Another DAC90A - UK Vintage Radio Repair and Restoration Discussion Forum

Pete_kaye · 25th Jul 2005, 4:37 pm

I have what appears to be an excellent white version It is extremely good at picking up all stations on LW and Mw >trouble is ..it seems too loud to me .The top gets warmer than I would expect although there is no damage to thge cabinet.When turned up to half vol it is fine but then it seems to be straining itself and the speakerand starts to distort with the panel bulbs dimming and flickering.I have put new 6V ,0.1 A bulbs in and basically cleaned it and removed the original plug and hardwired a mains lead to the volume control.Yes the dropper resistor is set to 240 V.Is the heater current too high ?I cannot measure it with a DC meter only.Any thoughts?

Nickthedentist · 25th Jul 2005, 5:30 pm

Quote:

Originally Posted by Pete_kaye

Is the heater current too high ?I cannot measure it with a DC meter only.

Even with the cheapest multimeter, you should be able to measure the AC voltage across one section of the dropper, and then use Ohms law to calculate the current flowing.

If the current is right, then maybe the lack of power reserve might be due to the smoothing electrolytics becoming low in value.

Don't forget that these run pretty hot at the best of times.

Nick.

Sideband · 25th Jul 2005, 5:33 pm

The scale lamps should be 3.5v at .15amp. As the lamps are in the neutral return lead, they must be rated correctly for correct operation of the set.

The sets do run hot but it would be advisable to, at the very least, change the coupling capacitor to the grid of the UL41 output valve. Very common for this to be leaky and will cause the output valve to draw excessive current, get hotter and give distorted sound. UL41s are getting expensive and more difficult to find so preserving good originals or ensuring that new ones are not overrun is even more important. The mains suppressor capacitor should be changed for safety. For reliable operation, all the old 'waxies' should be changed and you will have a good, dependable radio.

I repaired one of these recently and did not have any problems with flickering lights so check the points mentioned above.

Rich.

You beat me to it Nick! Good point about the smoothing cap...I forgot to mention it!! Also worth checking the HT in case it is low. Should be about 205v on the cathode of the rectifier.

Pete_kaye · 8th Aug 2005, 4:46 pm

Thanks for the advice .I have used the ideas and checked the current through the dropper resistor .It starts at .11A then quickly settles down to .099A (bulbs go bright,dim ,bright)and is unaffected by volume setting.I had already replaced the capacitor suggested in a previous post but it made no difference .I have since noticed that the distortion only occurs on a loud station and the flickering too.If I tune off then there is no effect .I am coming to the conclusion that the problem is mechanical and maybe the bulbs /speaker are loose. I will check it out soon.

Aerodyne · 8th Aug 2005, 7:29 pm

The excellent replies above should be heeded: this doesn't sound like a mechanical fault, rather one of the HT voltage fluctuating probably due to a low emission rectifier or low capacitance smoothing and reservoir caps. The output valve itself might just be the culprit, but I doubt it very much - I'd expect low volume with distortion when vol control is advanced, in that case.
-Tony

Pete_kaye · 18th Aug 2005, 2:56 pm

Latest news is that the original problem has gone after all wax caps replaced.Only remaining problem is now the bulbs are not bright enough even though new and 6v ,0.1A .I have measured res across shunt resistor and reading is 75R(same as its nominal reading) and voltage drop is 3V suggesting
0.04A through it(using cheap multimeteron 200AC ) which I think about right,but surely resistance of the shunt and 2 bulbs in parallel should be about 10 R,each bulb is 6R. I'm confused!! Could it be poor joints ? Shorting one bulb makes the other one come on bright

G0PKH - Pete · 18th Aug 2005, 3:12 pm

Hi Pete.

As Richard said earlier in the thread.

Quote:

The scale lamps should be 3.5v at .15amp. As the lamps are in the neutral return lead, they must be rated correctly for correct operation of the set.

I would suggest you change them from the 6v .1 A you appear to be using.

Pete

Pete_kaye · 1st Sep 2005, 1:09 pm

I have now tried the suggested bulbs(3.5 v,0.15A) which I picked up very cheaply (NOS)and can get more if anyone wants some . Unfortunately ,it made the bulbs even dimmer , as I expected. These have a resistance of 2.5 R while the originals were 5R(rated as 6.3V,0.1A).It seems to me that the current can't get through.I have measured the voltage as stated earlier and calculations don't make sense giving a current of 0.6A.I have tested all bulbs on a separate power supply at the voltages marked and they are fine.I am using a Maplin's cheap multimeter .Could this be the problem ? Incidentally I have used these same 6V bulbs in another Dac90A and they were fine .

ppppenguin · 1st Sep 2005, 1:28 pm

When measuring bulb resistance please remember that it can be considerably lower when cold (ie with a multimeter) than when hot (ie running normally). This also applies to ordinary mains voltage bulbs.

Kat Manton · 1st Sep 2005, 1:45 pm

Best bet is to calculate the resistance from the bulb rating:

Taking your 6.3V 0.1A bulbs as an example, the resistance when running at their rated current is 6.3 / 0.1 = 63R ( R = V/I )

At voltages/currents other than that marked on the bulb this will vary; if you've got two meters available (or a variable PSU with a meter on and a multimeter, etc) you can crank up the voltage from zero to the rated maximum, measure current and plot a graph to determine the charateristics of the bulb.

Kat.

