Troubleshooting Topward 7021 20Mhz Analog Oscilloscope

[Sam__]

So I was really excited to unpack my "new" Topward 7021 20Mhz analog oscilloscope today. I got it for cheaps off ebay and it was labelled as untested, but had picture with a line across the screen so I thought it might just be working. Of course it wasn't working! Don't be a fool!

Well I started it up and the first thing I noticed is a not so good burning smell. I thought this just could be dust burning off or something, maybe, hopefully, please be dust. I continue and play with the knobs, as any good man does with a new toy. Here is what happens.

The time division knob works well along with the horizontal position and CRT brightness. Nothing to do with either channel works, selecting a channel changes nothing on the scope, just a single blurry line is displayed. The voltage divisions don't work. So with my limit knowledge of scopes I would guess it's something to do with the input circuitry???

I have taken the case off and here is what it looks like:
top down view:


side view:


another side:


After some nasal investigation the smell definitely originated from the metal can on the underside of the top board, shown in the second and third pictures.

I am yet to find the service manual to figure out what's under the can. Any ideas what it could be?

Also, not shown well  in the pictures but there has been some definite reworking of the top board, there is a pot soldered on over the marking "focus" on the board. I'm not sure how many components have been replaced or changed but it looks like some work has definitely happened to this scope before. Looks like my hopes of a cheap first scope have now turned into a repair project. It's time to learn!!

[w2aew]

Did you apply any signals to the vertical inputs?
Did you check the input coupling controls, to be sure they weren't set to ground?
Do the vertical position controls do anything?
Does the focus control work?

[Sam__]

Did you apply any signals to the vertical inputs?
Did you check the input coupling controls, to be sure they weren't set to ground?
Do the vertical position controls do anything?
Does the focus control work?

I tried using the cal signal and also hooked it up to a 555 timer, on both channels. The display did not change.

I tried the input coupling controls on all settings, gnd ac and  dc. This made no difference to the output.

The vertical position controls did nothing either.

I have not tried the focus control. I will give that a go later.

[Sam__]

Anyone have any ideas?

[PA0PBZ]

Clearly the vertical amplifier is dead. Start by looking at the power supply, are there any points marked with a voltage? If so measure all of them.
I tried to find a service manual/schematic but nothing turned up. If you suspect that there is something smelly under the metal can take it off and have a look.

[Sam__]

Clearly the vertical amplifier is dead. Start by looking at the power supply, are there any points marked with a voltage? If so measure all of them.
I tried to find a service manual/schematic but nothing turned up. If you suspect that there is something smelly under the metal can take it off and have a look.

Thanks, I'll give that a go this evening.

[Sam__]

I just took the case off and powered it up (I stayed well clear of the CRT, being alive is fun) and before I could even probe anything I saw sparks between two tracks on the board. Not the best sign. It looks to be between a small 1/4W resistor and big blue 10M resistor. It seems to be around the part which has wires running to the display.

[Sam__]

There is the black burnt PCB from the short. The red arrows point to the short. The blue arrow is one of the legs of a big blue 10M resistor which someone seems to have already tried to replace as the solder joints are clearly done by hand, not overly well.

So I guess some kind of voltage is building up on the trace, enough to jump to one 2-3mm away. This doesn't seem good.

[MrsR]

In the first photo there is a red wire soldered onto the board. Check if it is the same as the wires used in the rest of the Cro. Also where the red wire is soldered onto the board there seems to be a burnt spot just before it is attached.
There shouldn't be that much voltage in the tracks to enable it to jump from track to track. Is there a big Electrolytic Capacitor near the short. The next thing is to find your voltage rails and check them. Have a look at Daves video on tracking down faults on a scope.
Basically it comes down to tracking down faulty components with an Ohm Meter and ESR Meter check for open and shorted circuit components you have found a place to start. I draw a simple circuit diagram of the section I'm working on and check the passives etc. Then move to the next section of the board but check your power supply first and see what voltages are and keep in mind what voltages the ICs run at.
All the best.
Rachael
PS contact the manufacture and see if they will send you a schematic .

[Sam__]

I sent Topward and email about the schematics... $40!! Looks like I won't be getting my hands on those.

This is going to be put to one side for about a month. Lets hope it hasn't magically got any new faults by then.

