Philips 141U

[DonaldStott]

Mindful of the facts that the set had a loud hum and that the reservoir and smoothing caps in the big can were reading 57μF and 105μF I decided to switch on with my lamp limiter. It took a long time to warm up but eventually we had some life and a fairly good station on MW. Left it for a while to ensure that nothing was seriously amiss - so far so good. Quite a bad hum but I knew that the large electrolytic can needs attention - more on that later.

I switched from Lamp to Direct and there was an almost instant uplift in sound volume and quality BUT the resistor R1 which is 1kΩ 1W and connected between pin 7 on V5 (UY41) and pin 2 of V4 (UL41) started smoking and throwing off blue sparks - switched off immediately!



We clearly have some overheating and something drawing too much current to take out the resistor - R1 is specified as 1W in the Service Sheet so something not quite right!

Back at that hum - the electrolytic can contains 50μF + 50μF caps and when disconnected one was now reading 56μF while the other was 35nF, yes that's nF so it was clearly problematic or as we say in Scotland gubbed! I assumed that this wasn't a candidate for reforming over time and it was simpler just to replace both by fitting two new 47μF 450V electrolytic caps to a tag strip under the chassis - plenty of room.

R1 was also replaced with a 1kΩ 2W resistor - just in case. Before all this happened, the section of the Dropper that was was giving no readings was replaced by a 240Ω 10W ceramic cement resistor.

So it's switch on time again with the Lamp Limiter same as before - left it running for a good 20 minutes in case something untoward happened but all seemed well. Switched from Lamp to Direct and no fireworks this time - progress!

First things first I checked the voltages on V5 (UY41):-

Pin 2 (anode) - 222V AC

Pin 7 (cathode) - 204V DC

Voltage drop across R1 27.4V
Current = 27.4/1000 = 0.0274A = 27.4mA
Wattage = 27.4 x 0.0274 = 0.75W

All these seem a bit high to me although with R1 being 2W we should be ok - just concerned that I'm over-running V5?

I checked the Dropper sections against the Service Sheet: -

R2 Spec 240Ω Measured 237Ω (new 240Ω 10W ceramic resistor)
R3 Spec 250Ω Measured 254Ω
R4 Spec 538Ω Measured 551Ω

Although the hum has now disappeared overall performance on MW is poor with only two good strong stations while performance on LW is very poor with only a very faint Radio 4? Sound quality is very trebly and sibilant?

[ColinB]

Daft question, but do you have the mains tapping set to the highest voltage option, so that all the dropper sections are in circuit?

[DonaldStott]

Originally Posted by ColinB

Daft question, but do you have the mains tapping set to the highest voltage option, so that all the dropper sections are in circuit?

No such thing as a daft question on this Forum - I thought I had the mains tapping set to the highest voltage option but I'll check again.

[DonaldStott]

Forgot to add that at time of measuring my mains voltage was 245V AC.

[DonaldStott]

The Voltage Selector only has two settings as shown here:-



Tried it in both positions but this made no difference to the voltages on Pins 2 and 7 of V5 - confused?

I checked the voltages on the various Dropper tags and working from the outside of the chassis in i.e. from R2 to R4 I'm getting:-

222V 243V 216V 159V

Even more confused??

[ms660]

Originally Posted by DonaldStott

The Voltage Selector only has two settings as shown here:-

Attachment 214175

Tried it in both positions but this made no difference to the voltages on Pins 2 and 7 of V5 - confused?

Study the schematic....

Lawrence.

[DonaldStott]

Originally Posted by ms660

Study the schematic....

Quickly glossing over the Voltage Selector issue I looked at the data sheet for the UY41 valve and feel that I may be over-running it: -

Pin 2 (anode) - 222V AC

Pin 7 (cathode) - 204V DC

If so, what are my options?

[ms660]

The only way to over run it is to increase the heater current or the DC load current beyond its maximum limit or exceed the maximum hot switching current, I wouldn't be too concerned about the voltages you measured, there's no curves in the valve data I'm looking at for 240 volts and a 250 Ohms surge limiter but a bit of rough and ready dead reckoning suggests the voltages you have aren't that far out.

The reason the voltage selector doesn't affect the voltage to the rectifier that you were querying earlier on is that the voltage into the surge limiter is independent of the mains voltage selector setting, the mains voltage selector only alters the dropping resistance for the heater chain.

Lawrence.

[DonaldStott]

Originally Posted by ms660

The only way to over run it is to increase the heater current or the DC load current beyond its maximum limit or exceed the maximum hot switching current, I wouldn't be too concerned about the voltages you measured, there's no curves in the valve data I'm looking at for 240 volts and a 250 Ohms surge limiter but a bit of rough and ready dead reckoning suggests the voltages you have aren't that far out.

Understood - thanks for that.

