Have been discussing various issues with my STC830 in Bowlers ASTOR NJ thread. This started because he had an issue with oscillation and so did I. Then got diverted to other issues, especially low filament voltage. So have decided to start this thread.

The filament voltage has always been low but because of generally high line voltage, ie always over 240V hasn't been an issue. However of recent times this has drifted down to the mid 230's especially in the evening when the solar arrays around the area (including mine) aren't able to push up the voltage.

However I have replaced the four dial lights with 150mA instead of the original 300mV (completely blackened), and don't usually have the indicator valve (6U5) plugged in. So that is a nominal filament current (6F6, 6J7, 6H6, 3x6U7, 6A8 + 4x 0.15A dial lights) of 3.1A instead of 4.6A (with 6U5 and 4x 0.3A dial lights). That would drag the filament voltage even lower.

The voltage has always been low however and the transformer doesn't show any sign of distress (smell or melted wax). There have however been shorts of some seconds in the dial lights which could have caused damage; also don't know the full history before 1988 when I got the radio. It is possible that the radio originally came with 0.6A dial lights which would push the current draw up another 1.2A over the 0.3A lights I got it with.

The rectifier heater voltage is close to 5V which leads me to think that the primary is OK. The HT is balanced for volts measured from the centre tap so that winding looks OK.

I looked up the history of line voltages. These were

Brisbane 1938, nominal 240 +/-10% ie 264 to 216 (no East coast grid then)

Australia 2021, nominal 230V +10% -6% allowable ie 253 to 216

Australia 2021, preferable 230V +6% -2% allowable ie 244 to 225

(Drop from 240V to 230V was to provide headroom for rooftop solar).

So peak line voltage was 9 volts higher in 1938. It is possible and likely that peak voltage in reality was even higher than this in 1938 and STC may have decided to go for a lower filament voltage. However could not find this sort of information on the web and would be restricted to STC technical bulletins.

However as the filament voltage would drop well below 6.0V even at the allowable line minimum now especially with a full valve lineup decided the transformer needed a closer look. So took it out.

It is sealed with wax but no melting or scorching visible. With the transformer out can see that the 6.3 V secondary winding is wound last, after 5.0 5Z4 heater winding, HT winding and 240V primary. Radiotron Handbook indicates that practice was to put the filament wiring between primary and other secondary windings to provide a shield - looks like STC had other ideas in 1938 and leaves open the possibility for inspection and maybe modification of the 6.3V winding.

I think it might be possible to expose the 6.3V winding by removing the outer layers of waxed cotton tape, without totally disassembling the transformer, and have a look for damage and possibly modify with a few more turns to bump the voltage up a bit.

My main question here is, is this exposure of the winding without total disassembly a good idea. And if so, how much should it be "bumped up". My thought would be 6.3V at 240V line, leaving headroom for high fluctuations. I have seen my solar cell system trip out when line exceeded the upper 255VAC line limit for the system as given at the inverter readout, which is slightly above the allowable max of 253 given above.

(Drop from 240V to 230V was to provide headroom for rooftop solar).

IIRC, the move to 230V was to harmonise with European manufacturing standard.

I'm not sure I'd be too worried with this voltage. There will be no issue with your transformer. It would draw excessive current and fail if it had any shorts present. The filament windings will most likely be the outermost and yes you could add additional turns if room permits. I'm not sure you have anything to gain by doing so though.

The voltage change was on paper only. You still get 240 volts at the GPO.

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A valve a day keeps the transistor away...

Thanks gents. The general verdict seems to be if it ain't broke don't fix it, a rule of thumb to which I agree.

Re the reasons for drop to 230, one of the references that I looked at certainly said this was to provide headroom for solar, but I probably forgot the other reasons given. Have had a look for this but can't find it again. I'll have another go.

Have searched out a 24VAC plug pack which I will connect to the 240V primary so I can monitor secondary voltages without risking a jolt, and make the arithmetic easy. Then poke around and see if I can get any sudden fluctuations in the 6.3V filament secondary voltage which would indicate a fault. If I assume the ratio between 6.3V and rectifier secondary voltages should be 1.26 I can't go far wrong.

Also megger the whole thing thoroughly and replace some of the old brittle spaghetti.

QUOTE: The nominal voltage in Australia was set at 240V in the 1920s. However, a change began in 1980 with the International Electrotechnical Committee (IEC) deciding to rationalise the 220V, 230V and 240V nominal voltage levels around the world to a consistent 230V.

This rationalisation was ostensibly made to improve the economics of making appliances by allowing manufacturers to produce a range of items with a rated voltage of 230V. In 2000, Standards Australia issued a system Standard, AS60038, with 230V as the nominal voltage with a +10% to –6% variation at the point of supply.

https://electricalconnection.com.au/when-voltage-varies/

The voltage change was on paper only. You still get 240 volts at the GPO.

Nearly 250V at my place at times, and I put that down to solar.

In the eastern part of Australia the nominal voltage may have been set in the '20s. But the Western Australian standard was 250v until the change in the 80s or 90s..

The nominal voltage here is now 240v +-6% - not the 230v seen in Eastern states.

