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Old 18th Nov 2020, 6:13 pm   #1
llama
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Default Dynatron Nomad - impressive!

I've just opened up a Dynatron Nomad which I've had for years, probably from a boot sale so would have been a couple of quid at the most.

I see from R&TVS (1960-'61) that my Nomad is a TP11 and the very similar TP12 is a table-top set.

The constuction of the Nomad is very impressive indeed and I hope the output is up to the standard of construction. Here are some of the high-end features I've noticed:

Separate, very long, ferrite rods for medium and long waves

Fixing self tappers for the chassis go into proper spring steel clips

Speaker leads just unclip

Really solid looking volume control / 2-pole switch (needed for the twin PP9s)

Outstanding tuning capacitor incorporating slow motion drive and anti-backlash gearing

PCB looks to be good quality.

My plans are to get this running and then fit a BlueTooth module for my grand-daughter to use as output for her tablet. The Plessey electrolytics are likely (I'm told) to need replacement so that'll be the first job. The Hunts little black capacitors will only get changed if they measure faulty. I can't say I like the resistors used throughout - metal caps pressed onto the ends of (I presume carbon composition) rods. Again, I'll only change them if they turn out to be faulty.

Observations from others welcomed.
Graham
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Old 19th Nov 2020, 7:26 pm   #2
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Default Re: Dynatron Nomad - impressive!

I have changed the 7 "red/black" Plessey electrolytics. No problem with the 100uF ones but I don't have any new 8uF ones so had to fit the two 10uF caps I've got and a 22uF to replace the last 8uF. I put the 22 in the signal path and the 8uF on the AGC (I presume) line is now a 10.

Much to my relief (having read previous posts) the speaker works very well. In fact the set seems very sensitive on Medium Wave. The fact that some stations crack up when tuned to central carrier makes me wonder if the AGC is working. However, I've only got PP3s at the mo so maybe they are flagging. I'll probably use multi-AAA holders ultimately.

The wavechange switch is jammed. I'll need to get the chassis out again to fault-find that. Even though LW won't get used much if at all I feel I should fix it if poss.

In line with other people's posts, there is significant hiss on zero volume setting. I might try an OC42 in the audio amp.
Graham
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Old 19th Nov 2020, 8:41 pm   #3
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Default Re: Dynatron Nomad - impressive!

The "stick" resistors can be a cause of noise - the pressed brass end-caps go brittle and crack along the tubular bit, meaning they no longer exert much pressure on the carbon-stick and the poor contact can lead to all sorts of fizzles.

I'd check the ones in the first stage of the audio-amp before you start replacing transistors.
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Old 19th Nov 2020, 8:51 pm   #4
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Default Re: Dynatron Nomad - impressive!

Hi Graham, I have plenty NOS resistors like that if originality is important.

Never used, never solderd so may be OK. Mail me if you want any

Ed
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Old 19th Nov 2020, 9:33 pm   #5
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Default Re: Dynatron Nomad - impressive!

Funnily enough I am working on a Nomad at the moment also.
I bought it at a car boot some time ago but only got it on the bench today.
As you say, very well built.

Mine has a problem with the local oscillator. If I start with the variable capacitor vanes fully out it starts, it continues to run (and tunes stations) until the vanes are about 2/3rds mshed when it stops.
However, to get it to start again requires fully opening the vanes again.
The tuning cap vanes are not shorting out at any point.

It is obvious that this is not a new fault, someone has changed several caps around the oscillator, the tuning cap vanes had an oily residue on them, looks like some has used switch cleaner on them. I have flushed away the residue with solvent.

I have changed the resistors around the oscillator transistor and the transistor itself but the fault is the same. I have left off for tonight and will resume tomorrow.

I also have found a lot of hiss on zero volume setting, I have tried changing the first AF transistor with a different type it made no difference. I have parked that issue until the oscillator is fixed.

When I eventually get it going I also intend adding a bluetooth module (in a way that could easily be removed and the set restored to its original condition).

