Aa5, No (-) voltage at 12SA7 Pin 5&

I've built an AA5 and if I screw out the IF can trimmers it will pick up WIND 560AM This is a weak 5kw station. WLS 890 is the strongest one around 50kw I think and I have to turn the screws in all the way. I'm certain the oscillator circuit is dead. The voltage at the 12SA7 pins 5&6 is only about -.3v I have tried other IF cans after testing them with my inductance meter, it just doesn't seem to be oscillating. Also the plate voltage is 74v and should be 85v. The 100 ohm resistor from the rectifier tube is getting hot. I've tried other tubes and the #47 bulb is not burning up. The voltage at the oscillator should be around -5v or so. I could provide a schematic but only have a picture and don't know how to post it.

This post has been edited by livinginthepast on February 14, 2013 02:04 am

You can't post/upload images directly (sorry), but you can use a free image hosting service (such as photobucket.com) and either use the IMG tags (if the pic isn't too large) or just post a link to the image.

Welcome to the forum.

12SA7 ... weeelll , this radio is well worth looking at !

the 100 ohm resistor getting hot .
if this is above the normal range of "hot" , there is excessive current being drawn downstream from it .
a leaky electrolytic can cause excessive current draw .
a tube can be going into grid conduction .
loss of bias (especially in the audio section) will cause a rise in current draw .

but this information is generic at best , as in "which 100 ohm resistor ?" .

yep ... post a link to the schematic .
a make and model would be good too (if you can't post the schematic) .
i may have a schematic around here *somewhere* .

I uploaded a pic of the schematic to photobucket.

http://s1339.beta.photobucket.com/user/mec...6328db.jpg.html

There is no make or model. It is a kit schematic from a 1950 INDUSTRIAL TRAINING INSTITUTE - Training Course. I got the instruction booklet off e-bay.

<a href="http://s1339.beta.photobucket.com/user/mechanic1996/media/kit_zps486328db.jpg.html" target="_blank"><img src="http://i1339.photobucket.com/albums/o701/mechanic1996/kit_zps486328db.jpg" border="0" alt=" photo kit_zps486328db.jpg"/></a>

This post has been edited by livinginthepast on February 15, 2013 01:10 am

Here ya go...

Schematic

Thats it. Tried to put it right on this page must be too big.

All the caps are new.

I had the P-C70-A and P-70-OSC from AES but switched them out with a loop and air core to reduce adjustments.

Loop = 191uH
Osc = 168Uh
Osc Gang is 147pf
RF Gang is 400pf - mounted on side original is 465pf

http://i1339.photobucket.com/albums/o701/m...zps5c6cd230.jpg

http://i1339.photobucket.com/albums/o701/m...zps1c1f7fd4.jpg

http://i1339.photobucket.com/albums/o701/m...zps04dd113f.jpg

http://i1339.photobucket.com/albums/o701/m...zpsd26e25b7.jpg

http://i1339.photobucket.com/albums/o701/m...zps41f5511d.jpg

http://i1339.photobucket.com/albums/o701/m...zps565bda1c.jpg

http://i1339.photobucket.com/albums/o701/m...zps0a70fb4f.jpg

This post has been edited by livinginthepast on February 15, 2013 02:18 am

Thanks Anyways Guys. It works! Just needs to be aligned.

glad you got it running .
you mentioned "replacing the caps" ... which units ?

did you replace that multi-section electrolytic with individual caps ?
the audio output cathode bypass cap is a 20uF/25V ... changing this will help improve the audio .

as for the 100 ohm resistor getting hot ...
this resistor passes all the current to run the set .
if the 30uf filter is leaky , it will cause the resistor to run hot .
if the audio output tube is going into grid conduction this will cause excessive dissipation (essentially shifting the grid bias) . this causes excessive current draw through the 100 ohm resistor ... check the bias voltage .
the 1uf (.1uF?) audio coupling cap (from the SQ7) can cause bias shift if it is leaky .

That's good, I was going to suggest that if you wired up the oscillator backwards, it won't oscillate. You'd exchange the wires only on the primary OR the secondary to correct this. Not both.