RF probe to work with amplified speaker

[Oldcodger]

Originally Posted by G8UWM-MildMartin

Or, if you don't have a signal generator, a signal injector consisting of just a simple 2-transistor multivibrator running at 1kHz or so will give useable harmonics from audio to well past medium waves.

An idea- I hooked one up from a 4000 Cmos logic chip. I'd got a problem with a loud radio years ago, and asking to turn it down meant it got turned up further. Blast of RFI and radio went silent. .
I used the same set up a few years later to fault a radio board I bought second hand. I'd got two- neither worked, but one had a problem in an IF transformer. Transformer changed- set worked for years, as part of a stereo system I built.

[60 oldjohn]

Originally Posted by David G4EBT

Sorry - don't know what went wrong there. I've re-posted it here, which seems to work:

http://www.nostalgiaair.org/Referenc...ost/post01.htm

Basically, it shows a radio circuit and traces the signal from the aerial to the speaker outlining what you should hear at key points.

Hope it's of help.

Link working for me.


John.

[mike1 ryan1]

Thanks I'm re-enthused, some really usefull information there, Thanks for all your replies. I've read and read about things but in the end for me I really have to do things before they sink in properly. Experience ultimately means you do less unnecessary things to get to an end point but to get that experience you have to do things you later realise might not have been necessary. So I suppose it becomes necessary to do unnecessary things while you are learning. Time for procrastination is over, fingers crossed I'm going in...might wait till tomorrow and read a bit more about it first though! BTW I'm making a box to connect my signal generator to with 2 x boc connectors so I can connect it to my frequency counter and the set at the same time with banana plug sockets also as I have some. What value blocking capacitors should I use in it?

[Sideband]

Yep...link is OK now.

[mike1 ryan1]

Printed out from link and think it will be really useful to me. Thanks David much appreciated.

[Andrewausfa]

Originally Posted by mike1 ryan1

BTW I'm making a box to connect my signal generator to with 2 x boc connectors so I can connect it to my frequency counter and the set at the same time with banana plug sockets also as I have some. What value blocking capacitors should I use in it?

Mike, without knowing what specific gear you have, you will probably find the low output level (signal strength) from your signal generator that the set requires for alignment won't be enough to drive the frequency counter concurrently. Unless, of course, it's got a seperate frequency counter drive, The sets receiving circuit needs microvolts, the counter likely needs several millivolts minimum to trigger a reading.

Assuming you have a valve based sig gen, connect your sig gen to the counter, whack the sig gens output on full and let everything warm up and stabilise for 1/2 hour, you can then set the generators dial to 455kHz IF or whatever needs to be per the indication on your counter. Then disconnect from the counter, wind the sig gen output down and start your sets alignment procedure.

Is it a DAC90 you're trying to align?

Andrew

[mike1 ryan1]

Ah yes thanks for that. It is a dac90a yes. I am going to check over everything again as suggested then follow the service data sheet procedure for alignment and see how I get on from there.
Mike

[Sideband]

Unless the alignment has been twiddled, I've never had to realign a DAC90A....and I've done a fair few of them over the years.

[mike1 ryan1]

All 4 cores on the if transformers are wound right out which just makes me a bit suspicious and seems to warrant a look and check.
Mike

[Mike. Watterson]

I've not seen any old signal generator for Radio Workshop use that can drive an ordinary bench frequency meter reliably.
On one generator I added a BNC via a 1kV 1nF ceramic capacitor and 1 K to the main osc output where anode feeds the attenuator. This pulls the frequency and adds a little distortion. So on the next valve generator I got I added this
https://www.radiomuseum.org/forum/simple_rf_buffer.html
In this case put a 50 Ohm terminator on when counter isn't connected to reduce leakage and avoid changing load, though the FET isolates pretty well. The J310 is easier to get, cheaper and maybe superior to the 2N3819 common in old articles.
Powered off the heater supply. Either mount at input to attenuator or on the back of the added BNC.
It's also a useful circuit to drive a scope or a pair of 1N60 germanium diodes in a peak detector for a DVM or wobbulator input, just to sniff a radio. Note that connecting direct with a scope probe will pull a radio IF or L.O., you need a short wire and even a series resistor on this buffer to avoid "pulling". Cable capacitance or gate capacitance is the problem. Only 10:1 probes switched to x10 at the clip end lightly load an IF or LO.

[Mike. Watterson]

Actually the Advance
https://www.radiomuseum.org/r/advanc...erator_e1.html
does have a high level output. However it was on a Car aerial type socket and not buffered. A 50 Ohm counter pulled the frequency and caused distortion.
This is also good:
https://www.radiomuseum.org/r/taylor..._generato.html
I added a BNC as described above, but it needs the FET buffer. Despite being made maybe 1946, the alignment was still good. It needed lots of caps replaced. The Modulation transformer died but was replaced with a small 110-0,110-0 = 220V & 9V transformer suitably wired. One primary is audio amp load and the other in series with Oscillator. The secondary is audio frequency feedback for the 800Hz modulation.