Meguro MO-1252A 25MHz Analog Oscilloscope - nothing on CRT - Page 1
Forum section: electronics / repair

author: huelter
author_rank: Contributor
timestamp: May 09, 2018, 02:06:58 am
content: 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/#msg442016Here is the link for the datasheet (Heath 4226):https://www.mods.dk/manual.php?brand=heathAny 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 OK6.3VAC OK17VAC OKPower Supply Outputs:+12V ±0.05V OK-12V ±0.2V OK+5V ±0.25V not OK, measured 7V+160V ±5V OK-2KV ± 50V not measuredEdit: Pictures:https://imgur.com/a/VFyOZZ1

author: oldway
author_rank: Super Contributor
timestamp: May 09, 2018, 08:06:48 am
content: 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.

author: huelter
author_rank: Contributor
timestamp: May 09, 2018, 11:36:28 am
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: May 09, 2018, 09:19:33 pm
content: 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-635JDR Instruments Model 3500Ramsey Model 3500Elenco MO-1252Tenma 72-330Aron BS-635Metermaster Model 65535Service manual:https://elektrotanya.com/hung-chang_protek_os-635_2ch_35mhz_oscilloscope.pdf/download.htmlThe 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

author: huelter
author_rank: Contributor
timestamp: May 10, 2018, 02:25:16 am
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: May 10, 2018, 02:37:53 am
content: The Meguro MO-1252 I got schematic pages fromRadiomuseumThe Heath schematic looks different, so I'm not sure what you are going off of.

author: huelter
author_rank: Contributor
timestamp: May 10, 2018, 02:41:34 am
content: Quote from: floobydust on May 10, 2018, 02:37:53 am1252 - 35MHz1252A - 25MHzBoth the internal and the panel are different. Just give me a minute, will take some pictures.

author: floobydust
author_rank: Super Contributor
timestamp: May 10, 2018, 06:51:55 pm
content: 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.

author: huelter
author_rank: Contributor
timestamp: May 11, 2018, 12:42:46 am
content: 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?

author: floobydust
author_rank: Super Contributor
timestamp: May 11, 2018, 01:37:18 am
content: 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 thoughR1014 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.

author: huelter
author_rank: Contributor
timestamp: May 11, 2018, 05:06:44 pm
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: May 11, 2018, 05:22:31 pm
content: Oops it's the-12Vreg 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 skyrocketOnly the 5V reg looks OK having R1021 as a minimum load.Could not find R330 on the sch

author: huelter
author_rank: Contributor
timestamp: May 11, 2018, 05:26:22 pm
content: 3xx is on page 26, vertical amplifier.Any recommended load? The 17VAC has a spec of 0.5A on the transformer.

author: floobydust
author_rank: Super Contributor
timestamp: May 11, 2018, 05:59:15 pm
content: 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 tougherMJE15033.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.

author: huelter
author_rank: Contributor
timestamp: May 12, 2018, 01:12:47 am
content: 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?

author: floobydust
author_rank: Super Contributor
timestamp: May 12, 2018, 02:00:30 am
content: The power supply, standing on its own - In addition to needing a large minimum load, I just noticed it also has no output capacitorsso 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.

author: huelter
author_rank: Contributor
timestamp: May 12, 2018, 02:09:29 am
content: Oh my. ACV is varying wildly.on 160VDC, I measured about 400VAC5VDC, 9VAC+12VDC, 25VAC-12VAC, 0VACDoes this make any sense? My multimeter is el cheapo, but it measures ACV from the outlet just fine.

author: floobydust
author_rank: Super Contributor
timestamp: May 12, 2018, 04:28:25 pm
content: 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.

author: huelter
author_rank: Contributor
timestamp: May 12, 2018, 09:26:32 pm
content: 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?

author: floobydust
author_rank: Super Contributor
timestamp: May 12, 2018, 11:29:33 pm
content: 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.

author: huelter
author_rank: Contributor
timestamp: May 13, 2018, 12:00:36 am
content: 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?

author: huelter
author_rank: Contributor
timestamp: May 14, 2018, 03:33:14 pm
content: 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?

author: floobydust
author_rank: Super Contributor
timestamp: May 14, 2018, 05:39:04 pm
content: For the HV deflection amp transistors2SB648A/2SD668A 160V 50mA 1W 60-200 hFEeasy 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 thereI 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.

author: huelter
author_rank: Contributor
timestamp: May 14, 2018, 06:22:11 pm
content: 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 ohms5V rail -> 5.12V+12V rail -> +12V-12V rail -> -12VI 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?

author: floobydust
author_rank: Super Contributor
timestamp: May 14, 2018, 07:43:42 pm
content: 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.

