Topics

Need 2N2207s and 2N2475s for 547


Delco1946@...
 

Hello all,

I am new to the forum and new to oscilloscopes. I am trying to fix a couple issues on my model 547, which seem to be centered around transistors. I don’t know anything about transistors and I’m much more familiar with tube audio. I need the transistors listed in the header, and I’ve been researching this a couple of hours and I don’t feel like I’ve made any headway. There doesn’t seem to be great replacements and they sometimes seem to be specific to the section that they are in which makes it more confusing.

At this point I’m getting rather exhausted and would love for someone to make a replacement suggestion for these transistors that is current. Some of the forums are 8 to 10 years old on the subject and many of their links or even the part number as a reference I cannot find today. I need to replace parts Q184 and Q173 (missing) and replace a broken 2N2475 which is supposed to be matched so I need two now for my vertical amplifier section. I’m open to modern replacements or new old stock that doesn’t suffer from tin whiskers. Thank you!

Christopher


Dave Wise
 

Hi, Christopher. Q184, Q173, and Q373 have been studied in depth; go to groups.io and read archived topic "Tek 547 A Sweep display during retrace".
Summary: A good replacement for these is BC556 or KSA992. Both cheap and abundant. Mind the pinout. Add 220pF across C170 to avoid slow turn-on.

I don't have a quick answer for the 2N2475. What are the circuit numbers of your matched pair, so we can look them up in the manual?

Dave Wise
________________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Delco1946 via groups.io <Delco1946=gmail.com@groups.io>
Sent: Tuesday, June 09, 2020 10:51 PM
To: TekScopes@groups.io
Subject: [TekScopes] Need 2N2207s and 2N2475s for 547

Hello all,

I am new to the forum and new to oscilloscopes. I am trying to fix a couple issues on my model 547, which seem to be centered around transistors. I don’t know anything about transistors and I’m much more familiar with tube audio. I need the transistors listed in the header, and I’ve been researching this a couple of hours and I don’t feel like I’ve made any headway. There doesn’t seem to be great replacements and they sometimes seem to be specific to the section that they are in which makes it more confusing.

At this point I’m getting rather exhausted and would love for someone to make a replacement suggestion for these transistors that is current. Some of the forums are 8 to 10 years old on the subject and many of their links or even the part number as a reference I cannot find today. I need to replace parts Q184 and Q173 (missing) and replace a broken 2N2475 which is supposed to be matched so I need two now for my vertical amplifier section. I’m open to modern replacements or new old stock that doesn’t suffer from tin whiskers. Thank you!

Christopher


Delco1946@...
 

Hi Dave,

Thanks for the reply. The others I’m more confident on, seems like the standard replacement might be NTE229. They are parts Q1014 and Q1024 in the push-pull part of the vertical input amp and need to match to get an accurate vertical position. I may have even found some originals which would be great to keep this scope 100% original.


 

On Wed, Jun 10, 2020 at 03:24 PM, Dave Wise wrote:


Hi, Christopher. Q184, Q173, and Q373 have been studied in depth; go to
groups.io and read archived topic "Tek 547 A Sweep display during retrace".
Summary: A good replacement for these is BC556 or KSA992. Both cheap and
abundant. Mind the pinout. Add 220pF across C170 to avoid slow turn-on.
Hi Dave,

More than ever, I am now convinced that this forum is A LIVING ENCYCLOPEDIA !!!

A few months ago when I was bringing up my 547 I had the problem that time base A was not blanking the retrace. I found that transistor Q373, 2N2207, which does the same function as Q173 in time base B, was gone. I checked its specs as Vcb = 50v max, Vce=70v max. From my calculations, with the time base not sweeping the Vce of this transistor is about -60v, and so its Vcb (which should be 50v max.). SO THERE IS A DESIGN FLAW, or a very daredevil design. I was going to bring this up to the forum, but... I had been over-critical of Tektronix before and I didn't want to be seen as cocky. So I let this pass.
Now that I see that I was not the only one with this problem, I am confident that the 2N2207 is MARGINAL in this application, both in time base A and B, and SHOULD NOT BE USED THERE again!

Coincidentally, I replaced this 2N2207 with the same you suggested, BC556, of which I got 25 pieces in my cheap 600 transistors kit ($9.99 at Amazon). This one is rated at 65v, so it can make it. After replacing this transistor, and bending two pins to match the pinout of the 2N2207 on its socket, the blanking worked fine, but I found that the un-blanking was much slower so that I had to raise the intensity to see the leading edge of the triggering pulse. I didn't worry much, and delayed the ultimate fix until I could find a suitable PNP silicon transistor with sufficient voltage and speed. But now, thanks to you post, I will try increasing the capacitance C370 (C170 in the B base) to speed up the turn-on of the transistor.

(Had I know of the history of this problem in tekscopes, I would have saved some time, but also missed some fun, ha ha)

Ernesto


Morris Odell
 

Q1014/1024 are the first balanced gain stage after the vacuum tube input cathode followers for the Y amp. The 2N2475 is a fast NPN silicon transistor described as a saturated switch with a Ft of 600 MHz and Cobo of 3 pf. Towers gives the 2N2369 or BSX20 as equivalents. The 2N2369 is a little slower with a Ft of 500 MHz and Cobo of 4 pf but should work OK in that circuit. It's not known what they would have been selected for but they are cheap and plentiful so it would be worth getting a handful and trying them till you find a good combination. Fortunately they are plugged rather than soldered!

I replaced Q184 in my 547 with a BC640 and it works perfectly. I haven't had to replace Q174 (yet).

Morris


Dave Wise
 

Sorry, I don’t have the manual handy at the moment. Are the vertical amp transistors operated common base? If so, their collector capacitance is less critical and the 2N2369 you suggested is almost certainly fine. High frequency peaking will have to be readjusted.

Q184 is common base, and just about anything will work. Q173 and Q373 are common emitter and quick turn on requires a speed up cap. Or rather, larger cap than 2N2207.

Dave Wise

On Jun 10, 2020, at 9:10 PM, Morris Odell via groups.io <vilgotch1=gmail.com@groups.io> wrote:

Q1014/1024 are the first balanced gain stage after the vacuum tube input cathode followers for the Y amp. The 2N2475 is a fast NPN silicon transistor described as a saturated switch with a Ft of 600 MHz and Cobo of 3 pf. Towers gives the 2N2369 or BSX20 as equivalents. The 2N2369 is a little slower with a Ft of 500 MHz and Cobo of 4 pf but should work OK in that circuit. It's not known what they would have been selected for but they are cheap and plentiful so it would be worth getting a handful and trying them till you find a good combination. Fortunately they are plugged rather than soldered!

I replaced Q184 in my 547 with a BC640 and it works perfectly. I haven't had to replace Q174 (yet).

Morris



Morris Odell
 

Hi Dave,

They are a common emitter differential pair and C1029 is a 1.5-7 pf trimmer connected between their collectors for adjustment.

Morris