Well I've just bought one of these for nostalgic reasons. Back in 1968 I bought one for a shilling from a jumble sale. The tuner was perfect and didn't need any work but the power supply was another story....I rebuilt it using the original 6X4 but with new smoothing caps and a couple of anti-modulation hum capacitors between the anodes and cathode of the 6X4. The tuner and power supply served as my first proper FM set-up for several years and as a cash-strapped teenager still at school I felt very pleased that I could listen to the latest pop music in decent quality albeit in mono. It was lost during a house move...I don't know what happened to it but by then I had acquired a Philips stereo tuner.
So 51 years later I spotted this on eBay for a reasonable price and couldn't resist it. I was the only bidder and it was also advertised as working. The seller also said he would throw in the original Jason booklet and the power supply.
Well it arrived today and I couldn't resist giving it a try out. I don't have an amplifier set up in the workshop yet but I do have the home-built headphone amplifier that I built a few years ago using two ECC82's in a transformerless circuit. After making up a suitable lead from the mono output of the tuner to the stereo input of the headphone amp I was ready to give it a try. Well it did work....sort of. It was clear that the tuner was mis-aligned since it was rather distorted and 'double-humping'.....The centre of the tuning point was very weak but either side you could receive a distorted signal. I suspected the ratio detector was off so experimentally gave the bottom core a tweak. There was an immediate improvement and I was able to get reasonable results. The 8uF cap across the two diodes appears to be OK so I'll get the signal genny out at some point and go through it. It certainly seems to perform quite well but the RF alignment is somewhat 'squashed' at the high end towards 108Mhz. Local station Radio Jackie is on 107.8 but it's appearing at 105. The other end of the scale seems to be quite reasonable. The advantage is that the Jason booklet has the alignment instructions and full circuits (it covers FMT1, FMT2 and FMT3) so there will be some action with the sig genny soon.
I have visions of having this tuner set up with a Mullard 3-3 (yet to be built) and providing background music in the workshop when I'm working in there. I wonder if it might be possible to add a stereo decoder to it since I have a spare one gathering dust.....may be worth trying just for fun.
It certainly sounds very good and the four EF91's are perfect. The power supply is very obviously home-brew but it works and has a good 6X5 fitted. Smoothing can is dated 1963 so that ties in well with the publication of the original circuit.
Chris.....just a thought. If you would like to borrow the Jason booklet to scan for the library, let me know. It's in excellent condition and contains circuits, alignment and construction details.
Chris.....just a thought. If you would like to borrow the Jason booklet to scan for the library, let me know. It's in excellent condition and contains circuits, alignment and construction details.
Hi Rich I've actually already got a copy just never bothered uploading it, but thanks for the offer. ? I will now upload the Jason FMT1/2/3 manual/booklet so at least others who are interested can follow along with your servicing thread.
I was given a CD year or so ago, its packed full of vintage audio service manuals, data, circuits, alignment info, leaflets and brochures for a fair few manufacturers. That's how I populated the vast majority of the audio equipment brochures up top.
I did load a lot of the quad amp circuits/service manuals and a few others into the data library, just not got around to doing the bulk of it as there never seems much call for it from members. Also as the site is already sitting at 85GB of data!!
CrustyTV Television Shop: Take a virtual tour
Crusty's TV/VCR Collection: View my collection
Crustys Youtube Channel: My stuff
Crusty's 70s Lounge: Take a peek
The Jason service manual booklet is now available via the free-to-view "Service dept" menu up top, under Audio.
CrustyTV Television Shop: Take a virtual tour
Crusty's TV/VCR Collection: View my collection
Crustys Youtube Channel: My stuff
Crusty's 70s Lounge: Take a peek
Having played with the tuner a little more, I've noticed some modulation hum on certain stations. It probably points to deficiencies in the rather basic power supply so I may rebuild it with silicon diodes or perhaps just try some more filtering in the heater circuit. The HT is also a little high so clearly some changes are required. Jason recommend 200V and it's currently at 220V which can cause tuning drift....it does drift slightly but I don't know what is normal for these. Anyway it's something to play with as the dark nights draw in.
The tuner and power supply served as my first proper FM set-up for several years and as a cash-strapped teenager still at school I felt very pleased that I could listen to the latest pop music in decent quality albeit in mono.
There wasn't a lot of pop music in FM in those days as I recall? (Radio 1 having to share VHF bandwidth with Radio 2 and only Radio 3 broadcast in stereo.)
No there wasn't but I used to listen to Pick of the Pops with Alan Freeman every Sunday on Radio 2 (I think it teemed up with Radio 1 which wasn't on FM back in those days. No it wasn't in stereo then either but that didn't bother me. The tuner, hooked up to my Philips tape recorder allowed me to make good quality recordings.
Well after the drama of the Advance generator in the 'Plunged into darkness' thread I was able to complete the alignment of the tuner. It's important to follow the instructions in the Jason handbook of course. The alignment was actually quite a way out and the gain was greatly increased once the I.F's were tuned. I always like to 'rock' the generator 75Khz either side of 10.7 Mhz to try and get the bandwidth as near to 150Khz as possible since I don't have a wobbulator. Basically clip a testmeter (preferably analogue) to the AGC rail, peak the IF to 10.7Mhz then 'rock' the generator 75Khz either side of 10.7Mhz and check the testmeter deflection either side of peak. You should be able to get a reasonably even 'dip' in the meter reading either side of the peak.
