Replacement transistors - Thurlby Thandar TG2001 function gen

[booyeah]

Hi.

I bought a Thurlby Thandar TG2001 20MHz signal generator
from 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 a
 
2N3019  https://ie.farnell.com/multicomp/2n3019/transistor/dp/1611571?st=to-39
for the MRF542/MRF544 https://www.datasheetarchive.com/pdf/download.php?id=69e6c81dc3ff92a60db52238272a4251bc457e&type=O&term=MRF542
 
50A02MH-TL-E https://ie.farnell.com/on-semiconductor/50a02mh-tl-e/transistor-pnp-50v-0-5a-sot-323/dp/2724347
for the MRF543/MRF545 https://www.microsemi.com/sites/default/files/datasheets/CDROMmini/MSC1315.pdf

Even though the ft is a bit lower, or would it just make a balls of it?

Cheers.

[CJay]

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

[KD4CCF]

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.

[David Hess]

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.

I do not envy you.

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.

Low 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.

Could I get away with subbing in something like a
 
...

Even though the ft is a bit lower, or would it just make a balls of it?

I think you can get away with that but watch out for power dissipation with those little surface mount parts.

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

CRT 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.

[edavid]

Could I get away with subbing in something like a
 
2N3019  https://ie.farnell.com/multicomp/2n3019/transistor/dp/1611571?st=to-39
for the MRF542/MRF544 https://www.datasheetarchive.com/pdf/download.php?id=69e6c81dc3ff92a60db52238272a4251bc457e&type=O&term=MRF542

That's too slow.  Buy some 2N3866s from UK eBay.

50A02MH-TL-E https://ie.farnell.com/on-semiconductor/50a02mh-tl-e/transistor-pnp-50v-0-5a-sot-323/dp/2724347
for the MRF543/MRF545 https://www.microsemi.com/sites/default/files/datasheets/CDROMmini/MSC1315.pdf

That 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.

[booyeah]

I ordered some of the 2n5160s and 2n3866s, hopefully they arrive next week and I'll update then.

[David Hess]

I ordered some of the 2n5160s and 2n3866s, hopefully they arrive next week and I'll update then.

That 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.