Another DAC 90 - UK Vintage Radio Repair and Restoration Discussion Forum

coopzone · 10th Apr 2019, 1:19 pm

Hi everyone,

Just started on another DAC90, i do like these sets! Just something about them...

Anyway before I go over to the wanted section for a mains dropper resistor has anyone tried the capacitive dropper in one of these radios. It's not something I'm sure of - it just does not feel right.

Derek

kalee20 · 10th Apr 2019, 2:18 pm

No - however Ed Dinning winds small autotransformers for the DAC90 which do the same thing, only do it better.

I run my DAC90 with its back off anyway, and things keep cool. There's nothing to disturb the asbestos.

Mike. Watterson · 10th Apr 2019, 3:18 pm

I've one to restore. I'll keep it original apart from paper cap replacement and soaking the asbestos in the sill/step repair rapid set cement.
It's not like it's going to be used often.
Though mine is missing the back, mains cable and a triangle of Bakelite at the side. I can make a socketed mains lead easily.
Obviously they will get nearly twice as hot as a DAC90A?

Sideband · 10th Apr 2019, 3:45 pm

Nothing wrong with a cap dropper. No heat (apart from the valves) and as long as the right type of capacitor is used it should run for may years. I have a Pilot radio running from a dropper cap and the heater chain in one of my TVs is now similarly powered.

All you need to do is add up the total heater voltage of the valves and use Paul's calculator to find out the best value.

Ian - G4JQT · 10th Apr 2019, 5:54 pm

Cap droppers are fine until they go short circuit...

10th Apr 2019, 6:35 pm

A diode and smaller dropper?

Ed_Dinning · 10th Apr 2019, 8:42 pm

Hi Gents, all these techniques are valid. If the correct type of cap is used it cannot go s/c, but will reduce in capacitance over time due to the self healing nature of the dielectric.

I do a small auto transformer that is suitable for both the -90 and the -90a that fits onto the chassis. It was originally developed for use with white/ cream sets that crack very easily with the heat from the dropper.

PM me for details

Ed

Mike. Watterson · 10th Apr 2019, 9:17 pm

Didn't the similar era FB10 or similar use an autotransformer to reduce heat in its small cabinet?

Sideband · 10th Apr 2019, 9:18 pm

Like Ed and I said..the CORRECT capacitor. Some people think that any old cap will do as long as the voltage is OK. Motor run caps are probably the best (not motor start...they are different). Even X capacitors can be used as they are designed to fail O/C.

However there have been many threads on Cap droppers and a search should provide all the information.

Davewantsone · 10th Apr 2019, 9:32 pm

I fitted a capacitor dropper in my DAC90 around 5 years ago. It is still working OK. I got the capacitor from Maplin(about£2.50) I can't remember what value it was. I know I did the maths and it worked out OK and runs cool.
I have just dismantled my DAC90 the capacitor is 3uF. See picture.

Glowing Bits! · 11th Apr 2019, 4:47 am

Personally, I would recommend Ed's auto tranny, it works well and only creates a little heat after 10 mins or so.
If the OP still needs a dropper, drop (no pun intended) me a message, one is available but would require a repair to the paxolin at the top, it's toasted!
The best, and possibly safest way to go, is to use an auto-tranny, it either works or it don't.
Forgot to ask, which section has failed (if it has) on the dropper? It is possible to use a separate resistor bridged across the dead bit if need be.
Rick.

coopzone · 11th Apr 2019, 11:52 am

The dropper resistor has failed in two places (i suspect it's the metal loop inside the ceramic coating between the 100 and 600 ohm sections). The end result is that the 600 section shows o/c and one of the 100 sections. The other is ok.

I'm going to try a capacitive dropper just for the fun of it! I have the value of 3uf using the spreadsheet calculator. Make up of 2.5uf 475 vac + 0.47uf 275 vac.

To test it i'm going to use some old resistors to pretend to be the valves (i know it's not a real test because the resistance changes with heat etc) but at least i can measure the voltage and pretend that the valves are up to heat.

I also found some 50watt resistors from china (rated at 1000v), so I ordered 2 x 100 and 1 x 600. Just in case I don't like the capacitive dropper.

