Dropper resistors for Marconiphone T18DA - what do I do?! - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

topboost · 7th Aug 2012, 1:02 pm

Steve - thanks for bringing Rapid online to my attention...not heard of them before, and it's always handy to have a few more potential suppliers noted!

Chris - good info again...not too bothered about the prices we're looking at here, but based on what you say and checking out the components I can find on line I think that the resistor option is not only easy, but also would at least squeeze into the space I have round the old dropper. I have a suspicion I'd struggle to find two x-type caps and cram them in on this one. Think I might have a couple of suitable 1k ceramic cased resistors in my parts boxes, but I'll check the values.

I'll also post a few photos here too...everyone loves a good photo, don't they?!

topboost · 3rd Jun 2013, 7:28 am

Well, it might have taken me a year to get round to (long story!), but yesterday I finally got round to lavishing a little TLC on the T18DA!

I replaced the two duff sections of the dropper resistor with ceramic "white block" style resistors. I replaced a number of the waxy caps, including the ones mentioned above. I also replaced the two 16+16 uF can caps (easier for me than reforming), and the 25uF can which had ruptured. With a little swearing I also managed to hook up a brand new dial cord, and clean and oil all the turning parts thereof. I also took all the valves out, cleaned each of the pins, sprayed contact cleaner over the sockets, and pushed al the valves home a few times to hopefully clean the sockets up. I fitted a new mains cord and plug too.

After putting the whole set back together again I powered up to test it. After some seconds of warm up the radio began to hum (sort of motorboating I guess) fairly loudly with the volume at minimum setting. This didn't really change as you turn the volume up, and no stations could be picked up by twiddling the tuning dial. So, I'm not quite there yet! However, I'm taking it as a good sign that at least the set powers up and makes a sound.

I have yet to source a set of new valves, which I think is worth doing, as these (particularly the potentially fragile output valve) ones look to have been in the set from birth! Some still have their Marconi stickers on. I'm hoping that fixes it, but I'll pull the set out again in the next day or so and check it over just in case. Any tips at all, or is it now a case of just very carefully visually checking and using the multimeter to find the fault? Assuming it's not as simple as duff valves.

TrevorG3VLF · 3rd Jun 2013, 10:59 am

Checking the DC voltages is always the first step.

Motor boating is due to low frequency feedback, almost certainly due to bad HT smoothing. Did you fit good capacitors and put them in the right way round?

topboost · 3rd Jun 2013, 2:28 pm

I'm now about to open the set again to check, but I think you might have hit the nail on the head there, Trevor! I'll check the caps first. They're brand new, but I have a feeling I might have missed a connection somewhere.

ukcol · 3rd Jun 2013, 2:51 pm

Quote:

Originally Posted by topboost

I have yet to source a set of new valves...........

The chances are the valves are fine. Always follow a logical path to solve the problem. Follow Trevor's advice and then come back to us for more guidance.

Valves sometimes cause problems but they should not be replaced without good reason.

Colin M

topboost · 3rd Jun 2013, 4:10 pm

Thanks for the good advice: I'll try and work in a logic pattern then.

As I had a few moments this afternoon I did a quick visual check. Sure enough I'd missed that the 16+16uF metal can caps originally present were connected to the chassis physically by their holders, as well as to the OT by a wire from the nagative connection from the rear of one of the cans. My new caps have insulated outers and a negative lead, so I'd connected to the OT by the original wire, but forgotten to add a connection to the chassis. I've now installed one and the motorboating has stopped. You now get a sensible level of mild hiss which increases as you increase the vol.

Currently there're still no stations coming through at all...not a sausage on that front. So I shall need to go through and check all the voltages to start with as suggested. Is this best done by compiling a chart showing all the valve pin-to-chassis voltages?

On the new valve front, I do have an old Sencor basic valve tester and will check the valves on that to make sure...but I do like to have a new set in there when I've fixed the radio, and a set or two in reserve if it's one I'm going to keep. I'll stick with the originals unless they're duff. Luckily I have noticed that three of the valves types in the T18DA are equivalent to the DAC90A, which means I have some NOS spares already.

TrevorG3VLF · 3rd Jun 2013, 6:49 pm

I have some leads with crock clips at each end. I connect the negative of my meter with one of these to the chassis. This means I only have to deal with one prod.

Make a table with cathode, grid, screen and anode for each valve.
Have one column for the documented values and another for your measured values.
Start with the grid of the output valve and if this is positive, switch off quick.

If the set hisses a little, the problem is possibly in the first stage. Maybe the oscillator is not running, the grid should show up as negative when operating correctly. Try tuning the radio, sometimes the oscillator will run at one end of the scale and not at the other.

