Transistor superhet local oscillator output level

[davidh2019]

This is my first post on this forum so please forgive me if I have posted in the wrong category.

It is many years since I dabbled in electronics howeverI am restoring an old transistor superhet radio I made in 1968 from a design in PW - The 208 Bandspread portable. I have the circuit diagram but there are no voltage levels stated (see attachments)

My test equiment included a Digimess RF Signal generator, a Tek scope and an RF probe + the usual multimeter.

Initally there was no output and so far I have fixed 4 problems and managed to get as the strongest signal, BBC 5 Live on 909 kHz. The sensitivity and selectivity are both very low and I cannot get any other stations. I have read somewhere that if putting a screwdriver on the base of the first RF stage transistor and then touching the metal shaft increases the volume of the station then this points to an issue with the local oscillator. I am not sure of this though.

Any advice on what I should do next please?

[paulsherwin]

If you can tune the radio then the oscillator is running.

Did you build the radio with AF117s? If so, are you aware of the dreaded AF11x tin whisker problem? Search the forums for lots of threads about this.

[davidh2019]

I had the tin whiskers problem on all 3 AF117s and managed to revive them by the charged capacitor trick. Before I found this out I temporarily replaced the AF117s with AF127s.

Yes the oscillator is running. Will the ampltude of the signal (approx .6V) matter to the selectivity / sensitivity?

[TrevorG3VLF]

I would have thought that 0.6V is sufficient to turn TR1 on and off and so give good gain.

Measure the voltages on all the r.f. transistors to ensure they are passing the right current. A zapped AF117 may have very low gain.

Try replacing the detector diode.

[G6Tanuki]

Also, check the voltage on the AGC line. I've had some really-strange issues on old radios in the past where the AGC line seemingly floated around at random - cause was a defective rivet in the volume-control pot (which also served as the detector-load resistor).

[davidh2019]

Trevor, there is no difference in performance with either the old AF117s after zapping the tin whiskers and the AC127 that I temporarily hooked up so I hope the gain of the AF117s are OK. Thanks for the heads up re the 0.6V. I will check the DC voltages on the transistors. I had to replace the detector diode as the glass on the old one had broken.

G6Tanuki, I will look into the AGC line voltages.

Thanks both for your advice. Looking forward to receiving any more tips as I am new to this restoring.

[unitelex]

Hi Welcome to the forum!
Do you remember if the IF section and also the RF section was aligned and you achieved good sensitivity and selectivity when you first built it? If so, assuming the cores and padding caps have not been disturbed since then no reason to suspect alignment to be an issue.

Otherwise if you know how to use an RF signal generator, then an alignment check would be my next step.
Best Regards
Chris

[paulsherwin]

Yes, that is an important question. Did the radio work well when you built it, and has now deteriorated, or has it never worked properly?

[crackle]

Have you checked the electrolytics, C6 & C8?
Mike

[davidh2019]

Unitelex: Thanks for the welcome.

When I built it I am sure the sensitivity and selectivity was better than it is now. Unfortunately the radio has has had a hard life in the intervening 50 odd years since I built it. For many of those it was stored in the loft. Initially it was aligned by listening weak stations and tweeking the cores and trimmers until I got the best response. When I was 16 I did not have the money to buy a sig gen.

Unfortunately the osc coil and IF coils cores have been twiddled with and the trimmers played around with too. I will take your advice re aligment.

Paul: My feeling is that worked well when I built it and has since deteriorated, but then my hearing response has dropped off too!

I have started making an attempt with IF alignment using my sig gen.

Questions:

1)With the Weyrad P51/1, P51/2 and P50/3V IFTs is the alignment freq 470 kHz?

2) Where in the circuit would be the best place to inject the signal? The base of the transistors?

3) I presume that I should connect the RF from the sig gen via a cap. What value would be suitable?

Any advice much appreciated.

Regards

David

[davidh2019]

Mike. I haven't checked those. I will replace them and see what happens.
David

[Sideband]

That design is by F G Rayer, a very well respected designer of the time. He would normally have given detailed alignment instructions unless it used a pre-set coilpack. Even then he would have given details on how to optimise it.

Full article is in the May 1966 Practical Wireless here, https://worldradiohistory.com/UK/Pra...PW-1966-05.pdf FG gives alignment instructions and it would be best to follow them

[trsomian]

Not relevant to the problem, but I notice that the output stage biasing is unusual; I can't remember seeing negative feedback around a push pull output stage like that before.

[m0cemdave]

Some of the carbon resistors may have drifted high in value, upsetting the biasing. I found this when restoring an old Heathkit Mohican receiver and was surprised how much difference it made after the bad ones were replaced.

[Chris55000]

Hi!

The general advice when aligning a transistor radio was always to inject the signal via a capacitor of no more than 1000pF and keep the signal injected to the absolute minimum needed to produce a just–audible output in the speaker, the standard output is 50mW which is about 0.4V r.m.s. across a 3 ohm speaker.

If you inject about half the maximum output of the signal generator (tuned to 470 kHz) via a 1000p capacitor to the base of TR3 (last i.f. stage) you should get a faint audible output if TR3, the detector diode and the a.f. stages are O.K.

Typical voltages in this circuit are:–

TR1:– e –0.95V, b –1.0V c –6.5V
TR2 and TR3:– e –0.7V, b –0.85V, c –6.5V
TR4:– e –1.0V, b –1.15V c –8.8V
TR5 and TR6:– e –0.05V, b –0.15V, c –9.0V

Battery Supply Current 8–10mA (no signal)

If the battery current is much higher than this, or the voltage at C8/R13 is much lower than the figure I suggest as a guide, there could well be AF117 "tin–whisker" trouble!

As is the usual practice in this type of circuit, the volume control is the detector–diode load resistor, and about +0.4V to +0.5V d.c. (with respect to battery positive) will be registered across the volume control when the receiver is tuned to a strong local station.

If you get this voltage on your nearest local station, then the loss of gain is most likely occurring in the a.f. stages!

Chris Williams

[davidh2019]

All, Thanks for the advice. I will have another attempt at getting this radio working properly by measuring the resistors, measuring the transistor voltages and having another attempt at alignment. Re tin whisker trouble, I did have this problem and resolved it