author: huelter
author_rank: Contributor
timestamp: May 14, 2018, 09:02:27 pm
content: Good call, Q1003 was shorted (again)!I think it is because I used 2sb546 (20W) instead of 2sb861 (30W), all other specs match. Now with only a 8.2K ohm load on the 160V rail and 100ohm loads on the 12V rails, all rails are within spec!I will now carefully insert one module each time and monitor the supply voltages.

author: huelter
author_rank: Contributor
timestamp: May 15, 2018, 03:47:27 am
content: OK, tested again with everything connected.There is a huge voltage drop on the 5V rail as soon as I connect CN2003 to the second board, becoming 2.75V. Connecting the rest of the cables/modules did not help.One thing to note is that all the other rails remained stable and within specifications, even with the added loads. Removing the loads also did not change the outcome.Now, what am I searching for? Looking every 5V component on the second board for shorts/opens?

author: floobydust
author_rank: Super Contributor
timestamp: May 15, 2018, 05:26:23 am
content: The 5V regulator could be weak, or there is an overload on that rail. I would expect R1026 to smoke and Q1004 to get hot.Shorted Q307/Q308 vert. amp could backfeed and overvoltage the 5V rail at R319.The 5V rail is mostly for the digital IC's so one may have shorted, and would warm up.You could take a bench PSU (with current limit set to low 100-200mA) and inject only 5V power to the main board and see who heats up. Or measure the voltage drop across some of the 10R resistors on the 5V rail to see how much current a part draws. Like IC202, IC201 with R254, R253.IC202 74LS00 vert alt/chopIC201 74LS00 vert alt/chopIC401 3/4 74LS00 horiz sweepIC601 74LS122 horiz sweepIC602 TC4053 mux for chopIC401 74LS00 triggeringIC4011 TC4011 CAL pcb square wave osc.I'd finger test these IC's and see if they are hot. Otherwise you'd have to lift a Vcc pin or guess and remove an IC. I'd put in a socket.

author: huelter
author_rank: Contributor
timestamp: May 15, 2018, 03:25:39 pm
content: IC602 was dropping the 5V rail. Replaced it by a hcf4053be, now rails are:160V OK5V not OK, measured 5.70V-12V OK12V OKEverything is connected again, no additional loads. CRT displays nothing, the filament is red and supposedly working...Another odd thing that I noticed is R635, it is way off. Gonna replace all these 1% resistors and capacitors on the region to see if it gets better.

author: floobydust
author_rank: Super Contributor
timestamp: May 15, 2018, 08:03:47 pm
content: Now the 5V rail is high? The 'LS TTL parts don't like over 7V and the 4000 CMOS to 15V. I'm not sure how to look for the reason, but it might explain why R1021 exists if the +12V or +160V can lift up the 5V rail. Or the regulator is not working. Voltage drop across R1026 will tell you the total current drain on 5V.If there's no sign of a trace, first I would look for the HV DC-DC signs of life. If Q2001/IC2001 are oscillating. Voltages on IC2001 would help.Next, I would measuring the CRT voltages (not HV) deflection plates etc.Is R635 off value in-circuit or out? Seems weird for resistors to fail there.

author: huelter
author_rank: Contributor
timestamp: May 16, 2018, 01:20:16 am
content: R635 was low off-circuit below 20%. Replaced some caps as well but the voltages remained the same.I socketed all ICs on the second board (IC401, IC601, IC602, IC603, IC2001), then powered the board with no ICs. the 5V rail remains at 5.7V, the same as with all the ICs.The voltages on Q1004 when the second board is disconnected: base 5.8V, collector 14V, emitter 5.1V; voltage drop on R1026: 6.37VThe voltages on Q1004 when everything is connected: base -9.15V, collector 19.7V, emitter 5.6V; voltage drop on R1026: 0VIf I understand correctly, something is shorted in the second board, and it is not one of the ICs, which completely shuts off the 5V rail?

author: floobydust
author_rank: Super Contributor
timestamp: May 16, 2018, 09:50:56 pm
content: Because load R1021 is on the output of the +5V regulator, the current from other rails returning into the +5V rail normally doesn't bump up the 5V.But if you have no R1021, then there would be enough current to push the rail to 5.7VThe only place I can see lots of current possible backfeeding is from the vertical and horizontal deflection amp output stages.Horiz. deflection amp at Q809/Q810 5V on the emitters and 160V on Q811/Q813, or if Q807 was on hard.Vert. deflection amp at Q307/Q308 5V on the emitters.So if there is still a problem in the deflection amps, they could push current back into the 5V rail.Which is the second board? At CN?Q1004 emitter voltage seems high, assuming R2008 is OK at 1R. But it would not affect the 5V issue.

