KB Radio MP151/2 - UK Vintage Radio Repair and Restoration Discussion Forum

Celestial · 2nd Dec 2020, 4:33 pm

Restroing this radio. Replaced all old Hunts paper capacitors as they were all out of spec. Looking at the power supply section both sections of the HT 50mfd smoother is reading below capacity, and one section of the 100mfd/25mfd LT Cap is faulty. They are both twisted lug types of which I do not have any replacements. Is it permissible to wire new separate capactitors on the exsisting tags of the old capacitors or should they be removed altogether?
Also I have not as yet tested the contact cooled metal rectifier. The old wax paper cap will certainly have to be replaced.
I would appreceiate and help with this. Thank you.

crackle · 2nd Dec 2020, 5:22 pm

I don't think I have had to replace the electrolytic caps on one of these radios.
It is not good practice to just tag new caps to the original tags because you will still have a leaky capacitor that could eventually go short circuit.
Have you tried slowly reforming the electrolytics via a 10k resistor for the HT one and say a 1k resistor for the LT ones. Leave them re-forming for 12 hours or so each.
Failing that if they are not reforming correctly you may be able to remove the cans from the chassis and re-stuff them.

Mike

crackle · 2nd Dec 2020, 5:45 pm

By the way it might be advisable to change that waxy C25, and the brown hunts C21 tone corrector. (manufacturers numbers)

Mike

Celestial · 3rd Dec 2020, 10:04 am

Good morning Mike,
Thank you for your replies. I have not tried reforming either of the can electrolytics.
I will replace the old wax paper capacitor as these are usually way off spec.
I can replace the old Hunts capacitor across the o/p tx primary with a ceramic capacitor of vitually the same value which is 1000v working as I know that high voltages can build up on the primary of the o/p tx. I obtained these caps form a supplier a little while ago as brand new discontinued types.
Thank you once again for your replies.

cathoderay57 · 4th Dec 2020, 10:10 am

Once you have reformed the caps, measure the HT voltage. If you are getting approx same voltage as shown in the service data and no hum then leave the rectifier as-is. If the HT has dropped and you are getting hum with the volume control at minimum then replace the rectifier with a diode bridge and a 100R 5W series resistor (because the replacement bridge will have lower series resistance than the contact cooled device). Jerry

cathoderay57 · 4th Dec 2020, 12:24 pm

Oops, very sorry to mislead - I should have looked at the circuit

The rectifier is half-wave so you mustn't try to replace it with a bridge; a single silicon diode would be required if the existing rectifier is bad. If you end up replacing it then test the HT voltage with all of the valves removed to check the offload HT voltage. Then by calculation work out how much additional series resistance you need to add in series with the diode to compensate for the internal resistance of the contact-cooled device. Before powering on with valves fitted then set the filament voltage adjustment R21 (Trader Sheet #1247 numbers) to maximum resistance. The filaments on these battery valves are very easily ruptured by overvoltage and surges. Also you should replace the mains input filter capacitor C26 (0.02uF) with a Class X. Jerry

Celestial · 5th Dec 2020, 8:45 am

Gentlemen,
Thank you very much for all your replies.

2nd Dec 2020, 5:22 pm	#2
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: KB Radio MP151/2 I don't think I have had to replace the electrolytic caps on one of these radios. It is not good practice to just tag new caps to the original tags because you will still have a leaky capacitor that could eventually go short circuit. Have you tried slowly reforming the electrolytics via a 10k resistor for the HT one and say a 1k resistor for the LT ones. Leave them re-forming for 12 hours or so each. Failing that if they are not reforming correctly you may be able to remove the cans from the chassis and re-stuff them. Mike Last edited by crackle; 2nd Dec 2020 at 5:31 pm.

2nd Dec 2020, 5:45 pm	#3
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: KB Radio MP151/2 By the way it might be advisable to change that waxy C25, and the brown hunts C21 tone corrector. (manufacturers numbers) Mike

3rd Dec 2020, 10:04 am	#4
Celestial Triode Join Date: Nov 2018 Location: Swindon, Wiltshire, UK. Posts: 43	Re: KB Radio MP151/2 Good morning Mike, Thank you for your replies. I have not tried reforming either of the can electrolytics. I will replace the old wax paper capacitor as these are usually way off spec. I can replace the old Hunts capacitor across the o/p tx primary with a ceramic capacitor of vitually the same value which is 1000v working as I know that high voltages can build up on the primary of the o/p tx. I obtained these caps form a supplier a little while ago as brand new discontinued types. Thank you once again for your replies.

4th Dec 2020, 10:10 am	#5
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,382	Re: KB Radio MP151/2 Once you have reformed the caps, measure the HT voltage. If you are getting approx same voltage as shown in the service data and no hum then leave the rectifier as-is. If the HT has dropped and you are getting hum with the volume control at minimum then replace the rectifier with a diode bridge and a 100R 5W series resistor (because the replacement bridge will have lower series resistance than the contact cooled device). Jerry

4th Dec 2020, 12:24 pm	#6
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,382	Re: KB Radio MP151/2 Oops, very sorry to mislead - I should have looked at the circuit The rectifier is half-wave so you mustn't try to replace it with a bridge; a single silicon diode would be required if the existing rectifier is bad. If you end up replacing it then test the HT voltage with all of the valves removed to check the offload HT voltage. Then by calculation work out how much additional series resistance you need to add in series with the diode to compensate for the internal resistance of the contact-cooled device. Before powering on with valves fitted then set the filament voltage adjustment R21 (Trader Sheet #1247 numbers) to maximum resistance. The filaments on these battery valves are very easily ruptured by overvoltage and surges. Also you should replace the mains input filter capacitor C26 (0.02uF) with a Class X. Jerry Last edited by cathoderay57; 4th Dec 2020 at 12:30 pm.

2nd Dec 2020, 4:33 pm	#1
Celestial Triode Join Date: Nov 2018 Location: Swindon, Wiltshire, UK. Posts: 43	KB Radio MP151/2 Restroing this radio. Replaced all old Hunts paper capacitors as they were all out of spec. Looking at the power supply section both sections of the HT 50mfd smoother is reading below capacity, and one section of the 100mfd/25mfd LT Cap is faulty. They are both twisted lug types of which I do not have any replacements. Is it permissible to wire new separate capactitors on the exsisting tags of the old capacitors or should they be removed altogether? Also I have not as yet tested the contact cooled metal rectifier. The old wax paper cap will certainly have to be replaced. I would appreceiate and help with this. Thank you. Attached Thumbnails

5th Dec 2020, 8:45 am	#7
Celestial Triode Join Date: Nov 2018 Location: Swindon, Wiltshire, UK. Posts: 43	Re: KB Radio MP151/2 Gentlemen, Thank you very much for all your replies.