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Old 18th May 2020, 8:00 pm   #21
davidgem1406
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Default Re: Home Built TRF radio

Hi PJL,

I will certainly try what you have suggested, but I have already tried adding a 0.1uf in parallel to C7 that produced no change.

I decided to look for any other valve in what I have that could replace the 6SJ7.
The 6SG7 was pin for pin compatible, so I tried that although it worked there was no change in the distortion.

As I had an 6AM6 available I decided to make up an adapter to allow that to fit the octal base. That produced a large increase in the gain available but did nothing for the distortion, that remained the same.

Regards, Dave
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Old 18th May 2020, 9:36 pm   #22
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Default Re: Home Built TRF radio

Most interesting thread. I have two circuits for the Premier, with component values. The first one is the radio I have and the second one is what another Forum member found. Premier obviously used whatever came their way.
Hope you can crack the problem, all I can suggest is you isolate the audio output stage and test in isolation then progress from there. You probably have already done this, how does the saying go, teaching Grandma ....
I do remember having distortion on a KB FB10, I tried everything then discovered that the speaker cone had distorted from the heat of the valves below it . So obvious but not to me at the time BUT lesson learned as I discovered this same problem on a number of FB10's since.
I have a copy of a bedside radio, cannot load it here as the pdf is too big, it looked like the forerunner to the Premier kit radios.
I can load it to my website where you can down load from there, sure the Mods will not mind.
Here we go, Page 4, The Straight Receiver covers a bit about distortion in Anode Bend detectors and then the Premier look-alike is covered.
http://www.g4cnh.com/public/Beginnings_perhaps.pdf
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Old 19th May 2020, 3:16 pm   #23
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Default Re: Home Built TRF radio

Hi PJL,

I have put R4 330K anode resistor back, changed R6 to 100K, added 0.1uf across R6, R3 remains at 3.3M.
That resulted in a loss of gain and the Anode and Cathode voltages have increased.
The Anode to 195V and Cathode to 7.6V, there is a loss of gain, the valve has gone microphonic.

However, did you mean to say replace R6 with 10K and not 100K?

Thanks, Dave
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Old 19th May 2020, 3:22 pm   #24
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Default Re: Home Built TRF radio

Hi Top Cap,

Thanks for all the information and the link to the magazine, I have saved that, and the second circuit for the Premier, that is identical to the circuit for mine apart from component values, and my own mod to the HT rails.

I have an EF36, I may try that if it is pin-compatible other than G1.

Many thanks, Dave.
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Old 19th May 2020, 4:11 pm   #25
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Default Re: Home Built TRF radio

PLJ,

I changed R6 to 10K we then had Anode 150V, Cathode at 2.3V.
Gain OK, valve slightly microphonic, but no change to the distortion.

Also, I tried the 6am6, which produced more gain but no change to the distortion.

Regards, Dave
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Old 21st May 2020, 8:06 pm   #26
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Default Re: Home Built TRF radio

I have now gone through this radio using a signal generator and scope.
Starting of I used my Tek 465B, after a sort while the trace suddenly went to a very bright spot about 6mm dia. at the right of the screen, then vanished. Following this there was the smell of burning but no smoke to be seen. Exit Tek 465B.

I then changed to the Telequipment D83 only to find that has decided to have a Trigger/Sync problem making it difficult to sync the trace. But we got bye.

Back to he radio and it’s problem.
I have added pictures of the scope traces with text referencing each.
There are 4 images on this posting and a further 4 on the following posting.

My findings were to say the least not what I expected at all. I had expected to see a reasonable AC style waveform at my carrier frequency of 650Kh for picture 03 to picture 06.

Picture 01:
I found that there was some 50Hz rubbish all over the place but at a low level of 125mV, this was present regardless of the set being On or Off, the aerial input being grounded or not, and from any earth point to any other earth point.

Picture 02:
Aerial input grounded. Waveform at HT 3 and V1 Anode. P t P 80mV.

Picture 03:
Input via aerial socket 650KHz carriers without modulation. Waveform at the junction of C4/L1-2.

Picture 04:
V1 g1

Continued on next posting.
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Old 21st May 2020, 8:10 pm   #27
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Default Re: Home Built TRF radio

Picture 05:
V1 Anode. The Anode voltage is also bouncing up and down by a small amount.

Picture 06:
V2 g1

Picture 07:
V2 Anode and junction of C9/R7

Picture 08:
V3 g1

As far as I can see the waveforms are a mess and what is supposed to be a 650KHz carrier is being modulated by the 50Hz as the peaks as shown on Pictures 03 to 06 all coincide with 50Hz.

I increased the value of C15 to 22uf, that reduced the 50Hz on HT 3 to what is shown in Picture 02.
Also as a shot in the dark I removed the AC switched connections from the rear of the gain pot just in case the 50Hz was being picked up by the pot track, but no change there.

Any further ideas would be most welcome.

Regards, Dave.
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Old 21st May 2020, 8:25 pm   #28
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Default Re: Home Built TRF radio

Did you use an isolating transformer or isolating capacitors?

Lawrence.
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Old 22nd May 2020, 1:42 pm   #29
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Default Re: Home Built TRF radio

Hi Lawrence,

If you are meaning capacitors to isolate the signal generator and scope from the DC content of the radio they are built in, also the matching pad for the generator has DC isolation.

Dave.
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Old 22nd May 2020, 2:47 pm   #30
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Default Re: Home Built TRF radio

I was meaning more from an earthing point of view if the receiver is a live chassis type.