author: huelter
author_rank: Contributor
timestamp: May 17, 2018, 12:48:53 pm
content: It is hard to say what that second board has, because the schematic doesn't follow. I think it has the vertical and horizontal deflection, 2KV high voltage circuitry, timebase selection, triggering selection.I checked every transistor on the second board. I found out that Q807, Q808, Q809 and Q3005 are busted. Just like you predicted, on Q807 there is backfeeding!So now, the hardest transistor to replace is Q807 2sc1907, because its spec has 1.1GHz... Is that really necessary? Q808 had a A838, with only 300MHz and easier to replace (nevermind the schematic, it is saying this PNP is also another 2sc1907, clearly wrong).Anyway, I swapped Q807 for a H945, Q808 for a BC558 (swapped base and collector legs), Q809 for another D669A and Q3005 for a C2371. The good news is that all rails are within spec now, and everything is connected! The bad news is that there's still nothing on the CRT.

author: floobydust
author_rank: Super Contributor
timestamp: May 17, 2018, 08:47:20 pm
content: 2SC1907 NPN UHF transistor 30V fT 1,100MHz popular with Asian scopes. Substitutes might be:SS9018or2N5770but both have EBC pinout. Both at Digikey.Next would be seeing if the HV DC-DC is running, and then measuring the CRT voltages (X+,X-, Y+, Y-) and CN2006 astigmatism.The cathode and control grid, screen grid are around -2kV so no DMM there, although you can make a cheap HV probe voltage divider with 1/4W or 1/2W 10MEG resistors, if you include the DMM's input resistance for the ratio. Or a neon lamp in series with some 10MEG resistors as a poor mans indicator. It's ok to push 1kV through a 1/4W resistor for a few seconds.Q3006, Q3005 voltages could be checked, to see if the blanking is not stuck on or intensity shut off.

author: huelter
author_rank: Contributor
timestamp: May 17, 2018, 09:25:33 pm
content: Thanks, will look into replacing Q807 into a proper one.Now, There are several diodes that are busted as well!D433, 1S1588, high speedD819, 1SS86, schottky UHFD3024, BAV21, Vrrm 250VD3013, 1SS83, Vrrm 300VThose are all old diodes, do you have any recommendations for replacements?Edit: gonna replace them for:2SC1907 -> same (found one!)1S1588 -> 1N4148 (fast switching)1SS86 -> BAT83 (low capacitance schottky)BAV21 -> same1SS83 -> 1N4005 (general purpose)Edit2: replaced everything that was faulty. Here are some measurements:Q3005: E=57.7, C=156.6, B=58.6 (active)Q3006: E=148, C=156.6, B=15.4 (inactive)Q3007: D=2.3, G=0, S=0.8 (active)Q3008: E=2.2, C=0, B=2 (still inactive?)VR3002 = 420 ohmsVR3031= 2560 ohmsD637, IC side = 5VD637, unblank signal = 0.4VI find really odd what is happening on Q3008. The Intensity adjustment VR3002 is set at 420 ohms, setting a collector voltage on Q3008 of about 2.7V. But instead I see 0V. Then there is VR3031, which would set an emitter voltage of about 2.7V, and instead it is 2.2V. I checked the region, found no shorts to ground. Have no idea what is going on.

author: floobydust
author_rank: Super Contributor
timestamp: May 19, 2018, 07:15:59 am
content: I'm not sure the best way to see if the beam is cutoff (blanked) all the time- if you can't measure the cathode and control grid voltages.You could try short the cathode and control grid, turning the CRT full on, which should be easy to see.On CN2005 jumper pin 3,4 (=CRT pins 3, 2) with a 100k resistor and turn on power and see if it lights up. Note the connector has -2kV so I would first power off the scope and discharge the CRT by grounding the cathode through a 100k resistor for a few seconds. Usually small CRT's don't bite if you power off and wait a few minutes.If you still have no display, then the HV is not there, or the trace is aimed at outer space, from a problem on X,Y deflection plate voltages.The beam blanking circuit is tough to work on, as the output floats at -2kV yet 180V-rated transistors are doing the switching.I'd have to be really awake to explain it.

author: huelter
author_rank: Contributor
timestamp: May 22, 2018, 03:17:49 am
content: Jumper 100K ohm on CN2005 pins 3,4: nothing on screen.Measuring -2KV with a /6 voltage divider: 0V.Testing D2010 and D2011 for operation: blown one with a 50ma current. Damn these 28V~40V forward drops!Any current replacements for these GHV06SSN diodes? 7KV peak Vrrm. I was thinking about putting some 12KV Vrrm 2A diodes used in microwaves.Are those neon lamps NE3001 and NE3002 for overvoltage protection, or for high voltage indication?Edit: replaced IC2001 and the diode. Now we finally have a dot on the screen!Now, the dot is shifted to the right. There might be a problem with the horizontal deflection circuit, right? Adjusting VR413 and the horizontal position knob doesn't move the dot.

