sparks-wit: Sparton 5-AW-06 low volume problem

I've been working on a Sparton 5-aw-06. The problem i'm having is very low volume. all the local am chanel's come in strong, everything seems to work the way it should except low out put on the volume.  I've re caped the radio, checked all windings, tubes,and risisters all seam within tolerance. any suggestions?

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Hi Bill,

Based on your description, it sounds like the problem is probably not in the RF or IF sections, but most likely in the audio section.  However, we can't be sure just yet.  Can you do the following to try to help narrow down the problem?

1.  Turn up the volume all the way and touch the center lug of the volume control with a screwdriver or a piece of wire (be careful not to touch this, or at least be aware that this is an AC/DC radio, and you can get a dangerous shock if you are not well insulated from ground yourself).  Do you get a fairly loud buzz or hum in the speaker when you do this?

2.  Check voltages on tubes #3, 4, and 5 as shown in the voltage chart at the bottom of the schematic you find on the model page.  Are you within 20% of the correct voltage for all readings on these tubes?  If you're not sure how to properly make these kinds of voltage measurements, just ask, and I can provide more detail.

Tom

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Hello Thomas,

I ran into a little bit of a discrepancy when looking at the schematics. On the actual schematic drawing, it shows the 35y4 as the rectifier tube and the 50a5 as the power output as is in my radio. But, on the voltage section it shows a 35z5 rectifier and a 50l6 power output. Does that make a difference in my voltage readings?  

 When I did a buzz test on the middle lug of the volume control it didn’t do anything.

Thank you for your help

Best regard,

Bill

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Bill,

Glad you noticed the problem with the voltage table on the schematic.  It does indeed make a difference.  The table assumes the output tube is 50L6, while the radio actually has 50A5.  These tubes are basically equivalent in function, but the pinout is different.  Same story for 35Z5 vs 35Y4.

Here's what to focus on for the 50A5:

Plate (pin 2) = 112 VDC (same as pin 3 on the 50L6)

Screen grid (pin 3) = 90 VDC (same as pin 4 on the 50L6)

Cathode (pin 7) = 6.2 VDC (same as pin 8 on the 50L6)

Control grid (pin 6) = 0 VDC (same as pin 5 on the 50L6)

If you make the above meaurements, it will not be necessary to check the voltages on the 35Y4.  However, if you want to try it, you can use the method I did above, checking which pins on the 35Y4 correpond to those for the 35Z5 in the table.

Looks like the 14B6 is straightforward, with no disagreement between the table and the schematic.

Best regards,

Tom

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Hello Thomas,

Okay, I took the following tests with my veriac set at 110vac.  I connected my meter ground to the negative of my electrolytic condenser. 

Hope this helps, here’s what I found.

50a5

(pin2)= 115 vdc

(pin3)75.5 vdc

(pin7)=4.5 vdc

(pin6)=0

14b6

(pin1)26vac

(pin2)=50.5vdc

(pin3)=9vdc

(pin6)=-.8

(pin8)=12.3vac

Best regards,

Bill

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Bill,

The only thing I really see amiss here is the 9 VDC you are measuring on the grid of the 14B6.  This should be zero or somewhat negative.  Are you sure it is reading +9 VDC (as opposed to -9 VDC)?

Is the 5.6 megohm resistor present, and does it read the correct resistance?  If that resistor is wired correctly and is good, then the only remaining possibility is a bad 14B6 with an internal short, or more likely, grid emission.  Do you have another 14B6 you can swap in there?

Grid emission is a problem that many tube testers cannot test properly for.  So even if the tube tester says it's a good tube, there can still be a problem.

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sorry, it does read -.9, my typing  isn't what it used to be. I changed the tube with the same results. Can i check that resistor without taking it out, or is it best to remove?

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You can check that resistor without taking it out.  If the voltage is in fact -0.9 VDC, then most likely the resistor is OK.

Assuming that the problem is probably not in that stage, I would suggest a couple more tests:

-  Is the speaker good?  If you connect the speaker to a working radio, does it sound OK?

-  Can you hear a low hum in the speaker when the radio is on and warmed up?

-  Is the output transformer good, at least to the extent that you can measure a very low resistance across the secondary with the speaker disconnected, and a few hundred ohms across the primary?  (your confirmation of proper plate voltage on the 50A5 suggests that the primary is OK)

-  If you touch a screwdriver to the grid of the 50A5, do you get at least a very faint hum in the speaker?

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Hello,

I re-checked pin 3 on the 14b6 and it’s bouncing between ­­-.9 and -1.

 I’ve checked the windings on the output transformer and have over 200ohms resistance on the primary.The secondary has around 7 if I remember right. I replaced the speaker anyway, with the same results (as a rookie, I’m not confidant in my meter reading results just yet).

I can get channels on the radio if the volume is turned up all the way.

I put a screw driver on the 6 pin on the 50a5 and get a loud buzz.

I tested the 5.6 ohm  resistor and my meter did nothing no resistance?

Hope this helps,

Bill

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Hi Bill,

If you get a nice buzz on pin 6 of the 50A5, let's trace it backwards one step to pinpoint the problem.

-  Do you get a similar buzz if you touch pin 2 (plate) of the 14B6 (it may be a little weaker than on pin 6 of the 50A5, but not drastically so).  If you don't get a buzz, then check component 9, the 0.002 uF capacitor.  Make sure it is the right value, it is good, and it is wired properly -- from pin 2 (plate) of the 14B6 to pin 6 (grid) of the 50A5.

-  Did you replace component 14, the 50 pF capacitor from pin 2 (plate) of the 14B6 to ground?  Generally you should not need to replace that component.  If you did, are you sure you put in a 50 pF (not uF) capacitor?

-  Is pin 7 (cathode) of the 14B6 properly grounded?

- Regarding the resistor omn pin 3 (grid) of the 14B6 to ground -- it should be 5.6 megohms, not 5.6 ohms.  If you have a digital ohmmeter, it should be able to read it correctly.  If nothing happens when you probe that resistor, it means the resistor is open (infinite resistance), not "no resistance."  Replace if its value is way off (its value is not particularly critical -- anything between about 2 megohms and 10 megohms should work OK).  If you have an inexpensive analog meter, then what you observe is correct -- the value of this resistor is too large to be measured by an inexpensive analog meter (a VTVM or FETVOM are examples of analog meters which could read that resistance properly).

Best regards,

Tom

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Found it!!! The .002ohm was connected to pin 7 not 6. Changed it and it works great!

In my defense, it was wired that way, when I changed that cap I simply clipped the wire and coupled the wires together. Looking at pin six, it doesn’t look like there was ever another wire soldered to it (looks factory like). Is that possible?

 Any way thanks for your help, much appreciated!  I’m glad I found this sight and hope I can contribute in the future.

Best regards,

Bill

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Bill,

You did a pretty good job of describing exactly what you were seeing with the radio, and in the end it mainly pointed to a lack of coupling between the first audio and output stages.  Indeed that turned out to be exactly the problem.  Glad you got it fixed.

Tom

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