Looking for help replacing OC44 with Si in a ‘recreation’ - Page 1
Forum section: electronics / projects

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: August 26, 2022, 07:37:59 pm
content: I’ve acquired a piece of vintage test gear - a Marconi “micro voltmeter” which was a battery-powered device using Ge OC44s. Measured down to 50 micro volts on lowest range.However, the entire circuit board is absent, but the switch assemblies with their ladder components etc., plus the panel meter (fully working) are present.I have the schematic.I’m minded to ‘recreate’ the circuit, but using modern components, not least of which being I’m not going to fork out for seven OC44s.Why - it’s in mint condition, I like old test gear and would relish the chance to bring it back to life as a learning project. Maybe even do an SMD circuit as that seems to be my go-to now I have found toner transfer PCBs…My questions are these:1. Without me poring through my transistor spec books, can someone suggest a broadly suitable Si PNP?2. Not expecting anyone to design it for me, but could someone point me to any suitable resources that would help me understand how I need to re jig the design (bias etc.) to accommodate Si vs. Ge?3. Am I correct that stability may be an issue?4. The original design I believe worked up to 200kHz. I’d quite like to extend that - possible?Thanks.

author: TimFox
author_rank: Super Contributor
timestamp: August 26, 2022, 08:18:43 pm
content: A reasonable Si PNP might be the 2N4249 or its equivalents with different prefix. The 2N4250 has higher beta, and I have used it for low-noise audio applications.See:https://www.mouser.com/datasheet/2/308/PN4249-1301574.pdfThe TO-92 plastic package may be obsolete, but NTE may still make them:https://www.nteinc.com/specs/original/PN4248_49.pdfhttps://www.arcade-electronics.com/PN4249-Transistor-NTE-Electronics-p/nte-pn4249-bu.htmI have also seen them recently on eBay.Have you considered replacing the "guts" with a suitable high-performance op amp, since you don't have the original circuit board?

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: August 27, 2022, 01:55:08 pm
content: That's a good steer, thank you.Yes re op-amp, and that is probably the better route - but haven't found a suitably good schematic yet. I've seen some old 1970s designs but they're 709-based!

author: TimFox
author_rank: Super Contributor
timestamp: August 27, 2022, 02:22:44 pm
content: You can probably replace the 709 with a more modern op amp, such as a TL071.https://www.ti.com/lit/ds/symlink/tl072h.pdf?ts=1661610137804&ref_url=https%253A%252F%252Fwww.google.com%252F

author: oz2cpu
author_rank: Frequent Contributor
timestamp: August 27, 2022, 02:44:02 pm
content: I collect, trade, restore, repair, document, test, teardown,and unfortionatly also rarely scrap old vintage equipmentwhen its state is not worth repairing,you have maybe seen my youtube ?https://www.youtube.com/user/alfa49alfa/playlistsI just checked my stash of old funny components, but did not find any OC44I also notified all in my team to keep looking, and to be sure we save and store parts like this.I just got a Marcony FM signal generator in for test and teardown, so that is what triggered my interest.Feel free to contact me, if you like me to look for something in the future