Tektronix 468 repair/refurb

[tautech]

Do you mean the 468 SM?   Volume 1 I assume.....    I will have a look at it.

No, 465 SM.

[Addicted2AnalogTek]

Anyone see any problem with using a 3.6uf cap to replace the one that's got issues?   The only Sprague 109D I am able to find is 3.6uf 150V.  I can find other ones that are 3uf 125V, but thats bout it. 

[MarkL]

Anyone see any problem with using a 3.6uf cap to replace the one that's got issues?   The only Sprague 109D I am able to find is 3.6uf 150V.  I can find other ones that are 3uf 125V, but thats bout it.

If it's just acting as a filter, close enough.  Standard tolerance is 20%, so Tek can't be expecting an exact value anyway.

Sprague tantalum division now owned by Vishay.  Seems that the higher voltages are discontinued.  You're lucky to have found a 150V one.

[Addicted2AnalogTek]

Good, I'm glad I was able to find them and that I ordered extra.
Ordered 10:
SPRAGUE 30D305F150CB3A AXIAL 3UF 150V -9.5%
Ordered 2:
Sprague 109D365X0125C2 3.6uF 125V 20%

The 109D's are the correct size, physically, and the 30D's are quite a bit larger.  Don't know if I will be able to fit it correctly, but at least I'll have them.

The power supply test points are way off voltage spec and waveform, and p-p........... so I am going to start isolating boards until it gets back up.  Unless it's actually the power supply, then I'll find that out last.   First thing I disconnected was the vertical preamp, and it changed nothing.    The grounding of the entire unit has loads of noise that fluctuates from 20-60mV p-p.  Im unable to even guess as to a frequency, and from what I see, it looks like I have multiple near-shorts into ground.  I have no idea how the scope even functions the way it does at this point.    The trigger board seemed to have been giving off a little bit higher amplitude noise, but that doesn't tell me much since I haven't confirmed... and running around poking the grounds isn't going to get me anywhere either! hahaha

One thing weird I noticed....  TP 159 drops to -1V and takes a min or so to climb back to zero once I power off the scope. 
-8V and 5V rails discharge immediately upon powering down.
55V and 15V drop like a capacitor connected to a dmm, as I would expect.

My assumption is that isolating boards is probably going to be the best way for me to go at this point, and then once I have the board or boards isolated, I can pull one lead of each cap and check them.  if I don't find the rotten egg(s) right away, I will re-connect it and probe away.  But I think that there are at least a couple issues to deal with that are causing the PSU problems, and I have a feeling that is why I was getting nowhere just probing test points and attempting to trace pulsating waveforms and the like  (there are multiple).    What do you think?     (I think I should order that assortment of tantalum capacitors now....... )

[Addicted2AnalogTek]

Oh, and Volume 1 of the 468 manuals has the flow charts and power supply isolation instructions.   Starts at page 354 for the main power supply.

[tautech]

Oh, and Volume 1 of the 468 manuals has the flow charts and power supply isolation instructions.   Starts at page 354 for the main power supply.

I can pull one lead of each cap and check them.  if I don't find the rotten egg(s) right away, I will re-connect it and probe away.

Away from the PSU ESR is less of a concern, but measured to labeled value is important. Any cap more than 5-10% from labeled value should be given "flying lessons". 

[Addicted2AnalogTek]

I finished the checks for the main power supply.. along with getting shocked by the 6kv CRT capacitor.   WHEWWWW.  I'm still alive, though, and so are both scopes.

I white-outed the parts of the waveforms that didn't closely match, and doodled in my own scriblets.   

Sooooo the +110V/+55V section has issues, especially since 110V rail has the jitters in sync with CH2... not sure how it doesn't affect CH1 but we shall find that out soon enough.

I think that the +110V is peeing in the other rails Cheerios.

[Addicted2AnalogTek]

Well, removing the trigger board cleaned up the waveforms on 158 and 159 a bit, but the voltages were still off.

Pulling one end of both CR727 & R633 unloaded the PSU rails for the most part, but now 159 is 5V high,  while 55V is on point.

Fun stuff.  This may take a while.   I may pull most of the boards off and repair each section/board one at a time until it's all done.  The obvious, noticeable problem of the CH2 jitter seems to be one of several problems I need/want to deal with.  If I only fix that one and nothing else, I'd likely be doing this again in the near future.  Better to get it all over with now, right? 

