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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 26th Oct 2019, 9:23 pm   #1
NickG0HIK
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Default MK128 working, but BFO unstable

I've recently obtained a MK128 receiver. It is not pristine, but war worn, how I like them really

It had all the valves missing and one screening can and a capacitor had fallen off, but that's about it.

It's working quite nicely and I've had it on CQWW this evening, it resolves SSB pretty well as long as the I.F. gain control is adjusted.
It seems stable enough and could handle the amount of traffic on the contest.
It's certainly more sensitive than my MCR310 and far easier to tune in SSB.

One small niggle is the BFO tuning, the radio appears to "Take Off" with some positions of the control, is this normal? It's really not a problem most of the time as I leave it in more or less one position, but it would be nice to know if I can get it sorted.

I'm not sure what to do for a PSU in the long term, 15 PP3's gets a bit pricey, so I was thinking about one of the Nixie driver SMPSU's. I've not have much luck with all the little Chinese SMPU's I've had so far, every one has failed.
I'm sure I could press into service a 555 driving a step up transformer. I've measured the H.T. draw @ 5.5mA.

Need to look out for the TX and accessories now

Nick G0HIK
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Old 26th Oct 2019, 9:37 pm   #2
G6Tanuki
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Default Re: MK128 working, but BFO unstable

On mine, the ceramic of the BFO tuning-capacitor was cracked, which caused an intermittent contact between the rotating spindle and the grounding spring/wiper thing. A new capacitor made a vast improvement!

I've built a HT-unit for mine including a 230-115V isolating transformer from an old 'over the sink' combined shaver-point/striplight, LT comes from a 3-0-3V transformer regulated by a small Ge power-transistor (AD161/162 type) whose base-voltage is provided by three forward-biased Silicon diodes (gives around 1.7V base-voltage, when the forward-voltage-drop of the transistor is included the resultant comes out within 5% of 1.5V).
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Old 27th Oct 2019, 7:13 am   #3
G3VKM_Roger
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Default Re: MK128 working, but BFO unstable

Hi Nick,

A group of six members of VMARS got a bulk order from the chap in Italy who makes 6 volt inverrters for ex-mil gear. We used the WS-18/48/68 model and fitting our own Jones plugs to mate with the RX. Not cheap at about 60 Euros but work well with the RX and TX.

pietronoto@tiscali.it is the last address I have but he's well-known.

73

Roger/G3VKM

PS: I may have shroud for that Jones plug if you need one?
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Old 27th Oct 2019, 11:02 am   #4
NickG0HIK
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Default Re: MK128 working, but BFO unstable

Hi Roger.

Thanks for the info, since having a natter with a mate on the air last night, I've revisited the idea of multiple PP3's.
Buying in bulk on the internet makes the price quite reasonable, about a £1 each.
So I could get around 60 PP3's for the price of the invertor and no switching hash to worry about.
As for the PSU housing, one of the tough cardboard boxes that all Air Ministry equipment came in would look pretty authentic for the period. Also picked up and old Cambridge instruments wooden case recently and thought that might make a nice battery box too.

As for the Plug shell, many thanks for the offer, I do actually have one, it's just been left off for testing, till I decide what type of cable to permanently fit to the plug.

Nick

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Originally Posted by G3VKM_Roger View Post
Hi Nick,

A group of six members of VMARS got a bulk order from the chap in Italy who makes 6 volt inverrters for ex-mil gear. We used the WS-18/48/68 model and fitting our own Jones plugs to mate with the RX. Not cheap at about 60 Euros but work well with the RX and TX.

pietronoto@tiscali.it is the last address I have but he's well-known.

73

Roger/G3VKM

PS: I may have shroud for that Jones plug if you need one?
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Old 27th Oct 2019, 11:08 am   #5
NickG0HIK
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Default Re: MK128 working, but BFO unstable

Thanks for that, I'll check the capacitor.
I'll also check out the alignment of the BFO, with the very helpful manual from the VMARS web pages.

I have a couple of the transformers I've pulled from old shaver lights, thinking that they will come in useful at some point, thanks for the info.

As mentioned, I might just stay with the battery box, it can be taken out portable then.

Nick


Quote:
Originally Posted by G6Tanuki View Post
On mine, the ceramic of the BFO tuning-capacitor was cracked, which caused an intermittent contact between the rotating spindle and the grounding spring/wiper thing. A new capacitor made a vast improvement!

I've built a HT-unit for mine including a 230-115V isolating transformer from an old 'over the sink' combined shaver-point/striplight, LT comes from a 3-0-3V transformer regulated by a small Ge power-transistor (AD161/162 type) whose base-voltage is provided by three forward-biased Silicon diodes (gives around 1.7V base-voltage, when the forward-voltage-drop of the transistor is included the resultant comes out within 5% of 1.5V).
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Old 27th Oct 2019, 11:17 am   #6
M0FYA Andy
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Default Re: MK128 working, but BFO unstable

Nick, I'm glad to hear you've got it working, I'd never have got round to it!

Andy
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Old 27th Oct 2019, 3:38 pm   #7
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Default Re: MK128 working, but BFO unstable

Also check the BFO on/off slide switch: those switches are not exactly of the highest quality. If there's an intermittent contact between the fixed and sliding contacts that's causing the HT voltage fed to the BFO to wander a bit, it could cause something very like what you're experiencing.
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Old 27th Oct 2019, 6:12 pm   #8
NickG0HIK
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Default Re: MK128 working, but BFO unstable

I've put some switch cleaner on the BFO switch, it has helped.

But I found the problem, I've set the BFO coil as per the instructions and it is now working without taking off.
The receiver appears to overload more now though. Shannon Volmet needs the aerial wire just winding around the terminal post, with a direct connection to my 80mtr Dipole it distorts. But these radio's were not designed for a full size dipole anyway I would think.

Nick

Quote:
Originally Posted by G6Tanuki View Post
Also check the BFO on/off slide switch: those switches are not exactly of the highest quality. If there's an intermittent contact between the fixed and sliding contacts that's causing the HT voltage fed to the BFO to wander a bit, it could cause something very like what you're experiencing.
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Old 27th Oct 2019, 6:37 pm   #9
G6Tanuki
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Default Re: MK128 working, but BFO unstable

Yes, they're a rather simple radio with only a single signal-frequency tuned-circuit - easy to overload with a 'good' antenna!

In their intended roles - operating from clandestine locations with maybe only 20 feet of non-resonant wire thrown into a tree as an antenna, and listening for your other-units who were only running half a Watt of TX-power - maximum sensitivity was really rather important. Shove the output of a high, resonant antenna into the poor thing's front-end, tune it to a transmitter running Kilowatts of RF - and it would be wrong to expect it to cope well.

Looking at your receiver, it has a lot of the same 'hastily-assembled-on-the-cheap' features of mine - in particular the rather-unprofessional quick-and-dirty sprayed control markings. My transmitter has a much-more-professionally produced photo-anodised front-panel. I wonder if the transmitters and receivers were manufactured by different companies/in different numbers ??
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Old 27th Oct 2019, 7:57 pm   #10
NickG0HIK
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Default Re: MK128 working, but BFO unstable

At every turn you find there is more to learn.
We will never know the full reasons for everything. At least we are giving them another life, for the time being anyway

Nick

Quote:
Originally Posted by G6Tanuki View Post

Looking at your receiver, it has a lot of the same 'hastily-assembled-on-the-cheap' features of mine - in particular the rather-unprofessional quick-and-dirty sprayed control markings. My transmitter has a much-more-professionally produced photo-anodised front-panel. I wonder if the transmitters and receivers were manufactured by different companies/in different numbers ??
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