Pye P131MBQ First Project Help - UK Vintage Radio Repair and Restoration Discussion Forum

Connor_B · 4th Oct 2018, 10:09 pm

Ok, so the story begins when i found a PYE suitcase radio at a car boot sale a couple of weeks ago. It took literally seconds for me to fall in love with its design so I bought it. after doing some reading around on the forums I saw that before powering up I should replace the wax and electrolytic based capacitors, so I purchased a selection of caps mostly polyester film and a couple of ceramics, and some modern electrolytic's to replace the combination one in the set. so after I replaced the old capacitors i made up a mains lead and powered up the set. and nothing happened, totally dead although after taking some voltage reading from the set it would seem that the transformers are working as they should. I then checked the valves and found that in some point in the past someone had swapped them all around to the wrong positions. so I put them all back into place. still nothing.
i then checked the valve heaters and found that the output valve had a very high resistance across the heater and also did not glow when connected to a power supply so I ordered a replacement. Once I installed the new valve something happened, now when I switch on the radio a low level hum comes from the speaker and there is some static too, although at quite a low volume the output is affected by the volume control but not by the tuning dial at all so I cant get any stations. also I cant see any glow at all coming from the valve heaters, even in a pitch black room.
there we go, essay over and if you've read this far your help would be greatly appreciated i'm all out of ideas and this being my first set very low on expertise!

Thanks all

julie_m · 5th Oct 2018, 3:12 am

Your first mistake was changing all the capacitors holus-bolus, without testing the set after doing each one. If you made a single wiring error, you will have introduced a new fault. They probably do need replacing, but generally they will sort-of, just-about work, and not do too much damage. There are three exceptions: Any capacitor between the mains Live or Neutral definitely needs replacing with a modern, "class X" rated capacitor. (A set will work without a capacitor in this position, but will be more prone to interference.) Any capacitor between mains Live or Neutral and Earth or any external connector or exposed metalwork definitely needs replacing with a modern, "class Y" rated capacitor. (Class Y capacitors are designed never to go leaky. They can be left in place temporarily if you first make sure the "accessible" end is not live, and the set is powered by an RCD.) And if the capacitor blocking DC from reaching the control grid of the output valve is electrically leaky, this can cause the valve to pass too much current; and in so doing, damage the loudspeaker matching transformer and possibly also the valve. (Don't even think about leaving the original capacitor. Originality be

. An ugly modern polyfilm capacitor is preferrable to a burned-out speaker transformer anyday.)

Your second mistake was trusting the set's own mains power supply. It's best to make up a temporary battery pack, using disposable batteries from a pound store: usually ten 9V smoke alarm batteries in series for the 90V HT, and five 1.5V, C or D size flashlight cells for the 7.5V LT. It's possible, otherwise -- unlikely, but by no means impossible -- for a fault to blow one or more of the delicate valve filaments. A battery supply is also inherently energy-limited: even in the event of a short circuit, the current will be cut off as soon as the battery is spent (in really egregious cases, the chemical reaction can be stopped prematurely due to the buildup of waste products blocking the charged particles from getting to the electrodes; but that's a Paper Three question).

Right, that's my "stern schoolmistress" bit out of the way. There is nothing wrong with make a mistake as long as you learn something from it. The next thing to do is to measure the voltages on the valve pins and compare with the values shown on the wiring diagram (available from the link "up top" for a very reasonable £1.99, which supports the running of the forum).

If the voltages seem OK (note that a modern digital meter may read a bit higher than an old-fashioned mechanical meter, because the latter type has to rob some energy from the circuit under test to move the pointer), you could also check for local oscillator operation. You will need a second set, preferrably a small transistor portable so you can move it about easily. Tune this to a weak station somewhere near the high frequency / long wavelength end of the MW band and place it near to the Pye. As you tune the Pye up and down the MW band, there should come a point where the second set goes silent or just emits a steady tone. This will be about 470 kHz below the frequency to which the second set is tuned (remember Hz * metres = speed of light, so kHz * m = 300 000). The local oscillator is always 470 kHz above the tuned station frequency, and inevitably emits some energy at this frequency; and when this happens to coincide with the frequency to which the other set is tuned, the leakage from the oscillator will obliterate the weak station. As the strength of this signal decreases with distance squared, the two sets will need to be close together for the effect to manifest. (If you can get a third, known working radio, you will be able to rehearse the manoeuvre and give yourself some idea what to expect .....)

boxdoctor · 5th Oct 2018, 9:51 am

Quote:

Originally Posted by julie_m

Tune this to a weak station somewhere near the high frequency / long wavelength end of the MW band and place it near to the Pye.

I think Julie is referring to the short wavelength end of the dial. (Around 200 metres, on the "medium" waveband. Both sets on the same waveband, of course.) Tony

julie_m · 5th Oct 2018, 5:17 pm

D'oh! Serves me right for posting while suffering from insomnia. Yes, I meant the short wavelength end.