TauTech - you are correct.  There is something rotten in the CRT circuitry.  Glad I'm getting to it before it kills my CRT.
The trigger board definitely has some surprises for me, and so does whatever is down the like from CR727 and R633 which I will now trace... well, once I finish the isolation procedures until I have a correctly functioning power supply.

Thanks for the help Tautech and MarkL.  I will keep on updating the thread with data and info as things progress and bad parts are singled out.  I'm anxious for that final pic of some parts laying on the table, with the caption " The Bastards". haha

[MarkL]

I white-outed the parts of the waveforms that didn't closely match, and doodled in my own scriblets.   

Sooooo the +110V/+55V section has issues, especially since 110V rail has the jitters in sync with CH2... not sure how it doesn't affect CH1 but we shall find that out soon enough.

I think that the +110V is peeing in the other rails Cheerios.

The causal effect may be the other way around.  Here's a plausible explanation:

The +110V unregulated supply is used to create the +110V regulated supply.  And the +110V regulated supply is used by the vertical deflection amp to drive the plates on the CRT.  When Ch2 changes position vertically on the CRT, the vertical amp is going to change its current draw to do it.  This will cause the unregulated +110V supply to fluctuate at the same time.


Now I do think from your waveform drawings and voltage readings that the unregulated +55V supply (TP159) might be loaded too much.  The unregulated +110V (TP148) supply is probably ok.  The fact that it's low and has a similar waveform is a side effect of TP159, namely TP158 equals TP159 plus the other half of the transformer winding that's common to both voltages.

It could also be the load is ok and filter cap C829 is in bad shape.

How is your +55V regulated (TP637) voltage and ripple?  It's important becasue all the other regulated voltages are derived from it.


I still don't think it's the power supply, but I would check the above before moving on.

[Addicted2AnalogTek]

the ESR of C603 is 6.3ohms.   Glad I ordered newww ones.   I think I may as well check the esr of all the big filter caps so I can make sure at least that part of the scope is good.   I  went thru the parts lists and wrote down all the tantalum cap values... I think I will be ordering enough to replace every one of them and then have some extra. 
Oh, I also ordered a Fluke 6kv dmm probe last night.   Wanted to get a 40 but I think I can live without being able to measure the CRT itself for now. If it works, it works.  I still have several hundred dollars worth of gear to pick up to do calibrations, so $120 for a probe is out of the question.  I'd rather use that for the PG506 & TG501.

[Addicted2AnalogTek]

The causal effect may be the other way around.  Here's a plausible explanation:

The +110V unregulated supply is used to create the +110V regulated supply.  And the +110V regulated supply is used by the vertical deflection amp to drive the plates on the CRT.  When Ch2 changes position vertically on the CRT, the vertical amp is going to change its current draw to do it.  This will cause the unregulated +110V supply to fluctuate at the same time.

Thus far I'm not getting any results that show there is anything wrong with any of the vertical boards.  When they are all disconnected from all of the rails, the jitter and loading issues are still present.

Now I do think from your waveform drawings and voltage readings that the unregulated +55V supply (TP159) might be loaded too much.  The unregulated +110V (TP148) supply is probably ok.  The fact that it's low and has a similar waveform is a side effect of TP159, namely TP158 equals TP159 plus the other half of the transformer winding that's common to both voltages.

It could also be the load is ok and filter cap C829 is in bad shape.

How is your +55V regulated (TP637) voltage and ripple?  It's important becasue all the other regulated voltages are derived from it.

I am in the process of isolating the +55V completely right now.   The only thing still connected to the +110V is the CRT, and I am under the impression that the CRT circuit is my main source of problems.  The A & B sweep gen appears to be what is causing the large majority of the loading. Once I fully isolated it, the loading almost completely went away and the BR's no longer heated up. 

However, now that I have the trigger board off, all the knobs on the bottom of the panel disconnected, A & B sweep isolated, and all the verticals isolated:

Regulated supplies:   121.5V;  54.99V;  14.99V;  6.78V, -8.02V    ( so the 110V and 5V DC voltages are off)   
All of the waveforms of the regulated supplies are a lot better, all being correct except for TP158, TP162, TP163 - they look like TP162 from the drawings I last uploaded.

