherwise of balanced modulators. I take it you haven't made any progress with that?

I have a few MW radios that it would be nice to have working, if just for fun. Also there is always Radio Caroline on 648kHz if that is to your taste, it has a surprisingly wide service area.

John

[top204]

The radio did not need any IF (Intermediate Frequency) adjustments as far as my ears could tell. The signals were clear and the tuning did not squark as it came into a channel, or drift, so for a 60 year old radio it is OK and did not need the setup of the wobbulator and scope.

I am in the process, or will be further into the process one day, of building a low power MW AM transmitter that will play audio files from an SD card and transmit them, using a PIC microcontroller as the oscillator/modulator/SD controller. I can then load the SD card with some of my favourite, old time, radio comedies, such as The Chlitheroe kid, The Navy Lark, Life with the Lyons, Educating Archie, The Huggits, Dad's Army etc, and listen to them on the few 1940s and 1950s valve radios I have here to fix that I have picked up from car boot sales (when their price is affordable. i.e. £5 or lower). :-)

I listen to the old radio comedies while I am working in the workshop, and swap the cycle of them every few months. They are such gentle comedies with no modern PC (Political Correctness) crap in them, so they are actually funny and are allowed to "take the piss" or use "double entendres" . I was born in the wrong era. LOL

[ken_k]

I am in the process, or will be further into the process one day, of building a low power MW AM transmitter that will play audio files from an SD card and transmit them, using a PIC microcontroller as the oscillator/modulator/SD controller.

Why not build a steam powered AM transmitter? http://www.dogstar.dantimax.dk/tubestuf/amtx-1.htm

[John Lawton]

That's very cunning. Valve circuit designers were masters at making their devices perform multiple functions. The idea to use a 1MHz crystal is nice, so then you have a calibrator as well.

[david]

1MHz?  Most receivers today are synthesized and would be stepping in 9kHz increments. (US 10kHz)
What I found interesting was the comment on the schematic about using capacitors across the diodes in the power supply.   This is something we routinely did back in the 70's and it's not about 50Hz fundamental.  I was fairly young at the time but from what I understood it was a low level but broad band RF interference with a 50Hz modulation caused by the reverse recovery time of the silicon power diodes.  Perhaps someone else could add to that.

David

[John Lawton]

Hi David,

yes I did encounter this on one occasion, a nasty buzzing noise, probably 100Hz as it was full wave rectification. The capacitors stopped it dead.

[charliecoutas]

Ken

We built a low-power AM transmitter at the museum, to do exactly what you describe. I used a DF-Player (mp3 SD) and an 18F26K22 plus the attached circuit. I'm not too good at attaching documents on the forum but let's see....

It transmits on quite a few frequencies, all at once! We transmit morse code audio and receive it on our rack of AR88 receivers. It's very low power but I would like to reduce the harmonics. I "tuned" the output PI filter by trial and error.

Charlie