The PHILCO Phorum
Testing a speaker and output transformer Trutone A2-G - Printable Version

+- The PHILCO Phorum (https://philcoradio.com/phorum)
+-- Forum: Various and Sundry Categories (https://philcoradio.com/phorum/forumdisplay.php?fid=37)
+--- Forum: Other Radios (Non-Philco) (https://philcoradio.com/phorum/forumdisplay.php?fid=12)
+--- Thread: Testing a speaker and output transformer Trutone A2-G (/showthread.php?tid=25756)


Testing a speaker and output transformer Trutone A2-G - murf - 04-27-2025

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

RE: Testing a speaker and output transformer Trutone A2-G - MrFixr55 - 04-27-2025

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/515/M0023515.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.

RE: Testing a speaker and output transformer Trutone A2-G - morzh - 04-27-2025

Any reason for the 2nd copy of the thread?

RE: Testing a speaker and output transformer Trutone A2-G - MrFixr55 - 04-28-2025

Hi Morzh,

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