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Electronics => Repair => Topic started by: booyeah on July 25, 2019, 01:40:11 pm

Title: Replacement transistors - Thurlby Thandar TG2001 function gen
Post by: booyeah on July 25, 2019, 01:40:11 pm
Hi.

I bought a Thurlby Thandar TG2001 20MHz signal generator (https://i.ebayimg.com/images/g/eN0AAOSwHglcUV3I/s-l1600.jpg)
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 (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 (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 (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 (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.
Title: Re: Replacement transistors - Thurlby Thandar TG2001 function gen
Post by: CJay on July 25, 2019, 02:08:46 pm
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
Title: Re: Replacement transistors - Thurlby Thandar TG2001 function gen
Post by: KD4CCF on July 25, 2019, 09:33:52 pm
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.
Title: Re: Replacement transistors - Thurlby Thandar TG2001 function gen
Post by: David Hess on July 26, 2019, 02:41:28 pm
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.

Quote
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.

Quote
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.
Title: Re: Replacement transistors - Thurlby Thandar TG2001 function gen
Post by: edavid on July 26, 2019, 02:53:48 pm
Could I get away with subbing in something like a
 
2N3019  https://ie.farnell.com/multicomp/2n3019/transistor/dp/1611571?st=to-39 (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 (https://www.datasheetarchive.com/pdf/download.php?id=69e6c81dc3ff92a60db52238272a4251bc457e&type=O&term=MRF542)
That's too slow.  Buy some 2N3866s from UK eBay.

Quote

50A02MH-TL-E https://ie.farnell.com/on-semiconductor/50a02mh-tl-e/transistor-pnp-50v-0-5a-sot-323/dp/2724347 (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 (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.
Title: Re: Replacement transistors - Thurlby Thandar TG2001 function gen
Post by: booyeah on July 26, 2019, 04:18:37 pm
I ordered some of the 2n5160s and 2n3866s, hopefully they arrive next week and I'll update then.
Title: Re: Replacement transistors - Thurlby Thandar TG2001 function gen
Post by: David Hess on July 26, 2019, 08:49:19 pm
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.