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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 26th Sep 2023, 12:11 pm   #1
kestrelmusic's Avatar
Join Date: Feb 2008
Location: Wrexham, North Wales, UK.
Posts: 466
Default Portadyne MU5

This was quite an interesting challenge. I spotted this nice little set on Ebay and got it for quite a reasonable sum. I thought the case looked very like a Pilot Little Maestro, and I was right. The real give-away was the second IF Transformer mounted under the chassis.

The original Line-Cord Dropper had been cut away, so the first task was obviously to replace that. I decided up a capacitive dropper with a series anti-surge resistor. The caps (2 off 2.2μF in parallel) were mounted in a little clamp which I made to fit under one of the mounting shoes [Pic 3].

The rectifier was missing, but the other valves all showed up reasonably good on the tester. Fortunately I had a spare 25Z4 in stock. So I powered up (slowly and carefully via a limiter) and all was well, but the HT was much too high. Put a resistance in series with the rectifier anode and tried again.

Audio stages were now working well but the IF was oscillating ferociously. Eventually tracked it down to the HT decoupling capacitor being open circuit. Replaced that and the power supply caps at the same time.

I then applied a signal to the IF valve grid. Not a dicky-bird from the speaker. Checked the IF circuitry and found the IFT secondary open circuit. Following a suggestion from a Forum member, I unwound the OC coil and re-wound it between “cheeks” made from fibre washers. Yippee! A little adjustment of the presets and there was a good output.
When I got the set the tuning gang was missing. I had one that would do, but had to make a mounting bracket for it and a pulley and drive cord from the spindle. The pulley was adapted from a Meccano wheel [ Pic 2].

Meanwhile I checked all the wax caps. Not one was usable. (I have a box of old waxies which one day I will clean up and re-stuff with modern polyesters.) The AVC decoupling resistor was open circuit. Replaced the lot.

I re-aligned the IF circuits to 451 kc/s which was correct for the Pilot, and then made a start on the RF end. LW came in quite well with only a little tweaking, but MW was miles out. Got that done, then of course had to re-do LW.

The case was in quite good nick, just wanted a good polish. There was only one knob when it arrived, but I sourced some suitable modern replacements from Jellyfish Audio.

It’s a really nice little set. Two variations from the Little Maestro: one is that the detector and AVC use the same diode (actually the 6Q7 diodes strapped together) and the AVC is decoupled by a 1MΩ resistor. The other is that in the Portadyne the aerial and grid coils are mounted above the chassis on two different formers, but the oscillator coils are under the deck side by side, where in the Pilot they are all above the deck.

So - a very satisfying result, I think. Now onto that pre-war Marconi...
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Old 26th Sep 2023, 2:35 pm   #2
Wijnand's Avatar
Join Date: Mar 2023
Location: Ugchelen, Netherlands.
Posts: 6
Default Re: Portadyne MU5

A beautiful Portadyne, restored to all its glory. You've had a lot of work with this project, but that's the challenge. Sometimes some creativity is required, because materials or parts are no longer available. Unfortunately, there is little information available about the Portadyne brand and associated schedules. Good luck with the next project, a pre-war Marconi, occasionally you come across these radio sets on the Dutch site 'Marktplaats'

Greetings Wijnand.
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