WEM Custom 15

[ppppenguin]

There's something odd about the ripple on the first trace. The timebase seems to be set to 10ms/div so the period is about 45ms. That corresponds to 22Hz which is impossible. If the EZ81 is working as a full wave rectifier the ripple will be 100Hz and period should be 10ms. If something is wrong and it's working half wave the period would be 20ms. What is the the actual timebase setting? Is the variable speed knob at the calibrated position? Is there an x10 multiplier that has been switched on? I can't see one but I'm not familiar with this scope. Let's hope the scope is close enough to accurate that we can rely on the readings. It's a decent but old scope so unless it's been "got at" or there's a real fault it should be OK.

As for amplitude, we don't know if you're using a x10 probe and the V/div setting isn't clear from the photo. Nor do we know if the variable control is in the calibrated position. It would be useful to know the actual peak-peak amplitude of the ripple. It's clear that the ripple is being smoothed in the 2nd photo. To measure the amplitude you would need to increase the gain.

All this is absolutely basic measurement technique when using a scope and shows how usefull it can be when diagnosing faults.

[Murphyv310]

Thanks Jeffrey.
I can now see the settings as I'm using my own screens now. Yes I concur with you Jeffrey, the thing though is there is no hum with just the EL84 pair and EZ81 in circuit and the smoothed HT looks fine. Only when the phase splitter is in circuit does the hum return.

To Rob there are many you tube tutorials on how to use a scope and its not easy to explain unless in a one to one basis. My opinion is also get used to an analog scope before you even think about a DSO.

[Murphyv310]

We really can't continue until I'm certain the PSU is 100% we need to know that the HT secondary of the mains transformer is OK and wired correctly, so the reading I have asked for between the two anodes of the EZ81 needs to be done, it is possible that one section of the winding is open circuit that would give just half wave rectification perhaps caused by the valve that flashed over. We also need to know the settings on the scope, were they set to the calibrated stop or variable.
At the end of the day if we can't do this here as before I will offer to repair this FOC with the exception of carriage and do a step by step write up and YouTube videos including use of the scope.

[Mike Watterson]

Yes, the resistance at pin 1 & 7 of the socket, not the actual removed valve, which should be open circuit between 1 & 7.
One winding / pin will be slightly higher resistance to the centre tap as one winding is on top of the other so for same number of turns has slightly longer path.

[Murphyv310]

Sorry I mean on the base, pins 1 & 7.

[Bushbaby]

The resistance across the base of the EZ81 pins 1 & 7 is 51.7R.
I did set the oscilloscope probe to x10 as was indicated. I ddo have to say that the T935a volts/div setting is hard to interpret. The manual indicates that the reading must be taken from the x10 (I assume the light grey) slot on that side.

[Murphyv310]

OK so the transformer is not open circuit on one leg.
Can you now check the voltages on the grid ones of the EL84s as I've asked with all valves in place except V1 and V2.

I'm actually expecting these to be OK as the G1 voltages would still be wrong if the coupling caps were leaky with V3 out.
But let's check this anyway.

[Mike Watterson]

It has to be a solder splash, a cracked track, one wire wrong or a coincidental failure as there was no hum before transformer changed?

I'm expecting something to turn up around the the ECC83 phase splitter stage as there is no hum without it. It's obviously not a PSU problem.

[Mike Watterson]

All this is absolutely basic measurement technique when using a scope and shows how usefull it can be when diagnosing faults.

Absolutely.
I couldn't fault find the "simple" battery valve AM radio I'm looking at without my 20MHz Hameg, a Pygmy Club 1957 approx. Though often I hardly need more than the DVM, my capacitor meter, Chinese universal component/cap/inductor meter, precision low value inductor/cap meter (uses internal reference SM cap) that does to 1uH and 1pF sensibly and my home made 600V via 2M Ohm plus neon capacitor tester, that also with the 20M input DMM lets you non-destructively measure diode PIV and transistor Vcb / Vce. Very large chokes or transformers need the AVO to measure winding resistance as the DMM gets confused by the inductance as it doesn't exactly use DC like an analogue meter.

If you are designing RF modulators, mixers and oscillators a spectrum analyser is more use than a DSO. I also use it for designing filters using a noise source rather than a tracking generator.

[Murphyv310]

Yes Michael this all points to a man made fault. The only other thing may be C2 being open circuit or not properly connected.
Also look for shorts or solder splashes between pins 4,5 & 9 to adjacent pins on V3 valve base.