ART-13, No Modulation

[W7SOE]

After finally getting a mic for the ART-13 I was disappointed to not see any modulation on the output.  I capacitively coupled my signal generator to the mic input and only when it was cranked up all the way did I see the carrier get modulated, maybe 5%.

I have tested the two 6V6's and the 12SJ7 in the audio section and they test ok, at least on my crummy tester.  The 12SJ7 is the first stage amp and I have ordered one anyway, because they are cheep.  ;-)

I have the audio section out and am checking resistor values.  So far, up to the first stage, they look ok.

Any advice appreciated.

Rich

[WA1HZK]

Get a scope and trace the audio all the way from the mic. to the 811's, No Scope? probably no chance to find it. Also unless the rig was a virgin someone might have already moused the audio. Again, scope. A cheap one will do. It's just audio.
Keith
WA1HZK

[Steve - WB3HUZ]

An audio amp with a hi-z probe will work just as well.

[WA2TTP Steve]

You could try the transmitter in the MCW mode and see what level of modulation you get from the internal tone osc. If that works ok that would rule out several stages of the audio system. I'm out of town so I don't have any docs to refer to but I think that would be quick test.

Steve,
WA2TTP

[W7SOE]

I have a scope and have tried to trace the signal.  I measure ~300 ohms from the mic input to ground which seems reasonable given the circuit.  The injected signal is almost completely attenuated even with the signal generator cranked up.

What I am afraid of is that T201 (see attached schematic, audio comes in from lower left)  is bad. 

I need to dig deeper.....

Testing with the MCW is a good idea, I will try that.

Thanks

Rich

[W2PFY]

That transmitter is designed to be used with a carbon microphone. I used a T-17 microphone on one I had and it worked fine.

[W7SOE]

That transmitter is designed to be used with a carbon microphone. I used a T-17 microphone on one I had and it worked fine.

Yup, That's what I am using also.

rich

[WA1HZK]

A lot of guys just take out the preamp/audio section and rewire it for normal 600 ohm in or a Hi-Z mic. depending on your setup. Probing around under the chassis is difficult unless you can make up some service cables. I would look at the preamp deck output audio with 1/2 a volt dumping into the mic at 1000 Hz. that should be a large signal coming out. If not then you know where the bad part is located, under that deck.
Keith

[W2PFY]

It's been a long time since I worked on one. Is it possible that you somehow have it in the CW position and shorting out the secondary of your mod transformer? Does it have a mod meter? like I said it's been a long time. If the mod meter is kicking up, then it's making audio but just shorted out else where. as I remember the audio was adjustable. The one I had almost went into talk back with the gain I had. Is there a place on line where I can download the schematic?

[KE6DF]

Here is the schematic:

http://www.kf9cm.com/Schematics/TS47-ART-13.pdf

[W7SOE]

Update:

I found a bad ground connection in the audio section, that helped a leeeetle.  What really helped was replacing the 12SJ7 first amp.  Now the gain is much better, I get good modulation with 1.0Vpp from the generator.

There is still a lot of attenuation.  If I couple the generator capacitively then It won't drive it much at all, I have to drive it directly.

Neither the carbon mic nor the D-104 will do any thing, with the transmitter set for either carbon or dynamic mic.

But if I drive it with a Beringer mic through a pre-amp, then it works great.

Trying the MCW mode was a good idea, that worked fine so it was relief that the problems seem to be in the audio section not the mod transformer etc.

Thanks all for the help and suggestions.


Rich

[WU2D]

Even if it works - The paper and Electrolytic capacitors are still suspect in the module.

C202 - 20 uF Electrolytic -  replace it!

C201 20 uF Electrolytic - replace it!

C207 20 Uf Cap - Replace it!

C203 .05 uF Screen Bypass - Maybe Replace - Use 0.1 uF 600V

C205 .001 Coupling Cap - Replace with 0.1 uF 600V

Mike WU2D

[W7SOE]

Mike,
   Those are all good suggestions, leaky caps are a distinct culprit.

Thanks

Rich