Originally Posted by ms660

The reason the voltage selector doesn't affect the voltage to the rectifier that you were querying earlier on is that the voltage into the surge limiter is independent of the mains voltage selector setting, the mains voltage selector only alters the dropping resistance for the heater chain.

Now it makes sense ... I wasn't able to discern that from the schematic.

[DonaldStott]

Thanks to Forum member Norman Raeburn I acquired a couple of UY41 valves to try in this set - the relevant measurements are:-

Pin 2 (anode) - 216V AC

Pin 7 (cathode) - 178V DC

Voltage drop across R1 29.2V
Current = 29.2/1000 = 0.0292A = 29.2mA
Wattage = 29.2 x 0.0292 = 0.85W

Unsure if these results are 'better' or 'worse' than those detailed in Post #21 above ??

Sound quality is still a bit bright and sibilant - would changing the value of C30 help? C30 (tone correction and spike suppression) is wired from the Primary of the Output Transformer to the chassis?

[paulsherwin]

These numbers don't seem particularly concerning to me. Rectifier performance does vary, and they generally drop more volts as they age. The HT voltage isn't critical in these sorts of sets.

You could increase the tone correction cap or add some top cut across the volume control, or even add some frequency selective NFB. The small Philips second sets of the 50s weren't noted for their high sound quality.

[ms660]

The cathode voltage looks worse than before, anode voltage is not much different.

Low voltage at the cathode of the rectifier.....low emission rectifier, excessive load current, insufficient reservoir capacitance or a combination thereof.

Lawrence.

[DonaldStott]

Originally Posted by paulsherwin

These numbers don't seem particularly concerning to me. Rectifier performance does vary, and they generally drop more volts as they age. The HT voltage isn't critical in these sorts of sets.

You could increase the tone correction cap or add some top cut across the volume control, or even add some frequency selective NFB. The small Philips second sets of the 50s weren't noted for their high sound quality.

Thanks Paul - think I'll just try to increase the value of the tone correction cap. I'd have to do more research into adding some top cut to the volume control but wouldn't know where to start with frequency selective NFB!

Originally Posted by ms660

The cathode voltage looks worse than before, anode voltage is not much different.

Low voltage at the cathode of the rectifier.....low emission rectifier, excessive load current, insufficient reservoir capacitance or a combination thereof.

Thanks Lawrence - thanks for confirming that a lower cathode voltage is 'worse' not 'better'.

Low emission rectifier - probably due to age as the valve is a pull from another set and not a NOS.

Excessive load current - not sure how to check for that?

Insufficient reservoir capacitance - both reservoir and smoothing caps were replaced by 47μF 450V components. Service Sheet spec says 50μF + 50μF so I'm doubting that this is an issue? Is -3μF going to make a difference?

[paulsherwin]

The rectifier is just getting on a bit. They can soldier on for years like that.

Paul Stenning wrote up adding NFB to a different Philips set in this old piece: https://www.vintage-radio.com/recent...ps-b2g81u.html

[ms660]

Originally Posted by DonaldStott

Excessive load current - not sure how to check for that?

Add together all the electrode currents that are given in the service data, give or take a few mA's that should be the normal load current when measured.

You can measure that current by disconnecting the feed from the reservoir capacitor to the rest of the receiver and inserting a test meter that's switched to an appropriate DC current range in series with the feed.

Lawrence.

[paulsherwin]

Unless I've misread your figures, you seem to have a load current of 29mA - normal or even a bit on the low side. Nothing to worry about as I said.

[ms660]

That's the load current excluding the anode current of the audio output valve.

Total load current should be circa 75mA.

Lawrence.

[paulsherwin]

Ah, right.

[DonaldStott]

C30, the tone correction cap, was originally a Philips 'black pitch' capacitor which was replaced with a 0.022μF 1kV ceramic capacitor but the sound quality was still a bit bright and sibilant for my tastes!

Decided to try adding another 0.022μF cap in parallel, not in series, bringing the total capacitance up to 0.044μF. This made the sound quality too dull and muffled the other way, so this was removed.

Replaced that with a 0.01μF cap in parallel (total capacitance now 0.032μF) and the sound quality is just about right - could spend a lot of time trying various values but time to move on...

[DonaldStott]

Time to move on or so I thought ...!

Don't know why but I've never been able to replicate the voltage figures for V5 (UY41) shown in Post #21 above :-

Pin 2 (anode) - 222V AC

Pin 7 (cathode) - 204V DC

After trying a couple of pulls from other sets I'm still getting: -

Pin 2 (anode) - 216V AC

Pin 7 (cathode) - 178V DC

This means that the anode voltages for all the other valves are lower than expected, although some of the grid voltages are not too far off, which is bound to degrade the performance of the set - thanks to David G4EBT for that explanation.

Is there a reliable substitute for the UY41 as a NOS is impossible to find?