Unregulated Supplies:  162.8V;  80.1V:  24.74V;  10.35V;  6.131V

R830, R728 both are getting fairly hot after a few minutes powered on.
R400, R834 both getting slightly warm.
Q838 seems OK temp wise, but I'm starting to think that either it's bad, or Q740 is bad.    The 110V was not folded when I checked before isolating anything, but It's worth checking again now.  The Vbe of Q740 was .5V, so I think it's likely good, but Q838 may be bad. 
I have re-removed C603 and temporarily replaced it with a measured 2.4uF electrolytic, and I am now about to power on and see what changed now that I took that bastard out again.  I had no idea that the ESR was that high!!  the C measured at 2.6uF - it's a 20%, but I should have realized that Tek isn't going to use parts that are ACTUALLY anywhere near their rated tolerance limit. It is about 13% off, which should have been a red flag.  Oh, the disadvantages to working on stuff late at night when I'm tired...

I still don't think it's the power supply, but I would check the above before moving on.

We will be finding out if there is anything wrong with the power supply very soon.   My guess is that yes, there are power supply issues.  If one cap is bad, then there are more bad caps and maybe bad transistors as well - such as Q838.   Fun stuff. 
It's a great learning opportunity for me, since I am rebuilding this scope from the supply up, and then am going to also calibrate it myself.   
I have a STP 2145 A Double OCXO on the way that I am designing and building a board for, and then an enclosure and LiIon battery system so that it stays on 24/7, and I am about to order a few voltage standard ICs as well.  Then to the cal gear. I'll have the engineering dept of the local college help me out with calibrations.

[MarkL]

the ESR of C603 is 6.3ohms.   Glad I ordered newww ones.
...

Looking at the Vishay datasheet for the 3.6uF @ 125V tantalum (109D365X0125C0) in the current product line, the max ESR is 15.0 ohms @ 120Hz.

C603 is 3.0uF @ 150V (109D305X0150C2).  So, it's max ESR will be even more than the one above because of 1) less capacitance, and 2) higher voltage.  But it's not.

Unless the capacitance is off, or there's goo coming out the side, I think your C603 is fine.

[Addicted2AnalogTek]

theres crust on one end.  Looks like it's dried goo.  That is what caused me to pull it in the first place, and then I put it back in because it measured within tolerance.  Took it back out because I wanted to make sure it wasn't part of the problem.  It is but only a little bit.
 
With the parts I already specified being isolated, here are the reg and unreg voltages as just measured, and the ripple.
The change in measurements between last night and now is due to the isolation of the Calibrator only..

Regulated:
(+110V)  121.4V   15mV ripple
(+55V)   55V    <1mV ripple, but has "noise" above 20mhz
(+15V)     14.99V   <1mV ripple, same noise
(+5V)    6.174V     40mV ripple
(-8V)   -8.02V    <1mV ripple, same noise

Unregulated:
(+110V)   162V    TP158:   .6V p-p, Waveform same as on scan posted previously.  Unstable (vertical+horizontal  jitter)
(+55V)     80V      TP159:   400mV p-p       Unstable (jitter)
TP160:     .8V p-p correct waveform - stable
(+15V)     24.8V    TP161:  +80mV -70mV   Correct waveform + slight amount of vertical jitter
(+5V)      10.3V   TP162:   -80mV +125mV   Bad waveform (same as TP158 scan posted previously)
(-8)     6.13V  -100mV +130mV    Waveform OK but also has vertical jitter.

So now I am going to isolate the horizontal amplifier from the +110 and +55 and see what that does, then isolate it entirely.  Next in line will be the CRT.   The PSU is going to end up being 100% isolated so I can repair that, and then add the CRT, Horizontal, Sweep, Vertical, Trigger, etc etc.   It's interesting that the scope still kinda worked before.   It was WAYYYYY out of cal... the calibrator was off, B intensity was bad, B gate was flatlined,  A gate's intensity output was bad.......  plus the CH2 vertical/trigger problem.     If I would have paid any more than I did for this scope, I'd be pretty mad right now.

[Addicted2AnalogTek]

Ah, yes, datasheets.       I was going to do that earlier, then I got carried away thinking a bit too far ahead of myself.  Trying to do too many things at once. 

That 3.6uf is probably close to the one that is in there now, since they are both 109D's, but the original is 3uf.   Probably near the same ESR tolerance, though.   

