Welcome to Australia's only Vintage Radio and Television discussion forums. You are not logged in. Please log in below, apply for an account or retrieve your password.
Australian Vintage Radio Forums
  Home  ·  About Us  ·  Discussion Forums  ·  Glossary  ·  Outside Links  ·  Policies  ·  Services Directory  ·  Safety Warnings  ·  Tutorials

Tech Talk

Forum home - Go back to Tech talk

 Astor Mickey, unusual variable capacitor?
« Back · 1 · Next »
 Return to top of page · Post #: 1 · Written at 10:32:24 PM on 16 March 2017.
JamieLee's Gravatar
 Location: Clare, SA
 Member since 27 March 2016
 Member #: 1894
 Postcount: 508


I have just bought a white/cream Astor Mickey and was hoping it was like my other one (dark bakelite), so I could use it as a template to fix it as it is working, but not very well, with the exception of ABC, which is loud and clear (Might be fine in the metropolitan area where signals are stronger)
Unfortunately, it is very different , the case shape and knobs are identical , but the chassis is very differently laid out with a slightly different valve lineup, however this isn't a problem really, the plug is missing and the original paper and electro caps are still there, somebody has fitted a different speaker, so I plan to re-cap and see what happens.
What is very odd about this set is that there is no tuning gang!
The dial moves this funny looking thing mounted above the chassis, comprising of this horizontal metal bit attached to two horizontal parallel cylinders, one fatter than the other, with what appears to be coils wrapped around them and pistons which move inside and out of the cylinders with the movement of the tuning dial??? Never seen anything like it!
Does anybody know what this thing is and how it works?

Cheers. Jamie.


 
 Return to top of page · Post #: 2 · Written at 10:37:43 PM on 16 March 2017.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6172

What you have there is called permeability tuning.

A radio's tuning circuit comprises an LC circuit. Usually the L is fixed and the C is variable. In your case, the C is fixed and the L is variable. Car radio tuners are/were often of a similar design.

Permeability tuning should be explained in one of those books I referred you to. If not, Google will have plenty on it.


 
 Return to top of page · Post #: 3 · Written at 10:47:53 PM on 16 March 2017.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1484

I have a few radios with that feature. They are adjustable coils. So, instead of 2 gangs on the usual tuning gang (variable capacitor), you have 2 tuned coils instead, one does the aerial coil and the other the oscillator coil. It's a neat idea and usually takes less room. They normally never break (at least I've never seen a broken one), and you shouldn't need to touch it.


 
 Return to top of page · Post #: 4 · Written at 10:52:51 PM on 16 March 2017.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6172

Another benefit is that they allow easy press button type tuning.


 
 Return to top of page · Post #: 5 · Written at 7:09:39 AM on 17 March 2017.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 923


Hi Jamielee, go to Special Projects on this site and click on 'An Astor Mickey HNQ'.
I bought that Mickey because it had the ferrite core tuner and the article discusses getting the whole set working again.
Cheers, Fred.


 
 Return to top of page · Post #: 6 · Written at 8:45:09 AM on 17 March 2017.
JamieLee's Gravatar
 Location: Clare, SA
 Member since 27 March 2016
 Member #: 1894
 Postcount: 508

Awesome, thanks guys, yes GTC, I have the first two of those books now and have read one and a half of them!

Ok Fred I shall have a gander, It would be great to have it working!


 
 Return to top of page · Post #: 7 · Written at 1:15:12 PM on 14 April 2017.
JamieLee's Gravatar
 Location: Clare, SA
 Member since 27 March 2016
 Member #: 1894
 Postcount: 508

Hi Fred, I've just got this thing ready for checking and realised mine is different again, it has a 6V6GT instead of the 6AQ5? Hopefully the rest of the schematic is the same, it's all that I have to go off...


 
 Return to top of page · Post #: 8 · Written at 1:27:04 PM on 14 April 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2013

Hi Jamie

All that means is you have a slightly earlier production unit. The 6AQ5 is a 6V6 in a smaller bottle. Same specs. Used to happen a lot in those days due to supply issues.

There is one Astor radio I've seen that has 7 pin "miniature" valves in all places except the IF amp, where there is a huge 6U7G complete with Goat shield! Looks most incongruous.


