Funny thing - it turns out that the Protek site has a P3502C service manual! It's the same scope as the P3502 except for one added feature for component testing. I bet this manual is just what you're looking for. Can't get a clear picture?

1. Protek 3502 Service Manual

You mean you can't focus the beam with the focus knob? If I had to guess. There are some service adjustments for focus covered in the manual I think. Also could be a problem with the focus circuitry. Could be a tube problem. Not sure what you mean by being unable to 'get things to move around ground?' Let me know if I can help.

I am starting to get a better handle on how these things work so maybe I can steer you in the right direction. Meanwhile here is a great troubleshooting guide that applies to all scopes even tho it is written by/for Tek. I couldn't make out the settings but that is ok. Interesting behavior!! Take a look at that PDF and just kind of skim through it and get the gist. Their approach is-diagnose from the front panel. That's what we'll try and do.

So, step one, diagnose the faulty section. The o-scope is made up of several main sections.

The power supply of course provides various voltage to all the other sections. The vertical section controls the position of the beam on the vertical axis. At present I'd put my money on a problem with the vertical section. The horizontal section is what sweeps the beam from left to right. It also is responsible for triggering when to sweep, blanking the beam when sweeping right to left to start another sweep. You seem to be having a problem getting the beam to show up below a certain point on the screen. The vertical position adjust is part of the vertical section circuitry.

So maybe that has something to do with it. In the PDF take a look at the chapter called 'Troubleshooting the Vertical Section' Let's see if we can isolate down to the block, circuit and then component levels. I am kind of winging it here.

First off, does this behavior only happen on channel A? Or does this also happen on channel B? Second, when you set Channel A coupling to AC and adjust the position knob, what happens? What about when you set coupling to DC? Third, when you set Channel B coupling to AC and adjust the position knob, what happens?

Protek 3502 Service Manual

What about when you set coupling to DC? Click to expand.Ok cool, nice work.

So whatever the problem is, it's common to both channels and all the coupling. That helps because it eliminates a bunch of blocks from the 'suspect' list. Here's the block diagram from the service manual. There's two obvious things common to both CHA and CHB, all coupling - the CRT itself and the vertical amplifier (the 'Output' triangle below).

This seems like an electronics problem, not a CRT problem. The other thing common to both channels is the block labeled 'Mode Control Logic / Vertical Select / Trigger Select' - I'm not 100% sure what these do, but the problem could lie there as well. I'll go look at circuit diagrams and see if I can get a sense of what these do.

Do you have an oscilloscope probe? If so, could you (1) plug the probe into the ChA input and then (2) hook the probe to the calibration pin on the front panel and (3) film the waveform and show what happens when you move the position knob and then post it to youtube for me to look? I'm curious to see what it looks like when displaying a waveform. Thanks, Michael. PPS: when you display the waveform, see what it looks like and how it behaves with Volts/Division at different settings from low to high. What I predict is going to happen is that the part of the waveform appearing above the top half of the screen will look normal while the bottom half will look wonky.

If you adjust the position and volts/div so that the trace fits in the top half of the screen, it'll look ok. But if you then change volts/div so that the trace takes up more vertical space, or if you reposition the trace lower, the bottom half will be wonky. Let's see if I am right.

Click to expand.Yup, just put the time/div indicator on X-Y Normally the horizontal sweep is controlled by a periodic, regular timing signal. In X-Y mode, CHB becomes the source for the sweep. That is to say, CHA deflects the trace in the Y axis, and CHB deflects the trace in the X axis (or maybe it is the other way around, but you get the idea). The result is you plot Lissajous patterns on the scope which is useful for diagnosing components, for example. You'd feed a frequency into a component and measure the signal on the input (CHA) and output (CHB) sides to measure phase shift. The actually goes into this in detail since the C model includes the frequency generator to input the signal. Otherwise, on the non-C model, you have to have a separate frequency generator.

So the lissajous pattern shows phase shift between two signals: (from ). Good luck with the exam!!! I saw the youtube comment. Think about when the waveform appears and doesn't appear - only when the trace deflects to a certain point.

And then consider the power supply is always supplying power regardless of what knob you select or where the trace is, etc. I think the fact that the trace goes haywire lower on the screen suggests something wrong with the voltage supplied to the Y- plate. Take a look at the vertical amp circuit diagram - some amplifiers (audio, for example) get a + and - supply but the vertical amp only has a +120V so there's no negative supply rail that could present ripple to Y- without also affecting Y+ - that is, you'd see the ripple everywhere. Finally, what ripple looks like is a 60hz sine wave where there should be a flat trace.

File name: Manual Type: Service Manual Pages: 65 Size: 5.86 Mbytes (6142660 Bytes) Language: english Revision: Manual-ID/Number: Date: 1979 Quality: Scanned document, reading partly badly, partly not readable. Upload date: 03 October 2015 MD5: ca77392c6dba5bca1729069e047ce81c Downloads: 683 since 03 October 2015 Information: 1 Features 2 Configuration 3 Precautions to be taken to ensure long life 4 How to display to trace 5 Specifications 6 Maintenance, Repair and Storage 7 Test Equipment Required 8 Initial Starting Procedure 9 POWER SUPPLY SYSTEM 10 DISPLAY 11 VERTICAL SYSTEM 12 HORIZONTAL SYSTEM 13 Electrical parts arrangment(with adjustment locations) 14 Block diagram 15 Schematic diagrams 16 Parts list Equipment information. Protek 6502 Oscilloscope Manufacturer Model: 6502 Date: 1979 Category: Group: Description: 20 MHz OSCILLOSCOPE Information: This compact, light weight model 6500 series oscilloscope has a frequency range of DC-60MHz, DC-40MHz or (DC-20MHz), and offers the following features: (1) Wide frequency range: Frequencies up to DC-60MHz, DC-40MHz or (DC-20MHz). (2) High sensitivity: 1mV/div (3) Large size CRT: Waveforms are easy to read, on the large 6-inch CRT with an internal graticule scale. (4) Scale: Waveforms are parallax-free with the CRT’s internal graticule scale (5) Alt Mag The normal( 1) and the 10( 5) magnified waveforms can be displayed simultaneously. (6) Alt Trig: Stabilized triggering is accomplished even with two unrelated signals.

(7) TV synchronization: Stable TV signals are displayed using new circuitry. (8) Auto focus: Focus deviation is automatically corrected.