Tektronix 7623A poser
 

Hi,

I have a poser with my Tek 7623A scope. It ceased to show a trace a while ago and I am attempting to gain access to the HV board in order to check the oscillator transistor, Q1195, together with some of its associated components. I have followed the instructions in the Service Manual and got to the point where I have removed the screws holding the HV board cover in place. The manual states that the HV compartment should be guided away from the chassis, being careful not to damage any components or connectors. The HV compartment is loose and moving, but only in a limited sense and I cannot get it very far out. Has anyone here ever gained access to this HV compartment or similar? I have asked about this on the Tekscopes Group, but all is silent so far. It seems unlikely to me that no-one has ever successfully done this job and that no advice is available.

The myriad connections around this area are daunting and I don't want to disconnect stuff if it isn't required. Can anyone offer some advice, please?

TIA, Colin.

Re: Tektronix 7623A poser
 

Hi Colin

I have done a 7603 & 7633 but not a 7623A ( so far). you can make some checks without removing the module - is there any voltage on the HV test point? ( -1475V, accessible through a hole in the cover) if it's low remove the black wire going into the potted eht module as when it fails it can load it down. there are also several high value resistors you can check from the top once the supply is discharged.

you can also check the T03 switcher in situ. I would check that the supplies are present before removing the module

if you want to completely remove the module take good pictures and be very careful with the white/ red & white/ brown single wires marked A & B. do not get these reversed on re-assembly. you need to remove the Z axis board mounted on top of the module so take pictures here as well. remove the eht plug to the tube at the side of the module.

remove the connector to the tube base and the other 2 ( or 3?) noting the orientation as the small arrows are not easy to see denoting pin 1.

there are 3 screws holding the module to the chassis, 2 are easy to see but the other one you must remove the plug ins and bottom panel - use a long screw driver with blu tak on the end

it's quite a lot of work so go carefully

good luck!
cheers
Tony

Re: Tektronix 7623A poser
 

there is fuse 814 in the unreg +15V line to check as well - I think it's on top of the power supply board

Re: Tektronix 7623A poser
 

Yes, that fuse keeps blowing, which is what has suggested that Q1195 or some associated components (Q1190, C1198, L1198) may be at fault. This advice came from someone on Tekscopes, but they have all gone a bit quiet at the moment.

The low-voltage values are all OK, but the HV at -1475 V is absent (zero if I remember, but I can't power it up in its present state).

Another anomaly is that the voltage at the "collector" test-point on the storage board is -1.4 V, when it should be +150 V. This problem initially started out as an inability to use any storage mode, but has now turned out to be a complete loss of the beam. I have a transistor checker and was intending to check Q1195 without dismantling too much, but that looks like a dashed hope. I can see Q1195 on the HV cover, but I can't see the P1196 connector - I think it is in the HV box. I have purchased four 2N3055H power-transistors (which I think are the correct hometaxial types) from Farnell, as I have pretty-much convinced myself that this is where the fault of the lack of -1475 V comes from, if not also the inability to store anything.

Thanks for the disassembly info, the Service Manual is woefully vague about it.

Colin.

Re: Tektronix 7623A poser
 

fuse 855 for the +130V line is on the reg board above the psu. if you have the upright 7623 you undo 2 screws on each side and the whole heat sink assy comes out ( no need to remove the plastic cover) if that blows the +50V & -50V lines will also be wrong along with the +5V line so I think it's best to find out what is blowing that fuse first before removing the eht module. ( test for a short or low ohms at the output side of the fuse and if you get one look where the +130V line goes and pull off one connector at a time to isolate the area. I think only the Z axis board and the regs at a quick glance)

you can do a rough test on Q1195 in situ

Re: Tektronix 7623A poser
 

The 'collector' test point on the storage board is powered by the +600V supply to the storage board, dropped to +160V on the Zener VR1831. The +600V comes from a winding on T1199 which is the source of all the high voltages for the CRT. So if you have lost the -1475V and the fuse to the driver transistor keeps blowing then you will lose the +600V to the storage board as well.

