Vintage Radio Cassette Supply Cable - UK Vintage Radio Repair and Restoration Discussion Forum

MajorWest · 28th Jan 2015, 5:49 pm

Here is perhaps a very basic question but, not being a car driver, I'm baffled. Why on earth do some of the classic radio cassette players I have use double red and double black cables? In fact, one has a large negative to chassis black cable and then two more black cables as well. Between 2 of them, resistance is 0. Add to that two large red cables. I know with alternators you do have 2 positive supply cables the B+ and D+ but I remain confused.

saxmaniac · 28th Jan 2015, 6:59 pm

I don't know what I am talking about!! but some doubling up of supply cables may be to maintain a constant unswitched supply as well as switched, and another possibility may be a feed to an electric aerial?

Nicklyons2 · 28th Jan 2015, 9:26 pm

Could it be thick black supply neg, other blacks earthy end of spkr ouputs (often connected to supply neg). One red - permanent supply (to keep clock running and/or tuner memory) generally low consumption/drain; other red - ign switched supply (turns on ouput stage and other power consuming bits which would flatten battery if left on continuously)

MajorWest · 30th Jan 2015, 6:42 pm

This refers to an old Radiomobile 8 track and radio I was trying to test for basic power supply. It doesn't look promising. For a start the set is dead and inside you can see an unpleasant rust area and what appears to be faint brown gunge covering some of the smaller wires.

I have found that one of the red cables shows some resistance between it and chassis which is normal, I think. I also tested a positive ground car radio and on this test got 0 Ohms on pos and some ohms on negative. (I recall how that puzzled me a lot some months ago).

So, this is now weird. The second red cable is showing 0 resistance to chassis.

I think at least 2 black cables are also showing 0 resistance.

Someone mentioned speaker wires and later I was a bit puzzled about that as I couldn't quite recall any figure for speaker voltage. So, I tested a set that worked and got a very low millivolt reading across the speaker terminals. Do some speakers bring the 12 volts down that much by the coil impedance?

I'm hoping battery acid hasn't leaked all over the place and knackered the radio. It doesn't look good and a shame really as Radiomobile are really nice to see.

'LIVEWIRE?' · 31st Jan 2015, 6:11 pm

In most Car radios, radio/cassette and 8-track players before the days of high power (25Wpc or more) amplifiers, there is no DC on speaker lines unless a fault oiccurs. This is nothing to do with the speaker impedance, but is because an electrolytic capacitor of (usually) 470 or 1000uf is connected between the amplifier's output and the speaker 'live' line. Older Radios, etc., used an output transformer to isolate the speaker from DC. That 8 track player you're testing wouldn't be either a 106S or 108SR by any chance, would it? These used red leads, with IIRC a stripe along them for speaker 'live' and black for speaker -ve, the latter being grounded(earthed) A Positive Ground radio would (or should) have continuity (zero ohms) between potive and chassis.

MajorWest · 2nd Feb 2015, 5:46 pm

It's a 108 NP. At the moment it's on my "serious cases" list so left aside when I find a day (or week) to do some major investigation. I tend to tackle the simpler cases first and the hard ones put to aside. This particular Radiomobile is nice to look at but I suspect it has major issues. Particularly worrying is the onset of rust in part of the chassis and the gunge on some wire looms.

Any idea what voltage the speaker wire is meant to be? !2 volts or less?

I tried bypassing the supply circuit altogether and traced a good negative point within the set. I connected between positive and negative and just a tiny spark at the switch. I fiddled with the switches but the set remained dormant. It's a nice challenge for when I get a bit more experience.

G6Tanuki · 2nd Feb 2015, 7:43 pm

Quite a few late-1960s/1970s car radios needed both sides of the speaker connection isolated from earth [to deal with the positive/negative-earth selection, and/or because one side of the speaker in a typical 'complementary symmetry' push-pull output stage hooked the zero-audio-volts side of the speaker to the amplifier's positive-rail].

Get this wrong and you risk destroying the output transistors.

The same was common in 1970s two-way mobile radios - inadvertently grounding either side of the speaker wires on Pye Motafones/Europas generally meant ~Goodnight Vienna~ for the TO3-cased-but-with-lots-of-legs audio-output chip.

28th Jan 2015, 5:49 pm	#1
MajorWest Retired Dormant Member Join Date: Jul 2014 Location: Stoke On Trent, Staffordshire, UK. Posts: 247	Vintage Radio Cassette Supply Cable Here is perhaps a very basic question but, not being a car driver, I'm baffled. Why on earth do some of the classic radio cassette players I have use double red and double black cables? In fact, one has a large negative to chassis black cable and then two more black cables as well. Between 2 of them, resistance is 0. Add to that two large red cables. I know with alternators you do have 2 positive supply cables the B+ and D+ but I remain confused.