The ratio detector is tackled slightly differently. In this case the generator is connected to the grid of the limiter valve and set to 100mV output. The meter is connected to one side of the electrolytic in the ratio detector....it doesn't matter which side....one side will give a negative voltage, the other will give positive. Both cores are peaked for maximum output....they were miles out in my case. I ended up with something like 9 volts showing on the meter from about 3 volts. The meter is then transferred to the junction of the de-emphasis network and the bottom core is adjusted for zero volts. You should be able to adjust either side of zero as the core is adjusted. This is important for correct balance of the ratio detector.
The difference after all this was done was huge. Much more gain, better selectivity and sensitivity and best of all, superb quality with no distortion or sibilance which is what I had last time.
The tuning scale calibration is also improved although it is still slightly 'squashed' at the very top end. This may have been a design quirk from day one since when the tuner was built, there were no broadcasts above 95Mhz so it wouldn't have mattered. I can live with it and only stations above 105Mhz are affected anyway.
Incidentally I fitted my Advance signal generator with a frequency counter so that I can adjust it accurately. It isn't finished yet and requires some sort of housing but it works and in particular I can adjust the frequency correctly for TV I.F's. (like 38.15Mhz). It just isn't possible to judge 150Khz from the scale reading at those frequencies.
Next I might see if it's possible to add a stereo decoder to the Jason. I have a working Philips decoder in a box so might try that and see what results I get. It will mean at the very least removing the de-emphasis network so that the pilot tone can be recovered.
Next I might see if it's possible to add a stereo decoder to the Jason. I have a working Philips decoder in a box so might try that and see what results I get. It will mean at the very least removing the de-emphasis network so that the pilot tone can be recovered.
You'll need more than 150kHz of IF bandwidth for stereo, Rich. Applying the principle of Carson's Rule, the required bandwidth is 2(Fd+Fm) where Fd = maximum deviation and Fm = modulation frequency, so you're looking at 2(75+53) = 256kHz.
I seem to remember a compromise for many sets was around at least 220khz but better sets went for 250khz. A wide band detector was advised as well so going for 1Mhz for better linearity, don’t know how many went that far.
Be interesting to see how well a stereo decoder performed with the Jason, as long as it’s not too difficult to hookup and try.
I don’t recall the Carson rule when at college, just equations with frequency, modulation index etc, they may have done though, I have forgotten most of the theory.
Frank
By the way, if anyone's wondering where I got the '53' figure from for modulation frequency, the difference signal that carries the stereo information is double-sideband suppressed carrier AM, so its bandwidth is 2Fm and as it sits on a 38kHz carrier, we get 38+15 = 53. This is why we regenerate the carrier from the 19kHz pilot tone.
Were we to have the SCA system in the UK, the bandwidth would have been even bigger - one of the Signetics PLL chips has an SCA decoder as a suggested circuit for it!
I don’t recall the Carson rule when at college, just equations with frequency, modulation index etc, they may have done though, I have forgotten most of the theory.
Like you, I've forgotten it too and I'm not in a hurry to retrieve my Evesham notes! When I was in training with Auntie the full mathematical analysis suggests that an FM signal has an infinite bandwidth, and a favourite calculation given to students is to prove at what mod. index the FM carrier disappears completely and all the energy is then carried by the sidebands. This is why Armstrong was told "it couldn't be done".
However, in the mots de jour, Armstrong said "hold my pint..." and proved that it could. Carson's Rule does fall down under some circumstances, but it provides a very good approximation for the bandwidth of the system.
Next I might see if it's possible to add a stereo decoder to the Jason. I have a working Philips decoder in a box so might try that and see what results I get. It will mean at the very least removing the de-emphasis network so that the pilot tone can be recovered.
You'll need more than 150kHz of IF bandwidth for stereo, Rich. Applying the principle of Carson's Rule, the required bandwidth is 2(Fd+Fm) where Fd = maximum deviation and Fm = modulation frequency, so you're looking at 2(75+53) = 256kHz.
Ahh yes...I knew there was something else....it's well over 40 years since I read up on anything to do with stereo decoding and I'm not one for maths anyway and when I came across +L+R and -L and -R, I was trying to work out how you could have a minus left or right signal....my mind then was thinking in terms of negative sound.....! Of course now I understand it all better (I think) but I was mighty glad when decoder IC's came along where effectively you shove an L+R signal in one end and get stereo out of the other.....
Good point about the bandwidth. That might be a problem. It depends if it's possible to broaden the IF response sufficiently without wrecking the selectivity.
It depends if it's possible to broaden the IF response sufficiently without wrecking the selectivity.
Well, it's the IF that defines your selectivity Rich so if you want stereo, it'll have to be broadened a bit. Try it though, you may be surprised. For some time now (so long, I think the owner has given up on me doing it!) I've had an Armstrong 526 tuner-amplifier on the bench. The biggest problem with these is redesigning the output stage to get rid of the unobtanium output transistors that are hideously unreliable, but also someone has been at the FM IF - and it has no stereo, although it should.
Some modern tuners use DSP to decode the stereo signal and these can retrieve quiet stereo information where a conventional decoder would produce a noisy signal.
