McMichael 135 Hums. - UK Vintage Radio Repair and Restoration Discussion Forum

Geoff 555 · 26th Feb 2008, 1:36 pm

Hi I am here again to bother you with further questions. The 135 has a hum problem or to be accurate 2 hums.It has been recapped through out apart from the 8 +8 mf . which I didnt have. I do have a 16 + 16 mf would that be OK? The mains transformer has dropped a little wax, it also seems to be very dry, the covering seems to be almost like old parchment fragile and crisp, it almost crackles if given a gentle squeeze. This may be revellent . Hum number 1 starts as soon as it is switched on, it is like a quiet mains hum that does not increase with the volume turned up. It's difficult to pin point the source as the trany is behind the speakers.
Hum number 2 is a more 'raw' sounding hum (buzz) which is quite loud when tuned to a station but when not on a station goes away. It does increase with volume. After running for 20 minutes the volume decreases to about a quarter of what it would be at full volume. If turned to full volume there is no increase in volume after the first 1/4 of a turn. I hope that makes sense.
Sorry for the long post but I was trying to lay out all the symptoms.
Any help would be most appreciated as always.
Thank you.
Geoff.

newlite4 · 26th Feb 2008, 3:25 pm

Geoff, replace the 8+8 (it could well be leaky!) with the 16+16 but check the data for the rectifier first just to make sure that it can handle the extra capacitance. The rectifier may have perished anyway if it has had been loaded-up with the extra burden of a faulty cap. Check to see if an RF bypass/modulation hum cap is fitted across the supply after the on/off switch. It may have been removed in the past because of failure, absence of this cap may be causing the tuning hum. If it is still there, replace it.
Neil

JoshWard · 26th Feb 2008, 8:46 pm

Hi Geoff, I'd say the 8-8 capacitor, If you need a replacement I'll dig two out, I think I've got some somewhere. Regards, Josh.

Geoff 555 · 27th Feb 2008, 2:16 am

Hello , Neil thank you for the advise as regards the valve data etc. I dont have any and to be honest wouldnt know what to do if I had. If someone could explain how or even better work it out then that would be great. It's a 442 BU valve,I think thats right.
There does not appear to be a RF bypass cap, the wiring diagram shows mains in via the switch on the live side, then straight to the transformer and the neutral to the voltage tapping. I did try the 16 + 16 as a tempery measure but it made no difference.
Josh I will take you up on the 8mf X 2 if the 16mf proves unsuitable.Thank you.
The other hum I took a chance on and tried a 1/4 turn on a couple of cores (and then returned them to where they were.) this was 90% better. So I am going to try to do an alinement , this leads to another question.It says about a injecting a 128.5 KC/S in which is fine I can follow that but then says inject a 214 m in , I take this to be 214 metres ? If the signal genny is in cycles then what is the conversion to metres?? or actually metres to cycles.

Sorry but I know nothing about this and am trying to learn as I go, or ask many questions and pick peoples brains, again.
Thanks a bunch.
Cheers.
Geoff.

wireful3 · 27th Feb 2008, 11:34 am

Hello Geoff,
Alignment instructions often give both wavelength and frequency because the signal generator has to be set at some fixed frequency and then the radio dial set to the same frequency before adjustments are made. As a lot of radios had the dial marked in wavelength and signal generators usually have the dial marked in frequency this causes the complication. Your first frequency 128 Kc/s relates to the IF and this was always given in frequency.

The conversion is straight forward divide the speed of light by the wavelength and this gives frequency. Divide 300 000 by the wavelength in metres and this gives frequency in KHz . At one time conversion tables were readily available and you might find one in an old radio magazine or even on the internet .

In your case 214m is about 1.4 MHz.

Geoff 555 · 27th Feb 2008, 12:52 pm

Hello wireful3 many thanks for that,it is in the book I have started for this kind of information.This is a fantastic forum, I would be totally lost without it.
I would never never have guessed the answer.
Cheers.
Geoff.

Geoff 555 · 28th Feb 2008, 9:47 pm

This is just to say a very Big thank you to you all. It's up and running!! It's got a few small things to do, a bootlace and the lid of a micro wave plastic container is the tuning cord that fell apart and the link to the dial pointer.
The bad buzz when tuned into a station was a 'funny' , I thought I would have a bash at alinement and found that for some reason it was well out, the cores etc. were well covered with dust but all required a tweek. Result no buzz and many more stations available. I am chuffed.
Cheers,all the best.
Geoff.

