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Old 22nd Apr 2019, 3:05 pm   #1
Celestial
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Default HMV Transistor Radio 1420

Dear All,
I am a new member and I am currently restoring this radio which I obtained from EBay. The set is completely dead apart from a little "mush" in the speaker.
I am replacing the old capacitors. The ones I have replaced so far have drifted a long way off their marked value.
The radio was designed to work from two 6 volt PP1 batteries. There is very little room in the bottom of the case to play around with. I was wondering if I use 2 lots of 4 x AA batteries in battery boxes, if they would supply enough current comparable to the old old PP1 batteries which are no longer manufactured.
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Old 22nd Apr 2019, 4:09 pm   #2
Nuvistor
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Default Re: HMV Transistor Radio 1420

The 2 lots of 4 AA batteries should cope ok, the manual states average current consumption is 20ma.

Did you perform any tests to determine which stages are faulty or have you decided to change the capacitors on spec? How did you test the capacitors and which have you replaced?

I am not a fan of just replacing parts without some fault finding first.

Edit. Should have said welcome to the forum.
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Old 22nd Apr 2019, 6:59 pm   #3
Cobaltblue
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Default Re: HMV Transistor Radio 1420

Hi Celestial and welcome to the forum.

Apart from the red black and yellow plessey caps most should be good enough and even those plesseys are not likely to stop it altogether.

Any crackling on the volume control or the wave change?

A cracked pcb is a possibility do you have a multimeter ?

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Old 23rd Apr 2019, 7:50 am   #4
Celestial
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Default Re: HMV Transistor Radio 1420

Hi everyone,
Thank you for your replies.
I replaced some large Wima branded caps. I have never come across these before. I presume that they some type of old paper cap. These plus the old paper types and the old Plessey types were disconnected and tested on a small digital cap tester that I have. According to that tester most were way off value. I know that the old Hunts caps are notorious for that. I have replaced these old caps with good quality polyester and new electrolytics. I never touch any mica or ceramic caps as they only very rarely go faulty. Having done all that the problem I am experiencing now is that I can trace a very strong station signal up to the collector of TR3 but no further than that. There is a possibility that the diode (OA70) has gone faulty. I cannot trace a signal on the base of TR4 (OC81D).
One other thing is that the switch on the volume control is very "spongy." The control could be faulty. I have not be able to remove the control knob to test it further.
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Old 23rd Apr 2019, 11:04 am   #5
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Default Re: HMV Transistor Radio 1420

Feed a signal into the audio stages, there should be audio from the cathode of the OA70 detector through the V/C, if that is ok then yes the detector diode could be faulty. One possibility that could upset that test is a s/c detector diode.
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Old 23rd Apr 2019, 11:50 am   #6
60 oldjohn
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Default Re: HMV Transistor Radio 1420

Quote:
Originally Posted by Celestial View Post


I was wondering if I use 2 lots of 4 x AA batteries in battery boxes, if they would supply enough current comparable to the old old PP1 batteries which are no longer manufactured.

Hi, You are replacing the old Zinc Carbon batteries with modern alkaline AA and are able to get a lot more power stored per cubic inch. As already said around 20ma drawn so a new "battery" should last a lot longer than the battery of the 1960s.


John.
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Old 23rd Apr 2019, 2:26 pm   #7
Celestial
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Default Re: HMV Transistor Radio 1420

Dear Nuvistor,
My mistake, I should have said that I did do some tests before commencing work on this radio. I give a radio an initial try to see if there are any signs of life. I listen for a switch on thump and any "mush" on the speaker. I give the speaker a very quick test by momentarily connecting a 1.5volt battery across the terminals to determine if the speaker is working. (I read that tip in a book somewhere a long time ago). Speaker is ok on this radio so I then did some signal tracing. Started with audio section and then worked back to the front end. I then start disconnecting caps and test all the values of them on my cap tester.
I use this methodology as I am merely a hobbyist. I do not have the expertise of a qualified electronic technician. I know a couple of technicians who have given me some guidance on these matters.

Once again thank you to everybody for your replies on this topic.
If I have any further problems I will come up again on this forum.

I have seen some very interesting topics being raised on the forum which I think is very useful and very helpful. I know that there is always somebody that can help with a problem!
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