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Old 8th Feb 2009, 6:24 pm   #1
Lee de Forest
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Default Advice needed on layout

This is a set I am trying to build at the moment.
I have modelled the layout to fit in an existing case.
Does the layout look OK.
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Old 8th Feb 2009, 7:41 pm   #2
Oldtimer74
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Default Re: Advice needed on layout

Yes it looks OK. Not sure of all the details as to what all the items are. The only thing I would watch is making sure the electrolytic smoothing capacitors are not near the rectifier or output valve as they will tend to get overheated and dry out. If you are using a resistor rather than a choke or energised loudspeaker in the smoothing circuit then allow plenty of wattage and make sure there is some air flow possible around it. If you are using a choke then make sure it is orientated at right angles to the mains transformer to reduce possibility of hum.
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Old 8th Feb 2009, 8:36 pm   #3
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Lightbulb Re: Advice needed on layout

Quote:
Originally Posted by Oldtimer74 View Post
If you are using a resistor rather than a choke or energised loudspeaker in the smoothing circuit then allow plenty of wattage and make sure there is some air flow possible around it.
For example, mounting above the chassis.

Al / Skywave.
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Old 9th Feb 2009, 12:15 am   #4
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Default Re: Advice needed on layout

Yes that would be fine but do keep the capacitors away from hot valves; even with ventilation there is quite a lot of radiated heat which will dry them out.
Pat
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Old 9th Feb 2009, 12:18 am   #5
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Default Re: Advice needed on layout

I would think that the second IF transformer and IF amplifier valve should be transposed in order to ease the signal flow. The second IFT is positioned between the IF amplifier and the detector.
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Old 9th Feb 2009, 2:03 pm   #6
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Default Re: Advice needed on layout

Thank you for the pointers.
I will transpose the IF transformer with the IF valve taking care that I can still adjust the transformer coils which are side mounted in this case.
The choke is mounted at right angles to the transformer.
Checking the design for problems caused by heat is a very valid point. I will position heat sensitive parts in more suitable places and ensure that there is plenty of ventilation.
As oldtimer74 says radiated heat will warm up parts in line of sight even with ventilation. I have seen reflective foil used on the sides of cases which I assume is to try and reflect radiated heat from the cabinet.

I have read some guidance points on good design;
Mount mains transformers on the edge to minimize vibration.
Mount variable tuning capacitors on the edge and use vibration isolation pads.
Keep leads short to minimize inductive pick up and keep the RF away from the mains.
Twist the heater wires to minimize hum.
There is probably a list somewhere.
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Old 9th Feb 2009, 3:45 pm   #7
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Default Re: Advice needed on layout

Good luck with your project, I think the advice you have already been given has covered most eventualities regarding component placement issues.

But without going O/T could I ask the name of the 3D modelling software you have been using to plan out your chassis? It certainly gives very impressive results!

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Old 9th Feb 2009, 4:03 pm   #8
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Default Re: Advice needed on layout

The rendered images have been made using Autocad Mechanical Desktop. This is a bit old in the tooth these days and has been replaced by Inventor which is also produced by Autodesk.
I am more used to working with mechanical desktop so I find it quicker for producing models and rendered images. I could of improved on the images if I had spent more time but you can go on for ever with things like this changing materials and lighting effects.
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Old 11th Feb 2009, 9:54 pm   #9
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Default Re: Advice needed on layout

I have been trying to think of a commercial receiver that has all the valves hundled together without a general flow from input to output. Usually the I.F. transformers form a screen between the stages. Looks like you are going to use the Mullard 'E' series red valves which are pretty stable so its only a thought. Good luck with it. A long project.
regards, John.
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Old 12th Feb 2009, 9:48 pm   #10
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Default Re: Advice needed on layout

Hi, I've posted a couple of pics to show the progress to date. The IF transformers shown are not the actual ones which will be used, I have just placed some small ones there to indicate the position of the transformers.
The variable tuning capacitor is mounted on two rubber vibration isolation bushes. Holes have been cut for the transformer and large electrolitic cap.
Some more holes are needed for inputs and outputs. I took the advise to transpose the valve and IF transformer as shown in the rendered layout.
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Old 20th Feb 2009, 12:19 am   #11
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Default Re: Advice needed on layout

http://www.youtube.com/watch?v=HLGs6xNxnRg

The above link shows the latest progress to date. The set is working but I think it needs some alignment to get the best out of it.
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Old 20th Feb 2009, 3:31 pm   #12
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Question Re: Advice needed on layout

Well it sounds fairly lively and with undistorted output and yes, tweaking the alignment may help - but what are you using for an aerial / earth?

