I've posted my major problem in the Introductions area, because I have just joined today.

Here's another question, after reading that Australian radios were not generally earthed until 1960+.

My Philips 123 has a two-core power lead, but there is an earth terminal wire - just a bare steel multi-strand - that comes from the detection area through a hole in the back.

I understand the principle that earthing the antenna circuit ought to improve reception of radiated signals. Is it a good idea or a bad idea to connect the signal earth wire to the earth pin of the power circuit?

Maven

In most cases it is neither here or there. The earth binding post is most often just bolted to the chassis directly and the chassis is also directly earthed by new electrical leads.

When earthing a radio it needs to be remembered that the MEN link in your main switchboard will bypass the isolation properties of the radio's power transformer, and this means that more care must be taken when working on the radio whilst it is turned on. However, if a short circuit does happen in the radio the earth will throw the circuit breaker that feeds the radio, reducing the adverse effects of the short circuit.

With the problem of the fading and boosting of volume on your Philips 123 try replacing C11 (30pf) mica. There should about 240 volts on the plate of the 6N8 (V2) and the mica cap (C11) there may be intermittently breaking down. When replacing it be sure it is rated 500 volts or more. It is generally considered that mica capacitors are reliable. However I have found a lot mica caps breaking down particulary in the H.T. sections.

Seems to be a week spot in those the 132L I have had a similar Mica cap issue, albiet one incident was caused by a large Arachnid commiting suicide on top of the oscillator coil.

That did throw it off frequency a bit..... fancy that!

Marc

Thanks for the tip on replacing C11.

What type of cap is best to replace mica in this situation?

Would you recommend replacing all the remaining mica caps?

Maven

The Mica caps that are the most likely to let go are those with DC B+ on them. Others do occassionally go.

Silver Mica types are still sold, otherwise use Ceramic but they all should be 500V or better.

Marc

Firstly replace micas in the H.T. section. If you are unclear about what I mean about the High Tension section - its the high voltages that usually feed the plates & screens of the valves. In your case C10 (100pf) which is off the triode plate on V1 (6AN7) and C11 (30pf) off the plate of V2 (6N8). Somehow I'd be more interested in replacing C11 because of the nature of your fault. I doubt you would have any problems with the other micas. You can use any high voltage (500V+) mica or ceramic caps. Altronics, www.altronics.com.au sell both 33pf & 100pf 3KV ceramics at about a buck a piece. However, that said, it is not a guarantee this will fix your fault - but it is a good start.

OK - I looked for silver micas online. Hard to find and wild variation in prices offered from different overseas vendors.

Saw a pack of 10 x 30pf 500v for less than $10 from a vendor in Israel.

However I decided to go for a quicker delivery and also to replace the 100pf on the other HT valve V1 6AN7, so I went to altronics.com.au.

They have the parts named by Monochrome, but they also have a minimum $20 mail order. I added some sundry items and ended up paying $28 including postage.

The things we do for love!

I'll post results here.

I have aquired some SM,s from RS components. You could also try EVATCO. the latter did not carry as big of a capacity range as RS, the last time I bought, but you never know until you look.

Marc

I replaced C10 and C11 as advised by Monochrome, but after the operation something is definitely wrong.

Powered on, V1 (6AN7) warms but V2,V3 and V4 remain stone cold and the speaker is totally silent - not the slightest crackle. V5 (6X5GT) warms.

At C10, replaced the 100pF mica with Altronics "ceramic 3Kv 100pF Y5P".

At C11, replaced the 30pF mica with "Cornell Dubilier MICA 30pF 500v +/-5%."

Both caps checked out with correct capacitance on my digital multimeter. I understand these types are not polarised.

Any help on what might be wrong and what checks I should do?

I have spare valves I can swap, but don't want to do that in case bad voltages are getting through somewhere..

Maven

Are you saying these valves are not lighting up? Check with your meter on AC volts range for the filament voltage. The 9 pin valves, pin 4 & 5; the 6X5GT pin 2 & 7. You should have around 6.3 volts. One side of the filament chain may be earthed to the chassis. Also check for shorts in the filament chain including the dial lamps. Your radio may have old rubber coated cable in the filament chain which can often crumble and short out when disturbed. You can replace the wiring with a modern plastic insulated type or in some cases you can sleave the old wiring with heatshrink. And most importantly, if there is a short in the filament chain, do not power up your radio until resolved or you risk damaging the power transformer or something much worse.

Maven,
I should of used the correct terminology in regards to valve filaments. Most AC operated radio valves (except some 5 volt rectifiers) have heaters, not filaments. The heater is enclosed in a sleeve, some round, some oval, others rectangular. This sleeve is the cathode and when heated emits electrons. That is why radios can take half a minute or so to warm up. The glow of the valve heaters is what you see either at the top or the bottom of the cathode. Valves with filaments are usually found in battery operated sets such as Brads current Radio of the Week.

Thanks again,

I checked for shorts in that 6.3volt circuit and found none. All the heater circuits seemed to be at about 5.99vac.

The 6.3vac pilot lights are on their own transformer tap, L16, of which one side IS tied direct to ground. The heater tap only grounds through resistors and caps. For a while I thought there might be a short in the light circuit. When I pulled out one of the bulbs, the radio came back to life. Then it died again.

After a few minutes probing I found that there is something intermittent that reacts to moving around the wires coming from the transformer. It may be in the power switch itself, or in the fabric of the transformer near the taps? The visible wiring is all in good shape, replaced not long ago, and all connections buzz out correctly. The volume potentiometer is not at all noisy so I think it is OK. There might be a break inside a component - say underneath a solder tab.

So this latest muting problem is still a mystery to me but at least I am confident that the new caps are not the problem. I'll know after a few hours' running if the original fading issue is finally fixed.

Correction to the above

The pilot lamps are on the 1.8A AC circuit from the tap L15, in parallel with the valve heater circuit XY. This rail does have one side grounded to chassis

L14 .6A feeds the rectifier V5 and the DC rail.

It seems to be V4, the 6M5 amplifier, that is not heating reliably. I've swapped valves, with same result.

Maybe the socket is not connecting reliably? Is there a tecnique to improve connection without damage to the socket?

OK, following up my own question, I think I have nailed it.

Pin 4 on the 6M5 V4 is earth/return for the 6.3VAC heater circuit. The solder joint was good, but the lining of the pin socket seems to have worn or moved back beyond firm contact range with the pin. The tab or pin seemed to move slightly with temperature change or with any movement of the valve, randomly breaking the contact and cooling the valve.

These sockets don't have any sleeve on them that I could pinch to grip the pin. To firm up the contact, I added a blob of solder to bulk out the tab to the edge of the relevant pin hole. Then I wrapped a layer of aluminium foil around the pin to thicken it, before pushing it in. So far, that has provided a consistent pressure contact between pin and socket.

The set seems to run at higher volume than before I changed the caps and fixed the heater pin. Would that be the 6M5 running hotter, or could new caps have that effect?

Maven