SGS IW 11700 Silicon PNP Transistor

[sarakisof]

I have to replace the master oscillator transistor in a divider / tone generator circuit in a 60s Farfisa Fast 4 combo organ. The first one next to the tuning coil, common circuit for all combo organs. Q1 PNP in the schematic.
It's an old silicon TO-105 "SGS IW 11700". No any infos about it on net indeed.



As we know, picking replacement transistors for such circuits isn't that critical at all.
I'm sure any BC55X PNP General Purpose one will do the trick fine.
But as a romantic head that I'm, always trying to keep originality of such old things, i would like to find a replacement as much close to the old original one. I'm not saying I'm looking for Lockfit or other problematic types, just a silicon T0-92 but an older version would be fine. It could be any Siemens, PH Philips or any other decent 70-00s you have in mind. No eBay / Mouser / modern supplier's shit.
Those BC55X's i have in hand right not seem to cheapo Chinese to me.

[sarakisof]

*** Update: If this helps, the other divider oscillator transistors for next octaves are 2N5172 NPN. From personal experience i have understood that master osc transistor has always nigger Hfe specs. So, as 2N5172 has about 100 Min. Hfe , if I'm correct, I'm looking for a PNP with Hfe bigger than 100, right?
And about 100-200 Mhz freq.

[mozz]

Have a picture of it?

[sarakisof]

If you just Google TO-105/106 transistor package you will find out many of them.
SGS-Thomson seems to be the most common brand. BC115/6/7, BC136, BC204/5/6/7, BC408 BF152, 2N3568, 2N3638, 2N3643 etc. are some TO-105/106 examples i found online.

The problematic key note in my case is the F#/FA# and the culprit transistor to be replaced the one i have marked with the arrow below.
Every transistor (PNP) next to the tuning coil on top pcb is the master top oscillator transistor for each note. The others (NPNs)following into pairs are the divider transistors for each octave (each small pcb = each octave that contains a pair of them). As you can see in my case for some reason only the FA# has 2N5172 TO-98's (black's) for divider transistors. All the other notes use SGS 9787 TO-105 (brown's). I have seen this in other Farfisa models too (to use some similar transistors / components in only one or two notes and not being all identical) but this has to do with what components they had in factory stock while producing. You can see that the last bottom pcb/octave uses the brown's too anyway again (same card with the other note's divider cards).





Also notice how every top master transistor is different from each other , proves that replacement choice isn't so much critical at all.


So, i want to replace the master Fa#. SGS IW - PNP - TO-105.
The divider pair transistors below it are 2N5172 NPN's.
200Mhz.
Hfe ~100. Does the master top oscillator transistor needs to be higher than 100 hfe?
What would be a decent close replacement PNP choice in a TO-105/106, TO-98 or even TO-92 old package with a 150-200 Hfe?
Don't want to use modern Chinese BC55Xs. 😛

[Rob Strand]

It's difficult the be sure but the way I look at the circuit is it only has a 5k6 collector load, and another 5k6 to the transformer, so it can't be working too hard.    You might want to consider the supply voltage in the transistor rating, it's not marked on the schematic.   I suspect a BC557 would do it.   For higher voltages BC556.

Back in the 60's a common TO-105 PNP was the 2N3638A.   The power dissipation on these wasn't any better than the modern BC547's but off hand they did have slightly higher current ratings.    I don't think the current rating is a big deal.   The BC series has the BC327 and BC328 for current higher ratings.   I reckon the higher gain of the BC547 would be a good thing to have up your sleeve.

In your pic I can see IW 9810/2 followed by some numbers which are probably date codes as they aren't the same number on each part.

[sarakisof]

Heh Rob, hope you're fine.
I think the supply voltage to be considered is +12V as stated at the point A in the scheme. Here's the whole diagram.


So i guess any TO-105/106 PNP (they start from 20V. Vcb,Vce) will be OK?
Mhz and Hfe aren't things to consider too? I thought Freq should be close and Hfe bigger compared to divider transistors.
For example in two other Philicorda organs i own goes like BF194 (Hfe~80) for master oscillator transistor and BF195 (Hfe~35) for divider transistors. Freq isn't close though so i guess it's not important.