[lowimpedance]

What ever else may be faulty, the one thing you must do is remove all traces of the carbonized PCB where its arcing (even if this means grinding a channel through that area).
then apply a suitable lacquer or non corrosive silicone blob to the area etc to prevent further arcing.
 Of course make sure there are no faulty parts in the related area that could cause high voltages to appear at those traces (not having the schematics will of course mean you will have to use best judgment and a little reverse engineering of the affected area!). Have you also confirmed the main power supplies are good.
 The focus control on the front panel did not do anything when twiddled??.
When you get back to it get the board out and remove the metal box to see whats going on in there. The 10 Meg and pot are part of the FOCUS cct and if your not getting any focus adjustment at all  something could be open cct allowing the focus drive voltage (will be high!) to maybe drift higher leading to arcing to the adjacent track (guessing!!!).
Good luck
LowZ
 

[Sam__]

What ever else may be faulty, the one thing you must do is remove all traces of the carbonized PCB where its arcing (even if this means grinding a channel through that area).
then apply a suitable lacquer or non corrosive silicone blob to the area etc to prevent further arcing.
 Of course make sure there are no faulty parts in the related area that could cause high voltages to appear at those traces (not having the schematics will of course mean you will have to use best judgment and a little reverse engineering of the affected area!). Have you also confirmed the main power supplies are good.
 The focus control on the front panel did not do anything when twiddled??.
When you get back to it get the board out and remove the metal box to see whats going on in there. The 10 Meg and pot are part of the FOCUS cct and if your not getting any focus adjustment at all  something could be open cct allowing the focus drive voltage (will be high!) to maybe drift higher leading to arcing to the adjacent track (guessing!!!).
Good luck
LowZ

Thanks a lot for the information . Looks like I'm going to be learning a lot!

[MelJ]

I have the same scope which I am trying to repair because it isn't generating any ht to the crt.  Topward were much more helpful and sent through the schematics free of charge as attached here.

If you look at the top centre of the first diagram you will see the crt hv supply components that are under the metal shield. This includes Op Amp U002, transistor Q003 (D880), hv transformer T001 and the voltage doubler diodes (CR09 and CR10) and capacitors (C015 and C016) etc. If you're lucky the heat will be from the transistor which does run very hot but mine tested ok. If you're unlucky it could the transformer which is not available anymore. If I can help with anything else let me know.

I am not an electronics engineer or student so I will post my problem shortly and maybe you or others can explain
some of the circuits elements to me so that I can diagnose my problem.

[Sam__]

Thanks a lot for the information and diagram Mel!!

I have another scope now but I may still have a go at fixing this one. Depends how much time I have.

[MelJ]

OK it seems that Sam is happy for now so here is my problem.

Symptoms and my observations on my problem on a Topward 7021 oscilloscope recently purchased. All these comments relate to components on the first diagram.

1) No trace at all, not even a flash on the crt during startup

2) The neons DS01 and DS02 (centre of diagram 1) were flashing and crackling every 30 secs or so which I took to mean they were going overvoltage. That has stopped now but they still flash gently on shutdown.

3) The mains transformer voltages are all good and the horizontal and vertical amps are generating good voltages that swing nicely when the pots are turned on the front panel. All good there so I started on the HT circuit.

3) I disconnected the socket from the back of the crt and inserted a 100M  Ohm resistor into the cathode socket but read zero voltage to ground (or from the grid) so I started looking at the 2000v supply

4) The HT transformer T001 is running cool and with no signs of burning. I have injected a small sine wave signal into the primary and I get 20 x the voltage on the secondary at 20kHz. I don't know what frequency the input is supposed to run at so I am guessing here but I believe this means that the transformer is ok.

5) The transistor Q003 (an oscillator I think) runs hot but tests fine but I replaced it anyway with no change. The signal at the base of this transistor (measured with my other Hitachi oscilloscope) makes no sense to me as I would expect to see a nice sine wave but I get a muddled trace apparently with some high frequency content that my scope can't display. I will try and take a photo to post here. I tested the components around the transistor (in circuit) while the transistor was out and all looked fine.

6) The 16v supply into inductor L001 near the transformer is running at around 18v and has 2v peak to peak ripple but I assume this may be acceptable. I would expect worse if the smoothing capacitors had gone.

7) The 12v supply into op amp U002 are fine until you get near pins 2 and 3 where the dc voltage picks up a lot of noise, making the trace blurry. But, if I hold the scope probe in the air anywhere near the transformer there is a lot of interference so this may be expected. Any opinions?