 
 Return to top of page · Post #: 9 · Written at 4:05:47 PM on 14 April 2017.
JamieLee's Gravatar
 Location: Clare, SA
 Member since 27 March 2016
 Member #: 1894
 Postcount: 508

Ok, well by looking at the 6V6 Data Sheet, I can see what pin is what. For some reason I had 190+ volts on pin 5 (grid) where I'm sure it should be negative? This voltage appeared to be coming across C15 (.0001mf), when I snipped it out I got minus 2 volts... The cap has a steady resistance of 900 Ohms, so I'm thinking it is NVG, I'm looking for one to replace it with.


 
 Return to top of page · Post #: 10 · Written at 4:25:47 PM on 14 April 2017.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 431

Probably meant to be around .01μF 630 volt.
The most common failing component in vintage radio repairs.
Also check the resistors off the grid.
JJ


 
 Return to top of page · Post #: 11 · Written at 4:49:53 PM on 14 April 2017.
JamieLee's Gravatar
 Location: Clare, SA
 Member since 27 March 2016
 Member #: 1894
 Postcount: 508

Really, bugger, I've just taped 9x .001 caps in series, soldered them and put in circuit, the one I removed was a square one, I'm sure the schematic says .0001???
Anyway I'm not getting any joy atm, it's not giving me even a glimmer of hope yet...


 
 Return to top of page · Post #: 12 · Written at 5:01:19 PM on 14 April 2017.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 431

I think the cap is quoted in farads, not microfarads.
So could be .1 μF, lots of audio coupling caps are .047 μF, which would be halfway between the two.
All will work OK at audio frequencies in this position.
JJ


 
 Return to top of page · Post #: 13 · Written at 6:18:34 PM on 14 April 2017.
JamieLee's Gravatar
 Location: Clare, SA
 Member since 27 March 2016
 Member #: 1894
 Postcount: 508

Well it's the one between the (1st) grid and anode, I could try clipping in a .047, see what happens. It turns out the 6V6 was NVG, as soon as I replaced it I got sound, but still no stations, just RF noise when tuning up the end where the pistons in the tuning thingy are nearly all the way in their cylinders, anyhow I'm nearly out of time for today, I've got to clear the table for dinner.
One thing is for sure, that square plastic cap is NVG.

I have -4 volts on the grid now where the schematic says I should have -8 volts, whereas I only had -2v before replacing the 6V6.
Funnily I only have 101v olts now on the 6V6 (pin3) cathode where I should have 165 volts, but I had 207 volts with the dud valve.
The second grid has 112Volts where there should be 175V, with the dud valve I had 214 volts there!
Pin 5 now has -4 volts where as with the dud, I had 200volts.
The 6AD8 has 42 volts on the anode, (pin 6) which is correct, but the control grid (pin1) has only 26V, not the 35V it should.

The 6BE6 anode only has 112 volts, rather than the 175 volts it should. There is a mere 19volts at pin six, which should have 42 volts.

It also seems the rectifier is screwy as without it I have 210 volts at each anode pin, but with it in, there's only 199 at one and 193 at the other, so that isn't good. I'm going to pop in another one and see what happens.

Hmmn that was wacky...

I found a 6X5GT/G Brand spanker, now I have 184V and 186VAC at the anodes and 186 volts DC at the cathode and 185V and 177V respectively at the 6V6, so much more in the ball park!
However, all put back together, nope... No music out of this little pig in a poke.
I think I'll just keep it for when I want to listen to loud white noise! That's the best it offers.
I've replaced the out of spec resistors. The speaker, which was dead when I got it. The white noise gets louder moving the dial clockwise... Disappointing, but something to come back to when I'm bored one day I suppose. My other Mickey has issues too, but at least gets stations, but that one I think just needs a new volume pot, strangely the volume across the range just affects the tone, with the volume remaining constant. It's bit quiet, but loud as a bomb with my transmitter, leading to believe these are more of a city radio, better where there's strong signal, but the one I've been working on has been got at previously, evidenced by a 5ohm 5 watt resistor across the antenna to earth, where the correct value is 20K ohm! Hmm...


 
 Return to top of page · Post #: 14 · Written at 7:03:48 PM on 14 April 2017.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 431

Because the coupling cap was leaky, putting positive volts on the grid of the 6V6.
The 6V6 would if left on for any lenght of time be stuffed.
Pin 3 is the anode, not cathode.
The fact that the anode volts are now down is that it is drawing current. Or the supply is not up to it.(rectifier).
With the old faulty cap in, very excessive current.
JJ


 
« Back · 1 · Next »
 You need to be a member to post comments on this forum.

Sign In

Username:
Password:
 Keep me logged in.
Do not tick box on a computer with public access.