Tektronix 7623A blowing mains fuses.

[ColinTheAmpMan1]

This is sort-of connected to my thread on my 7A26 with low gain. I had changed a resistor in the 7A26 and plugged it back in to my 7623A mainframe. The gain had been improved, so I was going to change the same resistor in the other channel. I turned power off and on again and the mains fuse blew. I replaced the mains fuse with another, which also blew. I then tried removing all of the plug-ins and powering up using a current-limiter. The bulb glowed brightly, so I thought it would be good to check a few more things. The resistance between both line and neutral pins on the IEC connector and earth was high with the power switch off, so any mains conditioner present wasn't to blame. When the power switch was on, there was again high resistance between line and neutral pins and earth, so no short to earth seems likely. However, with the power switch on, the resistance between line and neutral pins was 6R. Would this be normal (I can't find any data on the mains transformer) or not? Am I looking at a blown mains transformer? I hope not.

Colin

[tony brady]

Hi Colin

check the bridge rectifiers on the secondary side. particularly the one for the 15V supply. I had a shorted one on one of my 7603 and that blew the fuse. ( also check the supply lines for shorts to 0V)
I'm not near my 7623A at the moment so cannot measure the primary resistance but it sounds plausible

cheers
Tony

[ColinTheAmpMan1]

Hi Tony,
I have partially got the power-supply out (there are some taut wires and a couple of other obstructions). On looking in the Service Manual, the +/- 18 V bridge rectifier (which I think is the rectifier which eventually gives the +/-15 V regulated supply) is the only one which isn't a commercially-available type. It seems it was made by Tektronix and has the P/N 152-0406-00. It is described as "Semicond Device: Silicon, 200V, 3A". Assuming this particular device isn't easily available, is there an equivalent, even if a bit of lead-bending is necessary?

If you can measure the DC resistance of your mains-transformer, that would be good. My transformer doesn't look as though it has been damaged, but who knows?

Incidentally, the scope-cart cleaned up nicely.

Colin.

[MrBungle]

If it’s the square ones with 4 pins, Vishay do equivalents (KBPC series) for about a quid delivered from RS. 6A ones are cheaper than the 3A ones though. Check package dimensions but should be a drop in replacement.

Edit: RS PN 2278491

Absolutely notorious for blowing up the original ones are. Check the filter cap after them though. That may have gone high ESR to cause this to pop. I am suspect about putting a 6A one myself as that will probably survive longer than the transformer secondary!

[tony brady]

as MrBungle says. I changed all the large electrolytics on the 7603, painful job - nearly all were duff

glad to hear that the cart cleaned up well - must have been quite difficult getting it back out of the car!

[ColinTheAmpMan1]

Ok, I'm getting a bit frustrated trying to get CR811, the 18V bridge, unsoldered. It appears to have had its legs inserted through blank holes in the board, bent over to lie flat and pushed through holes with solder-pads, then soldered. My trouble is that I can't get the soldered holes de-soldered with my solder-sucker, possibly because there is a fair bit of oxidation on the soldered joint. Attempts to melt the solder and then mechanically pull the lead out of the hole just look as though I'm lifting the solder-pad with the lead. I think that could be close to disastrous. I realise that I could just assume that the bridge redtifier is toast and cut the legs off to extract it, but that goes against my nature. Does anyone have any hints or wrinkles on this job? I'm guessing that if there are any duff capacitors on this board, removing them will be a horrible task, too.

Colin.

[tony brady]

cut them one by one, it's makes it easier. be careful not to wreck the plate though holes

[MrBungle]

100% definitely cut them. Agree.

Motivational photo - here's a Tek board I found where someone tried to desolder the tants to reuse them...



Had to relocate the tant...



Still makes me feel dirty knowing that's in there.

[ColinTheAmpMan1]

That makes some sense, but I am a bit puzzled as to how I will remove the bridge rectifier once I've loosened the grip of the solder. The rectifier is bolted through the board into the metal framework of the power-supply. I have (with a bit of wiggling, the power-supply is still connected to the rest of the scope by various interconnects, the ends of some which I can't see to get at easily) removed the four screws fastening the A11 low-voltage regulator board but it remains gripped by something else, apart from the mains transformer wires. The CR811 is thus fairly well trapped between the board and the metal framework. Tek didn't set out to make this easy, did they?

Colin.

[tony brady]

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

[tony brady]

removing the caps is definitely not easy. cut the mounts and the legs to avoid stressing the board. but before this you need to find the short. it's not the easiest board to work on

[MrBungle]

I think being difficult to get into appears to be a tek feature. Their industrial design is utter rubbish (replacing 4x5 multipliers is not much fun)

[tony brady]

they're alright while they're working but keeping them like that is another story!

[tony brady]

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

[M3VUV51]

I think most Tek gear is total Bar Stewards to work on. I would imagine their repairmen spent one week in four in a looney bin after the trauma!!

[Skywave]

Tek. equipment being difficult to do maintenance on: agreed, absolutely.
And I've always wondered why. Was it simply because their method of construction and assembly was chosen to minimise the cost of all that, with zero regard for maintenance? ("We don't want people to repair our kit: that way we won't make any money. We want people to buy a new one!")
Moreover, IMHO, Tek. are not alone with that approach: most Marconi kit also similarly qualifies, although in their case, it's always seemed to me that the 'front panel appearance' seems to have taken a disproportional preference over maintenance and even style of construction.

Of course, others here may disagree with some - or all of that: it's simply my opinion based on a good deal of experience.

Al.

[Refugee]

Faults on the boards in 7000 series frames are a right pain.
When I worked at a service center I had to fix one that had bounced two times with an intermittent fault. I got a faulty reading on the control panel and a good reading on the board that did the function.
There was at least a foot of track over at least two boards to trace through and then a frame strip down to get to the other side of the board to solder the other end of the resistor lead clipping I used to fix the via hole once I had found it.

[Chris55000]

Hi!

At the risk of incurring everyone's wrath, I'd love to have a fault like that on a piece of Tek gear where I'd have to open a printed copy of the S.M. and dismantle it down to the last washer, nut and bolt!!

I've got a 465B and DM44 that only shows a faint haze on the CRT plus "3.3.3.3" on the DM44 bit!!

Chris Williams

[MrBungle]

Been there. You don’t want to have to do it. It’s hell. I replaced the HT transformer on a 465. And put it together properly. Then I found the damned attenuator switch was shot on channel A. That lurked in the cupboard for almost two years being scavenged for parts. I finally broke it down into parts and stripped everything off it a few days ago.

I now have a Rigol DS1054Z and a Philips PM3217 and boxes of tek parts to get rid of (and a 475 parts mule that is spoken for) and I’m not buying any more scopes (until next time ). I have to commend Philips - the scope is dead easy to work on.

[ColinTheAmpMan1]

This is all very well, guys, but I still have the problem of testing/replacement of the possibly blown bridge rectifier (and other components) and perhaps esr testing of the capacitors, but I can't get at them! I am very fond of this scope and want to get it back into good health to check the "repaired" 7A26, at least (I don't have another 7000-series mainframe).

Colin.