77 original radio - slight "hissing" sound

[Oliver B.]

Okay, my car's Stereo is the original from 77 - so (by nature) I do not expect modern "high-end sounds". But: it's hard to tune in a clear stereo-signal and even without receiving a good stereo, there is a slight "hissing" to be heard (which gets "worse", in the moments when stereo is received)...
This is the same, be it radio or music from an fm-transmitter hooked to mp3- or CD-player - and that one is lying in the luggage compartment directly below the antenna!

What might be the culprit? Bad grounding? Where - antenna base, maybe?

It's far from being as bad as to justify tearing too many things apart, but maybe it's just an easy one.....

[Pat K.]

Re: 77 original radio - slight "hissing" sound

Oliver - mine is a '69, and this may not be true for a '77. But, I've been going thru my AC restoration and I noticed there are capacitors throughout my car which I believe mitigate this problem. Check you AIM and see if you have them, then check the locations.

Pat

[Jeff G.]

Re: 77 original radio - slight "hissing" sound

You didn't say if the noise only occurs when the engine is running or not.

If while engine running - Radios of that era have a knack for picking up ignition noise from the power source. The sounds of popping or whining would rev along with the engine. Capacitors from power to ground are used for low popping sounds. For higher pitch or whining sounds, an inductor or choke needs to be installed inline.

If that high pitch sound is present even with the engine off, you have a different problem related to the radio itself. The degradation of components within the radio allow for a condition where it generates an oscillation that you can hear. You will need to remove the radio and have it repaired.

Let us know what you find.

[Jerry R.]

Re: 77 original radio - slight "hissing" sound

First the caveat Oliver: without having it on the bench this is best guess from previous experiences with that model...

Having said that, your radio is not acting all that different from others in that model series that I've worked on. 77 was the year Delco loaded the circuitry with almost every first generation integrated circuit available and fancy new gadgets (like voltage controlled tuning of previously mechanical tuning elements). It's logic decisions were not always smooth or quick by today's standards. And yes, you could hear the hiss (which is mostly internally generated component noise) in the radio circuitry. And, when the stereo FM sub carrier was detected, it switched in more noise generating circuits to split the second (stereo) channel over to the second audio amp.

The part that you described that sounds suspicious to me is the inability to tune in stereo that is clear. Amongst a dozen other potential problems, it points to an AGC (automatic gain control) problem which would be most pronounced on strong FM stations (or AM for that matter) or the voltage controlled tuning capacitors (varactors) which may not be fine tuning the circuitry based on the FM channel you are tuned to. Bottom line, FM stations (and stereo) should sound pretty good.

To clear up a common misconception about spark plug noise, alternator/generator whine, lightning crashes and such amplitude related sources of "noise"; those items will show up on the AM (Amplitude Modulation) band. But, the FM (Frequency Modulation) band was created to get around exactly those types of interference. It doesn't mean they can't show up by sneaking into the last amplifier stages of the radio (which ARE amplitude sensitive so that our ears can detect the music) but the chances of that happening are as good as winning a lottery.

So, to cut this short, I doubt you'll find an "easy" fix. Hope that helps... _Jerry_

[Oliver B.]

Re: 77 original radio - slight "hissing" sound

First: the problem is the same, engine running or not - so I would eliminate a relationship to "bad shielding" (actually I experienced that one before I put back the spark plug wires into their respective shield routings) or other engine influence...

Then: Jerry, is the 77 radio really of that "gadget-kind" you described? It works fairly simple with its turn-button-dial-in and push-button-station pre-set... Does the dial-in nonetheless work "voltage-controlled" instead of mechanically? Whoa!
So, if I understand right, a (bit of) hissing sound is to be accepted by nature of that design!? (Okay, I lived with that the last 17 years I own the car now - why not just carry on... )
And yes, stereo is only recieved "occasionally" - even with stong stations (and the fm-transmitter directly next to the antenna).
But - admittedly - that one is not an issue THAT pressing, that it would justify pulling the radio and send it (overseas - from Germany) for a check/rebuild.

Thanks again for everybody's input!

[Jerry R.]

Re: 77 original radio - slight "hissing" sound

If we're both talking about a model 70HFMT1/T2 then yes, it's full of technology devices that had evolved significantly from the 76 models. The first RF stage is a dual gate MOSFET with BG-190 varicaps on the input and output which are set by a changing voltage that varies from the high end of the dial to the low end. Although the inductors are of the traditional tuned ferrite core variety, if the varicap circuitry is dis-functional, the stage won't tune correctly and sensitivity will be sacrificed. That means (assuming your stereo circuitry is working correctly) that to get a stereo signal you'd have to be very close to the transmitter.

Some hiss on old car radios on FM is normal as the IF amplifiers are made to operate at maximum gain under weak or no signal conditions. You should compare the hiss in yours to another 77 radio at the next show you go to.

But if you are not receiving good sounding stereo signals, the FM circuitry is not working correctly. The 78 radio in my Pace Car sounds great (but does have noticeable hiss when turned up).

Try substituting a different antenna. Maybe this IS an "easy one". Good luck with it.

[Oliver B.]

Re: 77 original radio - slight "hissing" sound

Whoa, lots of tech lingo......

I have this stereo ( - see attchmt.)

And unfortunately I am one of the few over here in Europe with still the original radio in, I'd guess - so getting another original (and well working) 77 side by side with mine is something not to be accomplished too easily...