Silverspeed

So having replaced my head unit for a Blaupunkt SQR 46 and having lots of other bits done ,including headlight upgrade , new battery etc when I start my car I get a big static bang through my car speakers even if the head unit is turned off . I assume this is because the amp is not powered through the head unit and is always live ? Is this the amp possibly on the way out ? There is also a slight interference noise even with head unit turned off or muted . Checked the earths and all seems ok . Be interested to hear if anyone else has this issue.

jcorenman

This is a wiring issue. External amps are nearly always wired direct to the switched-12V bus and get power via that bus whenever the key is on, but the amp is actually switched on and "live" by the amp/antenna output from the head-unit. When you switch on the head-unit there is often a "pop" on its line-level outputs (particularly if amp and radio are not connected to the same gound-point; for the factory wiring, they are). The external amp will incorporate a short delay when switched on so that any "pop" on the input is not amplified. So this sounds like the amp power-on signal is miswired, and the amp is always switched on and live.

Is the interference noise related to engine RPM? If so, it is probably electrical noise from the alternator which gets into the line-out connection to the amp. Again, the amp should not be "live" with the head-unit switched off. It can still happen with the head-unit switched on and quiet-- for example, an aux-input that is not active. The factory wiring has a "suppressor" in the 12v connection to the head-unit to help with this, physically located next to the factory amplifier under the "hump". I would first make sure the same ground-point is used for head-unit and amp (see factory wiring diagram, "radio" page), and then add the suppressor if needed (and missing).

The factory harness routes 12v power to the amp from the CE panel as a red 4.0 wire. The 12v connection to the head-unit goes from the ignition switch (to the suppressor at the amp location, then to the head unit as a 2.5 red wire. The grounds (4.0 and 2.5 brown) both connect to GP-V (the "ground point from hell", above the CE panel for LHD cars). And a 1.0 red wire from head-unit to amp switches on the amp. Yours may be different, depending on who has been in there before you.

Silverspeed

Thanks for the info which is great - I will pass it on to the guys who fitted my head unit. Forgot to say my car is a 93 GTS. I know the fitter had an adapter to plug and play the new unit from the original connectors to the OE fitted unit . The other interesting thing is that the new head unit has a 60 minute on feature that if ignition is off you can still listen to the radio etc . When I do that the new unit powers up but can’t hear anything so amp is obviously not switched ‘on’ by the head unit or more likely would confirm the power for the amp is through the ignition ? ( if that makes sense ) .

jcorenman

I made a correction above-- the smaller red wire is unswitched 12v from the amp back to the radio head-unit, not the amp signal. The amp switch-on signal is part of the 8-pin DIN cable that connects the preamp line-out signals to the amp.

The SQR-46 has an ISO power connector and RCA jacks for preamp-out, while the original radio had the older 6-pin connector power connector and an 8-pin DIN for preamp-out. If you are working with supplied adapters then the DIN adapter will have four male RCA plugs for preamp-out, plus a wire to the ISO power connector for the antenna-amp signal-- not a direct connection to 12v. Check this first. And then check the main power to the amp, I am guessing that is connected to switched 12V (bus-15) and not continuous 12V (bus-30) as was originally done.

This is what makes radio installation so entertaining: You never know what a PO might have done, and I'm not convinced that the factory always did things per the drawings-- particularly with early/late '92/93 GTS's. This might be your opportunity to become a wiring expert.

Silverspeed

Thanks again . Just removed head unit and found this spade terminal just unhooked - it’s from the DIN lead that goes to the Amp . Any thoughts - looks like an earthing lead but can’t see where to attack it too ?

jcorenman

That is the cable shield ground connection. It connected to a ground terminal on the back of the original radio. The shield should also connect to the metal shell of the DIN plug as the primary ground, so in theory this ground lead is redundant. You can check that with a multimeter, look for continuity from ground-lead to DIN metal shell to the outer contact on each of the four RCA-plugs to the new radio. If that is the case, you can leave it disconnected, or look on the back of the new radio chassis for a grounded 1/4" male spade terminal.

