Help for Dougal Ekco AC 86

[bionicmerlin]

Hoping someone can give me a little help on a Ekco AC86.

After getting this radio in a non working state and replacing the output transformer and all wax condensers and leaky electrolytics I find the radio works fine powering it up on a variac. Now here's the problem.

If I up the voltage on the variac to over about 200 volts the radio sounds like a black and decker sander but works fine if I stick to 200 volts.
Even with the volume down I get the same noise. I believe the problem is to do with valve 4 that should be a 354V it has been replaced with a MH4, I first thought it was bad screening on the valve but this is not the case. The screening appears to be connected to the cathode. The interesting thing is this is fed from a tapped resistor but in the past it looks like someone has bypassed this connecting the cathode and screening directly to the chassis but then removing it leaving a pencil note on the chassis.

I have tried putting this non factory mod back in place and it does cure the problem. I have also tried it with a correct 354V valve and still has the same problem ie the sander noise. It must have this incorrect link in to work.

Question is can I use it like this or is there a better option or possible fault that I should be looking for?

Thanks in advance, Andy.

[ms660]

If with the original type valve fitted and the cathode connected as per the schematic there is a fault symptom then I would say there is a fault.

If it was a flawed design the manufacture would usually issue a modification, as to whether that was the case I wouldn't know.

Lawrence.

[Station X]

I see that the manufacturer's service sheet gives various suggestions as to what might be causing instability. Have these been checked out?

[jonnybear]

If you have a signal tracer or access to another amplifier you could attach it to the volume control and increase the voltage with your variac, this will prove where the oscillation is coming from IF or AF . the valves you mention are very similar and cross reference in one catalogue as equivalent types.
manufacturers data states R5,R11,C15,C18 will cause motorboating.
If you have the Manufacturers service sheet (on the VRSD disk) it goes through a few trouble shooting items.by shorting the cathode to chassis you are removing the positive feedback and changing the cathode bias of the output stage by shorting the 75 ohm section of R15.
John

[ms660]

To rule out mixer/IF instability, you could temp bypass the control grids of those stages to chassis with a hand held .047 or .1uf, instability is most commonly down to decoupling problems such as capacitors or poor chassis/screening connections or self inflicted (by whoever)

Good luck with it.

Lawrence.

[bionicmerlin]

Thanks so far guys I will have a late night in the workshop see what I can find with your advice