EKCO AD75 restore

[PJL]

You must use the isolation transformer if you are attaching any test equipment as the test equipment will be earthed (or should be!) and you would be shorting mains earth to neutral.

[thejazzageuk]

I've taken the traces. The two attached are V1 anode (pin 5). Just 50hz hum I guess?

When I attach the probe to V2 pin 8, the crackling stops and the speaker is silent and the scope finds no ascillating freq.

[PJL]

What type of probe are you using?
The V1 pin 5 trace is very strange as that is a huge amount of hum from somewhere. I guess it could be heater/cathode leakage in V1.
What do V1 pin 4 cathode and pin 8 hexode anode look like on the scope?

[thejazzageuk]

Thanks PJL. I'm using the 10X probe. Should I be attaching the probe gnd to chassis gnd? When I do this the radio outputs a faint 50hz audible hum.

It seems like something has changed in the radio in the last few days. The output volume is lower and I don't hear the frequencies that I previously heard when tuning through the dial.

I think V3 is damaged as the glass was rotating around 10 degrees on the base and will presumabley have damaged some contacts (it's glued now but the damage may have been done). Also V1 was becoming a bit loose as its a tight fit and I've pulled it in and out a few times.

All the traces seem to show only 50Hz. The only exception is on Pin 8 V1 when there appears to be a trace of 2 freqs -50hz and 1250khz (see pic).

[smiler411]

If you are getting 50hz at every test point I think you have a grounding problem you must connect the probe ground to the radio chassis are you sure you are using an isolation transformer that has no common earth connection hum when you connect the probe ground sounds like a grounding issue

[PJL]

Loose valves are common but rarely mean they do not work.

Can you show us a picture of the isolating transformer and how you have it wired.

[thejazzageuk]

Here is a photo of the isolation transformer. The 2 wires on the right are the radio power lead.

[PJL]

Looks OK.
Connect aerial earth to chassis and probe earth to chassis and try again.

[thejazzageuk]

Thanks pjl. I connected aerial and probe earth to chassis ground and did a trace from V1 pin 5 (anode). Unfortunately shows much the same, see photo.

[PJL]

And what about V1 cathode, pin 4, what does the scope image look like? Also check the value of R5 (100K) that should connect from pin 4 to pin 6.

[thejazzageuk]

R5 reads 221k in circuit. When powered there is 125mV AC across it mV DC). Pin 4 also just has 50Hz, see photo. 5

[PJL]

That's a serious amount of 50Hz on the cathode. There are three possibilities:
1. Your scope is not working correctly. Try connecting both the earth clip and probe to chassis.
2. The valve has a heater cathode short. Take the valve out and measure the resistance between pin 4 and pin 2 (or pin 3) on the valve.
3. There is a short somewhere in the wiring between pin 4 and pin 2/3. Either pin 2 or pin 3 is already wired to chassis so it would have to be the other pin. With V1 and V2 removed measure the resistance on V1 socket from pin 4 to the suspect pin (2 or 3).

[PJL]

The other possibility is the 50Hz is being injected into the triode grid. You can test for this by connecting your scope to V1 pin 4 cathode and shorting pin 5 triode grid to chassis.

[thejazzageuk]

Thank you PJL. Here are the results of the checks. Hopefully we are gettin closer to the issue now.

1. Your scope is not working correctly. Try connecting both the earth clip and probe to chassis. R: Scope seems OK. Gives a flat line with earth and probe to chassis.
2. The valve has a heater cathode short. Take the valve out and measure the resistance between pin 4 and pin 2 (or pin 3) on the valve. R: Valve seems OK. 6.2R between 2 and 3, O/C for pins 2 to 4 and 3 to 4.
3. There is a short somewhere in the wiring between pin 4 and pin 2/3. Either pin 2 or pin 3 is already wired to chassis so it would have to be the other pin. With V1 and V2 removed measure the resistance on V1 socket from pin 4 to the suspect pin (2 or 3). R: As far as I can tell neither pin 2 or Pin 3 is connected to chasses. Both give O/C to chassis when measured. Pin 1 and 2 are connected togerher on the socket. With the valves removed I measure O/C between 3 and 4, but 195R between 2 and 4.
The 50Hz is being injected into the triode grid. You can test for this by connecting your scope to V1 pin 4 cathode and shorting pin 5 triode grid to chassis. R Both traces seem similar whether Pin 5 shorted to chassis or not. (see pics showing 1. Shorted 2. Not shorted)

[thejazzageuk]

Here are the pics.

[PJL]

The scope is showing a minimum of -4.8V on the cathode and that should be impossible. Is it set for AC coupling?

According to the Trader V1 pin 2 or pin 3 should be connected to chassis. The valve heaters may be wired in a different order, can you please check the wiring.

Another possibility is that the hexode is causing the 50Hz on the cathode. Put your scope on V1 pin 4 again and short V1 top cap to chassis.

[thejazzageuk]

I think you have hit on one of the faults. When I traced the heater circuit it became clear that the V1 pin 2 goes to a ground point but the bolt on the ground point was loose. I've tightened it up and now the radio sounds much better. Lots of high freq whistles and hums as you tune rather than just a loud buzz. Still will not find a station though and scope traces give unstable waveforms with freqs rapidly changing. Here are 3 traces (taken using ac coupling).
Pic 1. V2 pin 8 anode
Pic2. V1 pin 5 triode anode
Pic 3 V1 pin 4 cathode

[PJL]

Oscillator is not running.
Have you cleaned the waveband switch carefully?
Measure the voltages on V1 again as they may have changed - pins 4, 5, 6 ,7 8 and topcap.
Check the resistance of the oscillator coils L4, L5, L6 and L7.

[smiler411]

isn't waveform 2 the oscillator running, if that is the anode of the triode

[PJL]

Originally Posted by smiler411

isn't waveform 2 the oscillator running, if that is the anode of the triode

819Hz so about 1000 times less than what we are looking for. I guess it is possible that a capacitor has been wired incorrectly.

There is also quite a bit of 454KHz on the hexode anode - not sure if this is instability.