Tektronix 475A Display problem [Horizontal?]

[(*steve*)]

That last image looks almost exactl like what you'd get if a regulator was falling out of regulation (you say it's unregulated though).

You get a fixed voltage until it falls -- and the dropoff looks almost exactly like a discharging capacitor. Then the charging side looks exactly like the leading edge of a sine wave.

Go back a little more and see if there is a bridge rectifier. If there is, check it. This looks a bit like your full wave rectification has gone half wave.

 

[MattyMatt]

That last image looks almost exactl like what you'd get if a regulator was falling out of regulation (you say it's unregulated though).

You get a fixed voltage until it falls -- and the dropoff looks almost exactly like a discharging capacitor. Then the charging side looks exactly like the leading edge of a sine wave.

Steve,
yup,the last image is the one that IS on the regulated line (+15V) the middle image is the unregulated (or what I think to be) line. I misquoted which images were which, but the actual image title is correct.

I was having a similar discussion with my friend and his father... they both also suggested the bridge rectifier... which I do have. My problem is that I don't know if I would be able to test it in the circuit itself... and removing that thing is going to be a royal pain.

At any rate, I should have the big capacitor soon, which the old one will probably have to come out for me to remove the bridge rectifier.

If there is a good way that I can test this in the circuit, please let me know.... I can get to all of the points on the thru-hole side of the board.

 

[(*steve*)]

Remove one lead from the transformer and see if you can see diode behaviour between each leg of the rectifier.

Using a diode test function you should see the forward voltage drop on each diode. In the reverse direction you will see another diode forward biased in series with the filter caps, so the voltage will slowly rise.

If you don't remove a wire to the transformer you can miss an open circuit diode, or have 2 appear as shorted. Removing 1 DC connection would be ideal, but probably difficult. If you're going to do that you may as well remove the bridge rectifier.

 

[MattyMatt]

Got the Big capacitor today in the mail! so now I should be able to go after the bridge with no problem and replace both at the same time....

Just for giggles I attached a picture of this mammoth now I've worked with 1F caps before in car audio... but I've never opened a piece of equipment to see anything this large before... I dunno just thought it was cool.

Matty-

 

[davenn]

Got the Big capacitor today in the mail! so now I should be able to go after the bridge with no problem and replace both at the same time....

Just for giggles I attached a picture of this mammoth now I've worked with 1F caps before in car audio... but I've never opened a piece of equipment to see anything this large before... I dunno just thought it was cool.

Matty-

yeah those 1F caps for car audio systems are physically pretty big huh. never had the need to work on any of them.

just a random thought about old caps, many yrs ago "in another life" I used to do a lot of old valve radio restoration. we always wanted the radio to look as original as possible and as you could imagine the electros of values like 10uF and 350V were just no longer available in the required style.
We (many of us) used to hollow out all the gunk from the old aluminium can and mount more modern caps inside then seal them up. once remounted on the radio chassis you would never know it wasnt an original cap

cheers
Dave

 

[MattyMatt]

Update:

So after a fun evening with my friend (who has a slightly better soldering/de-soldering technique than myself), we managed to replace both the bridge rectifier, and the GINORMOUS capacitor. however, when I was getting some of the camera shots in the previous post, I blew the mains fuse... so I stopped out and got some today.

So I popped in the fuse, plugged in the scope, crossed my fingers, and pulled the power knob.

To my pleasure the scope lit up, and I had a halfway normal looking trace.

I did a few adjustments with the astigmatism and the trace rotation, got the positioning correct and boom! it works.... WONDERFULLY.

I attached some pics of the scope, now appearing to be in complete working order... now just to get it calibrated (or the tools to do it myself!)


I want to thank all of you for helping me, and my friends with the troubleshooting process... I have learned quite a bit from this experience, and I greatly appreciate everything that was done.

The total cost for the scope, parts and shipping was $205 and honestly, its some of the best money I have spent... especially on test equipment, and it was a wonderful experience...
Thank you all !

 

[davenn]

outstanding, well done

hopefully it will give many more yrs of service

Dave

 

[(*steve*)]

Great outcome.

Did you test the bridge rectifier and the filter capacitor after you removed them?

Even though you've fixed it, it is always useful to know what failed.

 

[MattyMatt]

I believe it was the bridge rectifier, the cap seems fine, but I don't have a great way of testing it. From what I was reading on a few different posts about bridge rectifiers, I was getting some wacky results:

Negitive lead of DMM on: DC+ side
AC1: 5.90 Mohm
AC2: 4.30 Mohm
NEG: 1.3 Mohm

Negitive lead of DMM on: AC1
AC2: INF
POS: 4.39 Mohm
NEG: INF

Negitive lead of DMM on: AC2
NEG: 4.40 Mohm
POS: INF
AC1: INF

Negitive lead of DMM on: DC- side
POS: INF
AC1: INF
AC2: INF

Now all I have to do, is figure out if one of my old signal generators works, and I'm good to go

 

[(*steve*)]

I'd try a diode test function if you have one. Your meter may use a very low voltage on the ohms range.

 

[MattyMatt]

Okay, so, the DMM is now set to Diode testing mode all measurements I am assuming are in volts.

Negitive lead of DMM on: DC+ side
AC1: .508
AC2: .493
NEG: .935

Negitive lead of DMM on: AC1
AC2: INF
POS: INF
NEG: .492

Negitive lead of DMM on: AC2
NEG: .503
POS: INF
AC1: INF

Negitive lead of DMM on: DC- side
POS: INF
AC1: INF
AC2: INF

 

[(*steve*)]

OK, those readings are consistent with the bridge rectifier being OK. I guess it's possible that it fails under load, but that seems unlikely.

The output looked every bit like half wave rectification and I was sure you were going to see one diode open circuit.

Perhaps it was the capacitor.

In retrospect I should have asked you to determine if the frequency of the ripple was your line frequency, or double your line frequency.