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

[TMA-1]

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?

[coromonadalix]

Took me 5 secs to find services manual   

https://ia601301.us.archive.org/8/items/icom_IC725/IC725_text.pdf

https://manualmachine.com/icom/ic725/7620625-service-manual/


if you dont have experience with ham equipment, find reparators on qsl.net and other ham  websites


https://ham.stackexchange.com/questions/18165/troubleshoot-icom-ic-745-intermittently-shows-no-power-on-transmit

https://www.worldwidedx.com/threads/no-recieve-or-transmit-not-good.125570/

[TMA-1]

Uhm, sure.  Thanks for the additional copies.  (?)

I guess I'll just go away now.

[p.larner]

get your scope out and start checking drive levels on the lpf rf board etc.

[xrunner]

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. 

[TMA-1]

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.

[xrunner]

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.

[RFDx]

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.).

[TMA-1]

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,...

[TMA-1]

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?

[Hamelec]

which frequency you tuned? 13.764 MHz like in your picture?
..TX not working out of Band

[TMA-1]

No, that was just a "scan" at the time.  Sorry.

[TheSteve]

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!

[RFDx]

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.

[TMA-1]

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.

[TMA-1]

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",...}

[TheSteve]

Hopefully the part you get will be real and not some fake/remarked crap.

[TMA-1]

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.

[RFDx]

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.

[TMA-1]

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?

[Hamelec]

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

[RFDx]

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.

[TMA-1]

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.

[Hamelec]

any DC voltage on pin 6  ?

[TMA-1]

Good idea;  I'll add that one also.