[SOLVED] BWD504 oscilloscope blanking and square wave rendering issue

[gkmaia]

I could manage to bring this guy back to life but it is not still perfect.

Initially it had a faulty hex inverter, one transistor and a few capacitors. With that replaced I can now get it to trace sine on screen at all voltages and frequencies.

A few issues when rendering square waves and blanking is not working.

Bellow a video of it's behaviour and also a schematics with lots of test points done.

I am struggling to understand why Q42 is not oscillating. It's base should pull up every-time U40-2 is high. U40-2 pull high but signal does not flow to Q42. I already replaced the D46 & D51 but still the pulse does not flow as it stops on D46 cathode.

https://www.lost-manuals.com/manufacturer/BWD_Electronics/504

[pbarton]

Edit: Try lifting one end of C59 (BLANKING PIN2 CRT) it could be 'leaky' (holding Q42 collector permanently low). Then check to see if the 40V amplitude waveform on Q42 collector appears?
Otherwise, replace C59 and remove Q42…
what is the voltage at the junction of R90 and R54?
what is the voltage at the junction of R54 and D46?

[gkmaia]

Thanks for helping Phil.

I removed C59 and still no pulse. But then went back to Q42 and replaced it and pulse is back and blanking is perfect. Same issue as the Trio. Q42 was new and still pass off board testing. But in circuit it does not perform. I may end up buying a curve tracer to test these things properly. The "reversed" rise and fall on square waves was just a calibration issue.

What still persists as a problem:

At higher frequencies such as 1 Mhz up the square waves have some ringing on their rising edges and sweep seems to be incorrect and sometime irresponsive. The scope was supposed to go to up 6Mhz but is useless after 1Mhz.

At U40-9 modulation is 6 times of what it on manual. Pin 9 is the gate input and needs a 2v VIH and 0.8v VIL. I cannot understand how a modulation of 120mv pkpk can turn this gate one and off. It is a concept I cannot understand.

If you look at D45 cathode the modulation is only 25% of what is should be.


I updated the schematic test points on the first post for your reference. All greens are a close match to specs. Reds are the ones that are missing.



There are a few extra bits of info I would like to add.

C82-85 replaced with 10Uf
C86-87 replaced with 68Uf
U40 replaced with a Reneseas HD74LS04P instead of National DM74L04n. The LS apparently handles more current and has lower tPHL and tPLH times. The HD also has a lower VIH and higher VOL but pretty close in overall.

[pbarton]

Congratulations, you have repaired the blanking problem and at low sweep speeds the display does not look too bad.
However, you are now examining the higher/highest sweep speeds and clearly from your latest video, all is not well with the display. The latest video suggests a timbase problem at the higher speeds.
The waveform that gives me most concern is the one which feeds C46 through C50 in the timebase section. The measured waveform appears to be very different to the expected waveforms shown in the schematics.
The timebase consists of IC40 sections A and B plus Q43/Q44 and 7 diodes. I guess that you soldered the new IC40 (Reneseas HD74LS04P) directly into the PCB and it’s not going to be easy to change that one.
However, the amplitude from U40B pin 2 doesn’t look too bad (although you marked it as red), but can you check that the mark space ratio is similar to that shown on the schematic. (my guess: high for 35% and low for 65%?).
If that’s reasonable then the substitution (U40 replaced with a Reneseas HD74LS04P instead of National DM74L04n) is probably OK. Which eliminates a chunk of the timebase circuit.
More investigation, substitution and further testing is required to identify the remaining timebase fault at higher sweep speeds.

[gkmaia]

IC40 is on a brand new turned socket. Can easily change it if need to. Do you want to suggest it something?

Cool, that gives me some direction on what to investigate. I got a couple questions.

Q40 block is the trigger.
Q41 block with U40D/C is the auto block.
Q43/44 with U40A/B is the sweep block.

If I remember well my testing scope fails to trigger some of U40 pins at higher frequencies. Like if the periods were all over the place. Do you think it is worth trying to feed the scope with 5Mhz and go from trigger to auto to sweep and see at what point the signals start to become hard to trigger as a way to find where the fault may starts and then focus on that block? All the signals of 1ms should read 5MHZ @ 5MHZ and all the signals of 13ms should read 384KHZ @ 5MHZ. Sound like a good idea?

Also is it right to assume I have something that is not switching properly at faster speeds? Considering it is a switching problem that leads to semiconductors, therefor the probable issue is either a transistor or a diode?

