Philips 141U

[paulsherwin]

Actually, the HT being 20V or so down won't make a lot of difference. The heater voltages are much more critical.

There isn't any substitute for a UY41 unless you want to change the valve base. You could replace it with a 1N4007 and 100-200 ohm resistor, leaving the valve heater in circuit to keep the heater chain working and for cosmetic effect. If you do this, check that the smoothing electrolytics aren't being subjected to excess voltage before the valves warm up.

[DonaldStott]

Having concluded that I had taken the Philips 141U as far as I could while accepting that the set had a low emission rectifier I put it all back in the case and turned it on.

But two new issues:-

1. I now have a loud hum that wasn't there previously which is independent of the volume? Initially the set hummed badly but this all but disappeared when I replaced the reservoir and smoothing caps.

Took the chassis and speaker back out the case and there is still a residual background hum but which is probably acceptable. But when I try to reintroduce the speaker into the case the hum increases to an unacceptable level - it's as if the case is amplifying the hum?

So this is bizarre and I need to find a way to fix the residual background hum or work out the means by which the case is apparently amplifying this hum and fix that??

2. The set is running very hot with the Bakelite on the top and back being too hot to touch! You may remember that I had a broken section of the Dropper which was bridged with a 240Ω 10W ceramic cement resistor. The majority of the heat, however, does appear to be emanating from V5 (UY41) and V4 (UL41) - their respective heater voltages are 31.2V and 47.2V.

Although this set has a fairly standard valve line-up the case is relatively small in comparison with e.g. the Bush DAC90A, so this won't be helping? I need to investigate some means of reducing the heat output - is there such a thing as a heat resistant sheet that I can apply to the inside of the set? The intended recipients of this set have small children so I don't want to introduce a potential burning hazard to their home!


Any ideas warmly welcomed - pardon the pun ... ?

[paulsherwin]

Try a different UL41. Sometimes they develop an intermittent internal leak which causes humming.

All radios of this type run hot. There are a number of techniques you can use to reduce this:

Replace the dropper with a capacitive or diode dropper: https://www.vintage-radio.com/repair...per-calcs.html
Increase the UL41 cathode resistor
Bypass the UY41 with a silicon diode

I would be wary of using a radio of this type where there are small children around though. Some sort of repro fake vintage set would be a better choice.

[Sideband]

Originally Posted by DonaldStott

But when I try to reintroduce the speaker into the case the hum increases to an unacceptable level - it's as if the case is amplifying the hum?

So this is bizarre and I need to find a way to fix the residual background hum or work out the means by which the case is apparently amplifying this hum and fix that??

The bass will be increased when the speaker is introduced into the cabinet since the speaker grill and part of the cabinet acts as a baffle. This is intentional. Hum may well be more noticeable. The bass response of the speaker is greatly reduced when out of the cabinet since the sound radiated from the front of the speaker is partly cancelled by the sound from the back, most noticeable by a reduction in bass response. The baffle reduces the cancelling effect thus increasing the bass response. Ergo.....hum may be more apparent.

Originally Posted by DonaldStott

2. The set is running very hot with the Bakelite on the top and back being too hot to touch! You may remember that I had a broken section of the Dropper which was bridged with a 240Ω 10W ceramic cement resistor. The majority of the heat, however, does appear to be emanating from V5 (UY41) and V4 (UL41) - their respective heater voltages are 31.2V and 47.2V.

I used to have one of these sets, yes the cabinets do run hot but not too hot to touch....at least mine didn't. The UL41 and UY41 will run too hot to touch.

Measure the voltage across the cathode resistor of the UL41 and then work out the current. I think the UL41 maximum anode current is around 45mA and screen grid is about 10mA so measuring the voltage drop across the cathode resistor and then working out the current will be the TOTAL current of anode and screen grid. It will soon reveal if the UL41 is drawing excessive current. A total current of around 35 - 40mA drawn by the valve would probably be normal. If it's less, that's OK as well (as long as it's not too much less). Anything much more and I'd try another UL41.

With regard to using one of these near children....I would be VERY careful. They have wandering fingers and curious eyes. If it was me I would agree with Paul Sherwin and use a repro and keep this for use in another room.

[DonaldStott]

Originally Posted by paulsherwin

Try a different UL41. Sometimes they develop an intermittent internal leak which causes humming.

Thanks Paul - swapped the valve for a NOS UL41 and this has reduced the level of hum! Are there any other valve pins used as connecting points as in the DAC90A?

Originally Posted by paulsherwin

All radios of this type run hot. There are a number of techniques you can use to reduce this

I can certainly look at increasing the cathode resistor if required (see Post below from Sideband) and bypassing the UY41 with a silicon diode and resistor should be well within my skill set?

As for replacing the existing Dropper with a capacitive or diode dropper - I skimmed the document you provided the link to and this will need some serious consideration before I venture down that road! I understand that there is a bit of maths involved (not a problem), although a helpful spreadsheet is included, I forsee challenges in transferring the theory to my real world example where I have a Dropper with several resistive sections!

The important point about small children and vintage valve radios is well made here and in many other Threads - noted.

[paulsherwin]

The dropper calculations are made easier because very large numbers of 50s radios used the same valve lineup, so the work has already been done.

[DonaldStott]

Originally Posted by Sideband

The bass will be increased when the speaker is introduced into the cabinet since the speaker grill and part of the cabinet acts as a baffle. This is intentional ... hum may be more apparent.

Thanks Sideband - intuitively I sort of already knew that but you have articulated it much better than I ever could! Thankfully swapping the valve for a NOS UL41 has reduced the level of hum!

