27th May 2010, 12:35 pm | #61 |
Nonode
Join Date: Jan 2009
Location: Bristol, UK.
Posts: 2,062
|
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 |
27th May 2010, 1:19 pm | #62 |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
|
Re: Tektronix 475 scope: repair required.
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. |
27th May 2010, 1:31 pm | #63 |
Retired Dormant Member
Join Date: Feb 2010
Location: Sofia Bulgaria
Posts: 14
|
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! |
27th May 2010, 2:08 pm | #64 |
Nonode
Join Date: Jan 2009
Location: Bristol, UK.
Posts: 2,062
|
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 !! |
27th May 2010, 4:38 pm | #65 | ||
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
|
Re: Tektronix 475 scope: repair required.
Quote:
Quote:
Let me have another - and closer - look at that . . . . Don't g'way - I'll be back . . . Al. |
||
27th May 2010, 4:49 pm | #66 |
Octode
Join Date: Apr 2009
Location: Aylesbury, Buckinghamshire, UK.
Posts: 1,522
|
Re: Tektronix 475 scope: repair required.
Good luck Al, sounds like you're close.
Rob.
__________________
We have done so much, for so long, with so little, we are now qualified to do anything with nothing. |
27th May 2010, 6:39 pm | #67 |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
|
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 |
27th May 2010, 10:08 pm | #68 | |
Retired Dormant Member
Join Date: Oct 2004
Location: Bristol, UK.
Posts: 837
|
Re: Tektronix 475 scope: repair required.
Al,
No, but a straight diode go/nogo test with a meter may not tell the whole truth. Quote:
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. |
|
27th May 2010, 10:24 pm | #69 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,535
|
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.
__________________
....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O |
27th May 2010, 11:39 pm | #70 | ||
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
|
Re: Tektronix 475 scope: repair required.
Quote:
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:
Al. Last edited by Skywave; 27th May 2010 at 11:45 pm. |
||
27th May 2010, 11:44 pm | #71 |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
|
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. |
28th May 2010, 10:16 am | #72 |
Nonode
Join Date: Jan 2009
Location: Bristol, UK.
Posts: 2,062
|
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 |
28th May 2010, 1:18 pm | #73 | |
Retired Dormant Member
Join Date: Jan 2008
Location: Harviell, Missouri, USA.
Posts: 31
|
Re: Tektronix 475 scope: repair required.
Quote:
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. Dean |
|
28th May 2010, 5:29 pm | #74 |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
|
Re: Tektronix 475 scope: repair required.
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. |
28th May 2010, 7:50 pm | #75 |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
|
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. |
1st Jun 2010, 11:28 am | #76 |
Nonode
Join Date: Jan 2009
Location: Bristol, UK.
Posts: 2,062
|
Re: Tektronix 475 scope: repair required.
Must be the socket then
|
1st Jun 2010, 2:38 pm | #77 |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
|
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. |
1st Jun 2010, 4:02 pm | #78 |
Octode
Join Date: Jan 2003
Location: Ware, Herts. UK.
Posts: 1,082
|
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 |
1st Jun 2010, 6:17 pm | #79 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
|
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.
|
1st Jun 2010, 9:46 pm | #80 |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
|
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. |