I can get locally BC204, BC204B, BC205 and BC206.
They're all TO-106 PNP. Same Ic max @ 0.1A. and ft:100Mhz.

BC204: 45V - Hfe 75
BC204B: 45V - Hfe 240 !
BC205: 20V - Hfe 75 
BC206: 20V - Hfe 125 

BC204B seems to win, but considering that i want to stock them as i have three organs like this for future repairs so i may need about 50 of them, could the 4 cents BC205 do the trick, being 20V and only 75 Hfe?

[Rob Strand]

Heh Rob, hope you're fine.

Hope you are good too.  Doing better than the world is ATM.

Based on a 12V supply, the master oscillator transistor has a good bias point with a hFE of about 90 which lines up with your 80, however the other circuits could be completely different.   However it will bias OK over a much wider range.  Those oscillators are a little tricky since they don't ways require the transistor to be bias linearly as they can drive the transistor to saturation.  The problem is will it start-up every time!

Given all the other transistors have ratings of 25V or so I suspect you shouldn't have problems with the 20V units.

BC204: 45V - Hfe 75  (0,5$.)
BC204B: 45V - Hfe 240 ! (0,6$)
BC205: 20V - Hfe 75  (0,04$)
BC206: 20V - Hfe 125  (0,5$)

For the non-B transistors I have a feeling those gains are minimums.   I found this datasheet and the number don't quite agree.   The B transistors are gain sorted.

https://www.datasheet.directory/pdfviewer?url=https%3A%2F%2Fdatasheet.iiic.cc%2F15e349db%2Fst.com%2FBC205.pdf

The gains here are much higher.  Notice the non graded units have a wide spread of hFE.

You might as well go for the cheaper ones.   

As a side note I did some rough estimates for the transformer and simulated the circuit.   It seems to work on transistors from gains from 40 to 400.   Below gains of about 75 to 80 there was some slight shift in the oscillator frequency.   I don't know if you want to plug in transistor and the tuning is unchanged?   Do you tune the inductors yourself or leave them?

[mozz]

I've got a bunch of those blob top transistors.  Let me know what gain you are looking for. 

[amptramp]

It is amazing how little difference there is between this design and the Hammond Solovox:Model L:

https://antiqueradio.org/art/Solovox-technical.pdf

But in the Solovox, you have a set of inductors that are switched in to provide the frequency.  Other than that, you can almost swap in 6SL7's for the divider stages, run it off 280 VDC and it all looks like the unit I have right now.

[sarakisof]

For the non-B transistors I have a feeling those gains are minimums.   I found this datasheet and the number don't quite agree.

Yes those are min., B's included. I use the alltransistors.com site database.

Do you tune the inductors yourself or leave them?

Yeap they're tunable, like in every combo organ i have repaired but rarely tuning is needed.
The most common issue (same in this Farfisa too) is that you usually have one or two notes being distorted/sound wrong/not divided to lower pitch. You just replace the culprit transistors and gets fixed.
In this example here, a BC557 for master oscillator and BC547's for divider transistors would be OK. But I'd like to replace them with as much close to the originals in TO-105/106 packages.

So I'll go with cheap BC204's and use carefully after testing them.
For the pairs NPN divider transistors that i may need in the future (2x4=8 for each note, 8x12=96 for each organ) it would be nice to use the corresponding to PNP's matchings, BC207,208,209.

But seems hard to find them locally.

I've got a bunch of those blob top transistors.  Let me know what gain you are looking for. 

Wow, thank you for your kind help mozz, really appreciate it. I guess 100 Hfe min. for PNPs , 80 Hfe min. for NPNs. 100-300 average would be nice.
I'm based in Europe. Where are you? Just a small letter envelope would be enough.
If you also have any NPN's which i cannot find here, it would be great.

It is amazing how little difference there is between this design and the Hammond Solovox:Model L:

Heh, all the nice guys are here as i see. Interesting organ, beautiful archaeological construction amptramp, didn't know this, nice info you dropped here

[Rob Strand]

Yeap they're tunable, like in every combo organ i have repaired but they rarely tuning needed.