And I still have the option of the auto transformer.

Derek

Glowing Bits! · 11th Apr 2019, 3:31 pm

If them resistors are what I think they are, it may be a case of having to make loads of room to accomodate them.
The resistor method may work but still won't deal with the issue of excessive heat creation.
The tranny method still wins hands down at this end, the heat production is a quarter of what it would be with resistors.
Rick.

kalee20 · 11th Apr 2019, 10:57 pm

Quote:

Originally Posted by Sideband

Nothing wrong with a cap dropper. No heat (apart from the valves) and as long as the right type of capacitor is used it should run for may years.

Both a capacitive dropper and an autotransformer restrict the radio to AC only, but for me the transformer wins! Why?

* Transformer gives an output voltage sensibly independent of frequency; capacitor is highly dependent. If you take your capacitor DAC90 to France with 60Hz, you're stuffed!

* Capacitors usually fail short-circuit (Sod's law) and would over-volt the heaters; transformers usually fail open or shorted turn, in neither case would the heaters be over-run.

* Capacitive dropper still pulls the full heater current from the mains, albeit phase-shifted. Transformer will pull less current, but it will be nearly in-phase - the power factor is much higher.

TrevorG3VLF · 11th Apr 2019, 11:52 pm

* Transformer gives an output voltage sensibly independent of frequency; capacitor is highly dependent. If you take your capacitor DAC90 to France with 60Hz, you're stuffed!

Except that Europe has 50Hz. I like getting stuffed in France!
Of more interest is that we are often provided with a distorted waveform which contains harmonics which will pass more easily through the capacitor.
Turn on is an extreme version of this where the current could be quite high for the first cycle.

60 oldjohn · 12th Apr 2019, 10:23 am

I have also used Ed's auto transformer, The circuit is at the end of the DAC90 Trader sheet, It is then the same circuit as the AC90. I have had excellent results and use the set most days in conjunction with a FM / AM converter. The holes for the TX are already drilled in the chassis.

John.

Davewantsone · 12th Apr 2019, 3:49 pm

Quote:

Originally Posted by kalee20

Quote:

Originally Posted by Sideband

Nothing wrong with a cap dropper. No heat (apart from the valves) and as long as the right type of capacitor is used it should run for may years.

Both a capacitive dropper and an autotransformer restrict the radio to AC only, but for me the transformer wins! Why?

* Transformer gives an output voltage sensibly independent of frequency; capacitor is highly dependent. If you take your capacitor DAC90 to France with 60Hz, you're stuffed!

* Capacitors usually fail short-circuit (Sod's law) and would over-volt the heaters; transformers usually fail open or shorted turn, in neither case would the heaters be over-run.

* Capacitive dropper still pulls the full heater current from the mains, albeit phase-shifted. Transformer will pull less current, but it will be nearly in-phase - the power factor is much higher.

It's North America that uses 60HZ mains. The voltage of course is only 117V. So an auto Tx would be no good, but a capacitor may still work due to the reactance being less at 60Hz! Just joking. At 117V you would probably not need a dropper or an auto Tx or a capacitor!

Herald1360 · 12th Apr 2019, 8:41 pm

At 117V youi'll still need a dropper- the heater chain only adds up to 72.6V. That's the main reason why the dropper gets so hot- 200mA and 167.4V is about 33.5W.

Davewantsone · 13th Apr 2019, 9:48 am

On my DAC 9 I have left the dropper resistor in position. It was still in good condition and functioning. The asbestos I have painted to stop anything flaking off. The advantage of the capacitor is that they are readily available and I found a suitable spot under the chassis to secure it. Without the back on, the radio looks correct. Its just the dropper is disconnected and so no power is disssipated. The problem with the auto Tx is that it has to be made. I do not know the cost but I would imagine it would be more than £2.50. With vintage radio repair the idea is to keep the set functioning yet still looking original
Where would one fit a auto Tx?

Mike. Watterson · 13th Apr 2019, 11:58 am

There is a related model that is AC only, did I read that the mounts for the autotransformer are on the chassis? Must be underneath or are they instead of the Dropper because I can't see where it would go on my DAC90.

Ed Dinnning makes the AutoTx, send him a PM.

