24th May 2010, 2:46 pm | #21 | |
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Re: Tektronix 475 scope: repair required.
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Al. |
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24th May 2010, 3:59 pm | #22 |
Dekatron
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Re: Tektronix 475 scope: repair required.
I was assuiming that some of it would be biased OK. Can you fill us in on what you have discovered so far. Have you got the DC voltages for above? I am sure that the combined efforts of those on the forum should be able to crack this.
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24th May 2010, 5:09 pm | #23 |
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Re: Tektronix 475 scope: repair required.
Hi Al,
Unfortunately I'm swimming in a sea of 1960s radios at the moment, but if you're still struggling in a couple of weeks time, let me know. I have a fair amount of luck with these sort of things ("luck" being the key work there ), and I like a challenge! All the best, Mark |
24th May 2010, 7:41 pm | #24 |
Pentode
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Re: Tektronix 475 scope: repair required.
I hope I'm not telling you something you have tried already, but the standard way of tracing imbalance in these stages was to short the differential signals together, moving through the amplifier, thus forcing them to be the same potential and enabling you to find the faulty stage.
This was a Tek method and was certainly applied to the older 5XX series valved & Hybrid 'scopes. HOWEVER I'm not sure if would be safe to apply to a transistorised 'scope with presumably lower impedances and a large imbalance. Apologies if you've considered this already, I've had my share of battles with Tek gear, great when working but the occasional fault can be a right B***** Regards Mike A |
24th May 2010, 8:06 pm | #25 |
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Re: Tektronix 475 scope: repair required.
Mike -
Without going into the full-bloodied analysis, the appropriate circuitry starts with a single-ended drive source, a section of d.c.-coupled differential pre-amplifiers, ending in d.c.-coupled differential power amplifiers that drive the LH and the RH deflection plates. The single-source drive point normally sits at 0v d.c. I isolated the drive source and joined the amp. input point to gnd. / 0v. I disconnected both deflection plates from the O/P of the finals. This procedure was duplicated on a fully-functional 475: the O/P correctly sat at half mid-rail on this 'scope, but on the faulty unit the O/P stage remained turned 'hard-on', due to a biassing fault somewhere in the very complex d.c. couplings between driver input and final O/P. I was able to get close to determining where the excess current was flowing, but was unable to determine why. All active devices had been exchanged between the two 'scopes without revealing any faulty active components. All Rs and Cs checked out as good. Therefore, all electronic devices (in the affected areas) had been eliminated. All the PSU rails were established as O.K. & present. Obviously, I'm missing something. But my experience of fault-finding, so that I can proceed further, regretfully does not run to black magic! Hence, my 'shout' here. Al. |
24th May 2010, 8:09 pm | #26 | |
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Re: Tektronix 475 scope: repair required.
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Regards, Al. |
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24th May 2010, 9:10 pm | #27 | |
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Re: Tektronix 475 scope: repair required.
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Sorry if I'm teaching you to suck eggs, but sometimes saying something obvious (or stupid) helps! |
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24th May 2010, 9:42 pm | #28 |
Pentode
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Re: Tektronix 475 scope: repair required.
Hello again Al,
as previous post , trust I'm not teaching you to suck eggs (strange expression) , clutching at straws here, but I have had (Very painful) experience of switch leakage (in my case a Tek 546). Try lifting one end of R1109 (cct 9) this switches on the X10 mag neon, very slight leakage (from a 110v supply) here may upset the d.c. bias conditions. You never know. Regards Mike A |
24th May 2010, 11:10 pm | #29 |
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Re: Tektronix 475 scope: repair required.
I had some fun recently with dry joints on a scope tunnel diode trigger circuit. I wasted a lot of time measuring and looking at waveforms and finally realized that pressing the probe on the board was intermittently fixing the problem making the readings useless.
Anyway, how about a few voltage readings at the points suggested? |
24th May 2010, 11:59 pm | #30 | |
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Re: Tektronix 475 scope: repair required.
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Sentence 2: that, indeed, is a valid consideration. Sentence 3: Couldn't agree more! Apologies not required. It's a very rare human who has acquired a near-infinity of experience and that, as a consequence, can confidently pick up any item of electronics kit and bravely state: "Step aside! I can fix that! Just watch!" Al. |
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25th May 2010, 12:03 am | #31 | |
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Re: Tektronix 475 scope: repair required.