Peter
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Old 20th Nov 2020, 9:25 am   #6
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Default Re: Dynatron Nomad - impressive!

I have just returned to the bench and found the fault with the local oscillator.
It was X1 which forms part of the AGC system.
I replaced it with a NOS OA90.
Now to tackle the hiss.

Peter
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Old 20th Nov 2020, 4:06 pm   #7
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Default Re: Dynatron Nomad - impressive!

I think I have solved the excessive hiss.

To isolate the audio stage I temporarily lifted C31, the coupling capacitor from the wiper of the volume control. I measured the noise on the loudspeaker with a scope, it was approx 50mV p-p.

I initially concentrated on the first audio amp, TR4, as I expected the source of the noise to be in the front end before any gain was applied.
I changed all the resistors around TR4, they had all drifted well off the correct value and broke as I desoldered them. This had no effect!
I had already changed TR4 to an AC126 but there was no change so I changed it again to a silicon PNP device. I changed the bias by changing R16 from 180K to 82K, this restored the voltage on the collector to the correct value.
Unfortunately this also did not work.

I next lifted the coupling capacitor between the collector of TR4 and the base of TR5, C35. This removed any possible noise contribution from the first AF stage. Again, no difference.

Finally I changed the driver transistor, TR5, to a silicon PNP device. I made no changes to the bias. The base of TR5 is held close to 0V by R22, changing to a silicon device simply changed the emitter voltage from 0.2V to 0.6V, this had minimal effect on the collector current which is set by the voltage across R23/R24, this voltage changed by 0.4V in 9V so minimal effect.

When I switched back on I had to double check the amplifier was actually working so quiet was it.

Restoring the lifted coupling capacitors resulted in a working radio with no hiss from the audio stage.

I will now replace the electrolytics as a precaution, clean up the case and add the bluetooth.

Peter
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Old 21st Nov 2020, 7:07 pm   #8
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Default Re: Dynatron Nomad - impressive!

Emboldened by your findings and subsequent modification, I went straight for TR5 and swapped it for an MPS6516 - simply because it has EBC moulded into the casing. Unfortunately there was no improvement in the hiss level so I'm going to have to be more scientific about it.

I freed up the wavechange switch by dripping some light oil down the shaft from the top. Working the switch a few times and the back of the knob broke. My bad. It should super-glue together.
Graham
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Old 21st Nov 2020, 7:50 pm   #9
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Default Re: Dynatron Nomad - impressive!

I notice that it doesn't have any decoupling capacitors on the supply rails after the power switches and before the output transistors. I think this set may be a bit intolerant of rising ESR as batteries run down.

David
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Old 21st Nov 2020, 8:49 pm   #10
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Default Re: Dynatron Nomad - impressive!

Quote:
Originally Posted by Radio Wrangler View Post
I notice that it doesn't have any decoupling capacitors on the supply rails after the power switches and before the output transistors. I think this set may be a bit intolerant of rising ESR as batteries run down.

David
Agreed, I have added a 2200uF capacitor across the supply after the on/off switch.

Peter
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Old 21st Nov 2020, 8:55 pm   #11
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Default Re: Dynatron Nomad - impressive!

Quote:
Originally Posted by llama View Post
Emboldened by your findings and subsequent modification, I went straight for TR5 and swapped it for an MPS6516 - simply because it has EBC moulded into the casing. Unfortunately there was no improvement in the hiss level so I'm going to have to be more scientific about it.

Graham
Thinking about it I also left the change of TR4 to a silcon device in place so you may need to change both devices.
Try lifting one end of C35 to separate the 2 stages, if the hiss goes (with TR5 changed) then you will also need to change Tr4.

Peter
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Old 21st Nov 2020, 11:15 pm   #12
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Default Re: Dynatron Nomad - impressive!

Because it uses the centre point between the batteries for the cold end of the speaker, it really needs two decouplers.

David
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