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 Astor Mickey CN High B+
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 Return to top of page · Post #: 1 · Written at 7:52:40 AM on 10 February 2019.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 89

I have a Astor Mickey CN model which has a B+ of 149V. The schematic suggests it should be about 110V. This model uses a 32L7GT combined output - half wave rectifier valve, the plate voltage specification for this valve is 90V but it's running at 139V. The radio works very well.

With 130V input the schematic spec voltages are as follows, my radio actual voltages are in brackets:
Transformer output 210V (212)
Rectifier output after a 200 Ohm resistor and before the choke 160V (162)
After the choke which is B+ the voltage should be 110V (no voltage given but should be about right), mine is 149V

I don't understand how the voltage is expected to drop from 160V before the choke to 110V after. It doesn't have the original choke as it is date stamped 1961 but measures 500 Ohm resistance as per spec.

I can use a resistor to drop the voltage after the rectifier but it would get pretty hot and doesn't address the problem. My main concern is running the 32L7GT with a high plate voltage, the other valves are running at a higher voltage than the chassis design but are still within their operating envelope.

The link to the schematic is below and I have sent a copy to Brad for uploading.

Astor Mickey CN Circuit Diagram


 
 Return to top of page · Post #: 2 · Written at 9:30:49 AM on 10 February 2019.
Wa2ise's avatar
 Location: Oradell, US
 Member since 2 April 2010
 Member #: 643
 Postcount: 690

A possible solution would be to add a resistor, 1K 2 watts, and a bypass cap say 100μF at 50V, between the 32L7 cathode resistor and the radio ground (where that cathode resistor used to connect to). And also move the control grid's resistor to the point between the original cathode cap and the new 1K resistor with new bypass cap. What this is to do is drop the voltage between the 32L7 output cathode and the output plate (not the rectifier ones). By elevating the voltage on the output cathode. The rest of the radio will still see extra B+. but you said that the radio is working well this way, and the RF and IF tubes do work better with higher (but within spec) B+ voltage.

Diagram of my mod sent to Brad.

Circuit Diagram


 
 Return to top of page · Post #: 3 · Written at 12:14:26 PM on 10 February 2019.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 89

Thanks Wa2ise, appreciate your help. I'll wait for your mod drawing to be uploaded but I think I understand what you are getting at. Cheers!


 
 Return to top of page · Post #: 4 · Written at 7:54:44 PM on 10 February 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3284

Note the 210V is AC: Also note the position of the first 24μF cap. The 200 Ohm resistor makes it a choke input filter. There should be no cap directly on the cathode. If there is it will produce more voltage. The rectifier is quoted at 125V max 60mA. Capacitor input filter, which it isn't.

https://frank.pocnet.net/sheets/127/3/32L7GT.pdf The screen of the 32L7 is quoted at 90V in the data.

Do check where the filter cap is. Also the cathode resistor of 32L7. If its not a miswire of the rectifier caps, then the next likely is the OP tube (seeing its working) not drawing enough current. At 90V plate & screen one would expect 7V across Rk (150 Ohm) cathode positive.

Always check the plate resistor on 6B6 High fail rate same with grid resistor on 32L7 &10m is a resistor that also goes high. In Astor 50K & 500K resistors can have a high fail rate.

Marc


 
 Return to top of page · Post #: 5 · Written at 9:52:18 AM on 11 February 2019.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 89

Hi Marc, thanks once again for your help. I have checked the ideas you have raised as below:

The AC supply from the transformer is indeed 210VAC and the rating on the 32L7 is 125V at the rectifier plate but Astor have designed it that way so I don't quite understand. I connected an ammeter between the transformer and rectifier plate and it measures 95mA. I don't know if that's how it should be done. Swapping the 32L7 made little difference.

The filter caps and choke are all wired correctly. The 200 Ohm rectifier anode resistor was replaced with a 220 Ohm 3 Watt, The 32L7 cathode resistor 150 Ohm measures at 160 Ohm and I lifted the .005 cap off the anode to no avail (item 2 on the drawing). I assume it should be connected from plate to ground instead? The voltage drop across the 150 cathode resistor is 7.9V but at about 140V screen and plate.

The rest of the resistors measure Ok except the 10M grid resistor for the 6B6G which measures at 12M, I ordered a new one last week. The 6B6 plate resistor was replaced last week.

Edit: after removing the .005 cap from between the anode and the plate of the 32L7 and connecting it from plate to ground the plate voltage is now down to 124V the screen is still 138V.

David


 
 Return to top of page · Post #: 6 · Written at 1:52:03 PM on 11 February 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3284

Are those the old waxed paper caps or new ones? The plate bypass cap on the audio is the most likely to fail. If its old its probably leaking (a resistor) & should be removed ASAP before it turns into a short. If they are waxed paper and old. they and all of the old electrolytic's should be systematically removed & transferred to the bin.

