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Old 16th Oct 2020, 9:41 am   #1
ColinTheAmpMan1
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Default 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.
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Old 16th Oct 2020, 10:14 am   #2
tony brady
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Default 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
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Old 16th Oct 2020, 10:33 am   #3
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Default 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
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Old 16th Oct 2020, 2:22 pm   #4
ColinTheAmpMan1
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Default 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.
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Old 16th Oct 2020, 4:13 pm   #5
tony brady
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Default 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
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Old 16th Oct 2020, 5:20 pm   #6
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Default 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
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Old 16th Oct 2020, 6:26 pm   #7
tony brady
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Default 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!

Last edited by tony brady; 16th Oct 2020 at 6:34 pm.
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Old 17th Oct 2020, 6:00 pm   #8
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Default 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.
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Old 17th Oct 2020, 8:06 pm   #9
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Default 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
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Old 18th Oct 2020, 11:56 am   #10
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Default Re: Tektronix 7623A poser

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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
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.
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Old 18th Oct 2020, 1:14 pm   #11
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Default Re: Tektronix 7623A poser

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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.
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.
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Old 18th Oct 2020, 5:24 pm   #12
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Default 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
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Old 19th Oct 2020, 3:09 pm   #13
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Default Re: Tektronix 7623A poser

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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
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.
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Old 19th Oct 2020, 4:29 pm   #14
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Default 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
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Old 19th Oct 2020, 4:33 pm   #15
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Default 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
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Old 19th Oct 2020, 4:33 pm   #16
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Default Re: Tektronix 7623A poser

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Quote:
Originally Posted by RogerEvans View Post
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
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.
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.
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Old 19th Oct 2020, 6:24 pm   #17
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Default Re: Tektronix 7623A poser

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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
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.
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Old 20th Oct 2020, 4:51 pm   #18
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Default Re: Tektronix 7623A poser

Quote:
Originally Posted by tony brady View Post
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
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.
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Old 20th Oct 2020, 5:10 pm   #19
tony brady
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Default 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
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Old 21st Oct 2020, 3:48 pm   #20
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Default Re: Tektronix 7623A poser

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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
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.
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