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Old 26th May 2018, 10:50 pm   #1
egerton
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Default Philips 141U low DC voltage?

First post, first dip into vintage restoration! I bought a Philips 141U Bakelite set as non working for £20 because the case is in excellent order and I like the style. Looking inside I found the mains supressor cap. is blown so replaced with a X type and set now working on both bands can hear stations loud and clear! A quick easy fix! Having got the data sheet and measured the supply voltage at the smoothing cap. it is quite low - stated as VC1 should be 212V but measures as 185V this is the supply side of the o/p transformer. I just wondered what to check for this? Any pointers? The rectifier UY41, the smoothing cap or the dropper resistor, can all of these be responsible for low voltage? Thanks for any tips.
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Old 27th May 2018, 12:37 am   #2
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Default Re: Philips 141U low DC voltage?

It's not unusual for the HT voltage of AC/DC sets to be a bit low. The service data voltages assume that all components are new and the mains voltage is exactly what it should be. This radio should work perfectly well with 180V of HT rather than 210V.
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Old 27th May 2018, 1:08 am   #3
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Default Re: Philips 141U low DC voltage?

The coupling capacitor to grid1 (pin 6) of the UL41 ("that capacitor") is always suspect, even slight leakage over-running the valve and output transformer and consequently dropping the HT, as can a dodgy UL41. Worth measuring the G1 voltage and changing the capacitor if more than 100mV or so, then if still more than a couple of hundred mV, there are threads on here about treating UL41s with gas cooker sparks and possible replacement with a 10P13.
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Old 27th May 2018, 8:43 am   #4
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Default Re: Philips 141U low DC voltage?

Unless the Philips 'black tar' capacitors have already been replaced, these are all likely to be faulty. The most important one has already been mentioned above. If faulty it will overrun the UL41 (which will lower the HT). These are expensive and now becoming difficult to obtain so you need to make sure that all is well in this area.

The mains suppressor capacitor would not have stopped the set from working. If the HT is slightly low and the UL41 is not being overrun or faulty, most likely is the UY41 but as stated, it will work perfectly on an HT only 20V low.
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Old 27th May 2018, 8:46 am   #5
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Default Re: Philips 141U low DC voltage?

Quote:
Originally Posted by paulsherwin View Post
It's not unusual for the HT voltage of AC/DC sets to be a bit low. The service data voltages assume that all components are new and the mains voltage is exactly what it should be. This radio should work perfectly well with 180V of HT rather than 210V.
Your reply suggests that I may be worrying unduly and this voltage is to be expected. All part of my learning process. Many thanks.
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Old 27th May 2018, 8:50 am   #6
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Default Re: Philips 141U low DC voltage?

Quote:
Originally Posted by G8UWM-MildMartin View Post
The coupling capacitor to grid1 (pin 6) of the UL41 ("that capacitor") is always suspect, even slight leakage over-running the valve and output transformer and consequently dropping the HT, as can a dodgy UL41. Worth measuring the G1 voltage and changing the capacitor if more than 100mV or so, then if still more than a couple of hundred mV, there are threads on here about treating UL41s with gas cooker sparks and possible replacement with a 10P13.
Many thanks for this info. I will check out this. Treating a UL41 with gas cooker sparks!? wow that is intriguing! must look that up!
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Old 27th May 2018, 8:55 am   #7
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Default Re: Philips 141U low DC voltage?

Quote:
Originally Posted by Sideband View Post
Unless the Philips 'black tar' capacitors have already been replaced, these are all likely to be faulty. The most important one has already been mentioned above. If faulty it will overrun the UL41 (which will lower the HT). These are expensive and now becoming difficult to obtain so you need to make sure that all is well in this area.

The mains suppressor capacitor would not have stopped the set from working. If the HT is slightly low and the UL41 is not being overrun or faulty, most likely is the UY41 but as stated, it will work perfectly on an HT only 20V low.
Hi. The fuse in the mains plug was blown, and I noticed that mess of the remains of the cap. across the mains input - so replaced it straight off. New fuse and it all worked.
(I should make a lamp limiter for first power ups after reading about them on here)

Thanks
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Old 27th May 2018, 10:17 am   #8
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Default Re: Philips 141U low DC voltage?

Check the circuit voltages, and especially the grid of the UL41. Any +ve voltage there indicates a problem either with the coupling cap or the UL41 itself. You shouldn't continue to use the set until this is resolved.

That said, my original point in #2 is valid - AC/DC radios do often have low HT in the absence of any real fault. It's usually the result of low mains voltage and/or the UY41 getting a bit tired. The HT voltage isn't critical in itself.
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Old 27th May 2018, 10:25 am   #9
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Default Re: Philips 141U low DC voltage?

According to the schematic it's designed to have a voltage variation feed to the rectifier of 50 volts (200-250V) The mains voltage selector won't affect the voltage supplied to the rectifiers surge limiter.

