UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Gemini 80-Bus System

jncoleman · 13th Jan 2020, 1:13 am

John,

Before you do anything to that chip, check that you can obtain a replacement. I don't know about that one in particular but a lot of the LS series have been withdrawn. There might be a compatible one, but do check. The '221 is a 'multivibrator' which, once triggered by the input, delivers a short pulse of a duration determined by the external R and C.

If you're sure that R22 and C23 are correct, and that all the connections to the chip are OK, then it looks as though this chip is dead. If indeed there aren't any replacements then there may be a chance you could modify the circuit to operate without it, but that would need very careful thinking about first.

Nick.

13th Jan 2020, 1:13 am	#312
jncoleman Triode Join Date: Oct 2019 Location: Carlisle, Cumberland, UK. Posts: 11	Re: Gemini 80-Bus System John, Before you do anything to that chip, check that you can obtain a replacement. I don't know about that one in particular but a lot of the LS series have been withdrawn. There might be a compatible one, but do check. The '221 is a 'multivibrator' which, once triggered by the input, delivers a short pulse of a duration determined by the external R and C. If you're sure that R22 and C23 are correct, and that all the connections to the chip are OK, then it looks as though this chip is dead. If indeed there aren't any replacements then there may be a chance you could modify the circuit to operate without it, but that would need very careful thinking about first. Nick.