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Old 17th Sep 2019, 5:01 pm   #1
DaniTDS
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Default Blaupunkt Memphis SQR 88 Car Stereo - Speaker whistle while starting!

Hello everybody!

I'm facing a new problem with my old Blaupunkt Memphis SQR 88. Every time I turn on the engine of the car, while the starter is cranking, I can hear an high pitched whistle that seems to come from one of the rear speakers. I suspect of some nasty capacitor, but has anybody any idea about these kind of problems? I searched and found topics about alternator noise with the engine running, but nothing related to a starter noise.

Here is a pic of the diagram of the final stage:

https://radio-bastler.de/forum/attachment.php?aid=48357

Regards
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Old 17th Sep 2019, 5:31 pm   #2
G6Tanuki
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Default Re: Blaupunkt Memphis SQR 88 Car Stereo - Speaker whistle while starting!

I'd suspect some sort of earthing issue - and the 'whistle' you're hearing is commutator-noise as the starter-motor's brushes have to handle 1000+ amps to crank the engine.

From the 1970s onwards most cars I've come across have the radio's DC supply fed from a terminal on the ignition-switch which goes "dead" during engine-cranking, so turning the radio off for a second or so and preventing these unwanted "noises-off".
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Old 17th Sep 2019, 5:39 pm   #3
DaniTDS
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Default Re: Blaupunkt Memphis SQR 88 Car Stereo - Speaker whistle while starting!

I think it has to do with the unit itself because it doesn't happen with a modern Sony, but I noticed this problem on this car with this Blaupunkt and with another old Becker unit.
This radio seems to not turning off while cranking, as some radios turn off, others mute the outputs... But in this case, the whistle is heard.

With earthing issue, you mean inside the unit, or a earthing problem in the car?

Regards

Edit: As I see on the electrical schematic of my car (BMW E30), although it has a terminal for ignition and another with constant 12V for memory, the ignition wire is live even in "start" position. I think modern stereos turn off detecting a voltage drop maybe?

Regards

Last edited by DaniTDS; 17th Sep 2019 at 5:50 pm.
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Old 17th Sep 2019, 5:51 pm   #4
G6Tanuki
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Default Re: Blaupunkt Memphis SQR 88 Car Stereo - Speaker whistle while starting!

By "earthing problem" I'd be looking for anything that causes a significant [few hundred millivolts] voltage to appear between the car bodywork and the battery earth-terminal while the engine is cranking.

The battery -ve terminal *should* be connected to the engine-block via a really thick cable, with a separate somewhat-thinner lead from the -ve terminal to the body itself.

If there's an unintended 'high' resistance [which only needs to be a few Milliohms] in the battery-negative-to-block path, during crakling some of the starter-current may flow through the battery-negative-to-body lead and then through unspecified metallic paths back to the engine-block and the starter - this will inject ripple into the radio's -ve line giving the symptoms you describe.

Check and clean the engine-to-battery cable and also the battery-to-body cable; it only needs a tiny bit of corrosion here to cause all sorts of perplexing nuisances!

As an idea, when you're cranking the engine does the display on the radio noticeably dim? If it does it shows the radio's not getting its intended voltage.
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Old 17th Sep 2019, 5:58 pm   #5
DaniTDS
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Default Re: Blaupunkt Memphis SQR 88 Car Stereo - Speaker whistle while starting!

Hmmm yes, the radio and the rest of interior light dim a bit while cranking, but I though this should be normal due to the high consumption of the starter?

Anyway, now that you mention it... the thick ground wire that goes from the engine block/oilpan to the front suspension cradle/body is all rotten out, rusted and partly broken. I know I should have replaced it long ago... but it's that kind of things that gets lost in the priorities list. Because I didn't had this problem with other stereos in this car, I didn't thought it could be related to this. Maybe there are some more sensible units? Anyway it's a bit strange it seems to only happen on one speaker output.

Last edited by DaniTDS; 17th Sep 2019 at 6:11 pm.
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Old 17th Sep 2019, 7:46 pm   #6
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Default Re: Blaupunkt Memphis SQR 88 Car Stereo - Speaker whistle while starting!

A new piece of earthing strap isn't a major investment and it's probably the easiest thing to try to see if there's any improvement. Depending on where it it relative to the battery -ve and the starter, it might improve cranking speed too!
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