Agreed, I won't power up until I've replaced all caps that need to be and resistors and other components checked. I have variac and dim bulb setup with monitoring of current. Mains transformer will be checked on its own before rest of circuit.

As a lot of this stuff has to be "Tagged & Tested": If I am suspicious of a transformer & some times not not only will I check the primary I will lift the ground on the secondaries, or isolate all windings and check that there is no conduction (500V DC) between them nor to ground.

While I have a Jaycar Variac (aka slide transformer) (Variac is a trade mark I believe) and a Dimmerstat (India. Beautiful piece of work). I rarely use them on startup.

I have modified a 3A isolation box with downrated circuit breakers and pilot lights input and output. Also with MOV's and filter caps for RF. More importantly a "Kill Switch". Startup is always with an analogue meter monitoring "B" Voltage.

We all have different ideas. This has served me well for decades & several hundred sets.

Yes sounds like you are very well set up.

Hi everyone,

I have this powering up now and can get audio from radio and phono input.. The audio only lasts a couple of minutes and then starts to distort and rapidly is taken over by noise. I have found if I turn the mains voltage down to about 220V this problem stays away. Can any of the more experienced here (most of you), point me in the right direction? They didn't teach valves when I was an apprentice. I have replaced all capacitors now.

What does the HT rail do when the noise occurs.
Could be a cap breaking down which would show up as the HT dipping.
Is it the RF section causing the noise or the audio stage ie is there on both radio and phono switch positions.
Is it affected by adjusting the volume control.
Some places to start.

Interesting; The chassis has a serial number, so its a commercial unit. STC had a habit of using external binding posts and so did HMV.

Transformers look like Ferguson The black caps AWA and the rectangular pF ones tended not to fail. Trimmers are a Philips type.

The one meg resistor on the magic eye will likely be a dud, 99% are. Black mud caps (not pF ones) will need to go along with all electrolytic ones if the pink ones are not marked polyester they also go. Mains cable in its entirety also goes & earth the chassis (3 wire mains cable).

If the mains transformer is OK etc. It looks like a wet afternoon job. Circuit or no circuit. ARTS&P label can also help date it.

That only has a double gang so there is no TRF stage ahead of the frequency changer.

Hi Marcc,

I have replaced all the black caps which were cracked and the electrolytic capacitors. I have not touched the rectangle ones. Irext, the noise happens on both radio and gram. I will check for fluctuation of the HT rail tonight and let you know. Adjusting the volume control does affect it until the noise goes flat out and then all you can do is switch it off but as I said turning the mains voltage down to 220VAC keeps the noise away. I will test it for longer as I have more time.

The output from the rectifier valve sits on about 240VDC. It does not change when the noise takes over.

I'd completely forgotten about this topic, and had to read it again to refresh my memory.

It seems we never did get a list of valves, looking at the photo shows ???, 6N8, 6N8, 6M5, 5Y3GT, EM4.

My quick guesstimate would be to wait for the noise to start up, and then pull out the 6N8 that's next to the 6M5.
This will isolate which side of the 6N8 to concentrate on. The 6N8 itself could be the fault.

The valve line up is, 5Y3GT, 6M5, 6N8, 6N8 and 6AN7 plus a magic eye EM4.

I tried it with the 6N8 removed and the noise is still there. I swapped the two 6N8's but made no difference and tried with the other 6N8 removed also noise still there. I also noticed on radio the signal seems to disappear before the sound sets in.

Hmm, interesting. There's still plenty of possibilities. Seeing there's no schematic I'll have to guess.

When the crackling starts does it make the EM4 green display jump about?

Are there any mica caps near the 6M5? If so see if disconnecting them makes a difference - if so, replace them.

Otherwise I'd suspect some kind of power supply issue, and there's quite a few candidates. Anything from the power cord, power switch, transformers (power and speaker), rectifier, and filter circuits. The easiest to start with of course is to trying replacing the 6M5 and the 5Y3.

Oh, one thing that's often overlooked is if you're still using the old power cord, the screws on the terminals in the power plug will over time work loose and start arcing. This definitely makes a lot of noise.

This one sounds like a dart board job. Interesting that its voltage dependant. I have had issues with 6N8; that line up is very similar to a HMV chassis 42 (different rectifier) which appeared in several sets. There were others with that lineup. 6AN7 appeared in many SW sets.

I would actually start by looking at the wiring for a dry joint causing a make & break; resistors can also do that. do check voltages on the RF to second detector. If the 6M5 stops conducting I would expect a substantial voltage rise in "B". Shorts tend to drop the voltage.

I have gotten to the point where I test the electrolytics before use. I have had NOS and new electrolytic caps principally with surge. Get close to rated voltage & then start conduction at a hyperbolic rate. The Mica caps are normally reasonably reliable and its principally the one on HT that let go.

Did you remove the tubes? Often they will lose conduction from pin & socket corrosion. One removes them just to scape off any corrosion.

Do not run the set sideways. The 5Y3 can arc over if you do.

It seems some of the information I gave was incorrect. The noise looks like an oscillation and when it starts, if you pull out the 6N8 the noise disappears together with the audio obviously.

Oscillations can occur from faulty caps as the wax paper ones leak like sieves. However, when you change them, the modern ones do not have outside foil and along with lead dress, can cause trouble. Lead dress can be critical and I have had to install shielded wire.

Failing to have parts hugging the chassis can be an issue. The worst offending area is the 2nd detector first AF. Nudging things with a non inductive probe, like a chopstick can change the frequency of an oscillation. Bad lead dress can cause wiring to become antennas & radiators.

Also, "Motorboating" can be created (Philips especially) where negative feed back is taken from the secondary of the speaker transformer. The set may have a tapped volume control? If either primary. or secondary get reversed, then it will oscillate.

Marc