Author Topic: Icom IC-725 HF Radio Goes Into Tx But Emits No RF  (Read 5618 times)

0 Members and 1 Guest are viewing this topic.

Offline TMA-1Topic starter

  • Contributor
  • Posts: 26
  • Country: ca
Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« on: February 18, 2024, 07:07:17 am »
I dusted off my Icom IC-725 today, and found it is no longer working.  There is zero RF output on transmit.  This is true of both SSB and CW.  (Receive works normally in all modes.)

Here's the kicker:  This model provides for an optional UI-7 board, which adds FM transmit and receive capability, as well as AM transmit.  (The stock radio only receives AM.)  MY radio has a clone of this UI-7, which provides the FM capability but not the AM.

The radio continues to transmit FM!

Seems to me this should suggest where to start looking for the problem, but since I'm currently still in the denial stage of grief, it hasn't yet dawned on me where.

I can find no information in the user or service manuals or on the Net pertaining to this behavior--it goes into transmit mode, I can hear the relays switching, the LED comes on,... but zero RF energy is output.  I've tried resetting the processor but the behavior has not changed.

Has anyone else experienced this?
 

Online coromonadalix

  • Super Contributor
  • ***
  • Posts: 6089
  • Country: ca
« Last Edit: February 18, 2024, 07:35:16 am by coromonadalix »
 

Offline TMA-1Topic starter

  • Contributor
  • Posts: 26
  • Country: ca
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #2 on: February 18, 2024, 10:11:19 am »
Uhm, sure.  Thanks for the additional copies.  (?)

I guess I'll just go away now.
 

Offline p.larner

  • Frequent Contributor
  • **
  • !
  • Posts: 690
  • Country: gb
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #3 on: February 18, 2024, 11:26:57 am »
get your scope out and start checking drive levels on the lpf rf board etc.
 

Online xrunner

  • Super Contributor
  • ***
  • Posts: 7557
  • Country: us
  • hp>Agilent>Keysight>???
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #4 on: February 18, 2024, 02:22:37 pm »
Yea well ... sh*t happens right?  :-//

I've repaired lots of ham rigs and this issue is nothing especially new or unusual with older rigs. To go into all the troubleshootong here would be a class unto itself. I'd see if you can check the voltage levels of of the internal power rails. That's very basic but there are lots of places where this problem could be manifested. If you don't know how to do more advanced troubleshooting I'd just sell the thing for parts and buy a new SDR-based rig.  :)
I told my friends I could teach them to be funny, but they all just laughed at me.
 

Offline TMA-1Topic starter

  • Contributor
  • Posts: 26
  • Country: ca
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #5 on: February 18, 2024, 07:51:45 pm »
Yea well ... sh*t happens right?  :-//

Indeed it does.

Actually my intent WAS to sell the old girl to start with, but I'd much rather do it in working condition, so I was disappointed to find it suddenly lame.

Having progressed to the acceptance stage of grief, I will indeed warm up the 'scope and get to work.  The Service Manual graciously provides test point voltages, which seem a good place to start.

I'm still a little surprised by the fact that optional FM works while the "native" modes do not.  I had thought perhaps the processor was inhibiting Tx on those modes because it saw something it didn't like, but maybe not.

Thanks for the heads up about "lots of places where this problem could be manifested" xrunner.
« Last Edit: February 18, 2024, 07:53:44 pm by TMA-1 »
 

Online xrunner

  • Super Contributor
  • ***
  • Posts: 7557
  • Country: us
  • hp>Agilent>Keysight>???
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #6 on: February 18, 2024, 10:09:07 pm »
I dusted off my Icom IC-725 today, and found it is no longer working.

So it was working fine before it was put into storage? If so then I'd suspect the following -

1. A wire or connector was loosened or broken while the radio was being moved. This can happen with older equipment due to age.

2. Electrolytic capacitors can leak when they get old and fail.

3. Corrosion of wires causing them to have a high resistance or causing them to break when the radio is jostled around.

4. I have to say this though - possible operator error. Perhaps you are making a mistake with the controls of the radio. You know what HAL said in the movie right? "Things like this have cropped up before, and it has always been due to human error ..."

