[murf]

Hello guy's,
Have a Trutone A2-G teledial floor model radio that I am having issues with.
Suspecting maybe speaker or output transformer.
This speaker has a 5 pin connector, so I am not sure how to check it with my ohm meter. Used to 4 oin connectors.
Problem I am having is volume issues. Plays fine at lower volumes and goes south after mid range on the volume control.
Has been recapped and the transformer was replaced by previous owner.
Cadohm resistor is replaced as well as a handfull of resistors.
How do I check the speaker and output transformer on this 5 pin connector.
Tubes have all been verified as OK on my tube tester(Hikcock).
Checked voltage on rectifier tube and it is checking about 285v.
Checked on speaker socket and its about the same.
Idea's without getting to technical?
murf

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Sorry Murf, technical is what I do.

OK, Unlike the Western Auto Tru-Tones, this True Tone is a Wells Gardner set. The Nostalgia Air schematic is below:
http://www.nostalgiaair.org/PagesByModel...023515.pdf.

Unfortunately, they do not show the pinouts for the speaker plug. if all 5 pins are used, the pins (IN NO PARTICULAR ORDER) would be connected to:
Plate of one 6F6
Plate of the other 6F6
Center tap of output transformer and one leg of the field coil
Other leg of field coil
Chassis ground

If you take out the 2 6F6 tubes and ohm between pin 3 of one 6F6 and pin 3 of the other 6F6, there should be about 560 Ohm. Lift one leg of the secondary connection to the speaker voice coil. Resistance of the secondary, the voice coil and the hum bucking coil all should be quite low. I suspect that these resistance checks will pass and that your issue is elsewhere. My bet is that the wrong power transformer was used or there is an issue with the voltage divider (resistors R22, R23 and R24) that produces "C-".

One possible issue is that your B+ (from rectifier cathode to ground) of 250V is quite low. The schematic lists plate voltage to ground at the 6F6 plate (pin 3 of each 6F6) is 330VDC. The center tap of the rectifier connects to ground through resistors R22, R23 and R24 (the Candohm?) This develops the grid bias (C-) for the Output tubes, 1st AF and Phase Invertor (Balancing Excitor). There should be about -25V between chassis ground and the center tap of the power transformer. Measuring across C44 should yield about 355V. Note that t5h3 can case of C44 does not connect to Chassis ground, it connects to the junction of R24 and the HV Secondary center tap of the power transformer. This point, not Chassis Ground is the most negative point on the power supply. Note that the cathodes of the audio amp and outputs are at Chassis ground, but the 6F6s require a bias of about -25V for grid bias. This is taken from the junction of R24 and they HV secondary center tap.

If R22, 23 and /or 24 change resistance with high current, this may cause your issue. Low B+ (250Vvs 330V) will cause the same issue.

Hope this helps.

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Any reason for the 2nd copy of the thread?

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Hi Morzh,

From my end, no. I did not know that there are 2 threads on this topic. Maybe it should be combined.

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MrFixr

Honestly, I haven't done that yet and do not want to screw up the threads; maybe you could do it, or we could ask Gary or Nathan, or Bob.

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Sorry, my mistake.
murf

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Thanks for the replies, I will check these ideas out and report back.
murf

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Well I have found 315v on the rectifier when cold. Drops off to 280 after 15 minute warmup.
Checked pin 3 on both 6F6 tubes and found 280v when cold and 250v after 15 minute warmup.
THis is probably whats causing the low volume and distortion after a good warmup. Now I am guessing that I will be needing a power transformer? Will check again and verify my readings.
murf

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The transformer may be the last cause of voltage drop.
This level of voltage drop is not too big. Conduct additional testing at maximum volume. If the voltage drop is more than 30V from cold, it may be the cause of low emission of the rectifier tube. Try replacing it.
I have encountered similar problems on several of my radios.
Check voltage on rectifier 5Y3G catode, not on pin 6F6  when volume is on .

[GarySP]

Hello murf! I merged the threads. Please do not start new threads regarding the same radio. Take care, - Gary

[murf]

Would an output transformer if weak, cause the problems I am seeing with lower B+ voltage?
murf

[MrFixr55]

Hi Murf,

Do this:
Monitor between ground and one plate of the rectifier while playing the radio.  Do the same with the other plate.  If your meter can measure a high enough voltage, measure across the plates.  CAUTION!!  This is about 750 Volts or so.  If no voltage drop from start until distortion occurs, then the power transformer is good.

While radio is playing, especially when the distortion starts, observe the 6F6 tubes for "red plate" or any abnormal glow between elements. (Hopefully they are not metal).

Power down, unplug and immediately feel the transformers.  Feel the caps, C44 and C43.  Likewise, the speaker field coil.  They may be warm but not hot. 

What voltage rating did you use for the filter caps?  Fr C44 in particular, ensure that you used either 450 or 500 WV (Working Volts)

You can measure the current draw by placing the meter between the 5Y3 cathode and the rest of the circuit.  If the current increases with time as the B+ voltage drops, then suspect a component shorting.  If the current decreases as the voltage drops and the AC output of the HV winding of the power transformer did not change, then suspect the rectifier or the B- C- resistors (R22, 23, 24).  You can also measure the DC voltage drop between the chassis ground and the HV secondary center tap.  If this voltage increases with time when the distortion starts, then these are your culprits.  Ensure that you used sufficient wattage.  At least 5 W, preferably 10 or 20W.

I do not expect that output transformer to be the issue.  They either work or don't.  I don't think that it is shorting as it gets warm.  It is possible, but I don't think probable.  The speaker frame (and therefore the output transformer frame and field coil frame) are grounded.  If a ground develops, there will be a lowering of B+ along with a B current increase.  However, there would be smells, burning, etc. if intermittent grounding of transformer and / or field coil windings is occurring.

Hope this helps.