I cannot find the chart or info for what the correct voltage ranges are for the unregulated power supplies.   I know that the expected voltages are on the waveform sheet and the schematic, but that doesn't tell me anything about what is completely unacceptable for troubleshooting.  Should I just assume that they should all be within about 6-7%???

[PaulAm]

Pages 6-8 and 6-9 in Vol 1 under calibration has the LV power supply tolerances and ripple.  For ripple, 110 is 20 mv, for 55 it's 4 mv and the others are 2mv.  For voltages 55 is +/- 0.5%, 110 is +3 / -2% and the others are 1.5%

[Addicted2AnalogTek]

Pages 6-8 and 6-9 in Vol 1 under calibration has the LV power supply tolerances and ripple.  For ripple, 110 is 20 mv, for 55 it's 4 mv and the others are 2mv.  For voltages 55 is +/- 0.5%, 110 is +3 / -2% and the others are 1.5%

I'm talking about the unregulated supplies and what I should expect when I am removing power from various sections.

[tautech]

Apply the KISS principle, it's just a linear PSU is it not?
http://www.geofex.com/Article_Folders/Power-supplies/plus-9v1.gif

[Addicted2AnalogTek]

Pages 6-8 and 6-9 in Vol 1 under calibration has the LV power supply tolerances and ripple.  For ripple, 110 is 20 mv, for 55 it's 4 mv and the others are 2mv.  For voltages 55 is +/- 0.5%, 110 is +3 / -2% and the others are 1.5%

and for the 468 the regulated PSU tolerances are on page 4-31 table 4-10 and 4-11.

[Addicted2AnalogTek]

Apply the KISS principle, it's just a linear PSU is it not?
http://www.geofex.com/Article_Folders/Power-supplies/plus-9v1.gif

so what are you saying? that the waveform irregularities are from a reverse-biased diode that's gone bad somewhere?    I like the KISS principle, it helps, and it's the reason why I have put everything back together since I wasn't getting very far, and now the only thing off is the trigger board.   I am running some repetitive testing as it heats up to see how the rails change from cold to hot, etc etc.  Right now all of the unregs are showing instability, varying in voltage in a 1-3V range, some as they heat up and some back and forth. and varying ripple as well.  I'm just going to follow the flow charts and isolation procedures, but as I isolate something and nothing changes, I am going to hook it back up. Instead of isolating one voltage rail of any specific area, I am going to isolate it entirely, though. no more of this one V of one board here, another here, etc etc.   It gets me nowhere at all since I have so many rails that are having trouble.    As soon as I remove the main board to isolate the CRT, I am going to pull the filter caps and check the esr.   I am considering doing it in-circuit, but I'm not really sure about that.....

[tautech]

As soon as I remove the main board to isolate the CRT, I am going to pull the filter caps and check the esr.   I am considering doing it in-circuit, but I'm not really sure about that.....

For a linear PSU, I'd concentrate on "measured" value more so than ESR. Cross check against a similar value new cap and you'll likely find ESR values in the single digit Ohms range. In a linear application (working correctly) smoothing caps don't need to work as hard, generally the values are much greater to accomodate/manage the lower frequency ripple.

As I indicated a few posts back, overloaded linear PSU's will display much more ripple when operated outside "design".

Regulation "should" reduce ripple to intended levels IF the PSU is not overloaded.

And YES, you'll need to lift a leg.

[MarkL]

I cannot find the chart or info for what the correct voltage ranges are for the unregulated power supplies.   I know that the expected voltages are on the waveform sheet and the schematic, but that doesn't tell me anything about what is completely unacceptable for troubleshooting.  Should I just assume that they should all be within about 6-7%???

The unregulated voltages in a linear supply are directly related to the incoming AC line voltage, and the load being placed on 1) each of the individual voltages and 2) all of them as a whole (meaning a large load on one can drag them all down).

Tektronix specifies the measurements made with AC line voltage at 115VAC, and probably assumes a well behaved sine wave.  Hopefully they've thought of and specified all the other conditions that affect the measurement too, but I wouldn't bet my life savings on it.  These rails are called "unregulated" for a reason - they're not guaranteed to be steady or accurate.

If you have some unregulated loads disconnected it will have a large impact on the voltage reading and waveform shape.  Or, as I explained before, if you have changing loads on an unregulated supply, it can and probably will fluctuate.  It's completely normal.

