Modifying 90s car stereo to take Bluetooth/Aux

[jungle_jim]

Hi all

I am repairing and modifying the original stereo in an early-90s Volvo (stereo model HD-603) to replace the cassette with Bluetooth and Aux In.

I have followed the tracks from the cassette pre-amp on the main PCB to an audio processor chip (DIP20 Toshiba TA7764P), and cut in new audio inputs, leaving in just the caps (2.2uF 50v Elec.) immediately prior to the TA7764P. The audio from a Bluetooth receiver unit is working OK spliced in at this point. I cut the tracks near there to isolate and remove the cassette pre-amp circuitry.

My question is: those two electrolytic caps now between the Bluetooth receiver (outputting line-level signals) and the audio processor chip: was it right to leave them in, and, if so, is there anything else I could replace them with (given that they are 30 years old, and, were originally there to take audio from the cassette pre-amp)? Like-for-like LESR caps? Tantalums? Any recommended caps for audio?

Thanks
Jim

[stj]

Re: Modifying 90s car stereo to take Bluetooth/Aux

they are to block dc voltages,
i dont know how much space you have, but polypropylene film is best.

[redwire]

Re: Modifying 90s car stereo to take Bluetooth/Aux

You might have the wrong stage, TA7746 is the tuner FM-IF amp? The cassette head pre-amp has output coupling capacitors.
I usually look for the switch circuit that selects tape/tuner to see what caps ar e needed. Work back from the volume control.
Best fidelity is if you can fit i.e. 2.2uF 50V film caps. Polypropylene is harder to find, but polyester is good. Electrolytics are also fine like Panasonic FC or Nichicon Muse.

[jungle_jim]

Re: Modifying 90s car stereo to take Bluetooth/Aux

You might have the wrong stage, TA7746 is the tuner FM-IF amp? The cassette head pre-amp has output coupling capacitors.
I usually look for the switch circuit that selects tape/tuner to see what caps ar e needed. Work back from the volume control.
Best fidelity is if you can fit i.e. 2.2uF 50V film caps. Polypropylene is harder to find, but polyester is good. Electrolytics are also fine like Panasonic FC or Nichicon Muse.

Thanks Redwire

Well - following the tracks from the cassette pre-amp, it just goes into the L&R inputs of the TA7764P (via the 2.2uF caps) which is a simple audio processor that has volume, balance for a single pair of inputs, and single stereo pair output. Maybe the radio is also inputted in parallel across those input pins of the TA7764P - because it's possible to play radio and 'cassette' at the same time if the switches that (used to) trigger when a cassette went in are manually flicked. Either way - I have this aspect of the mod working ok.

But getting back to the caps - just to confirm - for the purposes of what I'm doing here - can poly/film caps generally substitute electrolytic caps (yielding better performance), if they are same-spec? (Though I realize that over a certain uF value, elecs are the only choice). And poly/film caps are not polarized - but are ok to replace elec caps which are?

This board has a number of sub-1uF electrolytic caps - would same-spec polyester caps (eg Mylar) be preferable to like-for-like replacement elec caps?

And - just one more question if I may: when caps are substituted - particularly with different types - is this likely to throw the biasing out? I don't have the schematic for this board, but there's 3 testing points, and 4 pots on the board.

Much appreciated,
Jim

[stj]

Re: Modifying 90s car stereo to take Bluetooth/Aux

film caps are better than electrolytic for audio path, but they are bigger and cost a lot more.

[jungle_jim]

Re: Modifying 90s car stereo to take Bluetooth/Aux

Thanks
Keeping it to one question per post:

So if I replace the electrolytic caps around this car stereo board - all with exact same uF values and mostly same voltage or a bit higher - but - replacing all electrolytic caps 2.2uF and below with polyester box caps - will this effect the biasing of the amp? Is that relevant here?

It's just that this board has 3 test points and 4 pots - which reminded me of biasing hi-fi amps. (I realize you usually need the schematic and amp specs for this - which I haven't got for this car stereo board).

Though this board is basically a pre-amp which outputs to a power amp elsewhere - it doesn't appear to have significant output transistors/fets or big caps on output which you might expect to see.

Thanks
Jim

[stj]

Re: Modifying 90s car stereo to take Bluetooth/Aux

i seriously doubt it has biasing because car stereo's use amplifier chips - not discrete circuits.
it shouldnt be a problem.
besides, i can be 100% sure that those 2.2uf caps are way off what they where when it was built.