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Old 20th Aug 2019, 12:48 pm   #21
MrBungle
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Default Re: Tektronix 7623A blowing mains fuses.

AFAIK this is a complete strip down job. Sorry for bad news. It looks like the intent of those power supplies was to actually be removable trivially but several design modifications later, no banana.

If you can gain access to the PCB at all without having to desolder anything you should be able to test the rectifiers and caps in circuit.

This is why I got rid of my 7603! I am afraid of effort these days
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Old 20th Aug 2019, 2:15 pm   #22
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Default Re: Tektronix 7623A blowing mains fuses.

This was on a 7633, but very similar to the 7623A apart from the extra option of doubling the CRT cathode voltage on the 7633. I also needed access to the psu but couldn't make enough space because the rear mounted BNC panel was on a very short lead. The troublesome lead was the remote erase for single shot operation and I traced its source to the storage board where the cable immediately disappeared underneath the board. Removing the storage board you could see the cable routed very neatly but not the shortest route. Wth this cable rerouted (above the storage board) I had enough room to get the PSU a bit farther out and then to remove some more ribbon cables and get decent access to the rectifier board.

As a side issue, I too had some dried out electrolytics but the easy way out is just to wire some (very much smaller) modern capacitors in parallel. It doesn't look original but it does work.

Roger
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Old 20th Aug 2019, 4:41 pm   #23
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Default Re: Tektronix 7623A blowing mains fuses.

I hope the power supply is not like some other TEK or HP stuff I have come across where each power supply is dependent on all the others working. Like no +5V unless -5V, +15V and -15V is working or no -15V unless +5V, -5V and +15V are working. This is simplified as there are usually more power supplies than just the four examples given. If you are into gaming then this is a dream else a ruddy nightmare, if the fault is not obvious like a burnt resistor etc. then best to try to emulate good inputs on each supply and check each supply comes up.
In my many years of servicing HP stuff, be wary of the little Blue ceramic decouplers that love to go either open or short circuit and which populate a lot of their vintage gear.
Believe it or not, that is a Golden tip I have given you!
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Old 20th Aug 2019, 5:34 pm   #24
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Default Re: Tektronix 7623A blowing mains fuses.

Quote:
Originally Posted by tony brady View Post
have you tested all the diodes in the rectifier? don't change it without doing this as it is rather hard. I've done one, not much fun
The bridge rectifier CR811 shows low resistance (short) between + output and both ~ inputs in both directions while in circuit. I'm guessing that means it has failed. Those caps that I can get access to show low esr, but "leaky or in circuit". I'm hoping that means that they are OK. The mainframe was made in Guernsey and has S/N 100195. It was owned originally by British Telecom.

Colin.
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Old 21st Aug 2019, 12:37 pm   #25
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Default Re: Tektronix 7623A blowing mains fuses.

After removing the bridge you might get more reasonable ESR on the caps.

I would try a resistance measurement, expecting to see the caps charge up giving me some confidence that they didn't short and take the bridge with them!

It's possible that it would be safe to apply some current limited DC as an alternative but that would need careful examination of the circuit ...

dc
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Old 21st Aug 2019, 2:09 pm   #26
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Default Re: Tektronix 7623A blowing mains fuses.

If the resistance of the transformer secondary is essentially zero then the bridge rectifier plus transformer secondary gives you two pairs of back to back diodes, one pair from AC to +ve and one from AC to -ve. So if you test using the diode test function of a DMM you should see the forward voltage drop but in both directions. If you see a very small voltage drop then would be sign of a failed diode.

Roger
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Old 22nd Aug 2019, 7:19 am   #27
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Default Re: Tektronix 7623A blowing mains fuses.

Please accept my profound apologies for the previous post, it is completely wrong! The transformer scondary makes a bridge rectifier look like two pairs of parallel diodes so always high resistance in one direction. It is a low resistance load that makes the diodes look as if they have failed on a simple resistance measurement. Not sure how I made this mistake but it is a bad sign of advancing years!

Roger
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Old 23rd Aug 2019, 11:39 am   #28
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Default Re: Tektronix 7623A blowing mains fuses.

I now have five of the bridge rectifiers from RS, so I can go ahead and snip the blown(?) rectifier out and check it somehow. I am beginning to think that the rectifier board is going to have to be removed using the procedure in the Service Manual, i.e. de-soldering the mains transformer leads. The connections that can be seen with the back panel removed look like they can be de-soldered and re-soldered ok using a de-soldering station ( I have a Duratool one), but the ones on the other side of the transformer (there are twenty of them!) might be a bit difficult to insert into the right holes when re-soldering. Does anyone have any observations on this? I had thought of removing the two hefty screws holding the mains transformer to the bottom of the framework and taking out the board and transformer as an assembly, but there is a fear that since the mains transformer is heavy, it would be easy to damage some of the still-soldered wires.

