Replacement transistors - Thurlby Thandar TG2001 function gen - Page 1
Forum section: electronics / repair

author: booyeah
author_rank: Contributor
timestamp: July 25, 2019, 01:40:11 pm
content: Hi.I bought a Thurlby Thandar TG2001 20MHz signal generatorfrom Ebay that appeared to be working.It powers up and the TTL output is good through all the frequency ranges, but the 50ohm output was showing a dc bias that it shouldn't and nothing else.So I opened it up and it obviously something had gone bad in the output amp, a number of the resistors are blown to bits. It also appears from the flux residue that somebody had previously replaced 3 of the transistors. Those 3 replacements are dead and inline with the blown resistors. There's also a dead capacitor but I think that's just from its close proximity to one of the resistors that blew. (*red lines in schematic is what blew)The issue I have is trying to source replacement transistors. Ideally I'd like to be able to order something that will arrive in a few days/week as opposed to say a month from the US, so looking for replacements on Farnell and Rs-online.The transistors all seem to have similar specs 70-100v, 1400MHz ft, 0.4amp, but looking at Farnell / Rs there doesn't seem to be that many with the higher voltage / higher ft rating.Could I get away with subbing in something like a2N3019https://ie.farnell.com/multicomp/2n3019/transistor/dp/1611571?st=to-39for the MRF542/MRF544https://www.datasheetarchive.com/pdf/download.php?id=69e6c81dc3ff92a60db52238272a4251bc457e&type=O&term=MRF54250A02MH-TL-Ehttps://ie.farnell.com/on-semiconductor/50a02mh-tl-e/transistor-pnp-50v-0-5a-sot-323/dp/2724347for the MRF543/MRF545https://www.microsemi.com/sites/default/files/datasheets/CDROMmini/MSC1315.pdfEven though the ft is a bit lower, or would it just make a balls of it?Cheers.

author: CJay
author_rank: Super Contributor
timestamp: July 25, 2019, 02:08:46 pm
content: I'd take a look at some old CRT TV schematics, I've a feeling you might get lucky with some CRT Gun drive transistors

author: KD4CCF
author_rank: Contributor
timestamp: July 25, 2019, 09:33:52 pm
content: Make sure your D49 thru D52 are good, The Q35 Q36 (complementary) 1/2 amp pair aren't doing any heavy lifting, so 25 to 50 V types would be sufficient. Even the outputs, 1/2 amp would be sufficient.There should be about 10VDC +/- at the resistors R229-R232 , so there should be some type of (large) capacitor where that wire goes off screen upper right of schematic if you want no DC offset.

author: David Hess
author_rank: Super Contributor
timestamp: July 26, 2019, 02:41:28 pm
content: Quote from: booyeah on July 25, 2019, 01:40:11 pmI do not envy you.QuoteLow demand means these parts are basically out of production. The same goes for almost all of the medium power RF TO-39 transistors. Sometimes I wonder that they even existed.QuoteI think you can get away with that but watch out for power dissipation with those little surface mount parts.Quote from: CJay on July 25, 2019, 02:08:46 pmCRT z-axis transistors are low capacitance but not particularly fast. Parts like the KSA1381/KSC3503 and KSA1142/KSC2682 may be acceptable if nothing better can be found. I might use them and just accept the resulting performance.A modern design would have to use a different topology to get the best use out of the parts which are available now.

author: edavid
author_rank: Super Contributor
timestamp: July 26, 2019, 02:53:48 pm
content: Quote from: booyeah on July 25, 2019, 01:40:11 pmThat's too slow. Buy some 2N3866s from UK eBay.QuoteThat one might work. You could also try posting a want ad in Buy/Sell/Wanted for 2N5160s. They were quite common and there should be some UK member who can sell you a few.

author: booyeah
author_rank: Contributor
timestamp: July 26, 2019, 04:18:37 pm
content: I ordered some of the 2n5160s and 2n3866s, hopefully they arrive next week and I'll update then.

author: David Hess
author_rank: Super Contributor
timestamp: July 26, 2019, 08:49:19 pm
content: Quote from: booyeah on July 26, 2019, 04:18:37 pmThat is where I would start since they are readily available.I would need to see the rest of the schematic to evaluate stability when replacing the driver transistors in the stripline packages. I have no doubt however that it could be made to work with slower transistors there.