I have been quiet over on Tekscopes simply because I have never stripped down the boards that Colin needs to get at and I know how awkward the access can be around the power supplies.

Sounds like Colin has some proper advice now.

Best of luck with the repair,

Roger

Re: Tektronix 7623A poser
 

ignore most of what I said Colin, I confused collector test point thinking you was talking about the switcher. I must be suffering a storage problem!

thanks Roger - I see where the collector test point is now!

Re: Tektronix 7623A poser
 

I have got into the HV module an all the transistors (Q1195, Q1190, Q1187) check out as good. However, the inductor L1198 (80uH) is open-circuit and has a cracked end where it contacts with capacitor C1198 (47uF), which itself shows a high resistance (several megohms) but a low capacitance of 13nF. I can get an 80uH inductors from Digikey and probably a 47uF, 25V electrolytic from my parts collection, but why would this all result in the 15v (unreg) fuse (F814, 2A) blowing?

Colin.

Re: Tektronix 7623A poser
 

Colin,

Is there any damage to the PCB? With the inductor open circuit everything else should be isolated.

My 7633 has a four layer PCB on the storage board with power traces on the internal layers, a right pain to fix when one of the high voltage transistors went s/c.

Regards,

Roger

Re: Tektronix 7623A poser
 

No obvious burn-damage that I can see, Roger. I have decided to buy some quality 80uH inductors and 47uF/25V electrolytic capacitors, but I will also try to lift one end of the currently fitted capacitor, as my measurements were decidedly weird to me.

Colin.

Re: Tektronix 7623A poser
 

Having now lifted the positive end of the 47uF capacitor (C1198) I can say that the resistance is a few tens of megohms (and rising) and a few nF. My Peak Atlas ESR70 doesn't recognise it as any kind of component. It looks like it needs replacing, doesn't it?
Maybe this was what was causing the storage not to work, too?

Colin.

Re: Tektronix 7623A poser
 

Colin,

If L1198 is open circuit and there is no board damage then I can't see why the fuse F814 bould blow, unless there is damage somewhere else. Can you measure resistance from the supply side of L1198 to ground?

Roger

Re: Tektronix 7623A poser
 

Of course, it is infinite. Probing around a bit at the earth/ground points, the pad that the negative end of C1198 is supposed to be soldered to doesn't have continuity to any other earth point (including C1193). It looks as though all of the other earth pads are connected together and to chassis. So perhaps a trace on the other side of the board has burnt out.

Is it possible that C1198 shorted, then took so much current (enough to blow F814) that it blew L1198 and then itself to lose all of its capacitance and become open circuit? Then the loss of continuity between the negative pad of C1198 and all other earth/grounds could be from burning-out the track?

Colin.

Re: Tektronix 7623A poser
 

Colin,

That sounds a very reasonable explanation, just a little bit of a coincidence that four points all blew open circuit at the same time (fuse, L1198, somewhere inside C1198, and a PCB trace). Did you replace the fuse and try a second time? That would give the already weakened 'weak points' a second chance to go fully open circuit. It would be worth checking that the pad at the earthy end of C1198 isn't now connected to anything else via some damage internal to thePCB.

Regards,

Roger

Re: Tektronix 7623A poser
 

Colin, did you see that C1193, C1198 & Q1195 all have a triangular symbol for ground? this goes back via pin 3 of the connector P1185, pin 3 of P1195 and pin 1 of P1180 to the power supply connector P870.

be careful that the connector P870 is correct because there is a mistake in the pdf of the manual that I Have where pins 1 & 2 both appear to be +15V unreg depending on which end you look. ( on diagram 7 pin 2 of P870 is +15V unreg whereas on diagram 8 pin 1 is shown as +15V unreg from the fuse). so check the orientation of P1180 and buzz the unreg +15V from the fuse to L1198 and the 0V from the psu to the triangle marked grounds on Q1195 etc

Re: Tektronix 7623A poser
 

A bit of a correction, here. This board is a tricky one to get meter probes to. The earthy ends of C1198 and C1193 are connected, but have no continuity to earth/chassis. Two other local capacitors do have their earthy ends connected and continuity to chassis; they are C1245 and C1250. It's looking more and more like a burnt-out track.

