Tektronix 7603 won't trigger

failure in one of the X-driver transistor tails?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

"N_Cook" wrote in news:gv0pjf$tqr$ snipped-for-privacy@news.eternal- september.org:

Or repaired. get a 7B53A service manual.

7600's are known for cracks on the regulator PCB,around the TO-3 studs where they solder to the PCB pads. also,aging 7600's have PS filter cap problems.

also,metal film resistors in the output amps often change value or open.

You have to check for a ramp coming out of the 7B53A,need that for a sweep on the display. It would help to have a different timebase to swap in to see where the problem is. Or you could put one of the vert PIs in the H slot and feed in a ramp or triangle waveform from a function gen. even a sine wave would work,to see if the MF is OK.

it's also possible the sweep control IC,155-0049-xx has failed.

FYI,you can check date codes on various ICs and other parts to see when your scope was made. format 8644 would be year 86,week 44

check a couple of PCBs because board-swapping is common.

See if you can borrow another timebase locally... That would be a quick diagnostic. If the timebase module is what is bad (probably is, but that's a WAG!) then you can get 'em on eBay for a low amount. I could even be convinced to send you one though shipping would be expensive...

Well, I got hold of a service manual and traced through the timebase with another working scope. Turns out the fault is on the horizontal preamp board, specifically one or both of the CA3046 xsistor arrays. I found the CA3086 as a readily available replacement in a cross ref, so I'll see how that goes.

Btw, the 7B53 isn't exactly cheap in Germany (not that anything else is). You can expect to pay up to 5-fold compared to the States for a used plugin on eBay.

Well, I replaced the CA3046s with a pair of CA3086s. Triggers fine now. Having said that, there's some noticeable noise (as in flickering) in the trace. I checked the power supply caps with a capacitance meter, and they look ok (so do the supply lines measured with another scope). Any ideas what else I should check?

Regards,

--Roland

FWIW, some 7603s were for sale for $10 at a local hamfest a few years ago and I believe that all of them had a bad can electrolytic in the power supply; the one I bought sure did and replacing it brought the scope to life but I too have complaints about noise (not flickering) on the trace with no input (7A18 dual trace vert). I have not investigated any capacitor issues in the plugins or elsewhere in the mainframe. Triggering worked for me.

Michael

Yep. I have a Tek 7623A storage scope, with 7A12 vertical and 7B53A sweep plugins. Lots of tiny push buttons to keep me entertained. However, I had to go through a pile of about 10 such mainframes before I found one that didn't have blown electrolytics in the power supply. I dragged home the one and only working one from the pile, ran it for about 2 months, and the canned electrolytics blew up.

Moral: If the cans are not blown now, they will be shortly. Pre-emptive replacement might a useful exercise.

Jeff Liebermann wrote in news: snipped-for-privacy@4ax.com:

heh,more than 10 years ago,I was replacing 7600 series PS filter caps because they were drying out/low capacitance.(at TEK-Orlando)

there's also some electrolytic caps on the mainframe interface board that affect PS operation. Also,the studs that the TO-3 xstrs screw into develop cracks around the solder pin(from thermal cycling).I used to file off the chrome plating on the pin,then resolder them all,and the socket for the transistor array on the +5V supply gets removed and the array soldered in.(TEK official mod)

One real worry is "gold-dot" connectors on the ribbon cables; there was a mandatory mod to replace all G-D ribbon cables on all 7000 series scopes that had them. they caused a lot of intermittents. they can be ID'd by the pale appearance compared to the good connectors. you cannot get new TEK ribbon cable assys anymore.

for 7600s that use spec analyzer modules,there was a mod that added a fan on the back,a larger shield over the heat sink to accomodate the fan,and changed a current limit resistor on one of the supplies. (by paralleling another R across it) (can't recall which,though...a current schematic should have it ID'd)

You also have to check the ESR of the electrolytics with an ESR-Meter. Often elytics have still their capacitance but the ESR goes up and than they are nearly functionless.

Jorgen dj0ud

Hmmm, even if I had an ESR tester at hand, I wouldn't have the reference values. Is there a "sane" range for these big-ass caps?

--Roland

Jorgen Lund-Nielsen wrote in news:h1fftq$5gc$ snipped-for-privacy@it-news01.desy.de:

on the 7600 scopes,I just checked the unreg voltages across the PS caps. If it's low,I replaced the bad filter caps. A scope will also tell you if the caps are bad.(ripple voltage)

Hi Roland,

the most ESR-Meters have a reference list printed on the housing as you can see here on some pics:

Jorgen dj0ud

Thanks for all the replies! I did check for ripple with a working scope, and found a few lines with obvious sine waves (like 4Vpp), but I understand the PS also has unregulated outputs, so I didn't investigate any further. The cap voltages themselves checked OK with a voltmeter.

Jeff, when you say the caps "blow", you mean they literally explode? :^O

The caps on the interface board would be the next suspects, if they weren't so tough to get at (on top of which I'd have to lift them out at one end to check).

This particular 7603 has the fan mod, btw. The TO-3s show no signs of cracks, and I didn't find the gold dot connectors you mentioned. I guess this is a revision. Mainframe serial# is B377720 with option 03, which according to the service manual means EMI shielding, though I can't find the bits mentioned there. Oh, and it has the readout. Trace is mushy and the scope stinks. ;^)

Regards,

--Roland

If the caps are bad, the voltage will be temporarily within limits, but the ripple voltage will be rather high. That causes blurred traces that can't be adjusted out with the focus and astigmatism controls. Use another scope to verify that you're getting DC on the PS voltages.

No. They got hot and eventually blew a series resistor or fuse. I forgot which. I got lucky and caught it before it destroyed any active components. The bakelite bottom of the can capacitors were obviously bulging and had turned from brown to black. My capacitance and ESR capacitor tester both showed that they were under value and with a very high internal series resistance.

I won't mention what I used to replace the caps, or how the replacements were installed. I'm not very proud of the work.

If it were easy, it would be no fun. If you don't want to do it twice, just find the replacement caps and install them instead of testing the originals. This way you're sure that you won't need to dive in again later.

The stink is a clue. It's probably boiled electrolyte.

Jeff Liebermann wrote in news: snipped-for-privacy@4ax.com:

I've seen 7600 filter caps leak electrolyte,all over the cap/rectifier board.

Well,if you can't get direct replacements,then you go with the next best sub. ;-)

I've seen all sorts of customer "repairs".

FYI,you can tell the approx. manufacturing date by checking date codes on several components,like filter caps,power xfmr,and ICs.

I'd say your 7603 is a fairly "new" one,that's a high serial number.

snipped-for-privacy@4ax.com:

Well, I see what *could* be date codes like 8352 and 8413 on a few trannies, and 8439L on a PS cap. I can't believe they built these clunkers well into the mid-80s! Everything about this scope screams

The stink is more like stale cigarette smoke, which is a lot worse than electrolyte. I know electrolyte when I smell it.

PS cap replacement is a challenge given the fact none of the readily available replacements I found have matching pin configurations (plus is usually centered rather than offset to one side). Sprague apparently had their own system, and bending pins is not an option here.

Regards,

--Roland

Oh, mine says "inspected by Jerry". I guess Tom had the day off. :^D

--Roland