25th Jul 2005, 4:37 pm	#1
Pete_kaye Octode Join Date: Jun 2005 Location: Sheffield, South Yorkshire, UK. Posts: 1,227	Another DAC90A I have what appears to be an excellent white version It is extremely good at picking up all stations on LW and Mw >trouble is ..it seems too loud to me .The top gets warmer than I would expect although there is no damage to thge cabinet.When turned up to half vol it is fine but then it seems to be straining itself and the speakerand starts to distort with the panel bulbs dimming and flickering.I have put new 6V ,0.1 A bulbs in and basically cleaned it and removed the original plug and hardwired a mains lead to the volume control.Yes the dropper resistor is set to 240 V.Is the heater current too high ?I cannot measure it with a DC meter only.Any thoughts?

25th Jul 2005, 5:33 pm	#3
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,580	Re: Another DAC90A The scale lamps should be 3.5v at .15amp. As the lamps are in the neutral return lead, they must be rated correctly for correct operation of the set. The sets do run hot but it would be advisable to, at the very least, change the coupling capacitor to the grid of the UL41 output valve. Very common for this to be leaky and will cause the output valve to draw excessive current, get hotter and give distorted sound. UL41s are getting expensive and more difficult to find so preserving good originals or ensuring that new ones are not overrun is even more important. The mains suppressor capacitor should be changed for safety. For reliable operation, all the old 'waxies' should be changed and you will have a good, dependable radio. I repaired one of these recently and did not have any problems with flickering lights so check the points mentioned above. Rich. You beat me to it Nick! Good point about the smoothing cap...I forgot to mention it!! Also worth checking the HT in case it is low. Should be about 205v on the cathode of the rectifier. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman..... Last edited by Sideband; 25th Jul 2005 at 5:40 pm.

8th Aug 2005, 4:46 pm	#4
Pete_kaye Octode Join Date: Jun 2005 Location: Sheffield, South Yorkshire, UK. Posts: 1,227	Re: Another DAC90A Thanks for the advice .I have used the ideas and checked the current through the dropper resistor .It starts at .11A then quickly settles down to .099A (bulbs go bright,dim ,bright)and is unaffected by volume setting.I had already replaced the capacitor suggested in a previous post but it made no difference .I have since noticed that the distortion only occurs on a loud station and the flickering too.If I tune off then there is no effect .I am coming to the conclusion that the problem is mechanical and maybe the bulbs /speaker are loose. I will check it out soon.

8th Aug 2005, 7:29 pm	#5
Aerodyne Octode Join Date: Jun 2004 Location: Hampton Vale, Peterborough, UK. Posts: 1,698	Re: Another DAC90A The excellent replies above should be heeded: this doesn't sound like a mechanical fault, rather one of the HT voltage fluctuating probably due to a low emission rectifier or low capacitance smoothing and reservoir caps. The output valve itself might just be the culprit, but I doubt it very much - I'd expect low volume with distortion when vol control is advanced, in that case. -Tony

18th Aug 2005, 2:56 pm	#6
Pete_kaye Octode Join Date: Jun 2005 Location: Sheffield, South Yorkshire, UK. Posts: 1,227	Re: Another DAC90A Latest news is that the original problem has gone after all wax caps replaced.Only remaining problem is now the bulbs are not bright enough even though new and 6v ,0.1A .I have measured res across shunt resistor and reading is 75R(same as its nominal reading) and voltage drop is 3V suggesting 0.04A through it(using cheap multimeteron 200AC ) which I think about right,but surely resistance of the shunt and 2 bulbs in parallel should be about 10 R,each bulb is 6R. I'm confused!! Could it be poor joints ? Shorting one bulb makes the other one come on bright

1st Sep 2005, 1:09 pm	#8
Pete_kaye Octode Join Date: Jun 2005 Location: Sheffield, South Yorkshire, UK. Posts: 1,227	Re: Another DAC90A I have now tried the suggested bulbs(3.5 v,0.15A) which I picked up very cheaply (NOS)and can get more if anyone wants some . Unfortunately ,it made the bulbs even dimmer , as I expected. These have a resistance of 2.5 R while the originals were 5R(rated as 6.3V,0.1A).It seems to me that the current can't get through.I have measured the voltage as stated earlier and calculations don't make sense giving a current of 0.6A.I have tested all bulbs on a separate power supply at the voltages marked and they are fine.I am using a Maplin's cheap multimeter .Could this be the problem ? Incidentally I have used these same 6V bulbs in another Dac90A and they were fine .

1st Sep 2005, 1:28 pm	#9
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Another DAC90A When measuring bulb resistance please remember that it can be considerably lower when cold (ie with a multimeter) than when hot (ie running normally). This also applies to ordinary mains voltage bulbs.

1st Sep 2005, 1:45 pm	#10
Kat Manton Retired Dormant Member Join Date: Jul 2005 Location: West Yorkshire, UK. Posts: 1,700	Re: Another DAC90A Best bet is to calculate the resistance from the bulb rating: Taking your 6.3V 0.1A bulbs as an example, the resistance when running at their rated current is 6.3 / 0.1 = 63R ( R = V/I ) At voltages/currents other than that marked on the bulb this will vary; if you've got two meters available (or a variable PSU with a meter on and a multimeter, etc) you can crank up the voltage from zero to the rated maximum, measure current and plot a graph to determine the charateristics of the bulb. Kat.