Now looking at the other wires I see one problem, but first a nudge down the radio-installation rabbit-hole:
The original radio had a 6-pin connector, common to European radios: Two large terminals, four smaller ones (or a gap and three).
From the "big end", terminal #1 is ground, #2 is +12v switched (ignition, bus-15), 3 is rarely used, 4 is antenna/amp power, 5 is +12v unswitched, 6 is an alarm contact.

The new radios all follow an ISO standard and have 2 or 3 connectors. Connector "A" is 8 pins for power, and connector "B" is 8 pins for speaker-out. (See picture).
A third "C" connector is sometimes provided for preamp-out, phone connections, CD-changers, etc-- three sections (C1, C2, C3), totaling some 20 small pins. This is not used for the SQR-46 so we can not worry about that. And you are not using the radio's speaker connection so we can forget about connector "B" also.
So, for connector "A", all you gotta do (tm) is connect the two 12v and one ground connection, and get the ant/amp power connected to the antenna and amp.

Looking at your photo, the ISO "A" connector is at the very right, with old pin-1 (black) wired to ISO pin-8 (ground), old pin-2 (red) wired to ISO pin-7 (+switched-12v). Old pin-3 is wired to ISO pin-6 (lamps), old pin-4 (blue) goes to ISO pin-5 (antenna), old pin-5 (yel) goes to ISO pin 4 (+12v unswitched). The anti-theft connection is not wired, that may cause alarm troubles, And I am not sure about the orange wire, the GTS wiring diagrams show pin-3 unused but somebody thinks it might dim the head-unit illumination.

The stock radio used one of the pins in the DIN connector to switch on the amplifier, the skinny blue wire in your photo. However, in your case, that wire is connected to the red switched-12v wire with a crappy black-tape splice, which is powering your amp whenever the key is on. This is guaranteed to pop when the head unit is switched on, and conversely, you won't hear anything with the ignition off.

The skinny blue wire from the DIN connector should be spliced (properly) to the blue wire between the old connector and the ISO connector for the SQR-46. That will power the amp on only when the head-unit is switched on, and should also avoid the pop or other noise.

Now one more possible wrinkle: On older radios, the antenna-power connection was used to literally extend the power antenna when the radio was on. For some radios, this was only powered when AM/FM radio mode was selected, and not if the cassette tape was selected. In that case a separate amp-power connection was provided, but I am not seeing that for the SQR-46. So hopefully the blue-wire "antenna" connection is powered whenever the head-unit is on. That is easily checked.

One more potential wrinkle: Traditionally, pin-2 on the old 6-pin connector provide the power to run the head-unit and any built-in amp, and pin-5 was just a keep-alive for the memory-- a new milliamps. And older 928s were wired accordingly, a fat red wire for pin-2 and a skinny one for pin-5. This gets into trouble when newer radios, which take their main power from 12v-unswitched, get wired to the skinny +12v wire on pin-5 of the old connector. I believe that for the GTS Porsche fixed this, with a 2.5mm wire for 12v-unswitched. For our '90GT I had to run a separate larger wire for this.

Here's the SQR-46 ISO connection diagram, pretty standard these days:

dr bob

Original radios had a theft detection contact in the original DIN bucket. It's shown as brown-with-yellow-stripe on sheet 8 of your 1993 wiring diagrams, and in my 1989 car it's a spade terminal as your picture shows. I can't see enough of the wire color in your picture to say that this is the one you are holding, so you'll want to confirm the insulation color. As broader guidance, sharing any wire insulation colors is very helpful when asking "what's this wire for?" anywhere at all in the cars.

If this is in fact the alarm connection, you'll want to insulate the terminal to keep it from triggering the alarm/de-mobilizer.

Silverspeed

Thanks - great information and really appreciated .

Silverspeed

Thanks for your reply - The cable with the spade terminal on is not insulated and connects in to the black Din cable that runs to the Amp . I have now wrapped it in insulation tape.

jcorenman

If it is connected to the DIN connector then it is a shield connection and was originally grounded to the radio chassis. The alarm wire was pin-6 of the original 6-pin connector, a brown/yellow wire.

Leaving it disconnected (and wrapped) is fine if everything works OK. That skinny blue wire the answer to your question is.

Silverspeed

Thanks for all your help - all sorted works perfectly . Just got to sort out a few more items now - getting to know the car though so all good .