OK, well at least you have the option.

As a passing note, I notice a very odd behaviour with that master oscillator circuit.   If I measure the frequency over two cycles I get a consistent number.  However if I measure the frequency over a single cycle it bobbles between two frequencies ie. the cycle periods go  TA, TB, TA, TB.   The transistor gains and the resistance of the inductor have an effect.

I haven't analysed why but I suspect it is related to the two resonant circuits formed by the two transformer windings.

It might not occur on the real units.   I have no data on the real inductor/transformer so I've had to invent the inductance, winding resistance and degree of coupling between the windings.

The most common issue (same in this Farfisa too) is that you usually have one or two notes being distorted/sound wrong/not divided to lower pitch. You just replace the culprit transistors and gets fixed.   To be honest I don't even know what frequency the oscillator is at (other than it's some high note on the keyboard).

I suppose that's just outright failure of the transistors.

In this example here, a BC557 for master oscillator and BC547's for divider transistors would be OK. But I'd like to replace them with as much close to the originals in TO-105/106 packages.

It's always nice to make it look like the original on those refurbishment jobs.

So I'll go with cheap BC204's and use carefully after testing them.
For the pairs NPN divider transistors that i may need in the future (2x4=8 for each note, 8x12=96 for each organ) it would be nice to use the corresponding to PNP's matchings, BC207,208,209.

But seems hard to find them locally.

I haven't seen or bought those TO-105/TO-106 devices for over 45 years!   It never ceases to amaze me people can still find sources/supplies.

[sarakisof]

Nice input as always Rob, greatly appreciated. 🙏

[sarakisof]

I learned smthng useful from the old masters of transistors, in places like vintageradio or antiqueradio forums. It's a treasure book guide, a bible for old transistors i didn't know before.
"Towers' International Transistor Selector". Can be found easily in pdf online.

Found some great infos in there.







EDIT: According to this book and considering Hfe's, it seems like the transistors I'm looking for are:

PNPs: BC204/205/206/225/400/417/418/419 (not sure if 4XXs with low freq 40-75Mhz will have any issues though).

NPNs: BC113/114/132/134/154/207/208/209/220/407/408/409.

[Rob Strand]

Yes, those old tabulated books come in handy.  I've actual got heaps and also old datatsheets but most of them I've archived away for now.

If you click on the .jpg link it has some small tables for the BC407... and BC417
https://es.web-bcs.com/transistor/tc/bc/BC417_cpl.php?ssize=desk&menu=select&stock=bipolar

In many cases the different BC numbers for similar parts correspond to different manufacturers or different packages.

I wouldn't be overly concerned about the transistor speed.   All those organs are running below 10kHz so pretty much any silicon transistor is going to be fine.  The transistor speed would have less effect on oscillator stability with temperature than the basic circuit design.

[sarakisof]

Finally, i was given some BC204B's (PNP) and BC209B's (NPN) from a good old repair man here. Gave me his last pcs and as he told me he was excited to see them going, cause it was kind of a bet between him and his ex co-worker there. The other guy always insisted that those TO106s would never being used or sold out. So he won and called his ex co-worker to announce the news in front of me, telling him that he should treat him to a cold beer soon.😂 So i made his day. 

[I haven't seen or bought those TO-105/TO-106 devices for over 45 years!   It never ceases to amaze me people can still find sources/supplies.

Called Rob's words in my mind while i was there.

Replaced the old Italian SGS master PNP with the new BC204B one, all FA#s got fixed perfectly in all octaves except for the last low  FA# note (bass octave).
So i dropped a matched (testing for close Hfe) pair of the new BC209B NPNs in the last divider board, replacing the old ones and... BOOM, the Farfisa runs 100% perfect as it was in her first day!

Thanks everyone who contributed here, it was a pleasure.

For the record, just a quick quiz. Can anyone tell the transistors' brand from the logo printed on them? Seems like "με"/"μe" or something similar is written there.




Cheers. 🥂

EDIT: "Google is my friend" so i found they probably came from a company named "Micro Electronics". Don't know anything about this brand though (any connection with ST?).