All of the above have been seen by me personally.
I told my friends I could teach them to be funny, but they all just laughed at me.
 
The following users thanked this post: TMA-1

Offline RFDx

  • Regular Contributor
  • *
  • Posts: 155
  • Country: de
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #7 on: February 19, 2024, 03:58:38 am »
I'm still a little surprised by the fact that optional FM works while the "native" modes do not.  I had thought perhaps the processor was inhibiting Tx on those modes because it saw something it didn't like, but maybe not.

As FM and everything else is working, the logical assumption would be the product modulator (SSB & CW) is not (e.g. µPC1037H modulator faulty, voltage supply for the modulator missing, no BFO voltage, TX diode switch dead etc.).
 
The following users thanked this post: TMA-1

Offline TMA-1Topic starter

  • Contributor
  • Posts: 26
  • Country: ca
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #8 on: February 20, 2024, 01:47:13 pm »
Interim post,... points well taken.  I haven't found the time yet to get back to this, but I appreciate all the input.

1. Not that I've found.  I had tried re-seating all the connectors that I can reach thus far.  Everything looks quite intact and tidy.

2. No caps have spilled or bulged visibly, but of course that doesn't mean they're okay, either.  I'm familiar with Bad Caps Disease from working on vintage home microcomputers, where due to heavy competition back in the day, the struggle to reduce production costs was usually front-and-center.

3. Same as 1. The unit was stored well and no deterioration is visible.

4. I don't think so, this time.  The radio is very simple to operate, which is perhaps fortunate since it has a simple operator.  :/

And I absolutely love the 2001 reference.  Always wanted to be able to debug with a super-duper logic probe and computer, like Dave & Frank on the AE35--one of my favorite gimmick scenes.

I think RFDx is on the right track.  I just need to make the time,...
 
The following users thanked this post: xrunner

Offline TMA-1Topic starter

  • Contributor
  • Posts: 26
  • Country: ca
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #9 on: February 21, 2024, 03:07:00 pm »
I've invested some time in this, but come up rather empty thus far.

I liked RFDx's assertion that I should start with the uPC1037H modulator.

The components are all so small and tight to the PCB, the only way to get a look with a scope was to undo all the connectors on the board, remove it, solder temporary wires to the underside for points I wanted to test, and re-insert & re-connect the board.

At the uPC1037H modulator I got no AC signal at pin 7, the carrier input.  "Ah ha!", thought I.

Need to trace back further to Q42, then Q31.  Unhook, flip, remove the RF shield, solder more wires, flip, reconnect.  Still no evidence of a carrier.  (The DC voltages from the schematic seem to be okay.)

Maybe the crystal has gone south?  Or it could be the switching diodes.  I pulled the 3 diodes (D67, 68, & 69) and checked them--they all seem okay.

Ran out of time and gumption at that point, and thought I'd check in for opinions before resuming.  I admit to limited experience at this level of diagnosis, and figuring out exactly what is happening, (or what SHOULD be happening) from a schematic is a stretch for me, so I'm somewhat lacking confidence that I'm even on the right track.  My alleged skills with a scope are somewhat new too.

At this point I think the crystal is not oscillating to produce the carrier.  This would seem to make sense given the symptomology:  receive okay, no RF needle movement on SSB or CW Tx, and FM Tx working fine (because it is a separate animal.)

Opinions?
 

Offline Hamelec

  • Regular Contributor
  • *
  • Posts: 156
  • Country: de
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #10 on: February 21, 2024, 06:28:06 pm »
which frequency you tuned? 13.764 MHz like in your picture?
..TX not working out of Band
 

Offline TMA-1Topic starter

  • Contributor
  • Posts: 26
  • Country: ca
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #11 on: February 21, 2024, 08:05:45 pm »
No, that was just a "scan" at the time.  Sorry.
 

Offline TheSteve

  • Supporter
  • ****
  • Posts: 3757
  • Country: ca
  • Living the Dream
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #12 on: February 21, 2024, 09:27:35 pm »
I've fixed several Icom HF rigs over the years. Can I just say how nice it is that a company still provides service manuals and supplies parts for the radios as long as they can source them.