Another idea that has been suggested to me (thanks, John_BS) is to leave the duff bridge rectifier in place, but cut the electrical connections and to re-position the new bridge rectifier to a different location on the metal framework.

Colin.

Note to self: I should really cut down on my use of parentheses.
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Old 23rd Aug 2019, 11:53 am   #29
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Default Re: Tektronix 7623A blowing mains fuses.

I get the impression you might have to go through what I described in post#17.
It applies to almost anything on the big boards in TEK 7000 frames.
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Old 23rd Aug 2019, 12:56 pm   #30
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Default Re: Tektronix 7623A blowing mains fuses.

According to the manual, power supply removal is 'remove six screws and slide the power unit out and on to the bench. The oscilloscope can be operated with the supply on the bench.'. I have never been able to do this due to the limited movement in the multi way cables but with all cables off except the mains cable it comes out a fair way and you would get much better access to the rectifier board.

Roger
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Old 23rd Aug 2019, 1:34 pm   #31
tony brady
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Default Re: Tektronix 7623A blowing mains fuses.

this is right, remove the 6 screws but I think it's 3 cables need to come off. a 2(or 4?) way going to the interface backplane board ( left side when looking from the rear) another on the right side and a 10 way or so going to the regulator board above - take pictures of these so you get the orientation correct ( not easy to see the marks on the board)

there is no need to remove the board from the transformer. remove the bolt through the rectifier and then snip the leads. clean the holes well and then the dexterous part comes in of bending the board up to get clearance to solder the rectifier. I might have undone something else when I did the 7603. cut the leads down to make it easier to fit to the board ( not too short!!)
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Old 26th Aug 2019, 6:45 pm   #32
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Default Re: Tektronix 7623A blowing mains fuses.

Quote:
Originally Posted by tony brady View Post
Clean the holes well and then the dexterous part comes in of bending the board up to get clearance to solder the rectifier. I might have undone something else when I did the 7603. cut the leads down to make it easier to fit to the board ( not too short!!)
Yes, Tony, I think you probably undid the two screws that fasten the power transformer to the power-supply metalwork. I have just snipped the bridge rectifier leads, removed those two screws after taking off the rear cover and after wiggling/rerouting some wires, the whole board, including the mains transformer, lifts sufficiently to slide out the bridge rectifier.

I have checked the rectifier and it does seem bad. So now to clean up the holes and mount the new rectifier. I have made some checks on the capacitors and they all seem good, but I will probably try lifting a few component leads to get some more reliable results. I will also check the ripple, so long as my repair of the blown rectifier gives the result that I want.

Colin.
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Old 26th Aug 2019, 10:47 pm   #33
tony brady
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Default Re: Tektronix 7623A blowing mains fuses.

hope it goes well Colin, the pth take a bit of clearing. obviously if you pull the modules out you'll get better cold readings. I tend to test with just the timebase in in case of disasters!

cheers
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Old 3rd Sep 2019, 8:58 pm   #34
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Default Re: Tektronix 7623A blowing mains fuses.

I believe that this scope is working fine again!

I had to wiggle some of the cables attached to the power supply a bit and one of them had to be unplugged to give a bit more room. With the cover which holds the output signal BNC connectors unscrewed, I was able to remove the two long screws holding the mains transformer down and when the four screws attaching the rectifier board to the power-supply metalwork were also removed, it was possible to raise the rectifier board so that the new bridge rectifier could be put into place. It was obviously necessary that this bridge rectifier was correctly aligned so that the legs could be soldered to the correct pads, but that wasn't too difficult. Replacement of the mains transformer attachment screws meant making sure that the insulating sleeves weren't lost or damaged. It wasn't too difficult to ensure that the plated-through holes were cleaned out by use of a desoldering station, checking that the legs of the replacement bridge rectifier could be pushed through comfortably.

The job wasn't too arduous and with the leads of the power-supply long enough to gain reasonable access, it was quite possible.

My concern that I had connected something incorrectly, or that there might be another fault, lead to my relative slowness at applying power, but all seems to be well.

Many thanks to everyone who gave me advice - I owe you!

Colin.
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Old 3rd Sep 2019, 9:11 pm   #35
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Default Re: Tektronix 7623A blowing mains fuses.

Nice work. Good to hear it worked out.
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Old 3rd Sep 2019, 9:35 pm   #36
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Default Re: Tektronix 7623A blowing mains fuses.

Well done, a lot of these jobs turn out to be less of a hassle if you have anticipated the problems beforehand.

Roger
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