The DAC91 and AC91 are export models that add Shortwave in a similar cabinet? The circuit looks similar. One schematic is shown for both in the 1946-1951 two volume R&TVS pg 127.
http://mikesradio.freeservers.com/bush_ac91.htm

10th Apr 2019, 1:19 pm	#1
coopzone Hexode Join Date: Jan 2018 Location: Cannock, Staffordshire, UK. Posts: 268	Another DAC 90 Hi everyone, Just started on another DAC90, i do like these sets! Just something about them... Anyway before I go over to the wanted section for a mains dropper resistor has anyone tried the capacitive dropper in one of these radios. It's not something I'm sure of - it just does not feel right. Derek Last edited by coopzone; 10th Apr 2019 at 1:35 pm. Reason: spelling mistake

10th Apr 2019, 2:18 pm	#2
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,077	Re: Another DAC 90 No - however Ed Dinning winds small autotransformers for the DAC90 which do the same thing, only do it better. I run my DAC90 with its back off anyway, and things keep cool. There's nothing to disturb the asbestos.

10th Apr 2019, 3:18 pm	#3
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Another DAC 90 I've one to restore. I'll keep it original apart from paper cap replacement and soaking the asbestos in the sill/step repair rapid set cement. It's not like it's going to be used often. Though mine is missing the back, mains cable and a triangle of Bakelite at the side. I can make a socketed mains lead easily. Obviously they will get nearly twice as hot as a DAC90A?

10th Apr 2019, 3:45 pm	#4
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,567	Re: Another DAC 90 Nothing wrong with a cap dropper. No heat (apart from the valves) and as long as the right type of capacitor is used it should run for may years. I have a Pilot radio running from a dropper cap and the heater chain in one of my TVs is now similarly powered. All you need to do is add up the total heater voltage of the valves and use Paul's calculator to find out the best value. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

10th Apr 2019, 5:54 pm	#5
Ian - G4JQT Octode Join Date: Jun 2004 Location: Reading/Fakenham, UK. Posts: 1,323	Re: Another DAC 90 Cap droppers are fine until they go short circuit...

10th Apr 2019, 6:35 pm	#6
Guest Guest Posts: n/a	Re: Another DAC 90 A diode and smaller dropper?

10th Apr 2019, 8:42 pm	#7
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Another DAC 90 Hi Gents, all these techniques are valid. If the correct type of cap is used it cannot go s/c, but will reduce in capacitance over time due to the self healing nature of the dielectric. I do a small auto transformer that is suitable for both the -90 and the -90a that fits onto the chassis. It was originally developed for use with white/ cream sets that crack very easily with the heat from the dropper. PM me for details Ed

10th Apr 2019, 9:17 pm	#8
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Another DAC 90 Didn't the similar era FB10 or similar use an autotransformer to reduce heat in its small cabinet?

10th Apr 2019, 9:18 pm	#9
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,567	Re: Another DAC 90 Like Ed and I said..the CORRECT capacitor. Some people think that any old cap will do as long as the voltage is OK. Motor run caps are probably the best (not motor start...they are different). Even X capacitors can be used as they are designed to fail O/C. However there have been many threads on Cap droppers and a search should provide all the information. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

10th Apr 2019, 9:32 pm	#10
Davewantsone Hexode Join Date: Dec 2011 Location: Wigan, Greater Manchester, UK. Posts: 291	Re: Another DAC 90 I fitted a capacitor dropper in my DAC90 around 5 years ago. It is still working OK. I got the capacitor from Maplin(about£2.50) I can't remember what value it was. I know I did the maths and it worked out OK and runs cool. I have just dismantled my DAC90 the capacitor is 3uF. See picture. Attached Thumbnails Last edited by Davewantsone; 10th Apr 2019 at 9:52 pm.