Good luck with the repair!
VE7FM
 
The following users thanked this post: TMA-1

Offline RFDx

  • Regular Contributor
  • *
  • Posts: 155
  • Country: de
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #13 on: February 22, 2024, 04:38:16 am »
At this point I think the crystal is not oscillating to produce the carrier.  This would seem to make sense given the symptomology:  receive okay, no RF needle movement on SSB or CW Tx, and FM Tx working fine (because it is a separate animal.)

The BFO feeds both product detector and modulator. If the BFO isn't working at all then receiving SSB/CW wouldn't work either.

~3V DC at pin 5 and 7 of the product modulator IC6 (µPC1037HA)?
DC voltages (3.2V / 4.6V) at the bases of Q31 and Q42 (both 2SC2458) OK?
Carrier frequency present at the emitter of the oscillator transistor Q31? Switch scope probe to 10:1!
Carrier frequency (+ 4V DC) present at the emitter of Q42? No matter what mode (USB/LSB/CW) is selected?
C237 (22p) cracked? This would prevent the carrier reaching the product modulator.
DC Voltage present at the anode of the USB switching diode D69 (1SS53) when mode=USB?
DC voltage at the anode of the LSB switching diode D67 (1SS53) when mode=LSB?

Btw., C-trimmer C294 (30p) is in series with R271 (10k) resistor which shouldn't be. Seems as a connection (dot) is missing in the schematics.
« Last Edit: February 22, 2024, 04:41:26 am by RFDx »
 
The following users thanked this post: TMA-1

Offline TMA-1Topic starter

  • Contributor
  • Posts: 26
  • Country: ca
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #14 on: February 22, 2024, 02:32:50 pm »
Thanks for the well-wishes, TheSteve.  And yes, it's nice that they provide test-point voltages on the schematics, too.  So easy to do given a working unit, but so few manufacturers seem to bother.

And a hearty "thank-you" to RFDx.  You certainly know this stuff!

Next session, I'll double-check these points, but my recollection is that the base voltages of the two relevant transistors were fine.  I may indeed have made a mistake with the scope in detecting the carrier.  I'll go back and test again.

C237 doesn't look cracked, but it's so darned tiny it's near impossible to tell.  And I agree that a dot is missing from the schematic; if they hadn't intended a join at that position, that part of the diagram would have been drawn differently.

I'll post again later when I've had a chance to do some more work.
 

Offline TMA-1Topic starter

  • Contributor
  • Posts: 26
  • Country: ca
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #15 on: February 22, 2024, 06:22:59 pm »
Hold on to your tickets folks, but I think we have a winner!  And yes, Hal, a short mis-diagnosis random stroll was due to human error.  (I wonder now if that original AE35 unit WOULD have failed in time,...)

I was making a mistake with the oscilloscope and not recognizing the carrier signal when I saw it (or didn't quite see it.)  The carrier survives long enough to go into pin 7 of the uPC1037H, but it does NOT come out.  No output at pin 3. He's dead, Jim.

So RFDx was right all along and it's the modulator which has died.  Part has been ordered.  I'll post again when it comes in, to confirm that the repair was successful.

Thank-you to all to chimed in help, and especially RFDx.  Thanks for holding my hand as I gain competence with this stuff; I owe you a beer or six if you're ever in Canada.

{... composes his Kijiji ad for a working IC-725,...  "recently serviced",...} :)
 
The following users thanked this post: xrunner

Offline TheSteve

  • Supporter
  • ****
  • Posts: 3757
  • Country: ca
  • Living the Dream
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #16 on: February 22, 2024, 08:36:10 pm »
Hopefully the part you get will be real and not some fake/remarked crap.
VE7FM
 

Offline TMA-1Topic starter

  • Contributor
  • Posts: 26
  • Country: ca
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #17 on: March 24, 2024, 03:20:21 pm »
Well, I said I'd post again when the part came in.  Unfortunately the news is not good.

The new modulator has made no difference in the symptoms.  So it was back to the o'scope to try again to figure out what is going on,...

