Meguro MO-1252A 25MHz Analog Oscilloscope - nothing on CRT

[huelter]

Hi, I got a Meguro MO-1252A 25MHz Analog Oscilloscope but it is not showing a trace on the CRT. I can see that the filament lights up, but no trace. On XY mode, not even a dot is shown.

I tried to hunt down the manual for this scope, but I had no luck. The user HalfSpace has found a similar model:
https://www.eevblog.com/forum/beginners/datasheet-or-manual-for-a-_gould-25-mhz-(rto)-325_/msg442016/#msg442016

Here is the link for the datasheet (Heath 4226):
https://www.mods.dk/manual.php?brand=heath

Any ideas where to start? Also, how could I measure the 2KV safely? Most DMM goes up to only 1000VDC, so I'm not sure what to do. The manual says to use a high input impedance probe to measure the voltage, but what about the DMM itself?

Edit:
Transformer Outputs:
170VAC OK
6.3VAC OK
17VAC OK

Power Supply Outputs:
+12V ±0.05V OK
-12V ±0.2V OK
+5V ±0.25V not OK, measured 7V
+160V ±5V OK
-2KV ± 50V not measured

Edit: Pictures: https://imgur.com/a/VFyOZZ1

[oldway]

Welcome to the forum.

Here are the schematics you are looking for:

Start checking the 4.5A fuse of the HV inverter and if it oscillate or not.

[huelter]

I don't think that schematic applies, @oldway. It is for a ELENCO 1252 35MHz if I'm not mistaken (check the bottom of the pdf, it says ELENCO). Checking a little closer, nothing matches with the board. Also, it is VERY hard to discern anything from it.

The Heath 4226 matches very closely. The voltages, the connections names are all correct. I'm attaching the schematic now.

[floobydust]

Post a pic of your scope.

These analog scopes (Meguro Japan) were widely copied by the Chinese and Koreans in the mid 1980's.
They are everywhere and came under many private labels.

The 35MHz model goes by:
Meguro MO-1252 (not MO-1251, not MO-1252A)
Hung Chang OS-635
JDR Instruments Model 3500
Ramsey Model 3500
Elenco MO-1252
Tenma 72-330
Aron BS-635
Metermaster Model 65535

Service manual: https://elektrotanya.com/hung-chang_protek_os-635_2ch_35mhz_oscilloscope.pdf/download.html

The 25MHz version is Meguro MO-1252A (OP's)

The 20MHz version with simpler timebase and component tester built-in, knock-offs of the Meguro MO-1251 discussed here: https://www.eevblog.com/forum/testgear/my-$50-scope-seems-doa-tenma-72-720/25/


For repairing your unit, I would measure all power supply voltages:  -8V, +5V, +12V, +20V, +65v, +150V. Check the HV power supply fuse 0.5A on transistor Q8 2SD401.
If your 5V rail is high at 7V, the Vreg may be bad or there is a shorted component running from a higher rail and backfeeding into the 5V rail. Hard to troubleshoot.
I would first get the CRT to glow, it needs 19V for Q8, +150V, +12V, -8V for the entire CRT circuit.


edit: confused sch/model numbers, OP's is MO-1252 so the MO-1252A schematics are different

[huelter]

Thank you floobydust. I've studied the schematic that you provided, but the Heath SO-4226 still is the closest so far for this MEGURO MO-1252A. The power supply voltages are -12V, +12V, +5V, +160V and -2KV for the CRT. I'll attach pictures on the first post soon.

[floobydust]

The Meguro MO-1252 I got schematic pages from Radiomuseum
The Heath schematic looks different, so I'm not sure what you are going off of.

[huelter]

The Meguro MO-1252 I got schematic pages from Radiomuseum
The Heath schematic looks different, so I'm not sure what you are going off of.

1252 - 35MHz
1252A - 25MHz

Both the internal and the panel are different. Just give me a minute, will take some pictures.

[floobydust]

Sorry about the model number confusion. 1252 is different from 1252A.

The Heath SO-4226 picture and schematic matches the PC board numbers and looks good to follow. I see Siemens electrolytics in there, so someone's recapped most of it.

Your 5V rail is high at 7V, record voltages on the 5V regulator op-amp IC1002 and Q1004. The circuit may be fine, just a short somewhere bumping up the 5V rail or an open 150R resistor R1021.

I would look at the HV DC-DC converter for signs of life. You don't have to measure the HV anode voltage- it can be inferred from the focus voltage-divider feedback to IC2001 pin 3. Measure the voltages on big Q2001 and IC2001. That will tell us what is going on there.

[huelter]

Something is wrong now. After probing some voltages, the resistor R1013 becomes extremely hot and starts to smoke. Shutting down and power on doesn't solve this, I might have done further damage. Even disconnecting all boards doesn't help.

