Author Topic: Icom IC-725 HF Radio Goes Into Tx But Emits No RF  (Read 5833 times)

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Offline TMA-1Topic starter

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Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #25 on: April 07, 2024, 04:21:07 pm »
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.
 

Offline RFDx

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Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #26 on: April 08, 2024, 12:25:49 pm »
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.
 
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Offline TMA-1Topic starter

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Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #27 on: April 08, 2024, 04:31:57 pm »
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.
 

Offline RFDx

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Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #28 on: April 09, 2024, 01:27:51 am »
Does the bypass capacitor on pin 6 have a short?
 

Offline TMA-1Topic starter

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Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #29 on: April 09, 2024, 01:48:46 am »
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.   |O
« Last Edit: April 09, 2024, 04:30:40 am by TMA-1 »
 

Offline RFDx

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Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #30 on: April 09, 2024, 07:10:32 am »
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.   |O

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

Offline dtxy101

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Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #31 on: May 23, 2024, 08:03:09 am »
 :)
« Last Edit: May 23, 2024, 08:11:15 am by dtxy101 »
 

Offline A.Z.

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Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #32 on: May 23, 2024, 09:40:41 am »
Does the bypass capacitor on pin 6 have a short?

Sorry for entering this way, but after reading the discussion I had a suspect; may some faulty capacitor (or other component) "kill" the IC ? If so, the previous one was "poof" and replacing it with the new one "killed" the latter as well

crazy ?
 

Offline vk6zgo

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Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #33 on: May 23, 2024, 11:17:26 am »
Have you tried CW mode?
You will have to "Key it On" in that mode, which hopefully has  some circuitry in common with both the FM & SSB modes.

AM probably has more circuitry in common with SSB---have you tried that mode?

How are you sourcing audio in SSB---just speaking into the mic or  supplying it with a tone source from a generator?
The latter may make it easier to trace the audio circuitry.

I've never worked on a IC725, but I have  on a IC575a, which, from the sound of it, uses a similar type of circuitry.

The IC575a generates the drive signal around 9MHz, then mixes it up to the transmitted frequency, in a similar manner.
The coax interconnection between boards are very prone to problems, as they use those horrible little push in connectors.
Your oscilloscope is your "ace in the hole", but you have to ensure that you know what the signals you are chasing look like.

That generation of Icoms are complex little shits of things, but they are easier to work on than their contemporaries from Yaesu.
 

Offline TMA-1Topic starter

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Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #34 on: May 31, 2024, 10:19:35 am »
First let me apologize for being away from this thread for so long.  My wife was out of the country for a spell, creating additional pressures on "life" which simply got in the way of me committing any more time to this.  I hope to change that this weekend.

I did previously have time to add the temporary resistor to Pin 3 of IC5 and test it, and the result was more reasonable, so back to IC6.

I also cross-checked many of my oscilloscope measurements with a multi-meter, and got no surprises.  They are consistent.  Whacky, but consistent.

Thank-you to the new contributors.  Like A.Z., I too have suspicions that some other component failure may have caused the second IC to die in exactly the same manner as the first.

I now have a 3rd copy of the chip which is still a virgin.  My task list is now:

A) Disengage the board again, and revisit my test wires for shorts or other problems, as I had previously planned.

B) From the solder side, test each neighborhood resistor and capacitor for shorts or opens.

C) Pull the current IC6, and perform the tests that RFDx so graciously and meticulously laid out.  It is my hope that by performing these tests on (1) the original IC, (2) the disappointing replacement, and (3) the newest virgin which has never been installed, that a pattern will emerge.  It should definitively prove whether or not the IC is the current problem, if not the catalyst.

I'm learning a lot from this, one way or another.  I just hope there's a happy ending eventually,...
 

Offline TMA-1Topic starter

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Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #35 on: June 08, 2024, 01:16:09 am »
Well they say no battle plan survives actual contact with the enemy,...

In revisiting all my test wires, I decided on the fly that it was all just too messy and cumbersome, and that (as RFDx suggested) I could be picking up stray RF on all those wires.  Bah.  I took them all off in favor of a better idea,...

Removed IC6 again, and soldered just 6 wires, through the holes of the board, in its place.  (One pin is unused anyway.)  This allows me to (1) take measurements without a chip of unknown characteristics skewing things, and (2) breadboard the chip (any copy) back into the circuit to test at will, at the chip, where it really matters.

The results were dramatic.  With no chip, the inputs were clean, and DC biases were reasonable.  By more directly comparing the original chip and its disappointing replacement, I could see that both were deficient, though not in the same way.  Throwing caution to the winds, I tested the 3rd, as yet untried, chip.

It works.  With modulation on the microphone, I get "SSB output" on both the power meter and my oscilloscope.  Well damn.  I will do a bit of a burn-in test before re-assembling everything, but I think its cured.

Conclusions:
  • The original diagnosis of a failed IC6 was in fact correct all along.
  • My "wire Medusa" produced unclear, conflicting, or downright bogus results.
  • The first replacement chip (from China) was poo-poo.  Just my luck.
  • The second replacement (from a different source in China) was clean.

Thank-you again to all who offered help and encouragement.  I learned a lot!
 
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