Marconi P20B portable valve radio

[Davey72]

Just got one of these to put in my old morris and would love to get it going.

Managed to cobble several 9v batteries in series and supply low tension side and speaker makes a little pop, but no volume.

A beginner's class in fault finding and how these work would be greatly appreciated. All solder joints look intact and power is going through I'm guessing by little spark and pop. Valves look bit black inside but unsure if thus us normal.

Got reasonable skills being a mechanic , but never set dwell angle on an old radio lol.

['LIVEWIRE?']

I didn't know that Marconi ever made car radios, but in principle, it's likely to be similar to other valved sets. Almost certainly, if it uses normal 'HT' valves, there will be a power pack containing a transformer & rectifier. These will be driven by a Vibrator, either sycnchronised or non-synchronised, fed from the car's DC supply (6 or 12v). Often the Power supply and output stage were in a seperate box, connected to the receiver via a multicored screened cable. If this was the case with the P20B, and you don't have both parts, it is unlikely you will be able to get it working again. On the other hand, if the components I mention are present (they may be built into the radio), the Vibrator may be faulty. Other components such as certain capacitors, will almost certainly need replacing. Blackened valves are not normally faulty. One simple check, which can be performed with a multimeter on a low ohms range (with no power applied to the radio) is for continuity through the valve heaters, either with them in situ, or by removing each valve in turn, and checking the appropriate pins (4&5 in the case of all-glas B9A valves). The service data for the P20B is available from 'up top' for £1.99. This will be useful when fault-finding.

[Cobaltblue]

The Marconi P20B is not a Car Radio its a portable valve radio

It uses B7G valves

When I first posted I mistakenly read P60B so hastily re-edited

Cheers

Mike T

[camtechman]

Mike T,

And there I was thinking I was having one of my confused turns PHEW (wiping my brow)

[Cobaltblue]

Hi Davey

And welcome to the forum.

These little battery valves are very delicate electrically and are not ideal for a first restoration.

The filaments are easily destroyed.

Its virtually impossible to see them light the filaments.

The Black is the gettering and should appear silver and shiny usually at the top part of the valve.

If any have a milky white appearance then the vacuum is probably lost

Do you have a multimeter and the sets circuit diagram?

First job is to care fully check for continuity of the filaments.

Use a 1.5 V battery and a meter on the current range.

Disconnect the HT supply when doing these tests.

Connect your meter in series with the LT supply having checked that the Battery is good.

The total LT supply for this set is 250mA but its approximate.

The Output valve which will be an N17 but maybe a 3S4 or a DL92 has a 100mA filament.

Removing this valve should drop the total current by 100mA.

Removing any of the others should reduce the current by 50mA

First check the filament then get back to us.

Cheers

Mike T

[camtechman]

Manufacturer's Service Data available from our own site (only £1.99) and, as the Wurzels would say: "B A R G I N"

http://www.service-data.com/product....61/4646/m13461

[Davey72]

Originally Posted by Cobaltblue

These little battery valves are very delicate electrically and are not ideal for a first restoration.

Really good advice. Thanks. How long should they be left on before valves warm up and are they easily obtainable as two are milky. Noted each valve specific code to slot too.

Thanks again. Set, physically, looks mint so well worth doing. Little gag burn by lid I believe v.common else mint

Thank you all. Will keep progress report here with pics if ok. Nice welcome guys. Thanks again.

[paulsherwin]

A milky appearance means the valve has lost its vacuum and will need to be replaced. There are specialist valve dealers and you can also buy them from eBay sellers. A forum member may be able to supply you if you post in the Parts Wanted section.

Battery valves like these don't really warm up. They work almost instantly.

[Cobaltblue]

Hi Davey

Paul is giving good advice these battery valves are directly heated and work within a second or so.

You can see what valves should generally look like on the Valve museum eample here
http://www.r-type.org/exhib/aaa0866.htm

Although a very different type of valve here is a picture of a valve with and without a vacuum.

The Getter goes white

Cheers

Mike T

[ms660]

Looking at the Trader schematic, V4's g1 is permanently connected to HT -ve when switched off....

Lawrence.

Nice little sets these, I think they where marketed as a makeup table model for the well off wife.

You can see the filaments glowing in the dark, real dark, at the right angle. Quite cute.

Does moving the wavechange switch make a crackle? And have you tried tuning it, they don't make much hiss when off station.

[Davey72]

Thank you I will try again.

Ideally like to fix things knowing how they work do if anybody fancies summing up in a sentence of be well pleased. Wouldn't rebuild an engine without knowing working principles after all

Thanks all.

Be ideal if anyone can summarise working principle if the radio and would help a lot.

After all, I don't rebuild engines without working knowledge.

An abc of operation would be great if anyone could oblige. I understand lt and ht circuits and have multimeter but that's it lol.

[Station X]

You're asking an awful lot. To describe how a radio works would take many pages, possibly a whole book. I'm sure someone will be along to recommend a book or books before long.

As far as the repair goes, concentrate on the LT circuit and getting the valves to light up first. The LT circuit is basically just five low voltage light bulbs in parallel.

[Davey72]

Thanks Graham. The valves being the bulbs I guess?

Didn't mean in detail, just an abc guide really. Totally understand the complexities are beyond summary, as would be an engine, but could give basics.

Looking forward to getting thus going and have a same sized cheeky Bose speaker i can fit without hacking and keeping original. Quite tempted but first things first.

[Station X]

Plenty of useful information here:-

https://www.vintage-radio.com/repair...ion/index.html

[camtechman]

Davey72

Have you downloaded the service manual yet, that I provided the link for in my post #6 ?