I get the same "carrier-like" signal on the base AND the emitter of Q42, which I thought was suspicious.  Perhaps an internal short?  If Q42 is supposed to amplify the carrier before going in to pin 7 of the modulator, maybe the present level is not high enough for the modulator to process?

...  Changed Q42.  No effect.

Checking on what RFDx was advising, most of his test points were satisfied.  For example, I get a solid 4V DC at the anode of D69 with mode USB selected.  Good.  I do NOT however get more than .5V DC at the anode of D67 with mode = LSB.  (It rises to 3V when mode is USB.)  Hm, not so good maybe.

...  Replaced diode D67.  No effect.

I confess I'm poking in the dark here, because I don't really understand how this circuit is supposed to work, or even if I'm in the right ball park entirely.  I'm learning, but still a bit over my head.

There are now a dozen test points wired to the solder side of the board so that I can use the scope with the board put back in place and all connectors re-attached.  I'm sure they paint a picture, but like those "Magic Eye" tapestries, you either see the special image or you don't.  I currently don't.

I'll keep blindly working on it.  If anyone has suggestions I'd be happy to hear them.  Thanks for reading.

« Last Edit: March 27, 2024, 03:13:25 pm by TMA-1 »
 

Offline RFDx

  • Regular Contributor
  • *
  • Posts: 155
  • Country: de
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #18 on: April 03, 2024, 06:01:14 am »
I get the same "carrier-like" signal on the base AND the emitter of Q42, which I thought was suspicious.  Perhaps an internal short?  If Q42 is supposed to amplify the carrier before going in to pin 7 of the modulator, maybe the present level is not high enough for the modulator to process?

Same level at base and emitter of Q42 is fine as this is a buffer stage (emitter follower) with a gain of 1.


Checking on what RFDx was advising, most of his test points were satisfied.  For example, I get a solid 4V DC at the anode of D69 with mode USB selected.  Good.  I do NOT however get more than .5V DC at the anode of D67 with mode = LSB.  (It rises to 3V when mode is USB.)  Hm, not so good maybe.

Do you have a counter to check the frequency of the BFO when switching between modes? At least USB seems to be working, for now that would be sufficient to check if the the modulator is working.

Is there any signal at pin 3 (DSB output) of the uPC1037? Switch to USB. Without modulation (AF) you get next to nothing as the carrier is suppressed at least 30...40dB relativ to the carrier voltage injected in pin 7. With AF applied to the MIC input there has to be a DSB output signal (riding on a DC voltage) at pin 3.






 
The following users thanked this post: TMA-1

Offline TMA-1Topic starter

  • Contributor
  • Posts: 26
  • Country: ca
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #19 on: April 05, 2024, 03:45:00 am »
Thank-you RFDx.  I'm so grateful that one of us knows what he's doing!

First, I have to make a retraction.  My disquieting results at the anode of D67 were bogus results.  I had soldered my "test point" wire to the wrong point on the solder side of the board.  (I am finding it very difficult with my aging eyes to locate the correct solder points for a given component, because they are so tightly packed.)

With that mistake corrected, I now get 7 VDC at the anode of D67 with mode set to LSB.

Okay, now to your latest suggestions,...

The input to the modulator at pin 7 is 9.013 MHz, according to my oscilloscope.  It is Channel 1 (yellow) in the photographs.  It seems to be stronger on USB than it is on LSB.

The output from the modulator at pin 3 is Channel 2 (blue).  It does not change visibly when in transmit, either with or without modulation.  (There seems to be no DC there either.)

It looks as if the uPC1037 is a black hole.  But then again, that's exactly what it looked like when I replaced that IC.  The new one performs exactly the same way.

VCC to the chip is 7 VDC.  Pin 5 does seem to be getting audio, riding atop 3 Volts DC.

(Sigh.)  Did I get a bad replacement chip?
 

