Background Hiss from Output Amp Vintage Car Radio - UK Vintage Radio Repair and Restoration Discussion Forum

Magical · 6th Nov 2025, 9:17 pm

Hello all,

I'm after some help please? I have noticed this phenomenon on a number of vintage car radios I've worked on over the last few years. Generally Radiomobile from the '60s and early '70s, model types typically 970, 980, 1070, 1085 etc. These are 12v negative or positive earth radios.

I've noticed on some that there is a very noticeable hiss, which does not change with the volume control. It can be present on push-pull or single output power transistors. I've never been able to identify what it is. I'm extremely confident it's the output amp as on the Radiomobile 970 models I've been able to easily swap output amps over and the his has gone away.

Has anybody come across this and have you been able to resolve the issue?

Many thanks in advance,

Nick.

paulsherwin · 6th Nov 2025, 9:33 pm

Old Ge driver transistors often become noisy.

Magical · 6th Nov 2025, 9:51 pm

Thx Paul, I’ll give that a look.

Silicon · 7th Nov 2025, 11:51 am

Another possibility is that the noise is coming through the power supply.

Perhaps some of the filter capacitors have gone open-circuit.

Magical · 7th Nov 2025, 4:18 pm

Hi Silicon,
Thank you for your idea. I've now found the culprit!

Thank you Paul,
You were right. I traced it down to the first transistor in the output amp on this particular radio. It's a Motorola 828 and it's the first Mullard OC82DM, which is indeed a germanium transistor.

Much appreciative of your suggestion and Silicon, I'll bear that in mind when this next comes up.

Cheers,

Nick.

6th Nov 2025, 9:17 pm	#1
Magical Tetrode Join Date: Jun 2020 Location: Stoke-on-Trent, Staffordshire, UK. Posts: 55	Background Hiss from Output Amp Vintage Car Radio Hello all, I'm after some help please? I have noticed this phenomenon on a number of vintage car radios I've worked on over the last few years. Generally Radiomobile from the '60s and early '70s, model types typically 970, 980, 1070, 1085 etc. These are 12v negative or positive earth radios. I've noticed on some that there is a very noticeable hiss, which does not change with the volume control. It can be present on push-pull or single output power transistors. I've never been able to identify what it is. I'm extremely confident it's the output amp as on the Radiomobile 970 models I've been able to easily swap output amps over and the his has gone away. Has anybody come across this and have you been able to resolve the issue? Many thanks in advance, Nick.

6th Nov 2025, 9:33 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 30,443	Re: Background Hiss from Output Amp Vintage Car Radio Old Ge driver transistors often become noisy.

6th Nov 2025, 9:51 pm	#3
Magical Tetrode Join Date: Jun 2020 Location: Stoke-on-Trent, Staffordshire, UK. Posts: 55	Re: Background Hiss from Output Amp Vintage Car Radio Thx Paul, I’ll give that a look.

7th Nov 2025, 11:51 am	#4
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,567	Re: Background Hiss from Output Amp Vintage Car Radio Another possibility is that the noise is coming through the power supply. Perhaps some of the filter capacitors have gone open-circuit.

7th Nov 2025, 4:18 pm	#5
Magical Tetrode Join Date: Jun 2020 Location: Stoke-on-Trent, Staffordshire, UK. Posts: 55	Re: Background Hiss from Output Amp Vintage Car Radio Hi Silicon, Thank you for your idea. I've now found the culprit! Thank you Paul, You were right. I traced it down to the first transistor in the output amp on this particular radio. It's a Motorola 828 and it's the first Mullard OC82DM, which is indeed a germanium transistor. Much appreciative of your suggestion and Silicon, I'll bear that in mind when this next comes up. Cheers, Nick.