Lawrence.
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Old 22nd May 2020, 9:02 pm   #31
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Default Re: Home Built TRF radio

Hi Lawrence,

Sorry, I missed the point there.
It is live chassis type but is connected the correct way round.
The scope and generator are AC grounded, maybe I should lift the AC earth from the plugs?

Dave.
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Old 22nd May 2020, 9:16 pm   #32
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Default Re: Home Built TRF radio

No, don't lift the earths from the mains plugs

Use an isolating transformer to power the receiver when connecting earthed test equipment to it.

Lawrence.
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Old 22nd May 2020, 9:20 pm   #33
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Default Re: Home Built TRF radio

Quote:
Originally Posted by davidgem1406 View Post
Hi Lawrence,

Sorry, I missed the point there.
It is live chassis type but is connected the correct way round.
The scope and generator are AC grounded, maybe I should lift the AC earth from the plugs?

Dave.
We used to do that sometimes in the bad old days (70s) but I can't recommend you do that now.

Yes, the neutral and earth will be bonded together at the substation or the cable head near your meter, it depends, but you should never regard them as being the same at the point of use.
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Old 23rd May 2020, 10:40 am   #34
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Default Re: Home Built TRF radio

Hi Lawrence,
I'm afraid that is something I do not have, and too expensive to think about buying.

Hello Graham,
Yes, that's what we used to do back then, none of the test gear was connected to earth.

Dave.
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Old 24th May 2020, 2:47 am   #35
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Default Re: Home Built TRF radio

Quote:
Originally Posted by ms660 View Post
No, don't lift the earths from the mains plugs

Use an isolating transformer to power the receiver when connecting earthed test equipment to it.

Lawrence.

So true, when I joined a Company belonging to the Marconi group, one of my first tasks was to set up a stack of Wien Bridge oscillators. The first instruction in the test spec was to remove all earth connections from the test gear to be used, i.e. scope, bench DMM etc.
I refused to do this as I felt it went against site safety etc and asked for an isolation transformer, which the company did not have. I went home to fetch mine which of course had to go through site services first to be checked .
I was thus able to set up the oscillators without any trouble. The worst part was trying to convince the inspection department that this was the correct way to do it and eventually the service instructions were quickly updated
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Old 24th May 2020, 4:03 pm   #36
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Default Re: Home Built TRF radio

Hello all,

I have now found that all the 50Hz rubish showing on my scope traces is caused by an AC earth loop between the generator used to modulate the carrier and the scope.
The disconnection of the AC earth from the scope resulted in producing a perfectly clean sine or square wave.
Also, the trigger/sync of the D83 is rock solid.

My expectation would be that the generator for the carrier would produce the same problem. Also maybe the AC earth between the 2 generators would produce the same problem.

However, that turned out not to be the case as connecting both generators produced a clean modulated carrier.

I will now have to repeat the earlier procedure with the radio and hopefully get some sensible results.

Dave.
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Old 26th May 2020, 4:53 pm   #37
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Default Re: Home Built TRF radio

There are a couple of articles from Radio Constructor magazine which relate to this thread.

“In Your Workshop”, April ‘63, Dick and Smithy discuss the circuit of a radio very similar to the OP’s, including the anode bend detector.

“New life for midget receivers”, October ‘69, discusses various detector configurations for this type of receiver including the anode bend detector, but the author finally settles on a low distortion infinite impedance detector, swapping the middle pentode with a double triode.

Both these articles are on the American Radio History site, and worth reading with respect to this thread.

Stuart
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Old 27th May 2020, 9:56 pm   #38
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Thank you, Stuart, I will certainly take a look at those.


I have continued my search for this problem over the last 2 days and trying to repeat my previous efforts that showed all the 50Hz rubbish.

This is what I have found yesterday, Thursday. Using a modulated signal as before I again found that I was having problems with the carrier being turned to rubbish by 50Hz.
This 50Hz was also on the HT rail, so I added additional smoothing to the HT3 rail. Although this reduced the 50Hz on the rail it did nothing for the signal at V1 Anode.

Next I checked the rest of the HT rail, also added additional smoothing to HT 2 rail, Still, no change at V1 Anode. I gave up as it was getting late.

I picked it up again today, Wednesday. I first looked at the output from the rectifier and added some more capacity to that bringing it up to near its allowed maximum. I had now added capacitors across C16, C12, C13, and replaced C15.

To start with I decided to try to look at the incoming carrier tuned to 601KHz (Smooth Radio) at V1 g1.
The peak waveform was around 40mV and the time-base frequency set at 5µS.
It was not possible to sync the scope as the waveform was constantly on the move due to the audio modulation.

Reverting to the signal generator I set a carrier of 601KHz and 40mV P t P with modulation of 1KHz.
The modulation was set to produce an approximate volume level as the tuned radio station. This was applied to the aerial input and again viewed at V2 g1. Time-base at 0.2mS, the waveform is 50mV P t P including modulation.

Again the waveform was dominated by the 50Hz.

I then decided to try to track down this 50Hz. I removed the signal from the aerial socket and grounded the input.
That 50Hz waveform was still on V1 Anode and the HT rails.

These are the results for the HT rails:
1/. HT 3 rail about 30mV full AC waveform.
2/. The junction of R10b, R3, R4 50mV full AC waveform.
3/. HT 2 rail a half-wave rectified waveform of 4V peak.
4/. V4 cathode a 35V P t P sawtooth.

With all valves removed except V4 in order of 1 to 4.
5mV, 5mV, 500mV, Not taken.

The only thing that crosses my mind now is that all valve heaters are fed from the same winding. Probably just clutching at straws now, but there must be some reason for this to be happening!

Regards Dave.
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