Marconi 382 - Barretter help please

[agardiner]

Hello,

I am repairing the above set which includes a Barretta. The set had previously been restored by someone in the late 80s, and had been in regular service until it recently broke down and ended up in my workshop.

No waxies in site! The previous restoration was clearly to a high standard. 2 of the main power supply electrolytics were completely open circuit, and the U30 rectifier had bitten the dust. Replacing all 4 electrolytics in the unit, carrying out various checks and replacing the U30 has brought back some life. However the HT is low and the set is struggling to function as a result. (a couple of stations getting through with low signal and audio)

Checking around, I noticed that all of the valve heaters are being under-powered; the entire chain is running at just over 50%. So my question is, what is the failure mode of a Barretta? I know its job is to control the current flow to the valve heaters, so why are they being 'starved'. I have confirmed that the remaining voltage is being dropped across the Barretta, so it is not going stray elsewhere! Unless I am missing something, the Barretta is the primary suspect. The filaments appear intact and it does glow ever so slightly. It is an ES style 301.

Should I try and find a replacement, or fit a capacitor dropper in its place? Or am I barking up the wrong tree?

Many thanks in advance for your assistance.

[Heatercathodeshort]

This is an odd one. They normally just go O/C. They only glow very dimly and are very easily overloaded. I would not suspect it is dropping excessive voltage as it would then glow brighter.

The 301 Barretter in the GEC data book says it's for .3amp circuits controlling between 138-221v. The best test is to calculate what resistance is needed by adding up the heater chain voltage and fitting a fixed resistor as a test. They used to pop off in the GEC television receivers and the replacement resistor became a permanent modification. John.

[Hoveradio]

Hi,I notice from the trader sheet that the set should use type 304 barretter not a 301 that you have.The voltage range may be off for a 301.
John.

[agardiner]

Strange indeed. The difference between the 304 and the 301 is only the max voltage range, and the set has been working fine with the 301.

The voltage for the heater chain is approx 100V, so the Barretta needs to drop around 140V to provide the 0.3A. Measuring across the Barretta, it is currently dropping 185V, hence the heater chain is low.

[Mr 1936]

Hi

This is a puzzle. If the barretter is working in its normal range the filament temperature will be about 700 degrees C and will be glowing dull red, so your description sounds about right.

I suggest you check the individual heater voltages to make sure there isn't any heater-cathode leakage going on. It might also be useful to pop in (e.g.) a 10 ohm series resistor just after the barretter and measure the voltage drop across it as a check on the actual heater chain current.

[Heatercathodeshort]

Type 304. .3a voltage range 95-165. I suppose all the valves are the correct types? J.

[agardiner]

Originally Posted by Heatercathodeshort

Type 304. .3a voltage range 95-165. I suppose all the valves are the correct types? J.

Yes, they are all the correct types and the set was working fine until breakdown.

[Heatercathodeshort]

I have turfed out by box of Barretters and taken a few resistance readings when cold.
301 255 ohms. 304 107 ohms and a NOS 305 [voltage range 40-90] 36 ohms.

You are more than welcome to one foc but once they have been used the filament becomes delicate and would NEVER survive the post!

I have several new 305s in boxes that should be ok post wise but the voltage range is a bit near the mark. John.

[agardiner]

Thanks for the info John, and the kind offer to send me one. Would love to try one just to eliminate it if nothing else.

I agree though that the 305 is a bit risky voltage wise. If you are prepared to send one of the others then would be worth the postage risk. Otherwise I am probably going to have to consider going down the capacitor dropper route; at least to try!

[Heatercathodeshort]

I can send you a new 305 and the 304. I doubt very much if the 304 will survive the post. They really are delicate. Let me have your details and I will sort it out post wise. John.

[PJL]

Including the 2x bulbs the total heater voltage is 95V. At 240V that gives a 145V drop. http://www.r-type.org/pdfs/301.pdf shows the 301 should work. The 303 and 305 are not suitable as they are for lower voltage drops.

[agardiner]

Originally Posted by PJL

Including the 2x bulbs the total heater voltage is 95V. At 240V that gives a 145V drop. http://www.r-type.org/pdfs/301.pdf shows the 301 should work. The 303 and 305 are not suitable as they are for lower voltage drops.

Indeed. And there is the mystery as the fitted barretta is currently dropping 185V.

[Mr 1936]

Hi

Maybe the filament in your barretter has aged and gone high resistance, despite the Hydrogen filling. An unused example might work better.

The original purpose of the Barretter was to stabilise the heater current against mains voltage variations in the days when the mains could be anywhere from 200 to 240 volts depending on where you lived. Nowadays, your mains may well be pretty constant, so a fixed dropper resistor would be an acceptable solution but will of course still be dissipating the same 40 watts or so.

A series semiconductor diode for halfwave rectification (plus a smaller series resistor) would reduce dissipation. A capacitor dropper would be even better from a dissipation point of view.

[jonnybear]

For a quick test you could if you have one try a 75 watt bulb in place of the barretter, and check voltages on the valves.only other thing I can think of you say you have replaced U30 could there be a leakage in the base or the valve?
Just a thought.
John

[agardiner]

Thanks John, I may give that a try.

[PJL]

You don't say if you have measured the AC current passing through the barretter?

[agardiner]

Originally Posted by PJL

You don't say if you have measured the AC current passing through the barretter?

Not yet; only calculated it. My multimeter doesn't support AC current, however I will be adding a series resistor and measuring the voltage across it as part of my ongoing tests.

[agardiner]

OK, I have spent some more time on this and am still baffled. I have taken voltage measurements across all valve heaters and also measured the current by inserting a 10R resistor and checking across it. Here are my findings... correct voltages in brackets

V1, X31 - 9V (13)
V2, W31 - 7.5V (13)
V3, D41 - 2V (4)
V4, N31 - 18V (26)
V5, U30 - 16V (26)
Lamps, 2 x 6.3V in series - 4V (approx 9)
Barretter - 180V (approx 149)

Current should be 0.3A but measures at 0.21A

There are two chokes in series with the supply, one on the live and one on the neutral. These appear fine and I measured the 237V supply on the secondary of them to be sure.

I also tried the suggestion of temporarily removing the barretta and inserting a 70W filament lamp instead. No change! The lamp dropped the 180V.

What on earth am I missing; this circuit should be so simple!!

[cathoderay57]

On the basis of the data you have already provided, the barretter seems to have gone high resistance since it alone is dropping higher than expected volts whereas the valves are dropping less
Cheers, Jerry

[Mr 1936]

Hi

As said above, if the present Barretter is dropping 180V at only 0.21 Amps then its resistance is 857 ohms which is much higher than it should be.

You seem to have been unfortunate with the 70W filament bulb, which is also presenting a similar too-high resistance.

Try a 100W bulb. At its rated 230V this will have a theoretical resistance of 529 ohms. When under-run at 150V (65%) I estimate the resistance will be about 440 ohms which is closer to the 500 ohms or so that you require. If it's now too low you might have to add something like another 56 ohm 5 Watt wirewound in series as a top-up.

This should then get your heater current close to 0.3 amps, so that you can assess the effect.