Early 303 Bias problem - help!

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16-May-2018 06:07 PM

PeteUK

PeteUK

Posts: 5

Hi, another bias issue! I've looked through old posts and cannot find the answer.

I've rebuilt a 303 (SN 10728), so it's got the 'old' bias system. I replaced all semiconductors including the IS920's for 1N4148's.

My problem is similar to previous posts, in that I couldn't get higher than about 1 or 2mV max. I decided it may be best to refit the IS920's.....but I've lost them!

So my question is: What can I use instead? the 1N4148's aren't near enough. Can I modify the circuit?

I'd appreciate any ideas.

Pete
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19-May-2018 04:30 AM

EJP

EJP

Posts: 1505

You are correct. You should not have changed the 1S920s, and 1N4148 is not a correct substitute for the biasing diodes in this version of the 303. You will have to source more 1S920s, or another diode that has the same forward voltage.

EJP
Dada Electronics Australia

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19-May-2018 09:18 AM

PeteUK

PeteUK

Posts: 5

Thanks EJP. Sourcing these now is extremely difficult, so I'm going to upgrade the boards to Mk2 spec.

I've designed a small board (12mm x 14mm) that holds the additional 5 components and fits in the holes vacated by the 2 diodes.

I wish I'd thought of this before, but I never really compared the differences between the boards until I had this problem.

Pete
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19-May-2018 10:04 AM

EJP

EJP

Posts: 1505

Very good. I was going to suggest that as another resort.

Don't forget to change the upper components in the bias chain - remove the 20K trimpot etc.

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