G Marconi 4142 transistor radio

[johnPH74HR]

Hello,

I'm looking for some help to possibly repair a small 60s transistor radio which my elder brother got new in 1968.

The radio is a G Marconi 4142 with SW/MW/LW.

My first challenge is open it up. I have removed all visible screws (2) but the radio seems to be still held together at the dial. I don't want to damage the dial...and I cannot see what holds the dial in place. Any help gratefully received.

Thanks,

John

[Simon Gittins]

Data available here:
https://www.service-data.com/section.php/4546/1/4142
This suggests the tuning knob originally could be pulled off; it may need gentle persuasion now of course.

[Mr 1936]

Hi John

I had one of these new, back in the sixties and had great fun exploring the short wave bands, given a steady hand ! From memory, the two case halves have some lugs joining them at one end, and two screws (which hold the swinging handle) at the opposite end.

The tuning knob has to come off first. The clear plastic has an extended boss at the rear, with the usual spring steel clip round it. It is a friction fit over the tuning spindle, and is supposed to pull straight off. To avoid cracking it, you need to arrange a straight pull. Some people have had success with looping strong fishing line under the disc at several places. Alternatively, you might find if you begin to open the case at the handle end that you can nudge the back of the dial with the case itself, at least enough to start it moving on the spindle.

The same case and knob design was re-used (I think) for other models so if disaster strikes you might be able to source a scrap set cheaply.

[johnPH74HR]

Hello,

Firstly, apologies to Simon Gittins and Mr 1936 for such a delayed reply..life got in the way.. and many thanks to both for their help..and yes, the tuning knob came off as they had suggested..and nothing broke!

I finally began to investigate the 'innards' of the G Marconi 4142 last week. First task was to repair/replace the battery terminal..and then connect a PP9 battery.

I have, as yet, to connect the aerial (not the original aerial my brother tells me).

Switching the radio on, the only available sound is a crackle when the Off/On/Volume switch is used.

Although a '68, the 'innards' look in reasonable condition..and I cannot see any damage..or dry solder joints.

Wondering what's the first thing I should do/check, on this old radio. I am a practical person, and have started to do some soldering..but have no electronics experience.

Thanks,

John

[AC/HL]

This radio uses AF115 and AF117 transistors, notorious for internal shorts. Search the forum for `tin whiskers` for further information. They can be cleared, but the long term solution is to replace them. A quick and dirty test to try is to tap them firmly!

[Simon Gittins]

The telescopic aerial is for Short Wave and doesn't need to be connected for MW and LW to work.
Crackles when the volume control is moved shows that the output stage is working to some extent.
Checking the voltages shown on the diagram is the next logical step but as Bill says, the AF115 and AF117 are highly suspect.
There is a short book by Les Lawry-Johns you can download. These transistors were troublesome even back in 1979 when the book was written; page 25 of the book (page 29 of the scan) shows how to cut the screen wire which used to work and just might still work! Replacement is better of course.
The book is Newnes - Radio Repair Questions and Answers
Les Lawry-Johns, Newnes Technical Books 1979
Near the bottom of this page:
https://www.vintage-radio.info/books

[johnPH74HR]

Hello,

I have been making very slow progress with my brother's Marconi 4142 transistor radio.

My last post was 7th Nov. 22.

Many thanks to AC/HL:

This radio uses AF115 and AF117 transistors, notorious for internal shorts. Search the forum for `tin whiskers` for further information. They can be cleared, but the long term solution is to replace them. A quick and dirty test to try is to tap them firmly!

..and to Simon Gittens:

The telescopic aerial is for Short Wave and doesn't need to be connected for MW and LW to work.
Crackles when the volume control is moved shows that the output stage is working to some extent.
Checking the voltages shown on the diagram is the next logical step but as Bill says, the AF115 and AF117 are highly suspect.
There is a short book by Les Lawry-Johns you can download. These transistors were troublesome even back in 1979 when the book was written; page 25 of the book (page 29 of the scan) shows how to cut the screen wire which used to work and just might still work! Replacement is better of course.
The book is Newnes - Radio Repair Questions and Answers
Les Lawry-Johns, Newnes Technical Books 1979
Near the bottom of this page:
https://www.vintage-radio.info/books

...for their very helpful advice.

I did download Les Lawry-Johns' book, which was very helpful.

I did as advised and gave each transistor a sharp tap (or two)...and 'knock me down with a feather'...the radio sprang into life. My elder brother in Oz was 'cock-a-hoop' when he heard his old radio playing during a zoom call.

As advised, my next task is likely to replace the AF115 and AF117 transistors...and I need advice on which transistors to acquire and where best to acquire them.

Thanks,

John

[Jez1234]

Japanese companies made a wide range of RF/IF germanium transistors which would be suitable....sorry don't have them downloaded to the soft drive between my ears and would have to research them to make specific suggestions. I'm sure members more familiar with portable radios of the period will be along with specific suggestions and there may well be UK or European ones that are reasonably easily available. Most of the one's I can think of are rather too good for this app and could oscillate at VHF/UHF (AF239 etc)

Beware that after changing them you will probably have to tweak alignment to compensate for differing internal capacitance's etc of the new transistors.

[Station X]

Threads merged.

[johnPH74HR]

Hello,

Thanks to Jez1234 for your helpful reply..and I note that you think that your suggested transistors might be 'too good for this app and could oscillate at VHF/UHF (AF239 etc)'.

Wondering if anyone could suggest a like-for-like transistor that would not require an alignment tweak (not that I know what alignment means!).

Thanks,

John

[Herald1360]

The best replacements for AF115-7 are the AF125-7 (same chip in a different package/pinout) but these are getting fairly expensive now. In a similar package but different pinout is the Russian GT322b but that's more or less disappeared now unfortunately.

Japanese types I don't know about though presumably any PNP Ge ones performing the same roles in a set would work OK.

[johnPH74HR]

Hello,

and thanks Chris for your reply..very helpful.

Reading of the ERT Service Chart 1649, informs that the 7 transistors are as follows:

Tr1: AF115 (as marked on original)
Tr2: AF117 (as marked on original)
Tr3: AF117 (as marked on original)
Tr4: AC155 (actually marked tfk ac122 on original)
Tr5: AC113 (unreadable on original)
Tr6: AC154 (unreadable on original))
Tr7: AC157 (unreadable on original)

I'll replace the AF115 (with an AF125-7)..and the two AF117s (with two AF127s) then see what I've got.

If the others need replacing, I shall do so.

Thanks again for all your help,

John

[Sideband]

Be aware that the AF125-AF127 can directly replace the AF115 - 117 BUT they are a different construction so the lead out arrangement is different. The leads are also shorter than the AF11x type which can be a problem in some sets.

It won't be necessary to carry out any alignment if you use the AF12x types.