[Eric Adams]

WooHoo! I just replaced the transistor and it works perfectly!

I can't thank you guys enough. Without you I wouldn't have had the knowledge or confidence to repair it.

Thanks!

[morzh]

Eric

1. Good for you.
2. A piece of caution: this is a transistor generator. Transistors work at low voltages and get destroyed by high voltages. i did not analyze the sch too much but right off I do not see any output protection.
The C14 (I think) cap is what decouples the DC from the transistor's output, but:
a) I am not sure what he voltage rating of that cap is, and
b) This is not even that important as when you connect it to a higher than allowed DC biased load initial jump in voltage will get to the Q3 collector, and L2 in the collector only exacerbates it.

So, when using this generator with tube equipment, be careful when using it to inject a signal to a place which may have large DC bias.

This is what you get when mixing the solid state with tubes.

[Eric Adams]

Thanks Morzh. Does it help if I put a .01 to .05 (or so) in series with the pos. generator lead?

[morzh]

Probably not.

But if you put a small 12V zener from Q3 collector to GND (remember the Zener goes "-" to Collector and "+" to GND) it will protect it, clamping positive surges to 12V and negative to 0.7V.

Make sure the zener has low parasitic capacitance.

Or you can do a clamp from the Q3 collector with two diodes, one to 9V and another to GND.

PS. Oh, you should use a small capacitor from the generator to your radio ANYWAY, ALWAYS.

[Eric Adams]

Would you happen to have a # for the diode?

[morzh]

Try any of these....use the one below 0.5W rating.

http://www.mouser.com/Semiconductors/Dis...bfZ1z0w7p6

This is one of them.

78-TZX12B


HOWEVER:

before you do this take a 100pF cap, put it in parallel to the transistor (C-E) and see if it degrades performance at high frequencies. You are about to get a scope so this would come handy.

There are low capacitance TVS type Zeners like NZL series but they only come below 6V clamp and you need 11-12V. Datasheets unfortunately do not list the capacitance for Zeners.

-----


Another way as Isaid,

take two 1N4148 or any small signal diodes

http://www.mouser.com/ProductDetail/Fair...bu6z6oTQTL

Put one from the Collector to +V (10V if it is convenient - it is better than 9V as the latter has resistor in series and a Zener) - put Plus to C and Minus to 10V.
Put another between C and GND - put minus to C and Plus to GND.

That is both diodes have to be reverse-biased.

What this achieve is it will clamp the voltage to GND/VCC should it exceed them by more than 0.7V. And because the capacitance of 4148 is 2 to 4pF you do ot have to worry much about degrading the performance.

[Eric Adams]

Sounds good.

Thanks Morzh.