Which way around did you put the back bias cap & Philips tended to use around 25mfd? The back bias cap goes from the CT to chassis (or common rail)

A metal Chassis is positive relative to the CT. when back biased. PCB may be floating?

Marc

25mF seems more realistic than 100mF.

Irrespective of what value it is, it will be fatally damaged if its the wrong way around. This could explain why you needed to change the back bias resistor?

Not unusual for VC to be grounded, but beware, that might be a misplaced negative feedback wire.

Philips sets get very unstable if its feedback & out of phase.

Marc

Well let's see what we have here.

- There is no negative feedback in this radio. It's too cheap for that.

- There was no capacitor across the backbias resistor originally, it's something I usually add. But it can be removed if you think it shouldn't be there. And no, it isn't in backwards, I'm not that dumb.

- It isn't a Philips, it's a Pye.

- The part numbers on the board (the ones that can be seen under the nicotine) do not correspond to the part numbers in the schematic. So when you say to change C14, is that by looking at the schematic or the PCB?

Robbert,

The decoupling cap value is important because it will determine overall gain.

Too low or no cap = low gain

Too high = more gain = distortion

The fact that there was no cap tells me the designer had sufficient gain at that stage.

Dumb has nothing to do with it: Electrolytic caps across any back bias is not common Astor JJR may have a similar circuit albeit that is a vibrator set and not back biased. Astor normally used none or 0.1mfd, or less across back bias. As pointed out, that can affect gain and change how the set behaves. Modifications have ramifications. Where there is an electrolytic across back bias the second filter cap is often also taken to CT; So that mod could have introduced distortion.

In over 50 years of fixing these things, I have learned that once you see any form of tampering, the guarantee of it having correct parts, or being wired correctly, goes out the window and I have had several this year like that. If it never had a back bias cap, then it was never meant to be there. The first filter cap in back biased sets is often wrong & put directly to chassis. Remember that these sets worked when they left the factory. On the odd occasion (mainly during the War) mods were put out. If it is put back to a factory circuit, I find that solves a lot of problems.

6AQ5 is a valve that traps the unwary constantly, as pins 1 & 7 are both grid one. Quite often I find one of those pins used as terminal of convenience, causing havoc. While AWA & Astor tended to use 6AQ5, 6M5 was a common OP mainly in HMV. 6M5 became hard to get & there were substitutions & any form of substitutions lead to mistakes.

One thing to remember with the early radios is that they all followed a similar plot & same brand parts were common to many. A lot of tuning gangs in Astor were Stromberg-Carlson & Philips made parts that others used. EMMCO, AWA & Astor assembled radios with different badges. EMMCO (later Email) also made parts for others.

My normal tactic where there is distortion and voltages appear to be correct, is to feed (superhet) an IF signal through it as per IF alignment and look at each stage with an oscilloscope.

Marc

Next time I open the set I'll remove the capacitor. Don't know when that will be though, as I'm currently having a lack of interest in this hobby. I'm even thinking of just getting rid of most of the radios. I've already thrown out a bunch of stuff while the forum was down. Perhaps the interest will come back some time.

I find a bit of diversification works wonders, the distraction from last Tuesday has seen me with a closer attachment to, one Tractor, a chainsaw & various devices required to remove the downed firewood from a 500m stretch of boundary fence: 5 damaged 13 trees damaged severely, one roof sprung. I have had to lop two trees & hard prune several others. Bees were no overjoyed when I pruned their tree.

The longest fence took 2.5 days just to get the firewood of it: But wait there's more........ abt. 10 tonne of it

Some pics on Facebook.

I mainly repair radio's so every one is a new challenge: When its fixed, bored sent it home & get another one.

However, this week I have also overhauled a bore pump & this year I did put a new cylinder head on a neighbours tractor: no boredom here, haven't got time.

Marc