The UL41 was designed to operate over a wide range of supply voltages without much modification as per the valve data:

http://www.r-type.org/pdfs/ul41.pdf

Lawrence.

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Old 27th May 2018, 10:48 am   #10
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Default Re: Philips 141U low DC voltage?

Quote:
Originally Posted by egerton View Post

Many thanks for this info. I will check out this. Treating a UL41 with gas cooker sparks!? wow that is intriguing! must look that up!
It's a 'kill or cure' method. Basically the idea is simple. The UL41 often has a deposit inside the glass around the base where the pins are (probably due to impurities that are slowly burned off the electrodes) and this causes leakage between the pins. The idea is to use a high voltage to 'burn' the deposit away and the ignition from a gas cooker is a good source. If you don't have a gas cooker, other methods can be devised. Ultimately everything carries a risk but if the valve is faulty anyway, there is nothing to be lost by trying to 'flash' away the deposits. Many UL41's have been recovered successfully and considering their scarcity and high price, it's worth trying.
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Old 28th May 2018, 2:11 pm   #11
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Default Re: Philips 141U low DC voltage?

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Originally Posted by paulsherwin View Post
Check the circuit voltages, and especially the grid of the UL41. Any +ve voltage there indicates a problem either with the coupling cap or the UL41 itself. You shouldn't continue to use the set until this is resolved.

That said, my original point in #2 is valid - AC/DC radios do often have low HT in the absence of any real fault. It's usually the result of low mains voltage and/or the UY41 getting a bit tired. The HT voltage isn't critical in itself.
I have taken some measurements around the UL41 in light of your comments. I find the DC voltage at the grid (pin 6 of V4) gradually rises and settles at 7.9V - not good at all! Checking other side of the coupling cap. C29 I get 47V which is on the anode of the previous stage, so I guess C29 is leaky.

Looking at the UY41 anode has 180V AC and the cathode shows 167V DC when settled.... which is feeding C1 in the attached diagram and 140V on C2.

I conclude I should change C29 would you agree?
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Old 28th May 2018, 3:17 pm   #12
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Default Re: Philips 141U low DC voltage?

If the voltage on the control grid of the output valve is POSITIVE with respect to the cathode, you should certainly replace the coupling capacitor.

Another fault finding technique is to disconnect the coupling capacitor from the grid and measure the voltage on the disconnected end of the cap with respect to the chassis. A voltage here indicates that the cap is leaking.
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Old 28th May 2018, 5:28 pm   #13
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Default Re: Philips 141U low DC voltage?

I will try that and measure the voltage. It seems the circuit I refer to in my previous post did not attach so I try again here.
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Old 28th May 2018, 10:09 pm   #14
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Default Re: Philips 141U low DC voltage?

As you say C29 is the one to check and most likely change.
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Old 28th May 2018, 10:23 pm   #15
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Default Re: Philips 141U low DC voltage?

It is a MUST change! The UL41 may survive the mistreatment for a while but the output transformer may not...and despite the scarcity of UL41's, the output transformer may be harder to source unless you are prepared to fit an alternative. Not worth risking the transformer for a capacitor of a few pence.
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Old 28th May 2018, 11:54 pm   #16
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Default Re: Philips 141U low DC voltage?

That capacitor is often referred to as That Capacitor here and is present in all domestic valve radios of this design. It is notorious and generally considered to be a change on sight component, even by people who like to minimise recapping. The only exception is when the cap is a type which is unlikely to be leaky, such as a ceramic or plastic film type. Philips started using ceramics in the late 50s and plastic films in the early 60s, so this radio won't have them, at least in that position.
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Old 29th May 2018, 9:06 am   #17
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Default Re: Philips 141U low DC voltage?

Thank you all for the advice. I will change C29. It's a black Philips type referred to by Paul Stenning in his useful guide. Any advice on what type to get as replacement? It's listed as a 10,000 pF 400V.
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Old 29th May 2018, 9:12 am   #18
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Default Re: Philips 141U low DC voltage?

10,000 pF is 0.01uF or 10nF.

This could be replaced with a yellow polyester type of suitable voltage rating. Other types are available.

http://uk.farnell.com/vishay/mkt1813...ngdom%2Fsearch
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Old 29th May 2018, 9:24 am   #19
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Default Re: Philips 141U low DC voltage?

In this set C30 (tone correction and spike suppression) is also a candidate for change. Its failure could also take out the output transformer primary or lower the HT.

Some sets put it across the primary to HT rather than to 0V. This makes its failure less likely to cause damage.
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Old 29th May 2018, 1:37 pm   #20
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Default Re: Philips 141U low DC voltage?

I think it's worth replacing this cap. and what about repositioning it across the primary HT? It seems a better place for it, but then I'm changing the original design slightly so purist may sqeal! I suppose it depends on ones point of view.
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