Pye P131MBQ restoration question

[MalkieX]

I'm trying to restore a Pye P131MBQ. It's an interesting "portable" from circa 1955. The set was owned by my grandmother and was working in the 1970's. It failed after that, progressively becoming unstable at one end of the tuning band (I don't recall which end).

I thought I would see if I could get it going again but face one initial problem. The green resistor towards top left of the photo has deteriorated and I cannot read the values to change it.

Does anyone have a similar model and could tell me the value or maybe a circuit diagram?

Any advice on working on this model would be appreciated too.

Malcolm Rix
Taipei, Taiwan (from England!)

[Brian R Pateman]

Hello Malcolm and welcome to the forum.

As a first step, you can download the service information for this set from the "Vintage Radio Service Data" link at the top right of this page. It doesn't cost very much and helps to support the forum.

These are excellent sets and give good results when restored. It is a relatively late example of this type of set and is mains and battery operated.

As well as changing the dodgy looking resistor you should also change the two wax capacitors and that little brown job with "Hunts" marked on it. The smoothing capacitor for mains supply should also be checked.

There's lots of information on this site and on Paul Stenning's other site here; http://www.vintage-radio.com/ where you will find this page; http://www.vintage-radio.com/manufac...-sets/pye.html which has some specific information.

Regards,

[GrahamN]

Hi

In mine, the resistor is 2.7k 5W

[Herald1360]

If the dodgy green resistor is wirewound and still shows the correct value, there's no need to replace it- just ensure nothing else is touching it! It looks like it has overheated at some point, though, so it's worth checking out any associated circuitry.

I don't know the set in question, but the only circuit I can imagine needing that much dissipation in a valve portable would be the AC power supply, so check out the rectifier and smoothing carefully.

I like the description "Jewel Case Portable" (just about readable in one of the service sheet trailers) - I've always thought of them more prosaically as "Attache Case Portables"

Chris

[yesnaby]

Hello,

I have one of these that has been sitting around for ages - works but probably needs restoring with new caps etc. If anyone wants one it could go!

Michael

[MalkieX]

Thanks to the people who replied. Here's an update on the repair...

I replaced all the capacitors, seems like a source of potential problems that is easily eliminated. Fifty years of development have resulted in considerable miniturization so I found myself double checking the values since the modern ones are all so small.

As a previous respondant mentioned the power resistor just has encapsulation damage. I measured the value at 3.5K ohm and replaced it with a modern equivalent. Not sure why it is different from the other set with 2.7K.

Now I applied power. Sadly no radio, just a light crackling sound of unclear origin. Probing the volume control gives a loud click at the speaker suggesting the AF is working well.

The power supply measures 80VDC; looks good since the HT battery that used to be used was 75V. The valves don't glow, but I guess that is normal for this type of low-power set.

So maybe it is back to the original problem with the RF/IF section. The DK96 measures: a=76V, g4=72V, g2=69V. G2 seems too high, but I'm not sure about that.

Well now it is off to swot up on superhet valve radio design. I located some similar designs and the valve data sheets. Sadly twenty years in digital electronics leave me poorly prepared for tackling this (well except perhaps for the soldering work, ha ha). Suggestions welcome!!!

MalkieX

[Mullard]

I have one of these and it is a nice useable radio. I have had problems with the contacts on the Mains/Off/Battery switch, and switch cleaner sorts this out. The set is a very conventional 4 valve superhet and checking the voltages on the valves will locate most faults. Check heater volts and HT line first. If you have a spare, try changing the DK96.
Good luck,
John

[Steve_P]

Very nice sets, if the valves are in order. These Dxx valves are said to be rather poor and they are weaker than other valves it must be said.

Take each valve out, check on ohms range between pins 1 and 7. (Either side of the gap.) Chances are one will have gone.

They do not glow in normal usage - if they have been in the past, then they are dead. Simple as that.

Might be worth changing the diodes MR. These do go funny - if there's a strange smell then they must be changed.

Cheers,

Steve P.

[MalkieX]

Good news! I traced the problem to deviation in the resistor values in the frequency changer section. Once replaced the set came to life. It even spoke good Chinese (I now live in Taipei) something it never did in London. All the valves seem to be operating fine. Now just the simpler things need attention - noisy volume control, cosmetics. I'm so happy it works again, it's a charming set, a cool piece of history and something of an hierloom.

I was concerned about changing the metal rectifiers for silicon. The charactaristics of the diodes are very different. With Si there would be higher inrush current at turn on and maybe higher HV since the silicon rectifier has a very low voltage drop. Couldn't this lead to problems like stressing the transformer or overvoltage on the valves?

MalkieX

[Brian R Pateman]

I'm glad you've got this working , nice little sets aren't they?

If you are changing the metal rectifier to a silicon one you will need to include a series resistor. there is lots of information on how to do this on the forum. I'd be inclined to use a 1N4007 with a 100 ohm wire wound series resistor.

Good luck,

[MalkieX]

I notice one other problem with the set:

When on MW and a station is tuned at 200m there is a stong background buzzing noise (sounds like mains power) but there is no buzzing on stations tuned around 500m. There is also no buzzing noise when off-station. Any suggestions where to start troubleshooting such a problem? Misalignment... AGC issue?

[Steve_P]

This could literally be anything. One thing which we have now but not then is switch mode power supplies. This sounds like a switch mode problem to me and if you are lucky it may be one of yours....!

Try switching various things off and seeing if it vanishes.

You could try changing the rectifiers as suggested above?

Cheers,

Steve P.