Hameg 203-6 low voltages +12v -12v +5v and +152vall reading low.

[JohnMcGivern]

Hi all hope you are well.
I switched on the above scope after months of inactivity.
Led came on short spark noise from scope the trigger led flashed but no gnd trace focus or intensity. The low voltages are all reading less than they should be.
Does anyone have a schematic would be most appreciated also any help appreciated, many thanks, John..

[ajgriff]

Hope the attached PDFs help.

Alan

[JohnMcGivern]

Thank you for the PDF’s. I’ll post results.

[turretslug]

These basic but handy 'scopes use mains-derived EHT, with a diode multiplier fed from a high-voltage winding on the small mains transformer. It may be that the spark noise and lack of trace point to an EHT problem that is loading this common mains transformer- does this transformer get hot, do you have a means of measuring the mains input power/current? An EHT winding breakdown would be a worst-case scenario, hopefully it's something less serious,

Good luck, Colin

[frsimen]

All the supply voltages that you have mentioned take their reference from the +12V supply. Assuming nothing nasty has happened to the mains transformer, the source of the problem will probably be found in the +12V section or something connected to it.

Paula

[JohnMcGivern]

Hi, thank you for replying Colin and Paula.
I found a 50milli amp fuse that was blown which I presume made the noise.
All voltages are good but still no trace heaters are working.
No screen activity focus or intensity.
Transformer not to warm.
Will check +12v tomorrow, many thanks again. I think the voltages were low as I was using a dim bulb setup dim me ha ha.

[JohnMcGivern]

Hi again, rechecked all low voltages all good.
Nothing from crt, focus or intensity controls heaters lit.
130v at R921 good.
151v at R815 good.
-1877v and -1900 difference 23v good.
Any help appreciated.

[ex 2 Base]

JohnMcGivern. I've sent you a PM. Ted

[frsimen]

It could be that the trace is being deflected off the screen, or that there is something wrong in the blanking circuit.

To check if the trace is off the screen, switch to X-Y mode and measure the voltages at points D1 and D2 on the X-Y amplifier board. They should be roughly equal and in the region of 70-75V. The X shift control should allow the voltages to be adjusted to be equal and that should centre the trace in the X direction.

Next check the voltages at D3 and D4. It should be possible to adjust these to be equal with the channel 1 shift control. The voltages should be around 75V when the trace is centred.

I gather that some of the transistors associated with the blanking circuit are plugged in to sockets. Try reseating these transistors, as a poor contact will upset the operation of the brightness control. The transistors in question are T501 to T505. Check that the EHT has discharged before touching anything in this part of the oscilloscope.

Paula

[JohnMcGivern]

Hi Paula, the transistors are soldered it’s the ic’s that have socketed pins. I tried to adjust D1/D2 also D3/D4 but was unable to achieve the measurements any ideas which components may be faulty.. Thank you for your help..

[frsimen]

Without knowing what you have measured, it's not possible to suggest anything.

If you set the oscilloscope to X-Y mode, measure the voltage on D1 with the X shift control fully anticlockwise, in the mid and fully clockwise. Repeat for D2.

Measure the voltage on D3 with the Channel 1 shift control fully anticlockwise, in the mid position and fully clockwise. Repeat for D4

Report back what you measure (12 measurements in all). It's quite possible that the fault lies elsewhere, these measurements will allow that judgement to be made.

Paula

[JohnMcGivern]

Hi Paula, these are the measurements.

D1= anticlockwise = 20.5v Mid=69.5 Clockwise =114.2
D2= anticlockwise = 112.7 Mid=68.9 Clockwise =26.5

D3=anticlockwise = 122.5 Mid=122.5 Clockwise =122.5
D4=anticlockwise = 32.8 Mid=32.8 Clockwise =32.8

[frsimen]

The voltages at D3 and D4 will cause the trace to be driven off the top or the bottom of the screen, which is why you're not seeing anything on the CRT. One further check will determine roughly where in the Y amplifier circuit you should be looking.

Switch back to normal timebase mode.

Switch the Y channel to Channel 2 only by pressing the I/II button, with the Dual and ADD buttons left in the out position.

Switch the trigger to AT (the out position).