The plates of 6J8 pins 6 & 3 should be 108V as should pin3 of 6U7 as the screen of 32L7 is also on that rail it should be 108V

How do you move 0.005 from anode to plate? Anode is the Plate. I would expect that cap to be from Pentode anode (pin3) to the cathode pin 8 and pin 8 to go to ground via Rk (150 Ohms) Octal base, wiring side, reads clockwise from the left of the spigot.

Also, what's the heater volts & make sure the mains primary is on the 250V tapping not the lower voltage one.

20% variance in voltage is considered within acceptable parameters.

Its pretty obvious that someone at Astor worked out that by using a half wave "choke input filter" they could get away with that rectifier.

Marc


 
 Return to top of page · Post #: 7 · Written at 5:52:25 PM on 11 February 2019.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 89

All the caps have been replaced as well as any of resistors that measured high. I rechecked and all the replacement components are the correct value.

"The plates of 6J8 pins 6 & 3 should be 108V as should pin3 of 6U7 as the screen of 32L7 is also on that rail it should be 108V" Agreed, that should be the voltages but they are all high because the B+ is high.

"How do you move 0.005 from anode to plate? Anode is the Plate" Sorry, that's a typo, I removed the 0.005 cap from the cathode and took it to ground from the plate, that's the way the radio was when I got it and I had corrected it. In your previous comment you said there should be no capacitor on the cathode but I think now you were talking about the rectifier cathode Sad I'll put it back as it should be.

The connection to the transformer is on the 240 tab and I checked it is correct with an ohmmeter.

I've also exchanged all valves but makes no difference.

I'll keep plodding along Smile


 
 Return to top of page · Post #: 8 · Written at 6:44:23 PM on 11 February 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3284

With the 32L7's pentode cathode, (pin 8) the only cap shown from plate to it. pin 3 to pin 8. Do make sure that you are reading pins correctly. http://frank.yueksel.org/sheets/093/3/32L7GT.pdf

Now! The valve specs are for a half wave rectifier with a capacitor input filter.

What as noted, Astor have done is put in a 200 Ohm resistor (30) between the cathode (pin1) and the field coil / choke (53) That will make it a choke input filter, which, as pointed out develops less voltage, than the capacitor input filter.

Therefore, there absolutely should not be any form of capacitor connected to pin 1 (rectifier cathode). If there is that could be the problem.? If not try, at the risk of hum; dropping the 24μF (14) to around 10uF.


 
 Return to top of page · Post #: 9 · Written at 7:16:56 PM on 11 February 2019.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 89

I have pin reading down pat Smile I have the 0.005 from pin 3 (plate) to Pin 8 (cathode) and then a 150 Ohm resister from pin 8 to ground just as in the drawing. When I got it the radio it had pin 8 to ground via the 150 Ohm resistor and pin 3 connected to ground via a 0.005 cap. It's correct as per the drawing now.

There are no caps on pin one... I'll try dropping the 24 to 10mf and see how it goes.

Thanks for your help... again Smile


 
 Return to top of page · Post #: 10 · Written at 8:13:05 PM on 11 February 2019.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 89

10mf wasn't a success, bit more hum and made little difference to the voltages. I upped the choke resistor from 200 to 300 Ohms and it brings the voltages within 20% of those stated without generating too much heat. The radio is working well and I'll leave it as is. I don't expect it will get much use.

Thanks for everyone's help Smile


 
 Return to top of page · Post #: 11 · Written at 8:31:28 PM on 11 February 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3284

Yeh! We can but only try. Normally if one want's to take the stress off of the cathode, which a too big cap can strip, they use a smaller cap first, or none.

Did you look at the AC volts on the heaters; That can be guide to an issue with the transformer. I would love to compare the volts with a 1000 opv meter, but with the HT & heaters, it should not be different with any meter type. That is a shielded transformer, I normally ground a chassis with on of them.

It is odd that a set has too much voltage if everything is as it should be I make it 51.4V AC for the sum total of heater voltages in series.

With all new caps, I do expect some slight voltage rise. Thinking aloud the modern Electrolytic cap, does not draw as much current as the old ones. They were considered useable if their current draw was less than 10mA & the power supplies were designed to cope with that.


 
 Return to top of page · Post #: 12 · Written at 9:37:58 PM on 11 February 2019.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 89

Hi Marc,

Yes the voltage across the heaters is 50 volts at 230VAC mains input. I used a DMM to check the voltages but I also have a 20,000 opv meter from the 40s, the voltages are much kinder using that although still a bit high and I don't know how accurate the meter is.

Interesting idea with the electros not drawing as much current, I wonder if that may be it. I sure can't find anything else wrong with it and have spent a fair bit of time checking it over. It's beyond me I'm afraid Sad


 
 Return to top of page · Post #: 13 · Written at 11:06:29 AM on 12 February 2019.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1177

A quick check of your meter can be made by measuring a new unused alkaline cell. You should get 1.62 volts.


 
 Return to top of page · Post #: 14 · Written at 6:05:55 PM on 12 February 2019.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 89

Hi Ian, thanks Smile


 
 Return to top of page · Post #: 15 · Written at 6:33:28 PM on 12 February 2019.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 5042

Circuit diagrams uploaded to Posts 1 and 2.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
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