How does this circuit work?

[Apfelmus]

Originally Posted by G0HZU_JMR

-13.2V at the base (i.e. pin 4 of the opamp) looks suspicious to me. That looks bad for Vbe of one of the transistors? I would just change both of the transistors now.

Bingo. Finally I found some time and replaced both transistors (against a BC141 and BC161), and it worked. I tested the transistors in my curve tracer, T4 was indeed bad.

So now I have, thanks to you all, a perfectly working audio output. I lowered the setpoint for the "inverse squelch", so its really usable now.

Still, there are things I do not understand in this unit. The 4040 has two RF inputs. One is RF, and one is "RF direct". According to the manual, "RF direct" does not have the 20dB attenuator in front and is used for weak signals.

Using the set as a radio you can put the antenna to either entry, BUT when connected to "RF direct", an additional 450kHz IF-filter with limited bandwitch of 20kHz is switched on. This results in very poor audio for FM reception when using "RF direct" instead of the normal RF input. I'm wondering what that additional IF filter is good for. Its mentioned just once in all the manuals, without hindsight or any explanation as to when and why it is used.

cheers
Martin

[karesz*]

Hi Martin,
The 20KHz filter is for noise reduction_ I think. On 450KHz IF you can not have a quality FM reception, maximum NarrowBand FM up to 3-5 KHz...
Karl

[Radio Wrangler]

These instruments are service monitors, intended to test transceivers. They are designed on the assumption that there is only one signal going into them and that it is quite large, if not several watts. The inputs which bypass attenuators are intended for checking signals extracted from earlier stages in a transmitter.

They don't make good receivers.

David

[Apfelmus]

Originally Posted by karesz*

The 20KHz filter is for noise reduction_ I think. On 450KHz IF you can not have a quality FM reception, maximum NarrowBand FM up to 3-5 KHz...

Hi Karl,

indeed, that would make sense. I know that this is not a perfect radio receiver. Now that its repaired I play around with it and wondered why it worked this way.

I aks myself if its useful to me, anyway. I have a good HP RF and NF generator, a W&G NFA, a Tektronix scope with FFT and a W&G spectrum analyzer with tracking generator. For what would I need the Stabilock?

I suppose its mainly intended for series work and quick verification of specs. For example squelch level, where the Stabilock sends different RF levels to see whether the receiver muted to determine at which level it takes place. I could do that by hand, ramping up and down the RF generator. Not as cool, of course...

cheers
Martin

[karesz*]

Hello Martin,

I could do that by hand...Not as cool, of course...

I think, you must see it on as a "Production Equipment", wher is time = premium...
Greetings, Karl

[Chaparal]

Martin,
the Stabilock Series is designed only to service Commercial Transceivers/Rx/Tx
(special set also for VOR/ILS systems)
This measurement system is not designed for broadcast receivers.
Commercial Transceivers working with max. 20 kHz bandwidth (now less)
and with very exact frequencies and not with travelling stones as quartz ref.
a FM radio works with 150 kHz bandwidth.
Same with input sensitivity of a Comm-Receiver is 5µV or less not 20 µV for a Radio.
For exact SINAD/Signal-Noise measurements it is nessesary to have a clear signal entry.

If you want only repair radios - this is the wrong instrument.

Peter

[Apfelmus]

Originally Posted by Chaparal

If you want only repair radios - this is the wrong instrument.

Yes, I guess you're right. Its an interesting tool, very interesting to repair - but I won't make much use out of it.

Thanks for all your advice and patience!
Martin