Ekco AD65 hum

[jelida]

Hi group,

I have finally got round to sorting an AD65 that I got in a local auction last year. I did some initial work at the time, I replaced all the capacitors (encapsulating in original cases where possible) and checked/replaced the resistors. The big dropper had to have several sections rewound with new resistance wire, and the output tranny had an o/c primary, and was re-wound by Ed Dinning. I sourced some tested valves. However when I wind it up on the variac I just get a loud hum that sounds mains frequency, and comes on after a delay while the valves warm up. The volume control has no effect on volume but does seem to alter the tone of the hum, and the pot can be heard to crackle with operation.

Can anybody give me some basic advice to track this fault? I have an AVO and digital meters, and a copy of the Trader sheet with circuit. It is not impossible that I have introduced the problem myself, though I have re-checked my work to the best of my ability.

Jerry

[Nuvistor]

First thing is to check the work you have done, maybe a simple wiring error.
Sorry should have read the last sentence.

Check the smoothing and reservoir capacitors, it could also be leakage between heater and cathode in the audio stages.

[G6Tanuki]

One time I had a similar fault was when I'd made the mistake of assuming that the can of a replacement dual-element smoothing-capacitor was connected to the common negative terminal and so would be grounded by the can-clamp.

it wasn't, so I ended up with the 2 capacitors essentially forming a nice AC bypass round the smoothing resistor!

[jelida]

Thanks both. I have substituted alternative smoothing/res caps, no change. Cap earthing is OK.

Jerry

[Nuvistor]

Does it have a mains energised speaker, if so have you left the hum bucking coil out of circuit or connected it the wrong way around.
I will have a look at a cct if it is on Pauls DVD later today.
Frank

[jelida]

Yes the speaker is mains energised.

[Ed_Dinning]

Hi Jerry, if you can link G1 and chassis on the output valve with a low value resistor (say 1K, most of the hum should disappear (do this with the set off) if the hum is in the signal chain up to the output valve. If it is at the same level it is in the HT, the valve or a faulty valve socket.

Ed

[jelida]

Thanks Ed, will try that.
I have checked the hum-bucker coil, its original solder connections were intact, so I swapped the field coil connections and the hum got louder, so they are returned to their original positions .

Jerry

[jelida]

The output valve PenDD4020 G1(TC) to chassis link made no difference, so the possibilities are narrowed. Will review the HT circuit. Could it be to do with the CY1 rectifier, the only valve I cannot substitute?

Jerry

[Nuvistor]

I let you check out the suggestions from Ed, I have never looked the circuit before but it is an interesting design with the reflex AF/IF stage and the volume control altering the grid bias on V1 and V2.
Frank

[jelida]

Cant find anything obvious so far. I have taken a couple of voltage readings with my Avo 8, do they give any useful info?

Jerry

[Station X]

When working on my own Ekco AD65 I found that all the voltage readings pretty well matched those in the manufacturer's service sheet.

The exception was the VP1321 screen grid reading where I got a false reading because I was using a DMM rather than an analogue meter of the period. Bear in mind that voltage readings will vary depending on the setting of the "gain" (volume) control.

Any problems I had with hum were down to the fluorescent lights in the workshop being turned on, including the bench magnifier lamp. This problem is not confined to AD65's.

These aren't easy sets to work on. The whole thing is built around the "tag" strip making it impossible to change one capacitor at a time and then check the results. Hence meticulous checking of all wiring and component inter-connections is required following replacement of components on the strip. The unconventional circuit can have you scratching your head too.

[jelida]

With another bout of enthusiasm I thought I might re-open this thread. Nothing has changed, I have tried a new CY1 rectifier and different smoothing caps, no difference to the hum - which is of the aggressive loudness that has you diving across the workshop for the off switch. As earthing the output valve grid made no difference, I presume this is a HT issue (the valve bases look clean) and as you can see in my diagram above there is a lot of AC in the HT supply, which I presume should be eliminated by the hum-bucker system? Is it possible that the fault lies in the mains energised speaker/opt combination? Is there a straightforward way of checking this? Or does anybody have a spare 8" Rola model F series speaker or parts that I can interchange?

Jerry

[Biggles]

Is it possible that AC is being introduced into the HT supply via R16? This seems to be quite an unusual set up to me. Can't quite understand the purpose of R16. I assume the wiring is correct for the different valve type options on the diagram. Have you got the correct valve line up?
Alan.

[ms660]

According to the trader sheet, the cursor bulb is rated at 300 ma, the valve heaters are 200 ma, also the bulb will get dimmer as the valve heaters warm up so to help compensate for that an extra 40 ma or so is supplied to the bulb from the HT via R16 so far as I can make out.

Lawrence.

[PJL]

There are a couple of odd things about the voltages.
1. The AC volts on the HT is greater than the rectifier
2. There appears to be a 45V DC drop on the output transformer primary which is 650 ohms according to trader 649 which suggests its taking 69mA
3. The energising coil is 400 ohms from the trader so the 40V drop gives 100mA which seems a bit high.

On the basis that the DC voltages may not be correct due to the presence of a large AC non-sinusoidal signal I would put the most likely culprit as C18. You say you have already checked this but have you tried adding an additional capacitor?

[jelida]

Thanks for the comments above. I will revisit the issues raised tomorrow.
Is it most likely than that the hum-bucker set-up is not the issue?

Jerry

[DangerMan]

You seem to have effectively proven that the hum-bucking arrangements are working properly.
Connecting grid of the output valve to chassis, as you have done, rules out the previous stages but does not rule out a heater-cathode short or leak in the output valve itself.

The easy way to check this is without trying a new output valve is to fit a temporary switch in the heater circuit... fit the switch in the feed to the "hot" end of the output valve heater from the dropper resistor. With this switch "closed", turn on the set and wait for it to warm up as normal and when the loud hum appears, open the extra switch (i.e. switch off the heaters).

If the hum immediately disappears, suspect a heater cathode fault inside the PEN40DD... if it dies away slowly as the heaters cool down... you have at least ruled out h-k shorts in the audio output valve, and can report back the findings.

HTH
Pete

[jelida]

Spent a little time on this problem this afternoon. No solution yet, but I can report the following.

The hum dies away slowly when the heater is switched out as suggested by Dangerman.

I have swapped C18 and C17 with new can capacitors, no difference.

Substituting another field coil loudspeaker/transformer combination makes no difference.

Disconnecting pin 7 (screen) of V3(output valve) from the circuit stops the hum, apart from very faint 'normal' loudspeaker hum. But of course there is then no signal eg from crackling from volume pot.

Any ideas?

Jerry

[jelida]

Bump ....I definitely need help with this one, any ideas?

Jerry