Icom IC-725 HF Radio Goes Into Tx But Emits No RF

[TMA-1]

Ok, I found a little time to hook up some more "test point wires".  The results are confusing me.  (Like that should be a surprise,...)

Measure the DC voltage at pin 2. The DC voltage at pin 3 has to be ~0.7V below the voltage at Pin 2.

No, there is no DC voltage at Pin 2, or Pin 3  And the AC output at Pin 2 is smaller even than at Pin 3.

As a reference you could also check the DC levels at the pins of the other uPC1037HA chips (IC5 in the main unit, IC16, IC17 in the PLL unit) although they don't have identical supply voltages.

Looking at IC5, it seems to have no DC present at Pin 2 either.  And Pin 3 is just plain weird,... there is about a -.2 VDC present there with the radio "on".

With the power supply active but the radio power switched "off", there is a +.4 VDC at IC5 Pin 3!  If the power supply is then turned off, the voltage drops, looking like a capacitor discharging.

Both IC5 outputs show a carrier of about .1 V peak-to-peak.  They do not seem to change perceptively when toggling to Tx or applying modulation.

any DC voltage on pin 6  ?

No, there is no voltage at IC6 Pin 6 (the bypass line).  It is at ground potential.  But it was still a good idea to check that off!

I'm clearly not understanding this circuitry, but wondering if it would be a worthwhile experiment to SWAP IC5 with IC6.  If the symptoms remain the same, it would indicate that the problem lies elsewhere, but it they change at all, it would tell me I'm still dealing with a defective new part.

[RFDx]

No, there is no DC voltage at Pin 2, or Pin 3  And the AC output at Pin 2 is smaller even than at Pin 3.

Looking at IC5, it seems to have no DC present at Pin 2 either.  And Pin 3 is just plain weird,... there is about a -.2 VDC present there with the radio "on".

No, there is no voltage at IC6 Pin 6 (the bypass line).  It is at ground potential.

This voltages are obviously all wrong. Something is off. Broken voltmeter? Measurement error? With all the long wires connected to the different pins you also cannot trust any RF measurements.

To be certain, if you find the time, take the old chip you replaced, solder it dead bug style on a small copper PCB for a DC test and compare with the ballpark DC voltage values shown in the attached picture. If you own a two channel signal/function generator do also the AC test. Test the new chip too if possible.

With the power supply active but the radio power switched "off", there is a +.4 VDC at IC5 Pin 3!  If the power supply is then turned off, the voltage drops, looking like a capacitor discharging.

Pin 3 is an open emiiter. It needs a resistor (2.2k...6.8k) tenporarily soldered to ground. Even without the resistor, the voltage is still far from what it should be. As you mentioned, the receiver is working which means IC5 has to be OK.

[TMA-1]

Yes it certainly seems to be that "too many" things are whacko here.   

My "next step" plans are:

1) Add the temporary resistor to IC5 Pin 3 and test it again.

2) Cross-check all previous oscilloscope measurements with a multi-meter.

3) (Re-)examine all my test point connections for soldering shorts or other "wildcard" problems.

4) Do the tests you suggest with the uPC1037HA.  I do have the old IC (with the legs intact), and enough signal sources for the AC test.

I suspect though, that since the replacement of IC6 made zero difference in the observed behavior, that both copies of the IC are possibly fine and the problem lies somewhere else entirely.

RFDx, thank-you so much for "hanging in there" with me on this.  I know it must be time consuming and I appreciate it.  I'll exhaust the list above before posting the results.

[RFDx]

Does the bypass capacitor on pin 6 have a short?

[TMA-1]

Uhmm,... it might?  I just noticed the "3V" there in your two test diagrams.  Is that how it is supposed to work--take it to ground means "bypass"?  Because when I measured it, it was "at ground".

NEW #1 on my list of "things to do"!

[Edit]  Ohm meter between my test point right on IC6 Pin 6, and ground, shows open circuit, just as it should.  (Damn.)  But in operation, Pin 6 is rock steady at zero anything vs. ground.  I'd like to know more about this chip, but the datasheets are all in Japanese.   

[RFDx]

Uhmm,... it might?  I just noticed the "3V" there in your two test diagrams.  Is that how it is supposed to work--take it to ground means "bypass"?  Because when I measured it, it was "at ground".

The mixer has a differential input (pin5 + pin6) but pin6 (bypass) is not used and is bypassed for AC signals, hence the capacitor to ground. The DC value should be the same or at least close to the voltage at pin 5 (signal input).

But in operation, Pin 6 is rock steady at zero anything vs. ground.  I'd like to know more about this chip, but the datasheets are all in Japanese.   

0V DC at pin 6 is definitely not right. For a tad more info in english take a look at the SL1640C (Plessey) datasheet.