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Old 27th May 2010, 12:35 pm   #61
dave cox
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Default Re: Tektronix 475 scope: repair required.

The conditions are such that Q1202 should be turned on.
Is it on a socket ?

Make sure you measure at the leads and not the PCB.
I think you are very close now
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Old 27th May 2010, 1:19 pm   #62
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Arrow Re: Tektronix 475 scope: repair required.

Quote:
Originally Posted by kvimbo1 View Post
Al, Q1202 has OC of the BE junction . . ...
Nope!
Q1202 checks out O.K. on diode test and also in a good Tek. 475.
Q1202 is fitted in a pcb-mounted socket; seperate sockets for each electrode. I removed Q1202 from its socket. I measured the resistance from the end of the resistor that connects to the Q1202 base socket: zero ohms. Ditto the resistor that connects to the Q1202 emitter socket: zero ohms.
The two resistors read correct resistance values on the DVM.


Al.
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Old 27th May 2010, 1:31 pm   #63
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Default Re: Tektronix 475 scope: repair required.

Then change the socket, Al ! You may just take it off the board and solder the transistor directly, if you don't have a replacement socket.
At least you must unsolder it from the PCB and observe for broken or oxidised contacts!
Good luck! You are at the end!
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Old 27th May 2010, 2:08 pm   #64
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Default Re: Tektronix 475 scope: repair required.

There CANNOT be 6 volts accross a working forward biased diode / BE junction.
The current would need be astronomical !!
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Old 27th May 2010, 4:38 pm   #65
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Arrow Re: Tektronix 475 scope: repair required.

Quote:
Originally Posted by Skywave View Post

I've just made the measurement . . .

Q1020/emitter = + 110v. The supply to R1202 = +110v. Therefore Q1202 is not passing any current.

Q1201/base = +104v
. The supply to R1201 = +104v. But, of course, the base current will be very small.

Nevertheless, Q1202 is cut-off. Its collector is at +4.3v (should be +52v).

Al.
Quote:
Originally Posted by dave cox View Post
There CANNOT be 6 volts accross a working forward biased diode / BE junction.
The current would need be astronomical !!
Err, quite! How come I missed the obvious!
Let me have another - and closer - look at that . . . .

Don't g'way - I'll be back . . .

Al.
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Old 27th May 2010, 4:49 pm   #66
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Default Re: Tektronix 475 scope: repair required.

Good luck Al, sounds like you're close.
Rob.
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Old 27th May 2010, 6:39 pm   #67
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Arrow Re: Tektronix 475 scope: repair required.

Various voltage measurements around the O/P transistors indicate that they have now all failed; probably brought about / encouraged by my swopping over the pair from the LH defl. plate to those of the RH plate during the earlier investigations. (One plate O/P stage was O.K. - initially ).

When all this started, I had another, 'known good' 475 on the bench in which, by substitution, these O/P transistors were proven good. This 'known good' 475 has now been boxed up and put to one side - and no, I'm not going to open it up and repeat this 'swop over the transistors' exercise again - one defective 475 to repair is quite enough, thank you!

However, there is one 'measurement' that contradicts the implied conclusions in para. 1 above: all the transistors (in the faulty 475) test "O.K" on the 'diode test' facility on my trusty Fluke 75 and my Fluke 77.

Can these instruments be fibbing? Surely no . . . But . . .

I'm now searching for some suitable replacement O/P transistors; high-voltage TO5 P-N-P ones are the scarcest, of course! (The ones fitted only carry Tek. type / part numbers.)

Aside: There are a number of 'non-interruptable / 'non-maskable' tasks that are now looming large on my radar. It may be quite some time before I'm able to come back here with an update.

But I'll be back!

Al
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Old 27th May 2010, 10:08 pm   #68
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Default Re: Tektronix 475 scope: repair required.

Quote:
Originally Posted by Skywave View Post

Can these instruments be fibbing? Surely no . . . But . . .
Al,

No, but a straight diode go/nogo test with a meter may not tell the whole truth.

Quote:
Originally Posted by Skywave View Post
I'm now searching for some suitable replacement O/P transistors; high-voltage TO5 P-N-P ones are the scarcest, of course! (The ones fitted only carry Tek. type / part numbers.)
Sphere in Canada sell NOS and recovered Tek parts. I haven't dealt with them, but they've been around a few years. I haven't looked into their shipping charges etc.

They also have a page of Tek to industry cross references on this page

http://www.sphere.bc.ca/test/tekequiv.html

which also has a link to a large downloadable Tek cross reference file.

Pete.
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Old 27th May 2010, 10:24 pm   #69
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Default Re: Tektronix 475 scope: repair required.

FWIW I've known transistors to be C-E short while testing OK as diodes B-E and B-C.
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Old 27th May 2010, 11:39 pm   #70
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Arrow Re: Tektronix 475 scope: repair required.

Quote:
Originally Posted by Skywave View Post
Can these instruments be fibbing? Surely no . . . But . . .
Quote:
Originally Posted by XTC View Post
No, but a straight diode go/no-go test with a meter may not tell the whole truth.
Quite; and as these are high-voltage transistors (Vce = 100v or so) & the battery in a Fluke is 9v; perhaps that is insufficient voltage for testing them as diodes.
I am now considering building a simple HV test rig for these transistors: nothing too complicated; just a few Rs and a meter or two. Only needs to be sufficient to conclusively establish if indeed they are breaking down under simulated HV conditions.


Quote:
Originally Posted by XTC View Post
They also have a page of Tek to industry cross references on this page

http://www.sphere.bc.ca/test/tekequiv.html

which also has a link to a large downloadable Tek cross reference file.