Biggles · 5th Oct 2018, 5:21 pm

I have one of these sets and am familiar with them so if you need to ask me particular questions feel free to do so. One thing you need to be very careful about is not to accidentally short pins out on the valve bases if you are taking test meter readings. Battery valves are not very forgiving in this respect. Also do not be tempted to remove valves while the set is switched on. Bad contacts on the bases can have a similar effect. Try and check for correct voltages across the valve filaments. Note I say filaments as they only have a very faint glow as opposed to normal valve heaters which are easily visible in daylight. It may be worth replacing the DK96 as these can be a bit temperamental about oscillating, if they are old and tired. Does it crackle when the wavechange switch is operated? If it does then the IF stage is probably ok. Lastly, and I only say this as I don't know your technical background; Make sure you are aware of the dangers from high voltages as this set is powered from the mains. Cheers.
Alan.

Connor_B · 6th Oct 2018, 1:43 pm

Ok Excellent! thanks for all your help Julie, I'll set about trying to measure the valve voltages today, hopefully i can find something amiss.

Cheers Alan, I'll investigate the set a bit more today and see if I can discover anything. As far as I can tell there is no discernible noise when switching bands at all, just a constant low hum and occasional interference. A previous investigator had swapped all the valves into the wrong positions, so i think more than one could be damaged. All the heaters seem to test fine on the bench, but have no glow at all in the set.

Mike. Watterson · 6th Oct 2018, 5:02 pm

D*96 tubes don't have any obvious glow, except maybe in total darkness.
It's normal to solder two wires to the DAF96 filament pins and make sure the voltage is between 1.32V and 1.38V (Mullard / Philips recommend 1.35V nominal for battery valves on a mains PSU. Total voltage on mains, about 6.8V, the 7.5V is a near "fresh" Zinc Carbon set of five cells and the DK96 stops working first, when battery gets to approximately 5V. Most DK96 need at least 1.1V on filament, a worn one maybe 1.2V and a good one with good circuit perhaps as low as 1.0V.

I had to replace one of the dropper resistors on my P131MBQ.

I prefer to run any battery / mains valve set with no mains transformer on batteries, or if the PSU is 100% tested, on a shaver transformer mounted in a box. I then put a 3 pin 5A plug and drill a 3rd hole to case side of the 230V outlet for the earth pin. I usually add 2 x XY rated 1nF 1kV caps to mains earth to the isolated outlet, or at least one to the CT of the transformer common to 115V & 230, otherwise you get a buzz. The earth pin of the 5A plug just ensures it's not plugged into a 3 pin to 2 pin "so called shaver adaptor".

4th Oct 2018, 10:09 pm	#1
Connor_B Retired Dormant Member Join Date: Oct 2018 Location: Ipswich, Suffolk, UK. Posts: 2	Pye P131MBQ First Project Help Ok, so the story begins when i found a PYE suitcase radio at a car boot sale a couple of weeks ago. It took literally seconds for me to fall in love with its design so I bought it. after doing some reading around on the forums I saw that before powering up I should replace the wax and electrolytic based capacitors, so I purchased a selection of caps mostly polyester film and a couple of ceramics, and some modern electrolytic's to replace the combination one in the set. so after I replaced the old capacitors i made up a mains lead and powered up the set. and nothing happened, totally dead although after taking some voltage reading from the set it would seem that the transformers are working as they should. I then checked the valves and found that in some point in the past someone had swapped them all around to the wrong positions. so I put them all back into place. still nothing. i then checked the valve heaters and found that the output valve had a very high resistance across the heater and also did not glow when connected to a power supply so I ordered a replacement. Once I installed the new valve something happened, now when I switch on the radio a low level hum comes from the speaker and there is some static too, although at quite a low volume the output is affected by the volume control but not by the tuning dial at all so I cant get any stations. also I cant see any glow at all coming from the valve heaters, even in a pitch black room. there we go, essay over and if you've read this far your help would be greatly appreciated i'm all out of ideas and this being my first set very low on expertise! Thanks all Attached Thumbnails

