Beginner and his Vidor 396A

[rayleask]

Having built my Velleman Signal Injector and worked my way through this nearly dead receiver I have worked out I need some more advice! I am getting no AF signal coming out of the secondary winding of my IF2 when I inject a signal into the primary.

IF I inject a signal on the secondary or any component after it I get a nice loud volume through the speaker. Any AF injection prior to the secondary winding I get nothing but the usual hum. Mind you I cannot find any signal going into IF2 using the Tracer either.

I have checked IF2 for relevant continuity and that seems OK. I could replace the internal capacitors but I dont want to risk damaging the fine wire. Can anyone throw some light on this? Is this normal? My understanding of IF1 and IF2 is that they restrict passing any frequency apart from the IF. If this is the case wouldn't that block an AF frequency as well?

Any help gratefully received.

Thanks, Ray

[Station X]

The IF transformers pass an RF signal of say 470kHz modulated with an AF signal. The function of the receiver's detector stage is to strip the AF signal from the RF signal and pass it on to the AF stages.

I'm not familiar with the test equipment you're using, but I'd typically inject a 470kHz modulated signal to the IF stages.

Don't even think about changing caps in or adjusting the IFT's until you're certain about you test equipment and testing procedure. Adjusting IFT's requires a signal generator which can be set to a specific signal.

[Station X]

Looking at the spec for the Velleman Signal Injector its frequency appears to be 1kHz which is an AF (Audio Frequency) signal so the signal won't get through the IF stages.

If the set's not working, then in the absence of an RF (Radio Frequency) generator I'd start checking valve voltages.

[ms660]

1kHz won't get through the IF stages unless it's a square wave, in which case the harmonics might, but even then it can be deceptive to some because the harmonics can sometimes find their way to the output by any whichway.

Lawrence.

[rayleask]

Thanks Graham for your quick reply. That does confirm my thinking that an IF should not be passing AF and hence the result I was getting.

I now see that V3 which is the detector should have a much lower anode and screen voltage than the others. ie. 11v instead of the 37v I have measured and 80v for the other three valves. So something is going on there I need to investigate.

I may have put back the two IF transformers incorrectly, ie. swapped the secondary and primaries. The Trader sheet for the radio just gives identical resistance for each so doesn't help. Is there a way of identifying each winding?

Thanks, Ray

[Station X]

You may have been mislead when taking voltage readings if you're using a modern DMM. Old analogue moving coil meters would have drawn current through the anode and screen resistors lowering the true readings.

A reading of 37V suggests that the valve is passing some current.

Inject an AF signal at the volume control. If that gets through V3 is probably OK.

[ms660]

The type of meter you use will affect some of the voltage measurements you make, the voltages in the service data were taken with a meter that has an input resistance of 500 Ohms per Volt for the range selected, the input resistance of modern DMM's is typically 1meg or 10 meg therefore the loading effect on the point being measure will be much less.

EDIT: post crossed.

Lawrence.

[rayleask]

Great, thanks for your replies. Time to get out my Avometer Model 8 and use it for the first time.

Ray

[Nuvistor]

To understand how that IF stage amplifies the 470khz signal and retrieves the audio, have a look at the section “AM detection “ about half way down the page.
Something to read while having a break from fixing your radio.
http://www.vintage-radio.com/repair-...rf-stages.html

[ms660]

If you're using the Trader sheet 981, be aware that the voltages and currents given for at least one of the valves don't tally up with the HT voltage, eg: V3's screen current , less of an error but still significant is V3's quoted anode voltage, which so far as I can make out is less than half of what it should be for the anode current quoted.

Lawrence.

[rayleask]

Thanks MS660, it is Trader Sheet 981 that I have. The Anode and Screen voltages really jump out for how different V3 is to the others. Everything downstream from IFT2 which includes V3 seems to be working OK so I will leave this question for now.

Cheers, Ray