This resistor R1013 (2.8K) is one of the few differences that I've seen from the Heath schematic. It is located between D1002 negative output and ground. I suppose it is some sort of constant load? Removing this resistor makes R1014 (also 2.8K) heat up and smoke in its place, so I don't know what is going on. Checked for shorts and even replaced IC1002 and IC1001 for some regular 4558d op-amps, but to no effect.

I think I need to understand the schematic more to debug this. What is Q1003, R1013, R1014 and R1015 supposed to do?

[floobydust]

Um, there is a problem with disconnecting the PSU from the scope at CN1007, CN1005.

This PSU does not like having no load on the +160V rail, so it will probably shoot up past 240VDC!

So you might have a shorted electrolytic capacitor on the 160V output side. C1013 would not be happy and probably fail, and C1018 too. I can't see any other reason for R1013/R1014 to smoke, although Q1003 should also be trying to output current and why didn't it contribute.

I'd say you need a dummy load for the 160V rail, to work on the PSU disconnected.
Hard to guess what current to draw, and not roast Q1003. Guessing ~20mA? 160V/20mA= 8k ohms but 3W so a bit of heat. Do you have a dummy load to use?

Use caution as 160-240VDC is dangerous. Careful C1012 can hold a HV charge if there is no load, always check voltage on it with a DMM (wait until it's dropped) before going in.


The 160V regulator:
Q1003 is the pass-transistor, but located on the return (GND) side. It seems to be only rated for 150V 2SB861...
R1013 sounds like doing the same thing as R1014; it's not on the Heath schematic though
R1014 is a boost-resistor to aid output current by taking some load off of Q1003, and it is a start-up resistor until IC1002 is controlling things.
R1015 is some current-limiting for the regulator.
IC1002 doing the regulation but not sure where it's getting power from.

Note the +12V -12V supply is the reference for the +160V and +5V regulators, so it must be working OK for the other rails to work.

[huelter]

Note taken. Put a load of 8.2Kr, 5W at the 160V rail, replaced those caps and gave it a go. Resistor R331 promptly melted.

After removing R323, R324, R325, R328, R329, R331, R332, C313, C330, C331, things stopped smoking. I can see that the 160V rail has 225V, even with the load connected. Also, +12V has 120V and -12V is on -110V.

I'm gonna investigate the 12V rail now, as it is the primary regulator.

[floobydust]

Oops it's the -12V reg that is the reference for the 160V and 5V regulators.

It looks the  +/-12V regulators also have those boost-resistors R1003, R1009 which means under no load, their output will skyrocket 
Only the 5V reg looks OK having R1021 as a minimum load.

Could not find R330 on the sch

[huelter]

3xx is on page 26, vertical amplifier.

Any recommended load? The 17VAC has a spec of 0.5A on the transformer.

[floobydust]

I'd go with at least 150mA load for the +/-12V rails, around 80R 2W or car #194 light bulb etc.
C330 is kind of scary 1uF 100V between 160V and 5V rails.

I think I would first get the power supply to work while disconnected from the scope.
If Q1003 has shorted, I would use tougher MJE15033.
There might be some shorted parts on the scope boards that would cause the PSU to fail. I don't know of an easy way to isolate that.

[huelter]

After putting a 100ohm load on both the +12V and -12V, both of these rails are stable and within margin.

However, the 160V rail is now at 188V, with a 8.2K ohm load. Is this because the load is still too small? It is drawing 23mA, and the transformer is rated 80mA on the 170VAC.

The 5V rail is sitting at 4.6V, a little bit below margin. Should I keep looking to stabilize this before adding more parts to the system?

[floobydust]

The power supply, standing on its own - In addition to needing a large minimum load, I just noticed it also has no output capacitors    so it could easily oscillate the 5V, +12,-12V regulators (160V has C1013). This makes it even more difficult to test, stand-alone.
I would check ACV or oscillations on the regulators, the 5V is 8% low which might be due to this.

I think worse outcome is if Q1003 is shorted. I expect the 188V to drop to 160V with heavier (scope) load.
If R1014 is 1.2k, then you need over 65mA load before Q1003 turns on to contribute additional output current. IC1002 needs to output around -2.5V at pin 7. Rough guess.

I hate to be the guy telling you to connect it and fire it up, after all this smoke already.
I would get confidence the PSU is not going to overvoltage the main board.

[huelter]

Oh my. ACV is varying wildly.

on 160VDC, I measured about 400VAC
5VDC, 9VAC
+12VDC, 25VAC
-12VAC, 0VAC

Does this make any sense? My multimeter is el cheapo, but it measures ACV from the outlet just fine.