5th Oct 2018, 3:12 am	#2
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Pye P131MBQ First Project Help Your first mistake was changing all the capacitors holus-bolus, without testing the set after doing each one. If you made a single wiring error, you will have introduced a new fault. They probably do need replacing, but generally they will sort-of, just-about work, and not do too much damage. There are three exceptions: Any capacitor between the mains Live or Neutral definitely needs replacing with a modern, "class X" rated capacitor. (A set will work without a capacitor in this position, but will be more prone to interference.) Any capacitor between mains Live or Neutral and Earth or any external connector or exposed metalwork definitely needs replacing with a modern, "class Y" rated capacitor. (Class Y capacitors are designed never to go leaky. They can be left in place temporarily if you first make sure the "accessible" end is not live, and the set is powered by an RCD.) And if the capacitor blocking DC from reaching the control grid of the output valve is electrically leaky, this can cause the valve to pass too much current; and in so doing, damage the loudspeaker matching transformer and possibly also the valve. (Don't even think about leaving the original capacitor. Originality be. An ugly modern polyfilm capacitor is preferrable to a burned-out speaker transformer anyday.) Your second mistake was trusting the set's own mains power supply. It's best to make up a temporary battery pack, using disposable batteries from a pound store: usually ten 9V smoke alarm batteries in series for the 90V HT, and five 1.5V, C or D size flashlight cells for the 7.5V LT. It's possible, otherwise -- unlikely, but by no means impossible -- for a fault to blow one or more of the delicate valve filaments. A battery supply is also inherently energy-limited: even in the event of a short circuit, the current will be cut off as soon as the battery is spent (in really egregious cases, the chemical reaction can be stopped prematurely due to the buildup of waste products blocking the charged particles from getting to the electrodes; but that's a Paper Three question). Right, that's my "stern schoolmistress" bit out of the way. There is nothing wrong with make a mistake as long as you learn something from it. The next thing to do is to measure the voltages on the valve pins and compare with the values shown on the wiring diagram (available from the link "up top" for a very reasonable £1.99, which supports the running of the forum). If the voltages seem OK (note that a modern digital meter may read a bit higher than an old-fashioned mechanical meter, because the latter type has to rob some energy from the circuit under test to move the pointer), you could also check for local oscillator operation. You will need a second set, preferrably a small transistor portable so you can move it about easily. Tune this to a weak station somewhere near the high frequency / long wavelength end of the MW band and place it near to the Pye. As you tune the Pye up and down the MW band, there should come a point where the second set goes silent or just emits a steady tone. This will be about 470 kHz below the frequency to which the second set is tuned (remember Hz * metres = speed of light, so kHz * m = 300 000). The local oscillator is always 470 kHz above the tuned station frequency, and inevitably emits some energy at this frequency; and when this happens to coincide with the frequency to which the other set is tuned, the leakage from the oscillator will obliterate the weak station. As the strength of this signal decreases with distance squared, the two sets will need to be close together for the effect to manifest. (If you can get a third, known working radio, you will be able to rehearse the manoeuvre and give yourself some idea what to expect .....) __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

5th Oct 2018, 5:17 pm	#4
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Pye P131MBQ First Project Help D'oh! Serves me right for posting while suffering from insomnia. Yes, I meant the short wavelength end. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

5th Oct 2018, 5:21 pm	#5
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: Pye P131MBQ First Project Help I have one of these sets and am familiar with them so if you need to ask me particular questions feel free to do so. One thing you need to be very careful about is not to accidentally short pins out on the valve bases if you are taking test meter readings. Battery valves are not very forgiving in this respect. Also do not be tempted to remove valves while the set is switched on. Bad contacts on the bases can have a similar effect. Try and check for correct voltages across the valve filaments. Note I say filaments as they only have a very faint glow as opposed to normal valve heaters which are easily visible in daylight. It may be worth replacing the DK96 as these can be a bit temperamental about oscillating, if they are old and tired. Does it crackle when the wavechange switch is operated? If it does then the IF stage is probably ok. Lastly, and I only say this as I don't know your technical background; Make sure you are aware of the dangers from high voltages as this set is powered from the mains. Cheers. Alan.

6th Oct 2018, 1:43 pm	#6
Connor_B Retired Dormant Member Join Date: Oct 2018 Location: Ipswich, Suffolk, UK. Posts: 2	Re: Pye P131MBQ First Project Help Ok Excellent! thanks for all your help Julie, I'll set about trying to measure the valve voltages today, hopefully i can find something amiss. Cheers Alan, I'll investigate the set a bit more today and see if I can discover anything. As far as I can tell there is no discernible noise when switching bands at all, just a constant low hum and occasional interference. A previous investigator had swapped all the valves into the wrong positions, so i think more than one could be damaged. All the heaters seem to test fine on the bench, but have no glow at all in the set.

6th Oct 2018, 5:02 pm	#7
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Pye P131MBQ First Project Help D*96 tubes don't have any obvious glow, except maybe in total darkness. It's normal to solder two wires to the DAF96 filament pins and make sure the voltage is between 1.32V and 1.38V (Mullard / Philips recommend 1.35V nominal for battery valves on a mains PSU. Total voltage on mains, about 6.8V, the 7.5V is a near "fresh" Zinc Carbon set of five cells and the DK96 stops working first, when battery gets to approximately 5V. Most DK96 need at least 1.1V on filament, a worn one maybe 1.2V and a good one with good circuit perhaps as low as 1.0V. I had to replace one of the dropper resistors on my P131MBQ. I prefer to run any battery / mains valve set with no mains transformer on batteries, or if the PSU is 100% tested, on a shaver transformer mounted in a box. I then put a 3 pin 5A plug and drill a 3rd hole to case side of the 230V outlet for the earth pin. I usually add 2 x XY rated 1nF 1kV caps to mains earth to the isolated outlet, or at least one to the CT of the transformer common to 115V & 230, otherwise you get a buzz. The earth pin of the 5A plug just ensures it's not plugged into a 3 pin to 2 pin "so called shaver adaptor".