Sony trannie won't tune in

[Goldieoldie]

I would suspect the caps as well .
If you don't want to replace all of them start by connecting in parallel a new cap across the cap from the wiper of the volume control ( middle pin ) to the base of x5 ( no need to solder just bridge it for testing )
Quite frequently these Low value caps ( 2uf ) tend to loose their capacitance so not coupling the output of the volume control to the audio amp
The next to bridge is the cap between the collector of x5 to the base of x6 ( 10uf )
For these tests any cap around 2 to 10uf will do
Perhaps you will be lucky and it will be just one cap and you will feel inspired to replace it !
Another one to check is the detector diode and try shorting r9 to see if it springs into life !!
Cheers
Pete

[stevehertz]

Thanks guys, plenty for me to go on there!

[Techman]

You've got the circuit information.

You've got a signal generator AF/RF.......?

[stevehertz]

Originally Posted by Techman

You've got the circuit information.

You've got a signal generator AF/RF.......?

Yes I have numerous sig gens for alignment purposes but have never done this type of signal injection thing at various points on radios and then looking on a scope for stuff. Haven't got a clue how to do it!

[stevehertz]

Ok, I have it apart. Doing my normal time honoured prodding around with a tiny screwdriver I've found that X4 is very 'wobbly'. That is to say, the plastic 'head' of the trannie seems very loose on its leads. Comparing with others, they too are curiously wobbly but probably as a result of their very flat lead wires, but nowhere near as wobbly as X4 is. It seems to me as if there is if one or more of the leads are severed where they go into the transistor, or just inside. Pressing on it produces crackles. I've tried simultaneously trying to tune the set while applying pressure but I'm not an octopus and in any case it didn't yield anything of note. Anyway, my plan is to temporarily replace it with anything that is likely to 'produce sound'. The spec sheet is below. I have a few Ge trannies that I need to check out their specs, but for now I have a Fairchild MPSH10 NPN (Si) that is a high frequency trannie, should I give that a go?

[PJL]

It's an IF amp but pretty much any small signal NPN germanium should do something. The difficulty is 'NPN' as most germanium are PNP.

[michamoo]

You could try injecting an audio signal at the wiper of the volume control. If that seems to work ok then work backwards from that point.

[Techman]

No need to worry about a scope unless you really have to - your ears are all you need.

AF in at the volume control and if that proves OK, then RF 455KHz(?) into the IF - you may have to rock the generator either side of the frequency to find the exact. Keep the output of the generator as low as you can get away with and play it by ear.

[stevehertz]

Originally Posted by Techman

No need to worry about a scope unless you really have to - your ears are all you need.

AF in at the volume control and if that proves OK, then RF 455KHz(?) into the IF - you may have to rock the generator either side of the frequency to find the exact. Keep the output of the generator as low as you can get away with and play it by ear.

You might as well be talking a different language Techman, no disrespect. I'm the sort of guy, if I don't know how to do something (like I said) I need a step by step, idiots guide. It's no use saying poke this into the IF! Sorry, that's me, terribly knowledgeable and capable at some things but a total duffer on others - including IT. I realise you mean well, thanks!

[poppydog]

This may help, i have it and its a excellent book

https://www.abebooks.co.uk/978085242...0852425694/plp

poppydog

[GeoffK]

The signal generator is set to the intermediate freqency of the radio, the fixed tuned coils. 455 kiloHertz and the internal audio oscillator of the signal generator turned on. Just hold the lead from the generator near the radio and tune the radio across the band. The signal will be heard if the radio I.F. is working. An audio tone also shows the detector diode is working as well as the audio stage.

[stevehertz]

A few points to report back on.

A 6kHz signal injected on to the volume pot comes through loud and clear. Injecting it as far back as the detection diode's anode does the same. It is adjustable in volume by the volume control. So the AF section, post detection seems to be fine.

On removing X4 it is marked C403 with 'mn3' below that. I take that to be 2SC403? But hey, a 2SC403 is a silicon transistor whereas X4 is shown on the cct diagram as type 2SC76 germanium?

BTW, on removing it I'm pretty sure that X4 did have an intermittent break or even a totally severed lead so I'm hoping that replacing it will at least help, if not cure the problem.

[Techman]

I should have made it clearer that when the signal generator is set to RF (455KHz) it should also be set to 'internal modulation', so you'll hear the 400Hz (or what ever it is) tone through the speaker, as obviously you won't be able to hear the 455KHz high frequency.

I just went back to the little radio that I mentioned earlier and below is a picture of the lead from the signal generator AF output with the ground connection clipped to a handy IF transformer can for radio ground and the signal lead with a capacitor (0.1uf in this case as it was to hand) in the crock clip and its other lead probing the volume control slider - the volume on the generator needs to be set to low with the volume full up on the radio for a loud tone to be heard from the speaker.

The second picture shows a quick loop of wire used for general IF injection which happened to be 470KHz on this radio and the AF tone from the internal modulation could be heard through the speaker of the radio. I don't have a circuit for this particular radio, so I'd have to work out where the different IF stages were if I actually needed to fault find down to a specific part of the circuit, but you've got a head start with having the actual circuit to hand. The IF modulated tone should come through the speaker regardless of the tuning position.

The above two test will tell you which half of the radio circuit the fault is in - you may end up only needing to do one of them.

[Techman]

I hadn't spotted your post #32 until just now, Steve. Sounds like you're a step ahead and may now have the fault pinned down.

[PJL]

If it is silicon I would expect the biasing to be different. R11=3.3K in the circuit diagram but I would expect something higher.

[crackle]

Isn't it a bit unusual to have all NPN Germanium transistors for this era.
I thought that all NPN in common use was at the advent of silicon.

Mike

[stevehertz]

Can people see my dilemma re post 32? Posts 35 & 36 are also 'in conflct'. Any views on what this grannie should be?

[Dickie]

Looking at the voltage and current info on the diagram I think X4 has to be a germanium npn. The base is fixed at 3.3/42.3 x9 =0.7V, so if the emitter voltage is supposed to be 0.35 volts we get a Vbe of 0.35V which would barely turn on a silicon device. Most strange.

Personally I'd pop a silicon npn one in just to see what happened, as I have plenty of them.

[PJL]

It is quite possible that early versions of the radio were built with Germanium and later ones with Silicon. Silicon transistors have different bias requirements so I would expect your set to have different bias resistor values (R10 and R11) if it was designed for silicon.

What are the markings on the other transistors? Is this the only one that is different from the circuit? The circuit is clearly for Germanium as R10/R11 set a bias voltage of 0.7V which would leave a silicon transistor with no collector current.

Either:
1) The set is designed for Silicon and you need to replace with a Silicon transistor. R10/R11 will not match the circuit diagram values.
2) The set is designed for Germanium and someone has replaced X4 with the wrong type and not changed the bias conditions (R10/R11).
3) The set is designed for Germanium and someone has replaced X4 with Silicon and made changes to the bias (R10/R11).

[Dickie]

So with X4 out of circuit Steve needs to measure the voltage applied to its base pad on the pcb. If it's more than (say) 1.0V he can fit a silicon device, if less than 0.8V it needs to be germanium.