author: huelter
author_rank: Contributor
timestamp: May 23, 2018, 06:15:25 pm
content: Replaced IC603. Now the dot is closer to the center of the screen, but not quite. Moving the horizontal position knob makes the dot move in the reverse direction, quite odd (clockwise moves it to the left). VR413 has not enough room for adjustment, so I cannot center it.I noticed that the vertical knob has no effect. The scope is in XY mode, but if I switch to another timescale, the dot remains unmoved. Don't know what to look at now...ICs that were replaced so far:IC401IC601IC602IC603No more ICs to be replaced on the second board, and the dot is still there on XY mode, but there is no more dot on any other timescale. I guess the sweep is faulty?Edit: using the signal generator from the calibration output, I was able to see the signal at CH1 OUTPUT with a second oscilloscope. Following the input, the signal becomes really weak at Q7 emitter, but at Q5 collector it is fine.Edit2: found the culprits. Q201 and Q202 are busted; any good 2sa711 replacements? I'm thinking about 2n3906, cheap and easy to find, but a little bit worse on the specs.http://pdf.icpdf.com/PdfOld/icpdf_datasheet_1/2SA71_datasheet_10346/171793/2SA71_datasheet.pdf

author: floobydust
author_rank: Super Contributor
timestamp: May 27, 2018, 07:43:12 pm
content: I would troubleshoot the H or V deflection systems separately in X-Y mode and get the V-amp working first, for both channels.Vertical deflection, you should be able to get a vertical line if you are injecting a signal like 1kHz probe cal or AC hum. A sine wave makes more of a line, a square wave makes top and bottom dots with not much in the middle.Replacing the transistors, the scope's alignment is probably out so the trace (dot) is not centered. There are trimpots as coarse adjustments to the front panel knobs. Like VR48/VR153 V-POS; with front panel knob centered, set VR48/VR153 for a centered dot. Or adjust it to keep the CRT V+ and V- voltages the same. You replaced many transistors in the diff amps so the balance trimpots may need to be adjusted, like VR147, VR148 too. Don't let the dot get really bright or you can burn the CRT phosphor.It's a bit confusing because there is one beam with one set of (vertical) deflection plates. For a two-channel display there are two vert amplifiers but one vertical deflection amp (driving the CRT) that is multiplexed by the alt/chop oscillator. i.e. in ALT mode it switches between Ch. 1, blanking, Ch.2, blanking and repeat again, with IC202.2SA711 (die alone is NEC NE71111) is PNP RF TO-18 fT=1GHz 40V 0.3W hFE=120 and run at higher collector currents here for speed.Equivalent parts arehttps://vetco.net/products/nte106-pnp-transistor-si-rf-if-amp?taxon_id=1872and harder to find metal can RF transistors. I think a TO-92 part might run a bit warm but you could try it to get things going.The horizontal oscillator is more difficult to troubleshoot. You have to know if the H-deflection amp, sweep oscillator, or the trigger sections are working.I could not follow CN602/CN404 (Y-input to H section) on the schematics.I suggest getting the vertical sections working, as that can help troubleshoot the H sections.

author: huelter
author_rank: Contributor
timestamp: May 30, 2018, 01:36:41 am
content: OK, using the scope on XY mode and on GND coupling, I'm able to see two dots on the screen now.Trying to center the dots vertically first, I adjusted VR48/VR148, but there is not enough room to reach the center of the screen. Tried replacing R49 with a lower value resistor, but now the front panel knob has even less room.A strange thing that I noticed is that adjusting the Y position panel knob makes the Y+ voltage vary wildly from 40V to 140V, but the Y- voltage stays the same at 150V. This happens on both channels. Maybe there's some leakage on the Y+ rail?

author: floobydust
author_rank: Super Contributor
timestamp: May 30, 2018, 07:32:35 pm
content: I think there's too much offset (difference) between the Y amps. The DC balance trimpots can help, but not fix a problem.The Y- can be high or the Y+ can be low, to make things off center (down). I think the Y- at 150V is high if there's only 160V available?The deflection amplifiers are identical (i.e same circuit for X+, X-, and same circuit for Y+, Y- (both channels up to alt/chop mux).So you can take comparison readings when the beam supposed to be right in the center.I would go back to the input stage and compare readings, to see if the imbalance starts there or is downstream.IC102 driving Q105 and Q106,IC2 driving Q5 and Q6, should be about equal output voltages. This applies for each stage, going to the CRT.A lazy transistor or bad resistor might be to blame.