Pete.
Thanks Pete: I'll check it out.

Al.

Last edited by Skywave; 27th May 2010 at 11:45 pm.
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Old 27th May 2010, 11:44 pm   #71
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Arrow Re: Tektronix 475 scope: repair required.

Chris; Herald 1360: "FWIW I've known transistors to be C-E short while testing OK as diodes B-E and B-C"

Yes - so have I - but in this case, the meter said "they're OK, C - E". But see my comment, the Post immediately above.

Al.
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Old 28th May 2010, 10:16 am   #72
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Default Re: Tektronix 475 scope: repair required.

What type are these devices ?

I would imaging you can use a 'cheap' parts to get it working (maybe even BD39/40 - but not sure on the Vce/Cob ?) before replacing with the proper items.

dc
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Old 28th May 2010, 1:18 pm   #73
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Default Re: Tektronix 475 scope: repair required.

Quote:
Q1202 is fitted in a pcb-mounted socket; seperate sockets for each electrode
Those "sockets" are Berg Miniserts. Tek used them in a lot of their earlier 1970-vintage scopes before switching to regular transistor sockets. The Miniserts were great for some of their custom IC applications where they would move one pin (e.g., a FET gate input) farther away from the other pins for isolation, guarding, leakage, etc. -- a custom array that was less expensive than a custom socket.

There are some digital boards that Tek had that used the Miniserts vs. IC sockets -- what a fantastic find for us packrats! Miniserts were easy to desolder. "Older" ones were uncovered while the "newer" ones had a tiny dip of RTV (silicone rubber) over the opening for better sealing with or without a component lead installed.

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Old 28th May 2010, 5:29 pm   #74
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Arrow Re: Tektronix 475 scope: repair required.

Quote:
Originally Posted by dave cox View Post
What type are these devices ?
dc
Q1202, Q1212: Tek. 151-0270-00 = a Tek. selected 2N3495 (PNP)
Q1204, Q1214: Tek. 151-0124-00 = a Tek. selected 2N3501 (NPN)

I put out a shout in the 'Wanted Parts' Section of the Forum; it seemed the most appropriate place:

https://www.vintage-radio.net/forum/...ad.php?t=54644

There's always a chance someone just might have a few in their 'bits box'.

Al.
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Old 28th May 2010, 7:50 pm   #75
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Arrow Re: Tektronix 475 scope: repair required.

Update, May 28th. @ 19.40 hrs.

So I built a little test circuit to check-out these O/P transistors: started with the NPN types. Nothing too elaborate: a simple common emitter config. with an Ic of about 10mA from a supply of 100v. d.c.; stabilized the bias current with a simple pot. divider.
Under these test conditions, the collectors were all at about half-rail and the Vbe was about 0.6v. Slightly increasing the base bias current to produce an Ic of about 15 mA lowered the Vc slightly - as expected. The Vbe did not significantly change.

I repeated this test for the PNP types - obviously with the polarity of the supply reversed! The results were much the same.

These results lead me conclude that these O/P transistors are not actually defective, after all. (Apologies to Mr. Fluke! )

Al.
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Old 1st Jun 2010, 11:28 am   #76
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Default Re: Tektronix 475 scope: repair required.

Must be the socket then
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Old 1st Jun 2010, 2:38 pm   #77
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Thumbs down Re: Tektronix 475 scope: repair required.

Hi Dave,
Yes, you'd like to think so, wouldn't you? But no.

I hard-wired the two transistors directly to the pcb, but I still get an impossible voltage difference between base & emitter on the PNP - just as before. So, I removed the transistors, checked them on the test rig: they're O.K.

What I would really like to be able to do now is to get to the underside of this main pcb and give it a good inspection: there are tracks on both sides of the board. Unfortunately, because of the design of a Tektronix 475 'scope, this is simply not a practical proposition, since it would appear that this board 'went in first' during the initial ass'y. Its removal will require almost a complete dismantling of the entire scope: not a job to be undertaken lightly - if at all.

Al.
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Old 1st Jun 2010, 4:02 pm   #78
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Default Re: Tektronix 475 scope: repair required.

Hello Al

Could the impossible voltage readings be due to the circuit oscillating at RF and confusing your DMM? I'm sure that a 'scope deflection amp could behave as an excellent RF power oscillator if it became unstable.

John
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Old 1st Jun 2010, 6:17 pm   #79
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Default Re: Tektronix 475 scope: repair required.

Q1202 is supposed to be a constant current source. Why not have a look with a scope as John suggested? Are you measuring on the transistor pads? Some CBE voltage measurements from the 110V supply might help too.
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Old 1st Jun 2010, 9:46 pm   #80
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Thumbs up Re: Tektronix 475 scope: repair required.

Thanks, jjl & PJL.

O.K. - I've now 'looked & measured'.

'Looked' with my HP1715A 200 MHz 'scope at the B, E, & C of the NPN & PNP: no signs of instability.

'Measured' with my Fluke 77:

Q1202 - the PNP 'top' transistor:
E = +110.5v from a +110.5v supply via R1212 = 1k82
B = +106.6v from a +106.6v supply via R1201 = 1k
C = +4.3v.

Q1204 - the NPN 'lower' transistor:
E = +1v
B = -4.8v
C = +4.3v

Transistor voltages were measured at the leads of the transistors & at the solder pads. No differences in each corresponding case.

Al.

Last edited by Skywave; 1st Jun 2010 at 10:10 pm.
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