Hi.
Today I did some chassis work on my Lekmek. I replaced the two electrolytic capacitors and the paper ones. I noticed the speaker looking ok, except that one side of the voice coil's wire went to a long wire about 6" long and clipped at the end. Likewise, the corresponding output from the speaker transformer also going to a 6" wire to nowhere? So no continuity there.. I theorised possibly another permanent magnet speaker may have been used in series with the field coil speaker voicecoil, so I put a small speaker between the two 6" wires just in case it's impedance was required, in any case it should only help, but as soon as I made continuity through the speaker, I got this ear piercing WWHweeeeeee!!! with a slight variance in pitch as I lowered either tone or volume. Also tapping the volume wiper gave me the Bbrrr indicating the audio section is working. There seems to be good voltages around the coils. A customary poking around showed me healthy voltages on plates, a nice -15 volt present from the centre tap of the transformer supplying the 40 grid, also 301 volts at the tone pot, the schematic indicates the full rectified potential, so I'm just getting a ,loud Wheeeeeee! and no stations?
I've packed it away for the day. Yesterday I used another metre of course sandpaper, trying to sand out some old stain marks and will need plenty more by the looks, but it would have been nice to tune into a station!
Any thoughts most welcome.
Cheers. Jamie.

Stain marks Jamie??
Pete

Like furniture stain I'm guessing, a round mark as if a small bottle sat with stain leaking? Same colour as the dark wood stain I've been sanding back to get to original light coloured wood.

3 tea spoons of Oxalic Acid in a small glass of warm water will return the veneer back to its natural colour and then you can stain it again or oil etc etc
The veneer will only be 2mm to 3mm think .!
Just paint the oxalic on with a brush over the unit.
When its dry wipe it off with metho to neutralise it.
Then just rub it down with 320 grit.
Put your grain filler in and use a bank card to scrap it off leaving it in the pores full, then sand again with 320 and add your stain or finish.
Oxalic acid is what all furniture restores use to return the veneer or timber back to its origanal colour.
It will all so remove the black ring marks and the dredded black spot.
How to buy it?
Pure oxalic acid is sold at bunning under the name of rust and stain cleaner by diggers $13
Its sound nasty but its no stronger than house hold bleach.
Infact that is the 2 bleaches we use in furniture restoring but never mix them together as the gas from doing so is toxic
Pete
Just a note , the clear top coats are off yes?

Yes Pete, top coats are off, I'm in bare wood, just stained. Thanks for that tip! I shall get some as there are other bits and hard to get into corners and that will help immensely! Cheers.

It will remove all the stains that have been put on the timber and it will return it back to its natural colour. Is the timber oak ?

I believe it is and I've bought oak stain to re-stain it prior to varnish coats, I just want it back to uniform bare wood. I've got to get some glue and filler for the rear bottom of the cabinet where the plywoods gone like bad wafers right at the bottom and glue and clamp it, then fill and sand again before staining.

Sounds like you have a feedback path in the audio section. Without a schematic I can only guess. Is there a connection from the speaker transformer secondary back into the amplifier? Do the wires from the primary pass close to the detector or volume control?

Must be English Oak!
Large long dark grains.
This veneer comes up beautiful. There is No more english Oak left in the world , very nice early Grain.
Oak goes black if it gets water deep into it it so that maybe your stain.the oxalic will remove that.
The other things that oak does, is it goes dark over night if not coated and the timber is bare.
This is how they got that black dark furniture 150 years ago. So if you dont want it darkened by time dont leave it bare wood for too long.
English oak Radios look fabulous when done ! That veneer suites them .
All the best Pete

No feedback coupling from the transformer, but as for the other I'll need to check, sounds like feedback oscillation.

Well I spent a few hours more on it checking and replacing resistors, but it's still going Phweeeeee! So I have some new valves coming so hopefully a new 6A7 and 6D6 might sort it out as the amp stage is working well, just nothing from the radio side? I have a brand new 75 valve in there and the 42 and 80 are good, so it's got to be something else. I've got an oscilloscope now, I just need to figure out how to use it to check for signal.

That sounds like a problem I had with my Little Nipper a while back. Is there a back bias supposed to be there but isn't. Can we have a picture of under the chassis?

Are you running that with, or without valve shields? 6D6 is actually the same valve as 6U7 with different bases & that also applies to 6A7 & 6A8.

6U7 is probably the worst valve ever invented for going absolutely crazy and causing massive instability in the form of oscillation: If the shielding is missing or faulty. 6D6 is no better.

When parts are replaced "lead dress" in the area of the 2nd detector is critical & many a time I have had to add shielded wire to shut it up, when you get feedback from radiation and /or induction. If you prod the wires & parts with a wooden or plastic chopstick and the frequency changes: That is normally where the problem is. Wiring errors also have to be ruled out.

Marc

Jamie, I agree 100% with Marcc, having had experience with 58/6D6 valves in superhets.
This valve has no internal shielding with a bare plate and WILL be unstable in operation without an external shield. I've has symptoms ranging from slight distortion through to downright squealing.
Cause is exactly as Marcc says.
Scoping the plate circuit will show oscillation whether there are symptoms or not.

Note: CHECK that the external shield can is actually connected to the chassis, use your ohms meter to prove a circuit exists!
If not the shield then acts as an antenna radiating the plate feedback better.

I had a clip on 2 piece shield can where the base part riveted to the chassis was isolated by corrosion and not grounding even though the can pushed firm over the base.
If it has a goat shield make sure the chassis clips make contact at both ends, sandpaper corrosion off.
Check ALL the shields.
Cheers, Fred.

Cheers, yes there are shields which seem ok, but I will check them as you suggested. With the oscilloscope, where do I check for signal/oscillation? I have a ground wire and a probe, can I connect the probe to the plate? Will the oscilloscope be ok with the high voltages? I don't want it damaged. I will take some photo's in the morning when I fire up the chassis with the speaker and new valves fitted and the antenna connected. Hopefully I'll get a station. It has a long wire round resistor with tappings, with 15,000 ohm written on it, yet can't get continuity trying to measure resistance anywhere on it, yet with power, I have 309 volts at one end, 4 volts at the first tapping and one volt at the next and one volt at the other end, maybe it's oc? not sure.. I'll photograph it.