Colin.

Re: Tektronix 7623A poser
 

Many thanks for that, Tony. I did notice the triangular symbols on those three components, but not on P1195. I just thought that Tek had got a bit lazy and used the normal chassis symbols and the triangles to mean the same thing, but it was me who was the lazy one! I will check all that stuff out; maybe the board isn't damaged - it certainly doesn't have any burn-mark evidence from the top. It would be nice if this can be repaired with just two new components. The ones that I have on order are off eBay, but the capacitors are Vishay-BC, even though only 85 degrees, unfortunately (I would have preferred 105 degrees). The inductors are Fastron and are capable of 2.5A, so might give a little more leeway (they are 77A-800M-00).

Colin.

Re: Tektronix 7623A poser
 

I have checked the continuity of these:

C1193, C1198 and Q1195 do indeed go back to the power-supply and not to an earth-point in the HV module.

I have checked that the +15V unregulated supply is continuous from F814, P1180 and P1185 on the X-axis board, through P1195 and the +15V end of (the O/C) L1198), so I am guessing that the A10 High Voltage board (and its connection to the +15V unregulated supply connections are fine.

My paper copy of the service manual looks to be accurate with regard to P870 and I have not disconnected it from the A11 regulator board, nor have I taken the LV power-supply out of the mainframe, though I will eventually have to when I replace fuse F814. I suspect that the problem is that you are reading the pin-numbers on the two-pin connector P1180 as GND is pin 1 (or index) and at the other end of the cable, the three-pin connector P870 has +15v on pin 3, GND on pin 2 and nothing on pin 1 (or index).

Hopefully, I will be able to fit the new capacitor and inductor and put the whole back together. I certainly hope it will then work, if I am lucky? (I usually seem to have poor luck these days).

Colin.

Re: Tektronix 7623A poser
 

you may be right as I'm going up and down a pdf...

it's good you have continuity on the unreg +15V but you should also have continuity of the ground from C1193, 1198 & Q1195 back to the power supply ground

Re: Tektronix 7623A poser
 

I thought that was what I wrote first; maybe it wasn't clear. I checked that the "earthy" ends of C1193, C1198 and Q1195 all went via P1195, P1185 and to P1180. You had me doubting myself, there, so I connected up P1180, which goes to P870 on the A11 Rectifier Board and buzzed-out the earth continuity. Since I haven't unplugged the connections to P870, I think that buzzing-out to the frame of the LV Power Supply verifies the earth continuity.

Colin.

Re: Tektronix 7623A poser
 

Colin: do you have an orignal Tek paper service manual for this?

John

Re: Tektronix 7623A poser
 

Yes, I do, John. I have replied to your email re this. It is the proper 7623A paper Manual, too. If you recall, I bough this scope off you before you moved out of London.
Colin.

Re: Tektronix 7623A poser
 

Well, this gets curiouser and curiouser, as Alice might say. I have removed the inductor L1198 and it was fixed to the A10 HV board at three points! When removed, there are two wires at one end with continuity between them (about 5 ohms) and the connection at the other end is only going into the ferrite and shows no connection (infinity) to the two connections at the other end. The end where the connected wires emerge has some broken "glue" around it, which convinced me that there was some damage underneath that I couldn't see. Not so; it looks as though L1198 is fine, but C1198 is no longer a capacitor. I will put a bit more Araldite around the cracked end of the inductor L1198 and remove the capacitor to give it a fully out-of-circuit check.

Who would have thought that an inductor would have both of its connections at one end and a support-wire at the other?

I will also have to re-think the checking of the connections to the inductor; I may have assumed that the support-wire was a genuine connection.

Colin.

Re: Tektronix 7623A poser
 

OK, so the capacitor measures ~3000pF and more than 20 Mohms out-of circuit. My Peak Atlas ESR70 doesn't recognise it as anything. I have put a little Araldite on the end of the inductor to strengthen and stabilise the end winding.
I have checked the connection between the 15V unstabilised pad that the inductor was soldered to and the fuse (which is still o/c and in place) and there is continuity.
I think I may be good to go on refitting the old and new components once the Araldite has set hard.
Colin.