7th Aug 2012, 1:02 pm	#21
topboost Retired Dormant Member Join Date: Jul 2011 Location: Ipswich Posts: 13	Re: Dropper resistors for Marconiphone T18DA - what do I do?! Steve - thanks for bringing Rapid online to my attention...not heard of them before, and it's always handy to have a few more potential suppliers noted! Chris - good info again...not too bothered about the prices we're looking at here, but based on what you say and checking out the components I can find on line I think that the resistor option is not only easy, but also would at least squeeze into the space I have round the old dropper. I have a suspicion I'd struggle to find two x-type caps and cram them in on this one. Think I might have a couple of suitable 1k ceramic cased resistors in my parts boxes, but I'll check the values. I'll also post a few photos here too...everyone loves a good photo, don't they?!

3rd Jun 2013, 7:28 am	#22
topboost Retired Dormant Member Join Date: Jul 2011 Location: Ipswich Posts: 13	Re: Dropper resistors for Marconiphone T18DA - what do I do?! Well, it might have taken me a year to get round to (long story!), but yesterday I finally got round to lavishing a little TLC on the T18DA! I replaced the two duff sections of the dropper resistor with ceramic "white block" style resistors. I replaced a number of the waxy caps, including the ones mentioned above. I also replaced the two 16+16 uF can caps (easier for me than reforming), and the 25uF can which had ruptured. With a little swearing I also managed to hook up a brand new dial cord, and clean and oil all the turning parts thereof. I also took all the valves out, cleaned each of the pins, sprayed contact cleaner over the sockets, and pushed al the valves home a few times to hopefully clean the sockets up. I fitted a new mains cord and plug too. After putting the whole set back together again I powered up to test it. After some seconds of warm up the radio began to hum (sort of motorboating I guess) fairly loudly with the volume at minimum setting. This didn't really change as you turn the volume up, and no stations could be picked up by twiddling the tuning dial. So, I'm not quite there yet! However, I'm taking it as a good sign that at least the set powers up and makes a sound. I have yet to source a set of new valves, which I think is worth doing, as these (particularly the potentially fragile output valve) ones look to have been in the set from birth! Some still have their Marconi stickers on. I'm hoping that fixes it, but I'll pull the set out again in the next day or so and check it over just in case. Any tips at all, or is it now a case of just very carefully visually checking and using the multimeter to find the fault? Assuming it's not as simple as duff valves.

3rd Jun 2013, 10:59 am	#23
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Dropper resistors for Marconiphone T18DA - what do I do?! Checking the DC voltages is always the first step. Motor boating is due to low frequency feedback, almost certainly due to bad HT smoothing. Did you fit good capacitors and put them in the right way round?

3rd Jun 2013, 2:28 pm	#24
topboost Retired Dormant Member Join Date: Jul 2011 Location: Ipswich Posts: 13	Re: Dropper resistors for Marconiphone T18DA - what do I do?! I'm now about to open the set again to check, but I think you might have hit the nail on the head there, Trevor! I'll check the caps first. They're brand new, but I have a feeling I might have missed a connection somewhere.

3rd Jun 2013, 4:10 pm	#26
topboost Retired Dormant Member Join Date: Jul 2011 Location: Ipswich Posts: 13	Re: Dropper resistors for Marconiphone T18DA - what do I do?! Thanks for the good advice: I'll try and work in a logic pattern then. As I had a few moments this afternoon I did a quick visual check. Sure enough I'd missed that the 16+16uF metal can caps originally present were connected to the chassis physically by their holders, as well as to the OT by a wire from the nagative connection from the rear of one of the cans. My new caps have insulated outers and a negative lead, so I'd connected to the OT by the original wire, but forgotten to add a connection to the chassis. I've now installed one and the motorboating has stopped. You now get a sensible level of mild hiss which increases as you increase the vol. Currently there're still no stations coming through at all...not a sausage on that front. So I shall need to go through and check all the voltages to start with as suggested. Is this best done by compiling a chart showing all the valve pin-to-chassis voltages? On the new valve front, I do have an old Sencor basic valve tester and will check the valves on that to make sure...but I do like to have a new set in there when I've fixed the radio, and a set or two in reserve if it's one I'm going to keep. I'll stick with the originals unless they're duff. Luckily I have noticed that three of the valves types in the T18DA are equivalent to the DAC90A, which means I have some NOS spares already.

3rd Jun 2013, 6:49 pm	#27
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Dropper resistors for Marconiphone T18DA - what do I do?! I have some leads with crock clips at each end. I connect the negative of my meter with one of these to the chassis. This means I only have to deal with one prod. Make a table with cathode, grid, screen and anode for each valve. Have one column for the documented values and another for your measured values. Start with the grid of the output valve and if this is positive, switch off quick. If the set hisses a little, the problem is possibly in the first stage. Maybe the oscillator is not running, the grid should show up as negative when operating correctly. Try tuning the radio, sometimes the oscillator will run at one end of the scale and not at the other.