Originally Posted by Sideband

Measure the voltage across the cathode resistor of the UL41 and then work out the current. I think the UL41 maximum anode current is around 45mA and screen grid is about 10mA so measuring the voltage drop across the cathode resistor and then working out the current will be the TOTAL current of anode and screen grid. It will soon reveal if the UL41 is drawing excessive current. A total current of around 35 - 40mA drawn by the valve would probably be normal. If it's less, that's OK as well (as long as it's not too much less). Anything much more and I'd try another UL41.

Measuring the voltage drop across the cathode resistor I'm getting 8.9V - as R17 is 150Ω this gives a current of 59mA (8.9/150). If this is too high I could increase the value of the cathode resistor to 200-250Ω which will allow the valve to run cooler, place less stress on the Output Transformer but with a slight loss in volume?

[paulsherwin]

That will reduce the maximum volume, but it won't be noticeable in normal use. Going from 3W to 1.5W is only -3dB.

59mA does seem a bit high. Have you checked that there aren't significant +ve volts on the UL41 grid? This can be caused by a leaky coupling cap, but also by internal leakage in the valve.

[DonaldStott]

Of course the audio coupling capacitor was the first thing that was replaced.

I will, however, go back and check for leakage.

Remind me, do I measure Grid to Chassis or Grid to Cathode?

[DonaldStott]

Quick update - I'm measuring 15mV (millivolts) on the Grid (Pin 6) of the UL41. And that's Grid to Chassis.

Seems reasonable to me?

[paulsherwin]

Yes, that's OK. I would certainly bump up the cathode resistor. Standard practice at the time was to run the output valve absolutely full pelt, but a small radio like this will be rattling like mad with 3.5W going into the speaker. 500mW would still be quite loud for normal listening.

[DonaldStott]

Increased the cathode resistor (R17) to 220Ω from 150Ω and the voltage across the resistor increased from 8.9V to 11.1V ?

This means that the current changed from 59mA to 50mA.

(8.9/150 = 0.059A and 11.1/220 = 0.05A)

I am confused though as this means that the Wattage increased from 525mW to 555mW, assuming my calculations are correct?

(8.9 x 0.059 = 0.525W and 11.1 x 0.05 = 0.555W)

I'm measuring 14mV (millivolts) on the Grid (Pin 6) of the UL41.

[paulsherwin]

The resistor is dropping more voltage so is dissipating slightly more power. You can try increasing it to (say) 300 ohms if you like. You probably won't be able to hear the difference, the radio will run cooler and the UL41 will last longer.

[DonaldStott]

Thanks Paul - understood.

I think we might have one last push with this set while it's out the case and increase the cathode resistor to 300Ω.

Of course I don't have any of that value in my supplies so on order!

[paulsherwin]

It's not at all critical. Have a play with whatever you've got.

[DonaldStott]

Originally Posted by paulsherwin

The resistor is dropping more voltage so is dissipating slightly more power. You can try increasing it to (say) 300 ohms if you like. You probably won't be able to hear the difference, the radio will run cooler and the UL41 will last longer.

Increased the cathode resistor (R17) to 300Ω and the voltage across the resistor increased to 12.8V ?

This means that the current changed to 42.6mA (12.8/300 = 0.0426A) - much better!

So not only is the resistor dropping more voltage but the Wattage (power dissipation) stays about the same (0.0426 x 12.8 = 0.545W previously 0.0555W).

As Paul predicted in his Post above I'm not able to discern any reduction in volume. Time to get everything back in the case, again, to see (feel) if the radio is running any cooler and hear how the hum is getting on!

[DonaldStott]

Finished ...

[Radio Tech]

Well done Donald, Just goes to show that perseverance and dedication comes to the fore in something like this.

Ken

[DonaldStott]

Can't believe that it was way back in July 2020 that I successfully restored this Philips 141U for a friend, with the help of Forum members of course:-

Unfortunately the radio was returned to me this week, the symptoms being completely dead and no sign of life whatsoever?

A quick perusal of my earlier thread gave me a 'step-to-a-hint' as to where the problem might lie as the set did run very hot and part of the Dropper had already been bridged. Sure enough another section of the Dropper was showing O.L. on my DMM - R3 should be 250 Ohms according to the Service Sheet! Temporarily bodged in a 2W resistor just as a quick test and the radio came to life but we need a more permanent and robust solution now that two of the Dropper's three sections had failed- it is only a matter of time!






So don't know if I should:-

1. Advertise for a replacement Dropper on the Forum?

2. Obtain a 250 Ohms 5W replacement to 'fix' the problem - yet another bridge?

3. Look at alternatives to the Dropper as this set does run very hot?

Of course I'm tempted by 3. as I've never tried that before! I did look at Paul Stenning's Dropper Calculations page and concluded that a Capacitive Dropper would seem to be most appropriate for this set but I am somewhat overwhelmed by the technical details and calculations, especially where we have a number of tappings to contend with?

Then I started thinking about what else could I do in addition to a Capacitive Dropper to reduce the heat output and I remembered that back in my original Thread Paul Sherwin had mentioned 'There isn't any substitute for a UY41 unless you want to change the valve base. You could replace it with a 1N4007 and 100-200 ohm resistor, leaving the valve heater in circuit to keep the heater chain working and for cosmetic effect. If you do this, check that the smoothing electrolytics aren't being subjected to excess voltage before the valves warm up.' - but I'm getting ahead of myself as usual!

[ms660]

For a capacitor dropper ignore the taps just use the nominal mains voltage, eg: 240 Volts for the calculation.

Lawrence.