IWATSU SS-5705 Oscilloscope Tear Down/Repair - Help me repair this thing please!

[lowimpedance]

Okay the transformer winding resistances don't seem unreasonable I guess!.
If you do not get any voltage readings at R34 relative to pin 5 of the hv transformer then something is stopping the oscillator from starting.
The resistance values measured don't point to any obvious problem with them.
 You mentioned that you used dt-830 to measure what exactly? CCT powered on or turned off, voltages etc?.
Maybe replacing the 4558 opamp anyway, Just a thought is the 10v rail present on pin 8 of this 4558?.

[David_AVD]

Maybe also worth checking / replacing any electrolytic capacitors located next to heatsinks.

[mianchen]

Okay the transformer winding resistances don't seem unreasonable I guess!.
If you do not get any voltage readings at R34 relative to pin 5 of the hv transformer then something is stopping the oscillator from starting.
The resistance values measured don't point to any obvious problem with them.
 You mentioned that you used dt-830 to measure what exactly? CCT powered on or turned off, voltages etc?.
Maybe replacing the 4558 opamp anyway, Just a thought is the 10v rail present on pin 8 of this 4558?.

I have no idea how to test the 4558 in circuit so I desoldered and tested it on a bread board with some basic op amp configurations. It seems that it is dead!

I've soldered an IC socket in the place of the op amp and odered a few 4558 and some LM358s today, hopefully that is the ONLY problem!!!

Maybe also worth checking / replacing any electrolytic capacitors located next to heatsinks.

All caps seemed ok when tested using my home made ESR meter...

[djsb]

This may be helpful

http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

It helps to diagnose the problem at the front panel level before removing the cover. The PDF explains the technique.

[lowimpedance]

The opamps are cheap and easy to replace so its an easy test to replace with a new one. Lets hope that's it.
Probably would have not suspected the caps so much (sure check if any are near a heat sink but where not talking switch mode environments here).
Keep us posted if the opamp fixes or otherwise.

[mianchen]

This may be helpful

http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

It helps to diagnose the problem at the front panel level before removing the cover. The PDF explains the technique.

Thank you, it's very helpful.

The opamps are cheap and easy to replace so its an easy test to replace with a new one. Lets hope that's it.
Probably would have not suspected the caps so much (sure check if any are near a heat sink but where not talking switch mode environments here).
Keep us posted if the opamp fixes or otherwise.

Put the op amp into the IC socket, powered up the scope - nothing happened... Too bad I don't have an other scope to poke around the circuit with. I used a frequency counter to measure the output pin of the op amp, it read about 14KHz. I guess this is the indication that the oscillator is working?

When I accidentally poked the lead of R13 connected to pin 4 of the HV transformer with a frequency counter probe. I could hear very high pitched squeaky noise coming out from inside the "Danger High Voltage" box (the transformer?). After the squeaky noise, I saw a green dot on the CRT, when the Beam Find/Intensity knob was pushed. The dot then quickly faded away when the noise completely disappeared.

I think there is hope! The CRT is still alive. Did the frequency counter work as a cap (measured 10nf with a DMM) and brought the circuit back to life temporarily?

[mianchen]

Ok, I became braver and kept the frequency counter probe on the 'squeaky' spot (pin 4 of HV) for longer. Now I could see some traces appear on the screen. It's moving slowly on the screen and the positions seemed random. I tried to take photos but they didn't come out very well (see pic):

[mianchen]

To make it easier for people to help, here is the schematic:

[David_AVD]

As well as replacing electrolytic capacitors in the suspect area, also check the value of higher wattage resistors.  Power resistors going high in value is reasonably common in enclosed spaces.  Also check for cracked solder joints.  Make sure you discharge any of the larger / higher voltage capacitors before poking around.

[mianchen]

Strangely, I can't replicate making the squeaky noise any more...Poked the output pin of the op amp with FC reads 50Hz (was 14kHz) and pin 4 of HV transformer reads ~400kHz and fluctuating.

Oh, well, that's it for today, try again tomorrow.

As well as replacing electrolytic capacitors in the suspect area, also check the value of higher wattage resistors.  Power resistors going high in value is reasonably common in enclosed spaces.  Also check for cracked solder joints.  Make sure you discharge any of the larger / higher voltage capacitors before poking around.

All the caps and resistors in HV area have been checked, could not find anything... 