Re: Tektronix 7623A poser
 

I have found that there are numerous circumstances where the ESR-70+ and similar ESR meters can not be relied upon. Further, to be certain of what I've noticed I've removed numerous electrolytics which tested fine for ESR with the ESR-70+ but fail when tested for leakage at their working voltages using my Sprague TO-6 cap tester.

I've also noticed that if the cap under test in circuit has a parallel path to ground, such as a parallel cathode bias resistor of anything less than 3K ohms, the ESR-70+ reads the low resistance as leakage. You can throw a low value resistor across an electrolytic and check out how your ESR meter responds to the ckt for yourself.

Because of these and related issues with my ESR-70+, I believe that such testers might help in a rapid service application to eliminate potential problems, but are not up to snuff in serious restoration or complex repair work...Of course, your position may vary from mine....!!!

I seldom use the ESR-70+ anymore, and I've only had it for 4-5 months. I get
more reliable accurate information from the TO-6 and a digital multi-meter...

Re: Tektronix 7623A poser
 

That capacitance is way too low for the ESR70, 1 uF is minimum!

To somewhat defend the ESR, it is actually very useful in its own domain. Measuring 'in-circuit' is always a bonus and need to be interpreted carefully. Out of circuit I have found it accurate and quick. The only limitation is that you can't alter things like the ESR test frequency but given its tiny size I can forgive that :)

dc

Re: Tektronix 7623A poser
 

I have been hanging back on this repair for a while because I've been busy, but I have put the original inductor back in circuit and replaced the capacitor with a nice new one. I fastened everything back together and turned it on and got - nothing but a turning fan! Even the power light didn't illuminate! I have now checked over all of the harmonica-type connectors and couldn't find anything amiss, but the voltages that should be showing on the Z-axis board aren't all there. I will have another check on those voltages, but it looks like I have either an incorrect or missing connection somewhere, or a fault in the low-voltage power-supply. I was rather hoping that it might all re-start working as soon as I turned it on, but no such luck. I am very fond of this scope...
Colin.

Re: Tektronix 7623A poser
 

The voltages as indicated on the harmonica-connector on the X-axis board, were well out-of-spec. I thought that I might be able to check them at the harmonica-connector with it disconnected from the X-axis board, but a few seconds after turning the power on, there was a sharp crack, so I turned the mains power off rather quickly. The odd thing is that there is no magic smoke, no tell-tale smell and no particular excess temperature on any of the components that I can see on the LV Rectifier board, the LV Regulator board, the X-axis board or the HT power transistor.

What could I have done?

Colin.

Re: Tektronix 7623A poser
 

One thing that I certainly have done, I have referred to the _Z-axis_ board as the X-axis board. Significant typo, I think....Apologies.
I am now thinking that having broken the link of the "Limit Command" I could have messed something up.

Colin.

Re: Tektronix 7623A poser
 

Colin,

Sorry for not replying earlier, I didn't have the facts to hand and needed to check manuals. The 7623A power supply relies on sense signals returning from boards further down the chain of PCBs rather than the supply voltages as measured on the low voltage regulator board. So if you break the chain for the various sense lines then the regulators may try harder to increase their non-existent sense voltages.

I am afraid it is a bit tedious but you need to use the manual to trace the multi-way connectors from the low voltage board via the other boards to the main interface board, schematic <1> , which appears to be the source for all the low voltage sense returns. Any board in the chain before where you did the disconnection may have had an overvoltage which probably caused one of the electrolytics to pop. There are some notorious instances where some boards have decoupling capacitors with little headroom between their rated voltage and the voltage they see in normal operation.

I would suggest you most likely have a shorted capacitor pulling one of the supplies down but due to the way Tek designed the regulators with almost every one relying on a regulated voltage from one of the others you can lose most of the supplies on a single failure. If you isolate each of the boards in the low voltage supply chain and check each supply for shorts to ground then you should be able to narrow the problem down.