11th Apr 2019, 4:47 am	#11
Glowing Bits! Octode Join Date: Oct 2011 Location: Wrexham, North Wales, UK. Posts: 1,457	Re: Another DAC 90 Personally, I would recommend Ed's auto tranny, it works well and only creates a little heat after 10 mins or so. If the OP still needs a dropper, drop (no pun intended) me a message, one is available but would require a repair to the paxolin at the top, it's toasted! The best, and possibly safest way to go, is to use an auto-tranny, it either works or it don't. Forgot to ask, which section has failed (if it has) on the dropper? It is possible to use a separate resistor bridged across the dead bit if need be. Rick. Last edited by Glowing Bits!; 11th Apr 2019 at 4:51 am. Reason: Added a question at the end.

11th Apr 2019, 11:52 am	#12
coopzone Hexode Join Date: Jan 2018 Location: Cannock, Staffordshire, UK. Posts: 268	Re: Another DAC 90 The dropper resistor has failed in two places (i suspect it's the metal loop inside the ceramic coating between the 100 and 600 ohm sections). The end result is that the 600 section shows o/c and one of the 100 sections. The other is ok. I'm going to try a capacitive dropper just for the fun of it! I have the value of 3uf using the spreadsheet calculator. Make up of 2.5uf 475 vac + 0.47uf 275 vac. To test it i'm going to use some old resistors to pretend to be the valves (i know it's not a real test because the resistance changes with heat etc) but at least i can measure the voltage and pretend that the valves are up to heat. I also found some 50watt resistors from china (rated at 1000v), so I ordered 2 x 100 and 1 x 600. Just in case I don't like the capacitive dropper. And I still have the option of the auto transformer. Derek

11th Apr 2019, 3:31 pm	#13
Glowing Bits! Octode Join Date: Oct 2011 Location: Wrexham, North Wales, UK. Posts: 1,457	Re: Another DAC 90 If them resistors are what I think they are, it may be a case of having to make loads of room to accomodate them. The resistor method may work but still won't deal with the issue of excessive heat creation. The tranny method still wins hands down at this end, the heat production is a quarter of what it would be with resistors. Rick.

11th Apr 2019, 11:52 pm	#15
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Another DAC 90 * Transformer gives an output voltage sensibly independent of frequency; capacitor is highly dependent. If you take your capacitor DAC90 to France with 60Hz, you're stuffed! Except that Europe has 50Hz. I like getting stuffed in France! Of more interest is that we are often provided with a distorted waveform which contains harmonics which will pass more easily through the capacitor. Turn on is an extreme version of this where the current could be quite high for the first cycle.

12th Apr 2019, 10:23 am	#16
60 oldjohn Dekatron Join Date: Dec 2004 Location: East Yorkshire, UK. Posts: 3,985	Re: Another DAC 90 I have also used Ed's auto transformer, The circuit is at the end of the DAC90 Trader sheet, It is then the same circuit as the AC90. I have had excellent results and use the set most days in conjunction with a FM / AM converter. The holes for the TX are already drilled in the chassis. John.

12th Apr 2019, 8:41 pm	#18
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Another DAC 90 At 117V youi'll still need a dropper- the heater chain only adds up to 72.6V. That's the main reason why the dropper gets so hot- 200mA and 167.4V is about 33.5W. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

13th Apr 2019, 9:48 am	#19
Davewantsone Hexode Join Date: Dec 2011 Location: Wigan, Greater Manchester, UK. Posts: 291	Re: Another DAC 90 On my DAC 9 I have left the dropper resistor in position. It was still in good condition and functioning. The asbestos I have painted to stop anything flaking off. The advantage of the capacitor is that they are readily available and I found a suitable spot under the chassis to secure it. Without the back on, the radio looks correct. Its just the dropper is disconnected and so no power is disssipated. The problem with the auto Tx is that it has to be made. I do not know the cost but I would imagine it would be more than £2.50. With vintage radio repair the idea is to keep the set functioning yet still looking original Where would one fit a auto Tx?

13th Apr 2019, 11:58 am	#20
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Another DAC 90 There is a related model that is AC only, did I read that the mounts for the autotransformer are on the chassis? Must be underneath or are they instead of the Dropper because I can't see where it would go on my DAC90. Ed Dinnning makes the AutoTx, send him a PM. The DAC91 and AC91 are export models that add Shortwave in a similar cabinet? The circuit looks similar. One schematic is shown for both in the 1946-1951 two volume R&TVS pg 127. http://mikesradio.freeservers.com/bush_ac91.htm