In the eastern part of Australia the nominal voltage may have been set in the '20s. But the Western Australian standard was 250v until the change in the 80s or 90s..

I well remember our engineers having discussions with Western Power in that era regarding power supplies dying in our Perth branch. We shipped the same equipment to branches in all states, but there were frequent problems in WA.

The nominal voltage here is now 240v +-6% - not the 230v seen in Eastern states.

Definitionally 'nominal' is what the (national) Australian Standard declares it to be. Actual (locally) is another matter, as we know.

Going slightly under on heater voltage isn't detrimental to most firebottles. Going over is another matter, it tends to shorten life dramatically.

In the late 60s, the company I worked for had a rental fleet of AWA 50 series TVs. The CRTs were wearing out with about 12 months use. AWA replaced the tubes and recommended we add 1 ohm (from memory) in series with the tube heater to drop the voltage to 6.0 VAC nominal.

Re the setting of the 230V nominal voltage, the comment about solar was in Wikipedia wrt Queensland Government decision.

https://en.wikipedia.org/wiki/AS/NZS_3112

See the section on voltage . The reference given is from qld.gov.au, but it can't be opened, but the qld.gov website says it can probably be found elsewhere on their website - I didn't try. So looks like making headroom for solar was a Queensland justification. It seems to me that they operate at 240V and if asked why not 230V can say that's because of solar.

In search for this also found this table on application of European Standard in various states in Australia

https://www.ewh.ieee.org/r10/nsw/subpages/history/Australian-AC-Line-Voltages.pdf

The lower limit varies from 207 in Tasmania to ~229 in Queensland, 228 in the Northern Territory; 216 in the rest of the states.

Another article says that a 5% installation voltage drop can occur (https://electricalconnection.com.au/when-voltage-varies/):
"The utilisation voltage range is particularly important for appliances and electricians. This is the full range of voltage an appliance should receive, and an electrician should measure it in an installation under normal conditions.

This range takes into account the voltage range allowed at the point of supply and the additional 5% voltage drop in the installation. This equates to a total range of 253V to 205V."

I guess what this means that in extreme circumstance quite low voltages can occur. Maybe all your high current appliance are on and your neighbour starts using his welder. If your radio fades right away in this circumstance, you turn it off. This is probably what you would do in any circumstance if it fades right away.

Re your comment on firebottles Ian, this is the exact sentiment I have been operating under with my radio for the last 30-odd years. Acording to the Radiotron Handbook valves were tested and work down to 5.7V. But it's getting a major renovation and I want to review the 6.3V filament voltage issue and especially see if there is a shorted turn on this secondary.

A shorted turn on the 6.3 volt winding would cause the transformer to melt.
JJ

The 10%/-6% tolerance is to cater for our "old" standard of 240 volts.

Re solar, there is a proposal to start charging people to connect to the grid:

QUOTE: Australia's electricity grid has traditionally operated on a one-way power delivery system, where big energy generators like coal-fired power and hydroelectric plants deliver electricity via poles and wires to homes and businesses.

But with the uptake of rooftop solar photovoltaics (PV), electricity is now not only being taken from the grid to the home, but is also being injected back into the grid.

This is fine up to a point, but if too much power is put into the grid it can push the voltage too high, according to electrical and information engineer Gregor Verbic from the University of Sydney.

I'm happy to see that. If the additional power isn't needed, then we don't want it shoved back up the network. Let rooftop solar systems pay their own way.

https://www.abc.net.au/news/science/2021-03-30/solar-power-electricity-should-owners-pay-to-supply-grid/100035198

If the transformer gets hot, then you worry. Normally I run them on the 240-260V tapping. The Pole Pig here got damaged changing wires & was heading past 250V output & throwing out oil. So one did not need to be a rocket scientist for cause & effect there.

Oscillation can be caused by modern caps, lead dress, poor shielding of wires & tubes, 6U7 being a horror. 6A8 can also be a problem as its the worst of the pentagrids and is apt to get fluttering. The reason for the second C8

On the load side of the transformer +/- 20% is considered normal. Upsizing the first filter cap can increase "B" voltage and add more loading to the transformer: Not a good idea in most cases.

In order to sort it you have to determine if its generated in the RF, or Audio, bad shielding around the 6H6 1st AF being a common area for induction & radiation to cause instability.

MBVC

There seems to be some mabiguity with regard to Australian Standards. An Australian Standard is not law and by themselves there is no obligation to comply with any standard.

EG: AS 3000 (SAA Wiring Rules) has been around in one form or another since early times and it is updated approximately every five years. The instrument that gives this standard some meaning is that each state and territory has made a law that says words to the effect of, "All electrical installations in this jurisdiction shall comply with AS 3000". If there is no such law or regulation then complying with a given standard is optional.

This will apply to AS 60038, as each state and territory is responsible for its electrical supply. WA tends to go it alone on many things due to its distance away from the other states and their voltage has always been slightly higher and at one stage the frequency was 40Hz rather than the now-nationwide 50Hz. It is ultimately up to each state to decide what its nominal voltage will be, though its a bit hard for the eastern states to digress from each other due to them all being interconnected.

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A valve a day keeps the transistor away...