If your shorted cap has then taken out part of the regulator circuitry then you do the usual checks on unregulated voltages and check the transistors in the relevant regulator. The main power transistor and its driver are the most likely to have failed.

Regards,

Roger

PS I don't think disabling the Limit Command has done any harm unless you saw any very bright flashes on the screen.

Re: Tektronix 7623A poser
 

Check the harmonica connectors are not plugged in one pin out. It is *very* easy to do, and not immediately obvious in a cramped Tektronix scope.

I pretty much destroyed my 577 curve tracer that way - one of the power supply connectors one pin out, which shoved 100V straight up the 15V line. Fortunately all the dead silicon (lots of it, and every tantalum) was not custom and easily available - even the dual jfet's. It is more or less running now after lots of work, with a remaining gremlin in the step generator or amplifier. It works but only at a fraction of the step amplitude.

Why Tek did not use connectors with an indexing feature heaven only knows.

Craig

Re: Tektronix 7623A poser
 

Well, they sort-of did.
There are index marks on both the connector and by the pcb pins. I do take your point, though, that something a bit more fool-proof might have been a great improvement. Consider that there are six of these connectors on the LV regulator board and two connectors on the Z-axis board of the 7623A which are pretty tightly packed. No doubt there are other places on this scope and probably other devices with the same awkwardness. I suppose that Tek felt that the only people likely to fiddle with these connectors would be their own operatives, not us five-thumbed bumbling hobbyists. Another unfortunate thing is that P1170 on the Z-axis board has nine sockets on the harmonica connector but only eight pins on the pcb. Couple this with the fact that it is inboard of P1171, which has eight sockets and pins and obscures P1170. The pins can be a pain, too, they are often quite thin and easily bent out of alignment, which can cause them to be missed and bent out of place when plugging the harmonica connector in.
Still, this doesn't get my scope working. I have checked the positioning of these connectors a few times, now, but I am still searching. Wish me luck...
Colin.

Re: Tektronix 7623A poser
 

Tek did make some concession to us amateurs by colour coding the harmonica connectors with the resistor colour code of the last digit of the plug number ie P1170 is black Pxxx5 would be green etc.

Nevertheless the previous owner of my 7623A had managed to find two four way ribbon connectors that, despite being on opposite ends of the storage board, could be made to mate up the wrong way round! Fortunately no long term harm was done but it certainly confounded the problems caused trying to track down the storage issues (three failed 74121 monostables and one other TTL device).

Regards,

Roger

Re: Tektronix 7623A poser
 

I think I have all of the harmonica-connectors correct now, but I still have no graticule lights nor the "power on" light working. I have checked the voltages on the harmonica-connectors on the Z-axis board (P1170 and P1171) and they are showing some correct voltages, but not all of them:

-15V +/- 0.1 (-15.05V)
+50V +/- 0.3 (+49.76V)
+130V +/- 5.0 (+128.2V)
-50V +/- 0.1 (-50.04V)

The other voltages are off; +5V +/-0.1 (-0.6V), +15V +/-0.1 (-0.74V) but GND is 0.0V and the LIMIT CMD connector is showing +14.99V.

I could (and should) look in the Service Manual to see where the +5v and +15V rails get their regulation from and if they are related to the missing +5V and +15V, but if any of you wise folk know what might be wrong, it could save me from searching.

TIA, Colin. :dunce:

Re: Tektronix 7623A poser
 

Hi Colin
it's probably easier to just check the voltages directly at the regulator - this sits behind the tube. you don't have to remove the white plastic cover at the back as that only covers the power transistors. there are 2 small screws at each side that can be seen with the covers off. remove them and the board can be swung out of the back.

check that no connectors have fallen off of the board

the +5V needs the +15V to be running so check the +15V output on connector P900 pin 5 and go from there.

if you have the plug ins installed it's possible that a tant has gone short - this is very common. with the power off you can test for a short on the +15V line

the graticule lights lights need both the +15 and the +5V to operate.

cheers
Tony