Al / Skywave.
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Old 20th Feb 2009, 5:03 pm   #13
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Default Re: Advice needed on layout

The aerial is a scrap length of wire about 3M long fixed around the window in the upstairs back bedroom (workshop). I get a better signal upstairs than downstairs.
The chassis is earthed. The coil pack is coupled to earth/chassis via several capacitors.
I found an article on the web for Converting a TR1196 Rx which uses the same coil pack.
http://www.vmarsmanuals.co.uk/archiv..._TR1196_Rx.pdf
The first version pre-new chassis had a shunt resistor connecting the aerial to the chassis which was not earthed. The output was improved with the shunt resistor. I fitted the same shunt resistor to the new earthed chassis but it did not need it as the sound was O.K.
I have some problems with instability at high volumes with the radio which take the form of a high pitch whistles (similar to when the mic. is to close to the speaker) . It sounds like positive feed back in to the RF stage. This does not occur when playing an audio signal from a CD player through the AF amp. The feedback noise was reduced after some adjustment of the aerial coils and only happens at high volume. What are the common causes/fixes of positive feed back in to the RF stage.
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Old 20th Feb 2009, 11:48 pm   #14
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Lightbulb Re: Advice needed on layout

That probable RF instability at high loudspeaker volume could be caused by sound waves from the speaker impinging on the vanes of the tuning capacitor. Try placing a piece of hardboard between the speaker and the cap. to test this. Also, mounting the capacitor on rubber grommets so that it is mechanically isolated from the chassis may help. Also check that the HT supply to the RF amp. & mix/osc. is adequately decoupled.

Al / Skywave.
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Old 21st Feb 2009, 10:33 pm   #15
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Default Re: Advice needed on layout

Thanks for the guidance. It has help me fix the problem of the high pitch whistle at high volume. The original chassis had what I assumed to be a tone control circuit. This consisted of an electrolytic capacitor connected between the anode of the power valve and the chassis via a pot. I have re-fitted this to the new chassis and problem has gone even with the pot set to maximum resistance. The tuning cap is mounted on rubber grommets and the speaker is remote from the set so the decoupling theory was the most likely. The addition of the new cap and pot must have had some de-coupling effect either that or I have moved something whilst fitting it. I left it off to start with because I wanted to get the set working first with only the minimum parts needed (kind of like a safe start). I also think there might have been some microphonic problems with the power valve which the capacitor may have smoothed out but this is only a theory and not having heard what a valve that is microphonic sounds like it is hard to say for sure.
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Old 21st Feb 2009, 10:52 pm   #16
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Question Re: Advice needed on layout

Glad it's fixed. But I'm intrigued about the cap. / res'r. tone control cct. Conceptually, this design approach is not uncommon, but I'm surprised that you found an electrolytic in that network. What is its value, uF, please?

Al / Skywave.
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Old 22nd Feb 2009, 6:04 pm   #17
Lee de Forest
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Default Re: Advice needed on layout

Bit hard to say at the moment as I have screwed the radio into the cabinet.
The capacitors are the two wired in parallel near the bottom right in the second picture.
This is the set running in the cabinet.
http://www.youtube.com/watch?v=cotI8i4MUjM
I am pleased with the sound.
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Old 22nd Feb 2009, 8:27 pm   #18
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Default Re: Advice needed on layout

I suspect that is NOT an electrolytic but a paper capacitor with inner and outer foil marked. Electrically, it doesn't matter which way it's connected BUT if it's used in an audio coupling or decoupling circuit, then the black (outer) foil should be connected towards the earthy (chassis) side. This ensures that the outer foil gives maximum screening effect. Think they were called Metalmite or something like that.



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Old 22nd Feb 2009, 10:55 pm   #19
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Thumbs up Re: Advice needed on layout

Ah, I see now! Thanks for the photo. Yes, they're 'Metalmites'. Prob. in the range 0.05 uF to 0.005 uF.

Al / Skywave.
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Old 23rd Feb 2009, 1:34 pm   #20
Lee de Forest
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Default Re: Advice needed on layout

I think it was the red and the black markings with the metal case that threw me. Thinking about it, I assume that if there was a large value cap here then you would lose a lot of the audio frequencies through the output transformer.
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