Marconi T69DA hum. - UK Vintage Radio Repair and Restoration Discussion Forum

reeltoreelrob · 15th Feb 2023, 8:28 pm

Hi all,
I have had this set for years and has never worked, with changing the hunts caps and a few out of range resistors (one open) I now have it working and pulling in stations well.
My problem is a hum mostly on mw and lw that varies with volume and does not vary across the waveband, it is there to a very small degree on fm but seems to tune out when on a station. The smoothing caps have been replaced and I have tried a spare set of valves with no difference. The set has a fair few red tubular caps, they appear slightly lumpy but I have pulled a couple and test spot on, not sure about the reliability of these. I expect some hum but this seems a bit too much.

Any ideas greatly appreciated.
Thanks in advance.

Silicon · 15th Feb 2023, 10:02 pm

Is there any sign of overheating on the printed circuit board?

A charred PCB might become conductive and introduce 50Hz AC voltages into the AF amplifier.

Also look for 'dry joints'.

Have the power supply electrolytics been replaced recently?

reeltoreelrob · 15th Feb 2023, 10:15 pm

Hi,
The board is in pretty good condition but is dirty in places I am trying to clean it as I go, the electrolytics have been done in the last few years (3 at most) and I have given all the joints a re flow.
All the best Rob.

Techman · 15th Feb 2023, 10:31 pm

Quote:

Originally Posted by Silicon

Have the power supply electrolytics been replaced recently?

The OP says that the hum varies with volume control setting, so we can pretty much rule them out.

Could be 'modulation' hum?

Silicon · 15th Feb 2023, 11:36 pm

If the hum increases as you turn up the volume it could be a lack of screening of the audio cable going to the volume control potentiometer.

Are the audio cables routed next to mains cables?
Is the mains switch on the back of the volume control?

reeltoreelrob · 16th Feb 2023, 10:42 am

Hi, thanks for the ideas, it is a pcb design so the volume pot is mounted onto that. The mains switch is part of the tone control and when I bought the set many years ago it had been detatched from the control for what seemed no aparent reason, if it were to be that causing the hum would it not be the same amount of hum on all wavebands rather than mw and lw, as I say the vhf tends to tune out the hum when on a station and there is virtually none above about 97mhz.
All the best
Rob.

Gabe001 · 16th Feb 2023, 1:03 pm

Are you sure it's not environmental? From LEDs or SMPS's plugged in nearby sockets for example.

Boulevardier · 16th Feb 2023, 2:35 pm

Quote:

Originally Posted by Gabe001

Are you sure it's not environmental? From LEDs or SMPS's plugged in nearby sockets for example.

I agree - have you investigated the “local electromagnetic environment”? In urban locations this can be an impenetrable nightmare of interference from routers, computers, wireless telephones, TV boxes, even wireless doorbells – just about anything, even something not in your own house.
As an example, a few days ago I was getting a low-level hum on one of my valve radios – which in my case was independent of volume setting, so getting in to the audio stages. After a lot of head scratching and fiddling about, I just happened to move our wireless phone (which was sitting quite close to the radio) a few feet. The hum disappeared!
Just a thought before you start ripping the radio to bits looking for the fault…

Mike

reeltoreelrob · 16th Feb 2023, 3:41 pm

Hi,
Thanks for the ideas, I had read about this problem on this site, I did move the radio to another location in the house but made no difference, that said we do have remote doorbells, cordless phones, alexas and all that stuff so definitely worth further investigation, failing that I'll have to suggest to the wife we move into the countryside.
I'll keep the post updated.
Thanks again
Rob.

15th Feb 2023, 8:28 pm	#1
reeltoreelrob Pentode Join Date: Nov 2010 Location: Birmingham, West Midlands, UK. Posts: 202	Marconi T69DA hum. Hi all, I have had this set for years and has never worked, with changing the hunts caps and a few out of range resistors (one open) I now have it working and pulling in stations well. My problem is a hum mostly on mw and lw that varies with volume and does not vary across the waveband, it is there to a very small degree on fm but seems to tune out when on a station. The smoothing caps have been replaced and I have tried a spare set of valves with no difference. The set has a fair few red tubular caps, they appear slightly lumpy but I have pulled a couple and test spot on, not sure about the reliability of these. I expect some hum but this seems a bit too much. Any ideas greatly appreciated. Thanks in advance. Attached Thumbnails

15th Feb 2023, 10:02 pm	#2
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,167	Re: Marconi t69da hum Is there any sign of overheating on the printed circuit board? A charred PCB might become conductive and introduce 50Hz AC voltages into the AF amplifier. Also look for 'dry joints'. Have the power supply electrolytics been replaced recently?

15th Feb 2023, 10:15 pm	#3
reeltoreelrob Pentode Join Date: Nov 2010 Location: Birmingham, West Midlands, UK. Posts: 202	Re: Marconi t69da hum Hi, The board is in pretty good condition but is dirty in places I am trying to clean it as I go, the electrolytics have been done in the last few years (3 at most) and I have given all the joints a re flow. All the best Rob.

15th Feb 2023, 11:36 pm	#5
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,167	Re: Marconi t69da hum If the hum increases as you turn up the volume it could be a lack of screening of the audio cable going to the volume control potentiometer. Are the audio cables routed next to mains cables? Is the mains switch on the back of the volume control?

16th Feb 2023, 10:42 am	#6
reeltoreelrob Pentode Join Date: Nov 2010 Location: Birmingham, West Midlands, UK. Posts: 202	Re: Marconi t69da hum Hi, thanks for the ideas, it is a pcb design so the volume pot is mounted onto that. The mains switch is part of the tone control and when I bought the set many years ago it had been detatched from the control for what seemed no aparent reason, if it were to be that causing the hum would it not be the same amount of hum on all wavebands rather than mw and lw, as I say the vhf tends to tune out the hum when on a station and there is virtually none above about 97mhz. All the best Rob.

16th Feb 2023, 1:03 pm	#7
Gabe001 Octode Join Date: Oct 2019 Location: Stockport, Greater Manchester, UK. Posts: 1,646	Re: Marconi t69da hum Are you sure it's not environmental? From LEDs or SMPS's plugged in nearby sockets for example.

16th Feb 2023, 3:41 pm	#9
reeltoreelrob Pentode Join Date: Nov 2010 Location: Birmingham, West Midlands, UK. Posts: 202	Re: Marconi T69DA hum. Hi, Thanks for the ideas, I had read about this problem on this site, I did move the radio to another location in the house but made no difference, that said we do have remote doorbells, cordless phones, alexas and all that stuff so definitely worth further investigation, failing that I'll have to suggest to the wife we move into the countryside. I'll keep the post updated. Thanks again Rob.