26th Feb 2008, 1:36 pm	#1
Geoff 555 Heptode Join Date: Jan 2008 Location: Bristol, UK. Posts: 981	McMichael 135 Hums. Hi I am here again to bother you with further questions. The 135 has a hum problem or to be accurate 2 hums.It has been recapped through out apart from the 8 +8 mf . which I didnt have. I do have a 16 + 16 mf would that be OK? The mains transformer has dropped a little wax, it also seems to be very dry, the covering seems to be almost like old parchment fragile and crisp, it almost crackles if given a gentle squeeze. This may be revellent . Hum number 1 starts as soon as it is switched on, it is like a quiet mains hum that does not increase with the volume turned up. It's difficult to pin point the source as the trany is behind the speakers. Hum number 2 is a more 'raw' sounding hum (buzz) which is quite loud when tuned to a station but when not on a station goes away. It does increase with volume. After running for 20 minutes the volume decreases to about a quarter of what it would be at full volume. If turned to full volume there is no increase in volume after the first 1/4 of a turn. I hope that makes sense. Sorry for the long post but I was trying to lay out all the symptoms. Any help would be most appreciated as always. Thank you. Geoff.

26th Feb 2008, 3:25 pm	#2
newlite4 Octode Join Date: Nov 2003 Location: Bath, Somerset, UK. Posts: 1,806	Re: McMichael 135 Hums. Geoff, replace the 8+8 (it could well be leaky!) with the 16+16 but check the data for the rectifier first just to make sure that it can handle the extra capacitance. The rectifier may have perished anyway if it has had been loaded-up with the extra burden of a faulty cap. Check to see if an RF bypass/modulation hum cap is fitted across the supply after the on/off switch. It may have been removed in the past because of failure, absence of this cap may be causing the tuning hum. If it is still there, replace it. Neil __________________ preserving the recent past, for the distant future.

26th Feb 2008, 8:46 pm	#3
JoshWard Octode Join Date: Dec 2005 Location: Near Stowmarket, Suffolk, UK. Posts: 1,962	Re: McMichael 135 Hums. Hi Geoff, I'd say the 8-8 capacitor, If you need a replacement I'll dig two out, I think I've got some somewhere. Regards, Josh.

27th Feb 2008, 2:16 am	#4
Geoff 555 Heptode Join Date: Jan 2008 Location: Bristol, UK. Posts: 981	Re: McMichael 135 Hums. Hello , Neil thank you for the advise as regards the valve data etc. I dont have any and to be honest wouldnt know what to do if I had. If someone could explain how or even better work it out then that would be great. It's a 442 BU valve,I think thats right. There does not appear to be a RF bypass cap, the wiring diagram shows mains in via the switch on the live side, then straight to the transformer and the neutral to the voltage tapping. I did try the 16 + 16 as a tempery measure but it made no difference. Josh I will take you up on the 8mf X 2 if the 16mf proves unsuitable.Thank you. The other hum I took a chance on and tried a 1/4 turn on a couple of cores (and then returned them to where they were.) this was 90% better. So I am going to try to do an alinement , this leads to another question.It says about a injecting a 128.5 KC/S in which is fine I can follow that but then says inject a 214 m in , I take this to be 214 metres ? If the signal genny is in cycles then what is the conversion to metres?? or actually metres to cycles. Sorry but I know nothing about this and am trying to learn as I go, or ask many questions and pick peoples brains, again. Thanks a bunch. Cheers. Geoff.

27th Feb 2008, 11:34 am	#5
wireful3 Retired Dormant Member Join Date: Sep 2003 Location: Bristol, UK. Posts: 808	Re: McMichael 135 Hums. Hello Geoff, Alignment instructions often give both wavelength and frequency because the signal generator has to be set at some fixed frequency and then the radio dial set to the same frequency before adjustments are made. As a lot of radios had the dial marked in wavelength and signal generators usually have the dial marked in frequency this causes the complication. Your first frequency 128 Kc/s relates to the IF and this was always given in frequency. The conversion is straight forward divide the speed of light by the wavelength and this gives frequency. Divide 300 000 by the wavelength in metres and this gives frequency in KHz . At one time conversion tables were readily available and you might find one in an old radio magazine or even on the internet . In your case 214m is about 1.4 MHz.

27th Feb 2008, 12:52 pm	#6
Geoff 555 Heptode Join Date: Jan 2008 Location: Bristol, UK. Posts: 981	Re: McMichael 135 Hums. Hello wireful3 many thanks for that,it is in the book I have started for this kind of information.This is a fantastic forum, I would be totally lost without it. I would never never have guessed the answer. Cheers. Geoff.

28th Feb 2008, 9:47 pm	#7
Geoff 555 Heptode Join Date: Jan 2008 Location: Bristol, UK. Posts: 981	Re: McMichael 135 Hums. This is just to say a very Big thank you to you all. It's up and running!! It's got a few small things to do, a bootlace and the lid of a micro wave plastic container is the tuning cord that fell apart and the link to the dial pointer. The bad buzz when tuned into a station was a 'funny' , I thought I would have a bash at alinement and found that for some reason it was well out, the cores etc. were well covered with dust but all required a tweek. Result no buzz and many more stations available. I am chuffed. Cheers,all the best. Geoff.