Hello it's me again. A gremlin has moved into my Emperor console! After fitting the new r-wound power transformer the old girl worked good for a solid 5 hrs 3 times a week before suddenly going extremely garbled, it's loud like a whistling kettle with an almost motorboating perceptible, but extremely garbled reception. If you know the song playing you can hear a resemblance, but very garbled with a loud whistle like an angry kettle boiling. If you turn it on theres 5-10 minutes of it sounding great, however you need the volume advanced about quarter of the way for a listenable volume and halfway it's sounding great, the after 5- 10 minutes it suddenly starts and lasts for about 30sec to a couple of minutes, however once it kept up its shenanigans for over 5 minutes before I intervened. For some reason, if I wiggle the wire on top of the 57 valve it instantly reverts to normal for either a while or as soon as I withdraw my hand from the valve. I bought two new 57 valves, yet after 10 minutes it started playing up with each one. The shield seems fine, I've had it out and in many times wiggled it around took it out and put it back but it's being a real bitch, I intervene, turn it off and on, wiggle the wire on the 57 and it comes good, I sit down and no sooner, off it goes again. After half an hour to an hour it happens less. After 2- 3 hours it stabilises and behaves. As soon as it is off and cools down, the cycle repeats. I'm guessing it may be a dry solder joint somewhere? Another thing as I can turn the volume to all the way off once it's been on for a while and it's as loud as quarter volume when the set has only been on less than half an hour. Another annoying thing is that over time it gets louder, you set it at nice level to begin with, but after an hour, bam it's blaring loud, then you adjust it down to a nice volume, then it drops out whisper quiet, then back to normal then quiet, normal again at random times?
Just wondering if this sounds like a familiar sort of fault and has any ideas?

Double-check the shielding in the IF section. You might have to reroute wires, as feedback can occur between nearby wires. Especially, keep the wires to the speaker transformer well away from the rest of the radio if possible.

Make sure the shield can on the IF amplifier valve is reliably earthed-

Normally when a volume goes to full without being touched, its the earth end going open, unless it one in the cathode.

It would have to be reducing the bias. Did you check the resistors?

No, I haven't had a decent opportunity to pull it apart yet, Its on the to do list but I thought I'd wait until I received some feedback, the resistors all look pretty new inside and the caps, but I'll have a look at them when I finally attack it. I have read about the IF valves shield going open and not conducting to the chassis, it seems strange as the cans are attached, but maybe corrosion broken continuity? I'm a novice with valve radio faults as my old radios are by and large so reliable, this one started to be after at first roasting one side of the HT winding after the speaker output transformer and voice coil went open, so rewound transformer. Thought maybe the 80 Rectifier might have been uneven after that breakdown, but a good one that works ok in other sets later and it still plays up. I'm wondering if maybe the wire from the valve cap clip which goes from there into the one of the coil cans might be dodgy? I'll be testing all of the wires for continuity, then resistors, voltages etc... I'm not sure if radios have standard faults like cars and robots lol. It's annoying, one of those faults which lets you think all is hunky dory and then &$*(($**$$#$^ etc etc. its off like a bride's nighty!

Valve top caps can fail, & how the fly lead is removed can play a part in that. I am constantly repairing them. The first thing to do when you get a radio is to inspect it to see if its been Monkeyed with. If it has that should instantly ring alarm bells.

It is never unusual to get a pretty little radio, that looks just so good and find that it is the devil in disguise. As I have pointed out this week I had a set that was untouched & it ended up with 5 new caps. Topped off by another, proving my point; it had been got at & that was about 5 resistors, 3 caps , wrong caps, and a wiring error that saw the control grid pin with nothing on it.

Chopsticks are good for prodding & poking, and do not cause aberrations by being inductive.

Methodology is everything when fixing. Knowing how the thing works (not just applicable to radio) can give a wonderful insight into understanding why it won't.

Cranky performance and volume varying seemingly randomly was par for the course for those autodyne sets with no AGC.

Volume changes can be due to RF coming in on the mains and is affected by various things around the house turning on and off. That's mostly fixed Smith a good earth. Does it have a correctly wired 3 core mains cable??

I can identify with Ian's comment. Always expect the unexpected. I always retrofit those old sets with an earthed cable. There are reasons; One, RF return to ground, which can help stabilise some fluctuations; You eliminate live chassis risk; and if it has a shielded transformer, it bleeds off the charge they put on the chassis.

Many of those old sets with no AGC had a "Local / DX" switch. I did have an STC 59 chassis missing its (and any circuit diagram). I found 68pF to be the best in series, with the antenna, for Local but be warned, I changed the bias on the first IF, from-3 to -4.5 to stop it overloading the plate detector. (Oscilloscope involved here)

Plate detectors should be checked as variance in the valve, failing resistors and voltage changes can all conspire to throw them out of whack: They're not renown for being wonderful at the best of times, & Triodes are the worst for distortion.

I have pointed out several times that there is a huge amount of RF coming in on the miles of 22KV aerials here, exacerbated by the number of close transmitters. 4G is not one of them, its signal here, like its predecessors, 2nd Go; 3rd Go & No go, leaves everything to be desired: Its been like that from day 1: i.e. abt. 2 decades.

I have capacitors (X/Y) approved line caps: On the mains feeding radios & In a couple of cases, put in the radio (repair going out) with a MOV, to reduce its effects & lightning surges.

0.1mfd (104) across N & A; Not more than 0.01mfd (103) from active to earth and same Neutral to ground.

Ian so funny you should say that! My old consoles ALL do funny things when taps are turned on an off and appliances, lights and exhaust fans are turned on or off, volume goes up or down! I had thought it might be par for the course perhaps, it's just the whistling and I'm thinking after reading your posts that I know where to look now! It's got to be something very minor. It does have a 3 core earthed mains cable. I'm doing some research now on "plate detectors" so when I pull it apart I'll be able to know where to look, maybe a resistor is misbehaving, possible heating up and affecting the bias? For these line caps, are 600V polyesters suitable?

Yes I'm very careful with the fly leads, I usually insert a small screwdriver between the top cap and lead and twist it gently to prise it off as not to pull on the top cap. Thank you Robbbert, Ian and Marcc for your input, much appreciated!

Do not use 600V polys as line caps. You MUST use proper X or Y rated caps, they are designed to fail 'safe'.

Ok thank you for that information! You wouldn't perchance know of a link to a supplier from whom I might obtain half a dozen of each at the correct value? Cheers,

I believe Jaycar have them, haven't checked tho.

Altronics have a far wider range of Mains rated caps than what Jaycar stock. Have a looky online at both suppliers & if you are not close to either supplier ordering online is dead easy & both supply quite quickly that way.

Lindsay

Buddy I have had this problem in the past but I have discovered that if I put the beer down while I am whistling then it doesnt sound so garbled lol. Sorry I have that one off my chest now lol.