[pbarton]

Q1. "IC40 is on a brand new turned socket. Can easily change it if need to. Do you want to suggest it something?"
A1. We have not identified that the substitute U40 is responsible for the display problems at higher sweep speeds so its a little premature to recommend a different substitute, right now.
Q2. "Do you think it is worth trying to feed the scope with 5Mhz and go from trigger to auto to sweep and see at what point the signals start to become hard to trigger as a way to find where the fault may starts and then focus on that block?"
A2. I'm not convinced that that's a good diagnostic technique to utilise.
Q3. "Your pretty sure the fault is on sweep block and I can consider trigger and auto to be fine?"
A3. Yes, I believe that you should be looking at the timebase which consists of IC40 sections A and B plus Q43/Q44 and 7 diodes.
Q4. "Also is it right to assume I have something that is not switching properly at faster speeds?"
A4. Certainly there is some sweep signal degradation at higher sweep speeds.
Q5. "Considering it is a switching problem that leads to semiconductors, therefor the probable issue is either a transistor or a diode?"
A5. That's entirely possible.

The waveform that gives me most concern is the one which feeds C46 through C50 in the timebase section. The measured waveform appears to be very different to the expected waveforms shown in the schematics.
The amplitude from U40B pin 2 doesn’t look too bad (although you marked it as red), but can you check that the mark space ratio is similar to that shown on the schematic. (my guess: high for 35% and low for 65%?).
If that’s reasonable then the substitution (U40 replaced with a Reneseas HD74LS04P instead of National DM74L04n) is probably OK. Which eliminates a chunk of the timebase circuit.

[pbarton]

Looking at your last video again, I now realise that you have a reasonable/decent display image while at the highest sweep speed (0.5uS) and the timebase vernier control is clearly responding correctly.
It is only when you subsequently switch to slightly lower sweep speeds (1uS and 10uS) that the triggered waveform (only) appears on the RHS of the display.
What’s on the LHS of the display? It appears to look like the sweep is in AUTO (trigger not present, free running).
Also, the number of square wave transitions displayed/graticule in the X axis remains constant, when you switch across 3 different sweep speeds!  Weird or what!

[gkmaia]

If it is weird for you with the experience you have. Imagine for me. I am pushing myself really hard to understand that given the fact I got no formal education on engineering. But everyday I learn something new and these things make more sense.

"The waveform that gives me most concern is the one which feeds C46 through C50 in the timebase section. The measured waveform appears to be very different to the expected waveforms shown in the schematics."

I will do a better video of this wave and sweep controls when I am back home trying to cover your last comment.

Can you also explain why you are concerned about the wave at C46 specifically? Is it just because it is different or does it tell you something of major importance?

[gkmaia]

There you go. Added two videos.

One going all the way from 100Hz to 2Mhz on a sine. And another on a Square.

@3Mhz sweep from 1us to .5us makes no difference.

[pbarton]

Your latest two video’s exercise the scope using sine and square wave signals and the display appears to trigger correctly, presenting a stable display over all the available timebase speeds.
The CRT display is acceptable, given that the scope was designed with economy in mind, as the front panel controls and circuit complexity/component count are very limited.
I note that the trace rotation needs horizontal alignment. In a more expensive scope this would be done electronically, but in this case you need to slacken a clamp on the CRT neck and turn the tube until the trace is horizontal.
The previous problem you identified “Bwd504 2Mhz input square wave issue” where you have a 2Mhz waveform displayed using the 0.5uS timebase and subsequently switch to slightly lower sweep speeds (1uS and 10uS) creates an artefact/artifact or undesirable phenomena, which is probably a consequence of the design economy. The same video appeared to display a 2.0MHz square wave, which is quite an achievement for a scope with a 6.0MHz bandwidth! In reality the scope is probably intended for audio or ultrasonic signal applications.
The latest videos suggest to me that the scope is working, as well as can be expected.

[gkmaia]

Awesome! So we fixed another one! I am not sure I will use this one for anything but it is so good to see something that was given as trash to be back to life.

It has a trace rotation pot in the back. The CRT is loosen so I could easily remove it to remove components. I will align it properly!

I did a read on the manual and the vertical bandwidth is DC 6MHz and the horizontal is DC 1MHz. Also looking that Trio we fixed previously the vertical is DC 15MHz and horizontal 1MHz.
How these numbers relate to the capability of the scope to display acceptably either sine or square waves?

[pbarton]

Here is an article about oscilloscope bandwidth in general terms, https://en.wikipedia.org/wiki/Oscilloscope#Bandwidth
although many more bandwidth articles are available with a Google search.
Here is an article which specifically relates to the display of either sine or square waves https://rigol.desk.com/customer/en/portal/articles/2268981-why-does-my-scope-show-a-sine-wave-when-i-expect-a-square-wave-

[gkmaia]

Thanks Phil!

Here are some photos after the full restore. Cleaned and retro brightened.