With everything connected and under normal load, I would personally not get stressed out if the unregulated voltages were off by up to 10%.  At 15% I would take note and if other symptoms pointed to a possible power supply issue I would start looking.  At 20% nothing else would get done until I found why.

Linear or switched, the specific design of the supply, how the unregulated voltages are used, and the AC input range specification can sway exactly how much slack I'd allow.

If you're seeing that they're all low by 6-7%, I'd move on.  For this scope it's right on lower bounds of the AC input range of 108VAC (115VAC nominal).  If you have low AC line voltage, there's a switch that allows for 90VAC to 110VAC operation.  But I wouldn't flip it because it's going to create more heat dissipation, and you've already noted that things are running suspiciously too hot already.

[Addicted2AnalogTek]

It's similar to a faulty diode.. since it is a N-P-N junction, but Q828 is toast.  I'm checking my lists to see if I have a temp replacement on hand, but I am ordering some motos to replace it and have a spare.  I know that there has to be more going on here, esp in the 5V section, so I would like to diagnose that as well so I can maybe do the whole combined shipping thing.... and not have to wait forever for one replacement to come every couple of days!!

[Addicted2AnalogTek]

I cannot find the chart or info for what the correct voltage ranges are for the unregulated power supplies.   I know that the expected voltages are on the waveform sheet and the schematic, but that doesn't tell me anything about what is completely unacceptable for troubleshooting.  Should I just assume that they should all be within about 6-7%???

The unregulated voltages in a linear supply are directly related to the incoming AC line voltage, and the load being placed on 1) each of the individual voltages and 2) all of them as a whole (meaning a large load on one can drag them all down).

Tektronix specifies the measurements made with AC line voltage at 115VAC, and probably assumes a well behaved sine wave.  Hopefully they've thought of and specified all the other conditions that affect the measurement too, but I wouldn't bet my life savings on it.  These rails are called "unregulated" for a reason - they're not guaranteed to be steady or accurate.

If you have some unregulated loads disconnected it will have a large impact on the voltage reading and waveform shape.  Or, as I explained before, if you have changing loads on an unregulated supply, it can and probably will fluctuate.  It's completely normal.

With everything connected and under normal load, I would personally not get stressed out if the unregulated voltages were off by up to 10%.  At 15% I would take note and if other symptoms pointed to a possible power supply issue I would start looking.  At 20% nothing else would get done until I found why.

Linear or switched, the specific design of the supply, how the unregulated voltages are used, and the AC input range specification can sway exactly how much slack I'd allow.

If you're seeing that they're all low by 6-7%, I'd move on.  For this scope it's right on lower bounds of the AC input range of 108VAC (115VAC nominal).  If you have low AC line voltage, there's a switch that allows for 90VAC to 110VAC operation.  But I wouldn't flip it because it's going to create more heat dissipation, and you've already noted that things are running suspiciously too hot already.

that sounds like a reasonable set of guidelines for dealing with unreg rails.   I have been measuring my line voltage here and it is fairly stable at 116VAC.  sometimes it gets up to 118VAC, but it's never been over that.

I found a variac on craigslist that is $10, but an hour away.  I need to go and get it..... it's only rated 150W and up to 150V, so at the 132max that the manual specifies, I would only have about 1.14A rated current, but it's an older one (not too old) so it can prob be pushed a little for a short time if need be.  I'm thinking it's worth the gas, drive, and ten bucks.  I have hooked my clamp meter to modified cables to the 568, and thus far even with the problems it draws less than 1A.

[tautech]

I found a variac on craigslist that is $10, but an hour away.  I need to go and get it.....

Never found the need for one.....just use a dim bulb tester

[Addicted2AnalogTek]

I made an octopus out of an old train transfo.     Now I can do in circuit testing..... and CR600 is bad and several others seem to also be bad, but I won't know until I pull a lead up and check again.    CR729, CR728, CR815...........................     
I made sure that this octopus DOES in fact work when I measure definitely good components, and I have found plenty of components in the scope that are good acc to the octo.   I know that I will have to check the schematics to see if there are any components that would interfere with the octo, such as C603 interfering with CR729.... but I know what a 3uf cap looks like, and it didn't look like that. 

I just wanted to put an update on here.

Another thing that seems strange to me is that now that the trigger board is the only thing removed, all the loading on all the regulated rails is gone except for the -8.  hmm