I don't think the volume pot would cause distortion.
Is it an AF or an RF issue?
A signal generator would help narrow it down.

What is the back bias voltage?

Ian Robertson that I dont know but I have found two resistors ( 23ohm and 350 ohm ) on the same tag ( on the pot ) that both measure at 23 ohms when connected but measure correctly when disconnected.
I know the problem isnt in the RF section and I think it is back bias related.

The AORSM circuit Voltage across R8 -2.1V Across R7and R8 chassis positive. +/- 10% The back bias is a diagnostic. If that voltage is not in spec. then something is not right as all cathode current passes through those resistors, especially if its the one to ground: That voltage is created by that current.

They used a 1000 opv meter; with low resistors like that The difference between DVM and Analogue should be insignificant.

If you heavy up the back bias resistors you are looking for trouble. They should "sail with the wind" and be of the lowest wattage (as specified) that you can get away with. They can burn on short, not the transformer.

Bad caps can cause hum as can an overload and worn out punched through, filter caps. There is a limit as too how much they are allowed to leak. They have to leak to maintain polarity. The first filter cap does not directly ground.

Many Philips sets take their negative feedback from the Audio secondary. If that gets reversed on either side of the transformer, they are liable to oscillate.

If there is an Oscilloscope and a Signal Generator? After you check to see if you have used a wrong part, or got the wiring wrong; Then those two make a formidable combination. You do an IF alignment procedure, however, you do as I do and use the oscilloscope as the meter. That not only shows the coupling wave form and its amplitude, it shows you distortion. The upshot being, that you then use the oscilloscope to pick where the wave form is going awry.

Shortwave limits the fault to that wiring common to SW that includes the tube as they tend to lose high frequency modulation as the age & fail, or run out of volts due to high resistances and low volts.

On the schematic, just to the right of the mains transformer, there are two resistors in series that go from the centre tap of the transformer secondary to ground.

You should measure from the centre tap to ground, first ohms when the set is off, then volts when it's running.

That will tell us a lot.

On switching this set on after a few days I notices the sound has deteriorated some what! Could this whole problem be due to very dirty contacts?

Well I have finally completed this beautiful big set. It works out that the chassis was slightly different but all it took was to dremel some of the fittings on the case and she looks like a bought one! This chassis needed to be fully aligned but now it's probably better then new.

As pointed out on numerous occasions (add infinitum) a Signal Generator with a good waveform and not overloading the system; In conjunction with an oscilloscope. Is a formidable combination for finding the source of distortion.

Marc

Photos sent

Photos uploaded. Apologies for the delay, I've just returned from a camping trip.

Thankyou Brad Hope you enjoyed the camp.

Now I can see the photo, I thought it looked familiar. I have a Mullard MAS1111 that looks the same, except that the 4 white dashes on the sides are a reddish-orange colour, and the dial glass is obviously different. The white patterning on the front is a different pattern too.

It has a shortwave band like yours does, with a bandspread control.

Valves: 6AN7, 6N8, 6N8, 6M5, 6X5GT.

There's a note saying that it takes a while to get to full volume, and also the bandspread drifts badly. I haven't used it for some time, so it has probably stopped working.

That tube combination often with a different rectifier, was somewhat common in HMV as well. 6N8 can be a horror tube. However, despite it being a tube with a separate exciter, 6AN7 was rarely used outside Australia and was principally a broadcast band tube.

To me that may well be part of its problem on SW. I would note that capacitive changes around the coils due to heat, or in the tube may have a lot to do with stability. Most Pentagrids like 6A8 were sensitive to voltage changes and anything requiring stability usually had voltage regulation on the front end.

Marc

Decided to try it out, in fact electrically it works fine, although a bit of jiggling of the band-switch is needed on shortwave or it cuts out.

But after a while the tuning mechanism froze and the dial cord jumped off its tracks. Something to look at later I suppose.

ROBBERT
I should point out that this case was meant for another model. Two of the speaker mounting points inside the case needed to be dremeled away as they made the chassis sit 15mm too far in. That was the only thing I needed to do. The proper chassis for it has the same valve line up so in reality everything is fine. That explains the different designs on the front.
It has worked out fine. The case I had sitting for years in my shed now is complete.
And Marcc you were correct the 6AN7 was the problem.