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I am refurbing a car stereo from an early-90s Volvo (stereo model HD-603) - plus replacing the cassette circuit with Bluetooth and Aux In. But this question is not about this modding per se.

The stereo itself, being 30-years-old, initially was failing due to the main input cap leaking its guts, and despite now working after that repair, there are a number of (quite a lot of) electrolytic caps on the board, and of all the ones I've pulled and tested off the board, they are drifting off and mostly high in ESR.

So - before reinstalling the stereo, I will replace the electrolytic caps with exact like-for-like, only LESR, plus there are several sub-1uF electrolytic caps which I will replace with polyesters of the same values.

My question is - might these new caps (despite being all exactly the same uF values as the originals) potentially affect biasing or possibly introduce new issues? (If so I won't do it!) (And I am a capable solderer, and won't put them in wrong!)

The car stereo board has 3 test points and 4 pots - though I don't have a schematic or much info about this board. I don't know if these pots are for bias adjustment or levels.

Also - I should add that this stereo unit appears to only output to an external power amp buried elsewhere in the dashboard, and, doesn't seem to have a 'power amp' stage itself, with larger transistors/fets and output caps. So I guess the matter of DC offset at the speakers isn't an issue.

Thanks for looking at this.

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  • \$\begingroup\$ Unlikely, but impossible to tell for sure without a schematic. I would just try it. \$\endgroup\$
    – winny
    Mar 21 at 21:52
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Capacitors in that application perform one of several general functions:

  • Power-supply smoothing or decoupling. If you're putting in the same values you'll be fine.
  • Audio decoupling capacitors to block DC going from one stage to the next. A wrong value here might affect the bass a little but, again, substituting the same values will be fine.
  • Tone control capacitors are unlikely to be electrolytic so you won't be replacing those.
  • Radio tuning capacitors are also too small to be electrolytic so you won't be touching those (and that's where you'd most likely cause problems).

The only way you'll affect bias is that if the old caps were leaking they may have pulled the bias one way or the other. By replacement you should be restoring everything to the original values.

Have fun!

You should be fine.

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You should be fine substituting similar types of capacitors. For example, 105C types for 85C types.

However, bias could be affected by the blind substitution of low-ESR types of capacitor that have significantly higher actual leakage than general-purpose aluminum electrolytic capacitors (which are really quite good). In particular, solid polymer capacitors have very high leakage, but other low-ESR types may also suffer from this. The difference may not be fully reflected in the datasheet values, so I think some caution is called for.

It’s probably unlikely, but there is also a small possibility that different parasitic characteristics of the polyester caps could cause oscillation.

And, for some circuits, a minimum amount of ESR (for example on the output of a regulator) enhances or enables stability.

Chances are fair that nothing will be greatly affected, but there are non-zero risks.

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