[floobydust]

I'd try adding some capacitors to the PSU outputs. Around 10uF-100uF on each +5V, -12V, +12V rails and see if things calm down. I know this is a big hassle, but the PSU does not work on its own, so the only other option is the big bang theory where you connect the scope's main board and hope the rails stay in spec. You should also check some main board parts, like the vertical amp Q312, Q310 if R330 did smoke.

I don't know your multimeter, but some freak out measuring ACV when there is a large DC component present.
Like 100VDC with 1VAC sine wave superimposed. I have seen some cheap multimeters get confused (internal) autoranging should choose 100V or 1V range- crappy firmware that saturates the A/D. You get a silly reading.

Getting a 400VAC reading on a 188VDC, that seems like the multimeter might be malfunctioning.

[huelter]

OK Thanks, very good insight into the multimeter readings.

I reattached all missing resistors and caps in the 3xx region, but again, another resistor melted. Found out that Q311, Q312, Q308, Q310 are all shorted, some were rated 25V and others 180V, so the 170VAC rectified to 240V killed them;  will replace them on Monday. This might get them killed again if the load doesn't bring the voltage down to sub 180V, so I'll double the load current.

Do you think anything else shorted on that region? Q307 and Q309 were tested and fine. What should be the V+ and V- outputs?

[floobydust]

The 160V rail powers the vertical and horizontal deflection amps, and Z-axis modulator.

Vertical deflection: I would also test Q307, Q308 and C330 1uF 100V I would replace. C353 is weird, if it's electrolytic I would test it.
Horizontal deflection amp: Q811, Q812, Q809, Q810 I would test too.
Z-axis: Q3006, Q3005, Q3004, Q3003 test as well.


I'm not sure why the 160V rail overvoltaged connected to the scope main board. Is that what happened?
I thought the rail went high while CN1007 disconnected (no load, no O/P capacitor).  Q1003 must have shorted.

The most important thing is to not have a short from 160V to a lower voltage rail like 5V or 12V. Like Q809, Q810 could maybe do.

[huelter]

That section of vertical deflection is very close to the power supply, no connectors attached. When everything was disconnected, that small region was still powered but with no load, so the rail shoot up the voltage.

The horizontal deflection and z-axis were on the second board and disconnected when the rail surged, so they should be fine.

That 353 cap, I don't seem to find it. Maybe it is not installed? There is not even a value for that. That region appears to be the filter capacitors that you said to install, right? They filtered the +-12V rails two times.

I'm starting to think this is a bad board implementation. I've seen other similar scopes with the power supply on its own board, while this one has the power supply sharing space with the vertical amplifier. Sure, my bad for not loading the rails, but frying some modules that you can't disconnect? 

[huelter]

Bummer. Did not find the required 2sb648a and 2sd668a. I could only find one 2sd669a.

Would there be any problem if I put an unmatched pair there? Also, would the Hfe need to be the same for the pair?

[floobydust]

For the HV deflection amp transistors
2SB648A/2SD668A 160V 50mA 1W 60-200 hFE

easy to find subs might be:
2SC2682/2SA1142 180V, 100mA 1.2W 100-320 hFE. Fairchild calls them KSC2682/KSA1142.
2SC3503/2SA1381, 300V, 100mA 7W 40-320 hFE. Fairchild calls them KSC3503/KSA1381. These are are popular in audio power amplifiers and have good linearity, I use them a lot.
2SC2690A/2SA1142A 160V, 1,200mA 1.2W 60-320 hFE. Fairchild calls them KSC2690A/KSA1142A.

I'm not sure why the 2SC2570 12V transistor is there 

I would replace them in pairs, top and bottom to keep symmetry, although the deflection amplifier has negative feedback with R315, R316- so it's not that critical.

[huelter]

OK, managed to get two pairs of 2sd669a/2sb649a.

Something that worries me is that the Vcb and Vce are rated 160V and 180V respectively. I measured the voltages now present after installing, both are sitting at 180V/185V. Am I screwed?

The supply voltages are now:

160V rail -> 185V, with a load of 72mA, 2.65K ohms
5V rail -> 5.12V
+12V rail -> +12V
-12V rail -> -12V

I can't load any more with resistors, the transformer is rated 80mA, so what would be best here? Connect all modules and hope for the best?

[floobydust]

The 160V rail seems too high, with a 72mA load. That's at over 13W!

Are you sure Q1003 and IC1002 are working? What voltages do you get on IC1002 pin 5,6,7?
I would remove Q1003 and test. There are resistors that will interfere with in-circuit testing. It's another wimp transistor, 2SB861 rated 150V but subject to more during power-up.

The 185V is a little above the deflection transistor's max. rating so I would not worry, there is usually some headroom.