[lowimpedance]

 Mianchen, I have noted that you have a shiny new Rigol from the 'multimeter thread'. Perhaps you could use that to look at the waveforms around the Iwastsu HV oscillator Primary and the opamp output to see if there is something sensible happening!. (make sure you use the x10 probe of course!, to reduce loading and to watch out for the 70v rail on the transformer and the switching transistor).

Nice HP multimeter for the price BTW

[mianchen]

Nice HP multimeter for the price BTW

Thanks, I really want to get a more modern DMM maybe 87V or something, but this one isn't too bad

Mianchen, I have noted that you have a shiny new Rigol from the 'multimeter thread'. Perhaps you could use that to look at the waveforms around the Iwastsu HV oscillator Primary and the opamp output to see if there is something sensible happening!. (make sure you use the x10 probe of course!, to reduce loading and to watch out for the 70v rail on the transformer and the switching transistor).

I was busy playing with the Rigol last week didn't have a chance to look at the Iwatsu

So I probed the oscillator opamp output pin one, all I have was noise (so I believe), please see pic 1.

Probing pin 1 and pin 2 of the HV transformer gave me some interesting waveforms (pic 2 and 3).

I think the opamp oscillator is not working at all? I have swapped the opamp with a few new 4558Ds, everything remained the same. These 4558Ds were actually working in the oscillator test circuit I built on a breadboard - so the IC can be ruled out?

I re-measured all power rails in the 'scope, all spot on except the +70V rail read ~65-66V.

I don't really have much of a clue what to do next :<

One thing I have found was that C46 in the schematic is missing on the PCB...

[mianchen]

It was very difficult to probe around the 'scope because there was no silkscreen or any marking on the solder side of the PCB. I have drawn the components in Photoshop to help me quickly identify them. I think I'll just post it here in case anyone out there also have one of these 'scopes with faulty HV circuit

[lowimpedance]

 The 70v rail is an unregulated one (nominal value),so it could be around the measured value and the waveforms at the transformer are the main PS ripple.
Is the '70v' dc rail present at pin 2 of the HV transformer?. Also could you measure the dc values on the pins of the op amp. DC on Q03 and Q01 C,B,E 's
Double check the HV transformer windings and be sure the other semi's in the oscillator circuit are in fact ok. (are you sure?). If yes recheck the passive components.
No cracked pcb traces?.
 Just one other part to check, test if R41,42, and pot R43 are ok!. (all meg ohm values!) which are part of the Focus cct.

[mianchen]

Is the '70v' dc rail present at pin 2 of the HV transformer?

Yes

Also could you measure the dc values on the pins of the op amp. DC on Q03 and Q01 C,B,E 's

All DC measurements are in the picture below.

Double check the HV transformer windings and be sure the other semi's in the oscillator circuit are in fact ok. (are you sure?). If yes recheck the passive components.

I've desoldered the HV transformer and measure the windings, it seemed ok - all resistance of the windings were similar to what I have measured in circuit and there were no shorts and opens.

No cracked pcb traces?

I could not see any visual cracks

Just one other part to check, test if R41,42, and pot R43 are ok!. (all meg ohm values!) which are part of the Focus cct.

R42 and R43 are what they meant to be in the schematic, R41 measured 2.2M (6.2M in schematic), however when I opened the metal can covering the HV circuit, R41 is indeed a 2.2M resistor according to its colour bands.

I've replaced most of the resistors just in case... the ones that were removed from the PCB were actually fine

I'm beginning to loose patience well I think I don't have enough knowledge to tackle this as a beginner .... I keep wondering that maybe time is better spent to learn more basic stuff first.

[lowimpedance]

 OK, maybe its down to stab in the dark approach now!. Have you replaced q1 and 3 with new parts?, if not do that (if available and not outrageous in cost!). The other possibility is the secondary is loading cct. , killing the HV oscillator from starting (wild guess!!). Try to measure the diodes on the tube bias cct's. and the resistors (note your multimeter may not be able to forward bias the HV diodes!).  Bit hard to tell if the x12 multiplier Mo1 is OK (given you got some sort of "spot" when probing with the counter, maybe its fine, as was getting some heater drive voltage).
 It is getting a bit hard to guess the problem remotely (nothing beats getting your hands on it to try some new idea if the previous one failed!!!).
As I said earlier put it away for the time being, but remember you have gained some useful experience which you can apply to something else in the future.
Good luck.