Tektronix 2235 problem

[lee_retzkin]

I recently acquired a tek 2235 oscilloscope that shows only a flat line. Both channels give a good flat line trace. Focus and intensity are good. Both traces can be moved up and down/side to side. I've set the unit up per the manual's instructions. All of the low voltages are there. The output of the vertical amp has the correct voltages. Any ideas on where to look next would be welcome. I know I can fix this, I'm just inexperienced with scopes. Thanks.

Lee

 

[lee_retzkin]

Oh and yes, I've tried the scope calibrator with a probe and a signal generator input. No dice.

L

 

[alec_t]

Obviously you've checked that the channel volts/div switches aren't set to 'GND' ?

 

[KeepItSimpleStupid]

dumb question: If your using a probe, is there a switch on the probe? Is it in the GND position?

 

[lee_retzkin]

I've tried AC and DC coupling. The probe I'm using has a 1x/10x switch, but's that's it. If I had a 2nd scope, this might be easier. It's an unusual problem. Could be an open solder joint somewhere, but it's strange that both inputs are zilch. Gotta love dem ebay specials.
L

 

[alec_t]

Given that you can adjust the vertical positions of both traces, at least the output stages of both Y amps would seem to be working. So it looks as though there is discontinuity between the probe inputs and the amps, or input stages of the amps are dead. Have you had the covers off the scope to check for broken/disconnected wiring? Do you have a schematic?

 

[KeepItSimpleStupid]

Check the probe with an ohmmeter. About 100K (I think) in x10 and very low in x1. Also make sure it's not shorted.

 

[lee_retzkin]

I took the probe to work today; a TEX150 and connected it to a 2445 we have sitting around. I was able to get the square wave calibration on all the channels. Also played around with the adjustment in the probe itself. I remember something from the 1st day I got this thing that might be a clue: it behaved the same way when I plugged my signal gen into the ext. input and switched sources. Since the external input goes directly into the trigger and it behaves the same as channels one and 2, it's logical to do some snooping in the trigger circuitry. Maybe my problem is there. One thing is certain: I'm getting to know scopes real well; a little frustrating, but fun. That 2445 feels like it's a lot better built than the 2235, probably much more complex inside, but what i have is good enough for now. Onward and up.

L

 

[lee_retzkin]

I have the tektronix service and user manual. So far, all my interconnections appear to be good.

 

[KeepItSimpleStupid]

Since your new with scopes, can you take a pic of the controls as you have it set up?

Will a DC voltage like from a battery move the trace?

With the scope at work, you might be able to do some signal tracing, but at this point it's odd because of both channels. You can trace with DC as input too. Even you presence and absence of the signal.

As always, you should always check the power supplies.

 

[KeepItSimpleStupid]

I think the 22xx series is the series with the U800 failures, but that's a horizontal IC, so it should not be your problem. It's important to keep U800 cool with the scope opened up. There are some fixes to better heatsink U800.

One guy has made a replacement board for U800.

 

[DerStrom8]

As requested already, a photo of the front panel of your scope would be extremely useful.

It could be as simple as a poor connection on the BNC jacks. If it's corroded, for example, it may not make good contact with the probe. Then again, it's also possible that it's something internal--a blown trace on the PCB, perhaps (I've seen that happen before). Would you be comfortable opening up the scope and snapping a few pictures of the inside as well as the outside?

Regards,
Matt

 

[lee_retzkin]

I'll get a pic out soon. Actually, I haven't represented myself all that well because I'm new to this forum. I'm kinda green on fixing oscilloscopes but i've used them on and off for at least 30 years; in school and later in aerospace. I've also used spectrum analyzers, function generators, various power supplies, etc. I'm also a radio amateur. At home today I wasn't in the mood to sit at it after 8 hours of work, but i did take an ohmmeter and measured from the center pin of each bnc connector to the attenuator board side of the 1st resistor: 62 ohms, which is what i should have. The scope is open; I've already checked all the ps voltages; they're all there and the voltages going to the vertical deflections plates are exactly what they should be. The scope was set up initially per the textronix manual. Yeah, something is open somewhere. I'll find it; it's a matter of principle at this point. I appreciate all of your ideas; they are helpful and it's nice to know you care. Sleep time; work in the am.

L

 

[rumpfy]

Congratulations on your purchase. I just love cro's and those who buy one know their value.
Can I suggest the problem may be in the switching of the vertical amp outputs. These cros have a single gun tube but have a high speed switch at the input to the vertical deflection system. The switch selects either channel or does an 'alternate' scan or 'chop' between the two vertical channels to give a 'dual trace' facility.
I have a block diagram; Fig 9.4. It shows in block 2 that there are inputs from 'a switch?' which has; Ch1, both, Ch2, ADD, ALT, Chop.
Fig 9.5 is the 'Detailed Block Diagram'. It shows the position control acting on Block 2. So maybe Block 1, the attenuators, are the problem.
Fig 9.5 says that the outputs from the Ch1 and Ch2 vert pre-amps (U 130/U180) goes to the channel switch (CR 200/1/2/3; Q440 and Q441).
I reckon the problem is there, but against this, if the position controls work then it may be the supply to Q102/103 and Q152/153. This is a+/-8.6 volt supply.
Hope this helps.

 

[lee_retzkin]

I knew I was taking a risk buying an ebay "powers up, no way to test" item, but the traces looked good and the manuals were included, plus it's a military version, so I went for it; if nothing else for grins. It's turning out to be money well spent because i'm getting the royal tour of the innards of this character; kind of like peeling an onion. It's a good, albeit frustrating education. I will investigate the areas you've outlined and report back. Ebay stuff can be very problematic because the vast majority of the time the buyer hasn't any idea of the history of item. Who knows where my little friend has been? Channel 1 might gotten blown, then down the road, 2 got whacked. Question: Shouldn't the external input show something if a source was plugged in and the trigger was switched to ext? Seems like this input isn't used that much. Anyways, onward and up.

L

 

[KeepItSimpleStupid]

Trigger usually has Auto, Norm, Ext, (line)

Auto - It's really used to get a trace. In the absence of a trigger event, it will create one, thus the trace may not be stable (rock solid). So, with a zero input, you get a line.

Norm/Internal - This is where, the slope, and level controls work.

Ext - an external signal (slope and level) work on that too.

Line - is the power line frequency,

 

[lee_retzkin]

So if pumping a signal into the ext connector gives me zilch as well as ch 1 and 2, is this another clue or am I going down a dead end?

 

[KeepItSimpleStupid]

Totally ignore EXT for now.

You should be able to trace a DC signal through the scope. Just make sure your set to DC coupled. Trace the vertical amplifier.
Use presence and absence of the signal. Binary search.

Still wait for a pic of the front of the scope/front panel in the mode your using it with the calibrator.

 

[JimB]

You should be able to trace a DC signal through the scope.

I understand your thinking, but just looking at a DC level it will be hard to decide if the signal is there or just the quiescent operating conditions of the circuit.

Much easier to follow a sinewave through the circuits.

JimB

 

[KeepItSimpleStupid]

Yea, that's why I said to use PRESENCE and ABSENCE of the DC input signal. it's time varying, sort of.

His "other scope" is "at work",