28th Jan 2015, 6:59 pm	#2
saxmaniac Heptode Join Date: Oct 2007 Location: Birmingham, West Midlands, UK. Posts: 618	Re: Vintage Radio Cassette Supply Cable I don't know what I am talking about!! but some doubling up of supply cables may be to maintain a constant unswitched supply as well as switched, and another possibility may be a feed to an electric aerial?

28th Jan 2015, 9:26 pm	#3
Nicklyons2 Octode Join Date: Jan 2013 Location: Barnsley, South Yorkshire, UK. Posts: 1,517	Re: Vintage Radio Cassette Supply Cable Could it be thick black supply neg, other blacks earthy end of spkr ouputs (often connected to supply neg). One red - permanent supply (to keep clock running and/or tuner memory) generally low consumption/drain; other red - ign switched supply (turns on ouput stage and other power consuming bits which would flatten battery if left on continuously)

30th Jan 2015, 6:42 pm	#4
MajorWest Retired Dormant Member Join Date: Jul 2014 Location: Stoke On Trent, Staffordshire, UK. Posts: 247	Re: Vintage Radio Cassette Supply Cable This refers to an old Radiomobile 8 track and radio I was trying to test for basic power supply. It doesn't look promising. For a start the set is dead and inside you can see an unpleasant rust area and what appears to be faint brown gunge covering some of the smaller wires. I have found that one of the red cables shows some resistance between it and chassis which is normal, I think. I also tested a positive ground car radio and on this test got 0 Ohms on pos and some ohms on negative. (I recall how that puzzled me a lot some months ago). So, this is now weird. The second red cable is showing 0 resistance to chassis. I think at least 2 black cables are also showing 0 resistance. Someone mentioned speaker wires and later I was a bit puzzled about that as I couldn't quite recall any figure for speaker voltage. So, I tested a set that worked and got a very low millivolt reading across the speaker terminals. Do some speakers bring the 12 volts down that much by the coil impedance? I'm hoping battery acid hasn't leaked all over the place and knackered the radio. It doesn't look good and a shame really as Radiomobile are really nice to see.

31st Jan 2015, 6:11 pm	#5
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Vintage Radio Cassette Supply Cable In most Car radios, radio/cassette and 8-track players before the days of high power (25Wpc or more) amplifiers, there is no DC on speaker lines unless a fault oiccurs. This is nothing to do with the speaker impedance, but is because an electrolytic capacitor of (usually) 470 or 1000uf is connected between the amplifier's output and the speaker 'live' line. Older Radios, etc., used an output transformer to isolate the speaker from DC. That 8 track player you're testing wouldn't be either a 106S or 108SR by any chance, would it? These used red leads, with IIRC a stripe along them for speaker 'live' and black for speaker -ve, the latter being grounded(earthed) A Positive Ground radio would (or should) have continuity (zero ohms) between potive and chassis.

2nd Feb 2015, 5:46 pm	#6
MajorWest Retired Dormant Member Join Date: Jul 2014 Location: Stoke On Trent, Staffordshire, UK. Posts: 247	Re: Vintage Radio Cassette Supply Cable It's a 108 NP. At the moment it's on my "serious cases" list so left aside when I find a day (or week) to do some major investigation. I tend to tackle the simpler cases first and the hard ones put to aside. This particular Radiomobile is nice to look at but I suspect it has major issues. Particularly worrying is the onset of rust in part of the chassis and the gunge on some wire looms. Any idea what voltage the speaker wire is meant to be? !2 volts or less? I tried bypassing the supply circuit altogether and traced a good negative point within the set. I connected between positive and negative and just a tiny spark at the switch. I fiddled with the switches but the set remained dormant. It's a nice challenge for when I get a bit more experience.

2nd Feb 2015, 7:43 pm	#7
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: Vintage Radio Cassette Supply Cable Quite a few late-1960s/1970s car radios needed both sides of the speaker connection isolated from earth [to deal with the positive/negative-earth selection, and/or because one side of the speaker in a typical 'complementary symmetry' push-pull output stage hooked the zero-audio-volts side of the speaker to the amplifier's positive-rail]. Get this wrong and you risk destroying the output transistors. The same was common in 1970s two-way mobile radios - inadvertently grounding either side of the speaker wires on Pye Motafones/Europas generally meant ~Goodnight Vienna~ for the TO3-cased-but-with-lots-of-legs audio-output chip.