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Old 18th Jun 2012, 1:35 pm   #1
Malcolm G6ANZ
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Default NAAFI receiver problem

Greetings all, I’m not sure if this should be under the military section or not. Anyway, I have been given a NAAFI type C receiver to get going. It is without its PSU and I’m going to make a mains power supply for it. So at the moment I’m powering it from a Radford Labpack. The heaters from the 6v3 supply and the ht from the packs ht via a dropper resistor to bring the volts down to 250v from 350v. The current drawn is approx. 60 mA. The audio stage works well. But there is no sign of any stations. The local oscillator is running at the right frequency as proved by a radio tuned to twice the IF frequency (930 KHz). If I inject a 465 KHz modulated signal into the detector stage a demodulated tone can be heard. The aerial tuning peaks nicely on the grid of the frequency changer but there is no signal on the anode, as observed by a scope. Looking at the IF cans it appears that they may have been twiddled as the sealing wax is disturbed. Even if its way off tune surely there ought to be something at the anode.

So is 250v too high and the bias way off or is the fault more subtle? I have changed the wax caps and some high value resistors. I’m at work at the moment so cant tell you ay oif the voltages around the valves. If neded I can do that tonight.

Malcolm

Last edited by Dave Moll; 18th Jun 2012 at 1:54 pm. Reason: font normalised and thread title corrected
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Old 18th Jun 2012, 2:11 pm   #2
Adjuster
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Default Re: NAAFI receiver problem

For such a "dead" result I might suspect more of a dead fault rather than just misadjustment - for instance a short across one of the IF coil windings, or maybe shorted turns within a winding.

Perhaps some of the smaller RF or IF capacitors which don't usually get changed in a re-capping may also deserve attention. I have found trimmer caps to be particularly dodgy, especially if Mr Fiddler has previously been on the case.

Last edited by Adjuster; 18th Jun 2012 at 2:12 pm. Reason: Typo
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Old 18th Jun 2012, 2:42 pm   #3
AlanC
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Default Re: NAAFI receiver problem

Malcolm,

When you say the local oscillator is running at the right frequency, where did you set the tuning to measure this? Assuming you're on medium wave, the LO will be running at 465kHz above the signal, so 530 + 465 = 995kHz at the LF end, to 1600 + 465 = 2065 kHz at the HF end. 930 sounds a bit low to me. It's just worth checking this again with the tuning set to 300 metres. This is 1000 kHz and the LO should pop up on another set at 1465kHz or about 204 metres on the dial.

What happens of you inject the modulated IF signal at the mixer anode, via a small isolating capacitor? ... it should be a lot more sensitive than your reading at the detector. In fact, just touching the mixer anode pin with a screwdriver blade should probably result in crackling and/or a 'hash' being received.

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Old 18th Jun 2012, 3:11 pm   #4
David Simpson
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Default Re: NAAFI receiver problem

Malcolm,
Any chance of a picture or two, please? How old do you think it is ?
I can just about remember the speaker units. Placed on the wall either side of the serving counter. The Rx, I suspect, would've been in the back office somewhere.

Regards, David
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Old 18th Jun 2012, 9:04 pm   #5
Malcolm G6ANZ
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Default Re: NAAFI receiver problem

Thanks for the pointers. I'll have another look when I get time! Hesre are some photos.
http://i131.photobucket.com/albums/p...z/DSC03148.jpg
http://i131.photobucket.com/albums/p...z/DSC03141.jpg
http://i131.photobucket.com/albums/p...z/DSC03158.jpg
The date on the brown electrolytic is 1945 so it was made right at the end of the war.

Malcolm
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Old 22nd Jun 2012, 1:29 pm   #6
Malcolm G6ANZ
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Default Re: NAAFI receiver problem

Hi more information. With the input tuned to 800 KHz as set by another radio, resonance measured by a scope on the top cap (G1) of the frequency changer the LO is running at 1475 KHz. This is measured by the frequency counter on the DVM. This gives a difference of 675 KHz. The IF is 465 KHz so it is way off. To get an IF of 465 KHz the LO needs to run at 1265 KHz for an input of 800 KHz
Measuring the tuning range of the LO with the DVM counter gives 940Khz 2.08Mhz.
Touching the top caps of the frequency changer or the IF give no noise from the output,. Touching the grid of the detector gives a lot of noise. Injecting a signal ,at IF frequency, on the detector gives a demodulated output. Injecting a signal at the anode of the IF valve or at the anode of the frequency changer gives no output, even if the frequency is swung over a wide range.
It seems as though the RF tuning is way out. I have tried adjusting the coil cores and the padding caps to no avail. They are now back where they started as I counted the turns I moved them!

Voltages:
Mixer. 6K8G
3 Anodehexode237
4 Screen Grid 167
5 Grid Osc 87
6 Anode osc 137 oscillating
8 cathode 96
t/c grid hexode -0.23

Vh
Ah
Va
Vs
Vg
mAa
mAs
ra
gm
6.3
0.3
250
100
-2.0
3.0
3.0
1.3M
0.65


IF. 6K7G
3 Anode 238
4 s/g 166
8 cathode 96
t/c grid 0.0

Vh
Ah
Va
Vs
Vg
mAa
mAs
ra
gm
6.3
0.3
250
125
-3
10.5
2.6
0.6M
1.65

The screen grid volts seem a little high and the grid volts too close to 0. Could there be something in the AVC line cutting the valves off?
I did try removing the detector valve to stop any AVC but it made no difference.
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Old 22nd Jun 2012, 11:31 pm   #7
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Default Re: NAAFI receiver problem

Hi Malcolm, the voltages don't look too out of the way, for now. Yes the LO is a bit off but you'd still hear something at that if was working. We need to find out why no IF signal is getting through- I think thats where your problem lies, not In the front end. The g1 voltage on the AGC controlled stages will be slightly negative, even under no signal conditions due to a phenomenon called contact potential, caused by ele trons landing on the grid. But all the theory aside, we need to find that IF problem!
Cheers,
Alan
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Old 23rd Jun 2012, 2:01 am   #8
G8UWM-MildMartin
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Default Re: NAAFI receiver problem

Might be worth trying injecting as signal at IF at the 6K7 IF amp's grid in the hope of giving a huge signal at its anode, since its DC conditions seem OK, but no response to injecting to its anode, then investigating , repairing, and aligning the last IF transformer for starters.
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Old 23rd Jun 2012, 1:39 pm   #9
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Default Re: NAAFI receiver problem

The cathode voltage looks a bit off to me on the 6K7G.
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Old 23rd Jun 2012, 2:59 pm   #10
G8UWM-MildMartin
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Default Re: NAAFI receiver problem

Refugee is right, and the same for the 6K8's cathode connected to it!
Please ignore my previous post. Sorry for adding to the confusion.
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Old 23rd Jun 2012, 3:29 pm   #11
Alistair D
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Default Re: NAAFI receiver problem

The 6K7 has a -96 volt bias on it. No signal will pass at that level.
Confirm if R3 is the correct value. If the problem remains you need to pull one of the valves and re-measure the cathode voltage. Repeat the process with the other removed. I would normally expect the cathode voltage to be somewhere between 5 and 10 volts.

Al
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Old 23rd Jun 2012, 3:35 pm   #12
AlanC
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Default Re: NAAFI receiver problem

Mmm, as the 6K7G would be cut off at that wouldn't it; hence the lack of IF? Didn't spot that! Yet the oscillator grid, at -9v wrt the cathode, looks good...

R3 is the common cathode resistor; but the value appears to be 15K according to the circuit, though its a bit difficult to read. That's a bit bizarre; I would have expected a couple of hundred ohms??

Malcolm, what value is R3 in the radio? Does it measure the correct value?

Also, please tell us what the common cathode voltage is with the 6K7G pulled, and the 6K8G. With both valves pulled, there should be zero volts.
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Old 23rd Jun 2012, 3:38 pm   #13
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Default Re: NAAFI receiver problem

Sorry Alistair; just seen your post!
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Old 23rd Jun 2012, 3:45 pm   #14
Alistair D
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Default Re: NAAFI receiver problem

As far as the mixer is concerned, operating at lower current makes it easier for the oscillator to drive it into non linearity and hence generate the mixed frequencies one of which is the I.F. signal. Not sure why the 6K7 shares the cathode resistor though.

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Old 23rd Jun 2012, 3:52 pm   #15
Alistair D
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Default Re: NAAFI receiver problem

No problem Alan. At least we came to the same conclusion. Great minds etc.

Al
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Old 23rd Jun 2012, 4:00 pm   #16
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Default Re: NAAFI receiver problem

Well, one way to achieve that condition would be to use the cathode current of the 6K7 to hold the 6K8 cathode positive, backing off the bias. Perhaps that was the intention.
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Old 23rd Jun 2012, 4:04 pm   #17
Alistair D
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Default Re: NAAFI receiver problem

That sounds a good explanation.

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Old 23rd Jun 2012, 7:00 pm   #18
Malcolm G6ANZ
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Default Re: NAAFI receiver problem

We are getting progress
At your suggestions I pulled the mixer and measured the cathode voltage. Now down to 32v. I then pulled the IF and refitted the Mixer. Cathode now at 96v. Both out 0v. With just the IF valve in and vCath at 32v I could inject a signal at IF grid (topcap) and hear a demodulated tone. So this brings us back to the mixer. The hexode coil (L8) measures about 2ohms. The screen grid R measures correct at 15Kand its decoupler (C9) is a new replacement. I put in a 0.1 as opposed to the origonal of 0.01. Reasoning that as its decoupling the value should not be that critical. Maybe i was wrong.
Dead valve?

Malcolm
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Old 23rd Jun 2012, 7:36 pm   #19
Alistair D
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Default Re: NAAFI receiver problem

I have just checked the diagram. Measure the voltage on the control grid of the mixer. If it is positive C14 could be leaky.
R10 could be open circuit, in which case connecting the meter to the control grid of the mixer will cause a reduction in the cathode voltage.

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Last edited by Alistair D; 23rd Jun 2012 at 7:43 pm. Reason: Added text
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Old 23rd Jun 2012, 9:40 pm   #20
Malcolm G6ANZ
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Default Re: NAAFI receiver problem

Hi Al, looking at the diagram, there appear to be 2 C14's! One is from the IF amp anode goning to the detector anode. The other is from the volume control to V3 grid. However I have substituted both and checked the associated resistors and all appear OK.
I'll check again tomorrow when I'm more awake

Malcolm
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