Offline Hamelec

  • Regular Contributor
  • *
  • Posts: 156
  • Country: de
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #20 on: April 05, 2024, 08:50:12 am »
Is there any signal at pin 3 (DSB output) of the uPC1037? Switch to USB. Without modulation (AF) you get next to nothing as the carrier is suppressed at least 30...40dB relativ to the carrier voltage injected in pin 7. With AF applied to the MIC input there has to be a DSB output signal (riding on a DC voltage) at pin 3.

check pin 5 if there is any modulation (AF) signal if you TX and talk to the mic
 

Offline RFDx

  • Regular Contributor
  • *
  • Posts: 155
  • Country: de
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #21 on: April 05, 2024, 04:19:10 pm »
The input to the modulator at pin 7 is 9.013 MHz, according to my oscilloscope.  It is Channel 1 (yellow) in the photographs.  It seems to be stronger on USB than it is on LSB.

9.013MHz is OK. The IF is at 9.0115MHz so carrier frequencies should be 9.013MHz (USB) and 9.010MHz (LSB).

1Vpp carrier level in USB? A bit much imo but we are not going to mess with it. The datasheet of the uPC1037HA doesn't mention the max. allowed level but in the datasheet for the SL1640C (same chip from Plessey) the limit is 200mVrms.

The output from the modulator at pin 3 is Channel 2 (blue).  It does not change visibly when in transmit, either with or without modulation.  (There seems to be no DC there either.)

No DC at pin 3 is a problem. Measure the DC voltage at pin 2. The DC voltage at pin 3 has to be ~0.7V below the voltage at Pin 2. 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.

Is R181 (2.2k from pin 3 to ground) OK?

Pin 2 (IC6) is also an (unused) output that is connected directly to the internal mixer. Make the same AC measurements (TX, with and without audio modulation) as for pin 3.

VCC to the chip is 7 VDC.  Pin 5 does seem to be getting audio, riding atop 3 Volts DC.

Good to know. I forgot to ask if audio is present at pin 5 during TX.
 

Offline TMA-1Topic starter

  • Contributor
  • Posts: 26
  • Country: ca
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #22 on: April 05, 2024, 04:22:40 pm »
Pin 5 does seem to be getting audio, riding atop 3 Volts DC.

The audio does appear to come through to the input pin of the chip, but the output is AWOL.  No DC component, and the modest carrier that is there during receive, drops in amplitude when transmit is engaged.  See the video here:  https://youtu.be/SYR7KdMpxW4

I'd like to find an alternate explanation, but everything still points to the replaced uPC1037.


(RFDx replied while I was typing.)  Yes, R181 is fine at 2.21K.  I shall remove the board again to connect the other test points you mention.
« Last Edit: April 05, 2024, 04:35:43 pm by TMA-1 »
 

Offline Hamelec

  • Regular Contributor
  • *
  • Posts: 156
  • Country: de
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #23 on: April 05, 2024, 04:32:12 pm »
any DC voltage on pin 6  ?
 
The following users thanked this post: TMA-1

Offline TMA-1Topic starter

  • Contributor
  • Posts: 26
  • Country: ca
Re: Icom IC-725 HF Radio Goes Into Tx But Emits No RF
« Reply #24 on: April 05, 2024, 04:41:12 pm »
Good idea;  I'll add that one also.
 

Offline TMA-1Topic starter

  • Contributor
  • Posts: 26
  • Country: ca
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

  • Regular Contributor
  • *
  • Posts: 155
  • Country: de
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.
 
The following users thanked this post: TMA-1

Offline TMA-1Topic starter

  • Contributor
  • Posts: 26
  • Country: ca
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

  • Regular Contributor
  • *
  • Posts: 155
  • Country: de
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

  • Contributor
  • Posts: 26
  • Country: ca
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

  • Regular Contributor
  • *
  • Posts: 155
  • Country: de
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

  • Newbie
  • Posts: 7
  • Country: cn
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.

  • Frequent Contributor
  • **
  • Posts: 893
  • Country: it
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 ?
 

Online vk6zgo

  • Super Contributor
  • ***
  • Posts: 7635
  • Country: au
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

  • Contributor
  • Posts: 26
  • Country: ca
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

  • Contributor
  • Posts: 26
  • Country: ca
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!
 
The following users thanked this post: xrunner, Hamelec, pienari


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf