emerson: (AX235); AX-235 Little Miracle

I am currently restoring an example of one of these radios and I have already a fair amount of experience with Emersons. This one has me baffled at present!

The chassis is more or less as factory spec but the tuning coil T1 has been chopped about in the past probably to add LW to the bands. Im not even sure that the coil is correct however I have removed all the added parts and tried to rig up the coil as intended.

The rest of the chassis components appear to be OK and resistors etc seem to buz out fine. As I am in the UK and the sets were 110AC, the line cord (missing) had to be rewired. I do this by feeding 110VAC into the set but put a series 220R resistor in line with the heaters. This allows the HT to run plus also the LT. Im getting approx 5V across each tube heater. Little bit low but still OK.

The problem is that once powered up, the audio stage is good plus the 6Q7 stage but Im not getting any signal. Touching the caps if the 6A8 and 6K7 tubes produce a click but thats it. HT is running at 82V before the speaker field coil and 66V after. Im starting to wonder if I have a bad 6A8 but any help would be useful. What is the HT that I should get?

Attachments:

• Diagram (97 KB)

To thank the Author because you find the post helpful or well done.

According to Rider your HT should be 100V after the field.

To thank the Author because you find the post helpful or well done.

Hi Alan,

Looks like its low then. It does start about 110 but then drops as the valves warm up. The smoothing caps are new so could it be a bad rectifier tube?

To thank the Author because you find the post helpful or well done.

Hi Alan,

OK I've measured the HT again after the field coil and im getting 66V. No where near the 100V it's supposed to be. This wont help. Also the schematic I got off RM indicates a Production change. Any idea what this is ?

To thank the Author because you find the post helpful or well done.

If I can remember how to do this, there should be an attachment with the changes taken from the Emerson service manual. 

Attachments:

• Production changes (30 KB)

To thank the Author because you find the post helpful or well done.

Mark,

the low B+ voltage may be caused by poor emission of the 25Z6 rectifier probably due to the low heater supply voltage. If the rectifier reads good on a tube tester, try to add a resistor in parallel to the 220R one on the heater string, so to rise the voltage readings across heaters close to their nominal values.

Low B+ reading could also depend upon wrong bias of the audio power amplifier, the 25L6. You should read about 7.5 volts on the cathode, pin 8, and anyway zero on the control grid, pin 5. If grid is positive, replace C17.

Regards, Emilio
 

To thank the Author because you find the post helpful or well done.

Hi Emilio,

Many thanks for your help. OK I have now improved the LT supply and the HT improved. I then checked the bias voltages as you said. The Cathode seemed OK but then the grid was +3V which suggested C17 as you correctly stated. This cap is tucked away under a pile of other parts so not easy to spot at first. I have now removed it and tacked in a similar cap just to see what happens.

Voltages now are as follows.

HT before Field = 119V

HT after Field = 94V

Cathode of 25L6 = 5.6V

Grid of 25L6 = 0V.

 

There is now much more gain in the output stage and the 6Q7 stage is healthy. Touching the cap gives a good buzz out the speaker. The 6K7 stage is OK too with similar results but the first RF stage (6A8) is still very quiet. Touching the top cap only gives a slight click. No reception is there and tuning makes no difference.

I have also hooked up a scope loosely on the 2nd Grid of the 6A8 and I seem to have a LO. Tuning the set sweeps the LO from approx 666KHz to 1.6MHz.

Here are some more voltages measured!

All for the 6A8

Anode = 94V

G1 = -6V

G2 = 94V

G4 = 48V

Also the end of R4 feeding the Tuning coil is -1V

 

To thank the Author because you find the post helpful or well done.

Ok,

I have finally fixed this thing! Seems like the volume control was open circuit so there was no load on the detector. This then meant no AVC voltage and hence no output. I have temp fixed a 220K resistor across the pot to give it a load which seemed to work. On top of this, C17 the audio coupling cap was leaky. I've also replace the main HT smoothing caps plus figured the tuning coil that I'm not convinced is the right one. However after all this work and getting the limited stations on AM that there are, what am I greeted with...... football chat! I think this radio will be a display object only!

 

To thank the Author because you find the post helpful or well done.