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Old 28th Oct 2020, 3:53 pm   #21
John_BS
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Default Re: Tektronix 7623A poser

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

John
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Old 29th Oct 2020, 11:24 am   #22
ColinTheAmpMan1
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Default Re: Tektronix 7623A poser

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

John
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.
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Old 29th Oct 2020, 7:24 pm   #23
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Default 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.
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Old 31st Oct 2020, 7:45 pm   #24
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Default Re: Tektronix 7623A poser

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. 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.
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.
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Old 5th Nov 2020, 7:11 pm   #25
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Default 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...
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Old 6th Nov 2020, 11:47 am   #26
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Default 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
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Old 1st Dec 2020, 2:18 pm   #27
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Default 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.
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Old 3rd Dec 2020, 7:41 pm   #28
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Default Re: Tektronix 7623A poser

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. 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.
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.
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Old 4th Dec 2020, 7:34 pm   #29
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Default Re: Tektronix 7623A poser

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Quote:
Originally Posted by ColinTheAmpMan1 View Post
. 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.
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.
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.
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Old 5th Dec 2020, 1:08 pm   #30
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Default 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.
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Old 5th Dec 2020, 2:07 pm   #31
Craig Sawyers
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Default 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
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Old 8th Dec 2020, 4:52 pm   #32
ColinTheAmpMan1
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Default Re: Tektronix 7623A poser

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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
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.
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Old 8th Dec 2020, 5:58 pm   #33
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Default 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
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Old 10th Jan 2021, 3:16 pm   #34
ColinTheAmpMan1
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Default 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.
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Old 10th Jan 2021, 4:20 pm   #35
tony brady
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Default 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
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