G'day everyone.

I'm trying to restore a chassis fro a Pye Radio Gram, model TS2, Mark2, its a hybrid setup with valves for the audio preamp and power amps, also the rectifier, but the AM radio part is based on PNP germanium transistors.

I've successfully rebuilt the valve based part and now that seems to work fine.

The AM tuner has me stumped.

It worked a little bit just once but with squealing and massive amounts of distortion.

Now its dead, I've established that the IF seems to be working as I can inject 400 Hz modulated 455kHz into the antenna and hear the modulated output ok.

I'm pretty sure the self oscillating mixer is not oscillating.

I've tried replacing the 2 ceramic caps associated with the oscillator and checked the oscillator coil, the transistor checks out ok on my Jaycar transistor tester, but I guess that doesn't check that it has gain at several hundred kilohertz though.

My question, has anyone been able to substitute a silicon transistor for a germanium in a circuit like this?

Thanks,

Raff

Ill post up a scan of the circuit I have, Ill do it again as I'm missing the middle bit of the circuit, but the tuner section is there ok.

If your tuner board has the ceramic Ducon Redcaps, replace all of them. They can often turn into resistors. Start with all the 0.047 μF first and see what happens.

The minimum value of forward voltage required to allow current to flow in a germanium transistor is 0.2 volts, whereas a silicon device is 0.6 or 0.7 volts. So it may not be possible to use silicon devices in their place.

Kevin Chant has the full circuit here:-
http://www.kevinchant.com/uploads/7/1/0/8/7108231/ts2_601s.pdf.

Hi MonochromeTV.

Thanks for the link to the circuit diagram, that version is much better laid out than mine, and mine is the genuine PYE one.

Ive already tried replacing the 0.047 (C2) ceramic cap and also the 0.01 (C3) with no change.

What do the Ducon Redcap ceramic caps look like? The ones in this circuit look like standard ceramic caps to my old tired eyes.

Do they have a red tip on the yellow ceramic?

Regards,
Raff

Hi Raff.

Yes, Ducon Redcaps do have the red tip on the yellow ceramic. I have found values of 0.047 μF and higher to be most troublesome, but I'd still replace all of them regardless. 0.047 μF 25 volts is the most common value you'll see in 1960's Australian made transistor gear.

I'll post some pictures here of some Ducon Redcaps to give people an idea what to look out for in 1960's transistor gear.

It may pay to post a photo of the boards so many of can see what is on it.

Albiet I fix few transistor sets; What I am finding is that many Germanium oscillator transistors are failing and I did have a number written down of a HF silicon TV transistor that will work in some.

There are some black & brown axial caps "Hunts" are one (not the only one), abt 3/8" long 1/4" dia are failing regularly & I have seen a couple in car radios blown in half. Electrolytics are common couplers in transistor sets and these are drying out & reducing coupling. Often manifesting as low volume: voltages correct.

Some early ceramic & tantalum caps, had issues. There were some interesting ways of mixing signals to achieve a super-hetrodyne, similar to when we only had thermionic Triodes.

Similar rules apply to fault finding, with solid state, as applies to valves. Once the Voltages are sorted out you then prove each stage. The most expedient method I have found for finding lost signal is to use a Signal Generator & Oscilloscope as you can feed in appropriate signal to either aerial, or stages to find the dead one. The oscilloscope will also give an indication of the oscillator working or not.

The other way that can help find a dead oscillator is to use another radio. Put it on say 1000kHz then get the good set and place next to it. You can expect squealing at the IF frequency on hetrodynes & around 1445 with a 455 kHz IF frequency.

An audio signal into the volume control should prove the audio and on a stereo one should have only refitted one amp at a time, if both were working or not. (Cut the power to the dead one, fix it first)

The injection of modulated IF frequency into each IF/ RF stage, will prove them even if the oscillator is dead. Squealing other than that, is liable to be an unstable stage that is itself, oscillating.

Marc

Hi Raff,

re substituting silicon for germanium? yes, but only once ....... one transistor only ..... and only an RF amp ..... one has to be mindful of voltages, biasing, pnp vs npn, and other transistor characteristics. .... not an easy 'swap'. Much simpler to address the existing problem as my limited experience with these old transistors has been that other components fail first.

Cheers,
Ian

That also applies to Valve circuits. Find the faulty stage then deal with it.

I have found in practice (several hundred sets) that there is a higher probability of discrete components being out of spec., caps leaking , or shorting & audio transformers failing & general wire faults (deterioration) and problems caused by Monkeys who think that they can fix, than the failure of Valves.

Sometimes one just has to deal with the fact that you have a dead & obsolete Transistor, that is not procurable and that is when you need to understand the circuit that it is in.

Marc

Hi everyone,

An update, she works!

I replaced all of the ceramic caps with red ends as suggested, it still wouldn't work though.

I then decided to crack out my "big gun" Motorola R1200a Service Monitor as its much more stable and accurate than the el cheapo oscillator I initially used to do an IF alignment.

I found that the whole front end and IF strip is very prone to oscillation and just stopping from working, maybe its blocking, I don't know.

Anyway, after fiddling with the IF and oscillator slugs its now working just fine with the original germanium mixer transistor back in it.

Thanks everyone for the valuable suggestions etc.

Raff

The front end stopping is interesting. It may be a bad solder joint, or an intermittent component. Do check base resistors as best can if it persists. A bit of prodding & poking with a chopstick may be in order. Bad lead dress (sloppy wiring) has always assisted instability.

Why I suggest the base is that they affect the bias & if that goes awry, some germanium transistors will suffer thermal runaway and that can destroy them.

Not a thing that can be ignored is that the unwanted oscillation in a stage. It could cause the generation of sufficient AGC voltage to be a problem. So that is a circuit to be looked at. If the problem persists.

Another thing is the failure of decoupling capacitors that are fitted to supply rails to rid them of RF.

Marc