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However, in my case, since I had disconnected the 'signal source' from the I/P to the diff. amp. for fault location analysis, this seems unlikely, since the switch to which you refer is prior to the point of disconnection. Al. |
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25th May 2010, 9:13 am | #32 |
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Re: Tektronix 475 scope: repair required.
I don't know if your scope has PCBs but I have had a fault that cleared by application of heat (hair dryer). Component changes made no difference and it turned out to be the PCB had become conductive (probably due to being kept in an outside shed ).
Just a thought ...Good luck.
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25th May 2010, 1:30 pm | #33 |
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Re: Tektronix 475 scope: repair required.
Yes, David - I have met that problem before & it may well explain the fault and fit the 'known history' of this item. There are points of distinctive high-impedance in this part of the circuitry which would, therefore, cause problems.
OTOH, I do recall that when it was first switched on from cold (many months ago), the trace did initially appear - then it coughed & spluttered for a bit - then the X-deflection collapsed entirely. I wonder: damp in the pcb that was then 'moving about' / drying out? However, it's been in the warm for quite a long time since, so . . . . Al. |
25th May 2010, 4:23 pm | #34 |
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Re: Tektronix 475 scope: repair required.
Al, unlike some tektronix manuals this one doesn't have any expected signal annotations so I've put some estimates down for the first part of the H amp with what I would expect the quiescent / no signal conditions to be. I’ve assumed the first LTP is going to be balanced with the spot in the centre of the screen. Does this look sensible / similar to what’s happening in the real world ?
By the way, does the beam finder switch have any effect on the display and / or operating points ? dave |
26th May 2010, 9:12 am | #35 |
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Re: Tektronix 475 scope: repair required.
Dave -
The base of Q1172 & Q1182 were found to be correctly at 0v. d.c. with the fault present. In the faulty unit, I joined these points to 0v. / gnd. to investigate the subsequent stages: the fault condition (as previously described) in these subsequent stages remained the same. Under 'normal conditions' - i.e. that hook-up, as above, not fitted - the beam finder had no effect. Al. |
26th May 2010, 3:50 pm | #36 |
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Re: Tektronix 475 scope: repair required.
I guess that leaves the two deflection plate drivers.
Don't you just knew the fault will be in the most complex part ? Q1182/1184/1188/1254/1212/1214 look like an X20 inverting amplifier with an adjustable offset controlled by R1256 feeding some current into the 'virtual earth' between R1253 and R1258 (which seems to control the other driver as well) I was guessing the upper driver is similar but how it works has me stumped at the moment. There doesn't appear to be any drive to the output stage ... I would be looking very carefully at that biasing / cross feed circuit around R1256, especially the current in R1258. dave |
26th May 2010, 4:34 pm | #37 | |
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Re: Tektronix 475 scope: repair required.
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If you're not happy to let us look at it, then can you give us the electrode voltages for Q1172/74/82/84/88/1202/04/12/14/54/94 under the fault condition. I know this is a lot of readings to take, but without a feel for how the circuit is sitting, then any guesses are going to be stabs in the dark. Rob.
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26th May 2010, 5:03 pm | #38 |
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Re: Tektronix 475 scope: repair required.
Quite: with regards to the layout on the cct. diag., it is the O/P stage to the upper plate that is turned hard on. The lower plate is biassed correctly. The actual deflection plates were temporarily disconnected for fault investigation.
Al. |
26th May 2010, 5:11 pm | #39 |
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Re: Tektronix 475 scope: repair required.
Nope. I can't figure that upper driver.
Am I missing something or is the schematic incomplete ? It really needs something driving the bases of the OP devices to set the dc operating point or it will just flop-over to one of the rails. dave edit. OTOH, bleeding current into R1209 could alter the balance in the OP stage with the feedback loop operating as normal ... Last edited by dave cox; 26th May 2010 at 5:15 pm. Reason: correction |
26th May 2010, 5:17 pm | #40 | |
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Re: Tektronix 475 scope: repair required.
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I'm quite happy for anyone who wants to 'have a look at it'; hence my OP! . No, seriously though, I've boxed it back up for now (having taken a deep sigh & made that OP of mine ). Right now, the bench is fully occupied whilst I complete an overhaul of a Bush DAC11 wireless; hopefully, not much more to do on that. Then I'll get back to this 'scope and make measurements / re-read my notes, etc. - come back here and reveal all - well, almost all! Then the fun can begin. Al. |
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