My attention is now on the blanking circuit to see if that is stopping the HT. I get a small dc voltage just before Q16 where unblank signal A comes in (far right of the diagram) but I don't know what to expect there. The diagram shows another unblank signal B but those components are not present on my board. The unblank signal on my Hitachi is an alternating square wave signal but i don't know if this should be similar. Any ideas on whether failure to generate an unblanking signal would stop the circuit generating HT? I don't understand what the circuitry is doing after the unblank signal comes in and it looks like high voltages so I am not probing around in there.

Any ideas on what to do next would be very much appreciated.

[MelJ]

Progress!

Probing the oscillator circuit with my other scope now shows good sine waves but still no high voltage to the CRT. This wasn't the case before so something has changed but what? It look a while to work out that this time I was testing with the CRT heater connection unplugged (from the input transformer to the CRT supply board). Sure enough when I plug the heater back in the oscillator signals become a mess again. The CRT connection on or off makes no difference but when the heater input voltage is connected it all goes wrong. Even so there is still no HT to the CRT. The heater supply voltage looks fine at 6.3v ac.

With the scope off injecting a small sine wave signal into the HT transformer primary produces good AC voltage on the secondary which is being rectified correctly and is arriving at the output connector that goes to the CRT. But, when the scope is powered up there is no HT!!!

Some resistance testing after the transformer shows that R042 (1K0) in the centre of diagram 1 has failed open circuit. I will replace that but can anyone tell me what this does? It connects the 2kV CRT supply to the heater supply - why? I thought it might be a bleed resistor but 1K0 seems a low value for that. When it failed open circuit why would it stop the HT reaching the CRT as it isn't in the direct path to the CRT? Could it have failed as a result of a failure elsewhere? If so what might that be? I would like to avoid replacing R042 and having something else blow.

Any advice appreciated.

[MelJ]

Back to square one!

With resistor R042 replaced the original symptoms are back. On switch on the neons DS01 and DS02 start flashing and arcing continuously suggesting that the neons are going over voltage. I have to switch off immediately before R042 blows again.

The CRT heater supply comes in to the HT PCB on a 2 pin connector from the mains transformer and goes out again on a 5 pin connector to the CRT together with the CRT Cathode and Grid connections. If I disconnect the 5 pin connector but leave the heater 2 pin connected the arcing starts as soon as I switch on. If I connect the 5 pin and disconnect the 2 pin heater connection all is quiet. So, the neons only go over voltage when the heater supply is connected and the arcing does not appear to be associated with the CRT itself (good news).

It seems that the reason the 2kv HT supply is connected to the heater circuit (via R042) is to bias the heater supply to near the cathode voltage to avoid insulation breakdown between cathode and heater. The heater winding insulation on the mains transformer therefore has be able to tolerate 2kv. One possibility is that the heater winding insulation is breaking down and pulling the HT supply to ground through R042. This may be what is causing the overvoltage across the neons. Does this make sense or does anyone have any other theories?

There is space to fit a separate 6.3v secondary transformer and they are not expensive but non of them specify that they can cope with 2kv so this may not be as easy a fix as it seems.

Any contributions appreciated.

[SeanB]

You could get heater booster transformers that did both isolation and voltage boosting for aging CRT tubes.

http://www.ebay.com/itm/Tektronix-120-0036-00-Transformer-For-CRT-Supply-/300844306644?pt=LH_DefaultDomain_0&hash=item460bb7d0d4

Here you go for one, and genuine TEk part as well.

http://www.ebay.com/itm/LOT-VINTAGE-RADIO-TV-TRANSFORMER-TELEVISION-TV-PICTURE-TUBE-BRITENER-/290736249445?pt=LH_DefaultDomain_0&hash=item43b13b1a65

Here are a batch of CRT boosters, they probably also do isolation as well.

[prebblet]

"One possibility is that the heater winding insulation is breaking down and pulling the HT supply to ground through R042. This may be what is causing the overvoltage across the neons."

Correct. I assume you eventually installed a 6.3V tranny. I have had the same problem and repaired with a new transformer.

Thanks.

[torittog]

  Hi, I have a problem with the horizontal deflection of a Topward 7045 oscilloscope.

I only see the right half of the wave.

I'm looking for the power supply and CRT circuit diagram.

Can you help me?

Thank you very much, G.