Adjust the Y Pos II control while measuring the voltage on D3, repeat for D4. If the you see a voltage change now, the trace should appear on the screen, if the voltages don't change you will not see a trace.

The result of this test will narrow the fault area down a little. Report back with what you find. If the voltages do change, the range of voltages would be useful to know.

If there are any plug in transistors or ICs in the Y amplifier part of the circuit, try reseating them.

Paula

[See_Mos]

Also try pressing the component tester switch in as this eliminates the time base and front end amplifiers. X-Pos still functions but Y-1 and Y-2 pos are disabled.

[JohnMcGivern]

New readings.
D3 = anti-clockwise 122 Mid = 58 Clockwise = 33.7

D4 = anti-clockwise 33.7 Mid = 97 Clockwise = 122

Reseated two IC’s. and pushed in component tester no difference.

[frsimen]

The bulk of the Y amplifier circuits seem to be working, whatever is wrong in that area is limited to channel 1.

Before getting too involved, just operate all the switches a few times to clean them up. Dirty switches are the most likely cause of the odd behaviour on channel 1 and may also stop the trigger/timebase circuits working properly.

If after operating all the switches there is still no display, this is pointing
towards a fault on the CRT board.

Great care is needed when working around the CRT board. Do not attempt to measure the voltage on G1 (pin 3) of the tube, as doing so will damage it. A safer approach is to carry out some resistance measurements with the oscilloscope unplugged and the EHT discharged.

Check the resistance from terminal 3 on P4-5/2 to the -1900V rail and from terminal 2 on P4-5/2 to the -1962V rail. They should both measure less than 500k ohms.

Also check from terminal 4 of P4-5/2 to terminal 3 on P4-5/2. Adjusting the INTENSITY control should change the resistance smoothly from close to 0 ohms to 500k ohms. If the control doesn't operate smoothly, try turning it over the full range a few times.

Repeat the test between terminal 4 of P4-5/2 and terminal 2 on P4-5/2.
That should also change smoothly from 0 ohms to 500k ohms.

If all these are correct, confirm that you have continuity from terminal 4 of P4-5/2 to pin 3 of the CRT.

If all the above are correct, the INTENSITY circuit is not faulty.

There are a few other voltages which can be measured. The voltage across C733 (should be about 73V) and C727 ( should be in the region of 50-100V).

If all these are correct, there is probably a fault in the blanking circuitry.

Paula

[JohnMcGivern]

Hi,
Resistance from terminal 3 on P4-5/2 to the -1900V rail = 360kohms.
Terminal 2 on P4-5/2 to the -1962V rail = 177kohms.

Terminal 4 of P4-5/2 to terminal 3 on P4-5/2 = 0 ---204kohms.
Terminal 4 of P4-5/2 and terminal 2 on P4-5/2 = 0 ---665kohms

C733 = 75.6v
No access to C727.

[frsimen]

The intensity circuitry should work given these measurements.

Earlier on, you reported that the channel 1 shift didn't do anything to the voltages at D3 and D4 when channel 1 was selected. Is that still the case, or has the problem gone away? If the fault remains, it's probably best to sort that out before going any further. Once that's working, the blanking issues can be investigated.

The blanking may not be working properly, but several different areas feed into the blanking circuit and could be forcing the display to blank. Fault finding in that area will be limited to resistance checks on the transistors and diodes as it isn't safe to go poking around the -1900V area. There is a way around this, involving setting up a 12V supply and a 24V supply, which will allow testing to be done safely. You can use batteries here, as the currents needed are quite low.

One last question, when the oscilloscope is switched off does anything flash on the screen, even if only briefly?

Paula

[JohnMcGivern]

Hi Paula, I have a power supply, Isolation Transformer variac and dim build setup. The Hameg is powered through I/T, Variac at 240vac. I removed front panel had to desolder component tester, 0.2v/2v calibration signals to remove front panel. I have a Fluke to test transistors and diodes I have tested all accessible diodes all good which transistors would you like me to check obviously with no power supplied.

[JonSnell]

I note you found a blown 50mA fuse. Did you find the cause as that may well be the issue here.
As I normally say to those fault finding, don't get in too deep and keep it simple.
Easy to say, I know but try to keep an open mind.