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Crystal off Frequency

IT may not be a bad idea to "track" the LSB and USB votlages BEFORE they get to IF mixing. If the jump occurs and it's DC is steady, then rule out power rails - but if intermittent - that means parts that are loose or bad routing or mounting.

I have dealt with SEVERAL SMD radios that got "epoxied" (sprayed) sealed from the factory only to see soldering "balls" and poor - cold joints in certain areas - again - looking for fresh work. It's a good clue as to what they were looking at before they sent it out.

The "dip" process is not as foolproof as many would think - the parts do fall off, and "cold spots" form when a board is "touched" by humans to re-align it from shifting or a simple flux wipe didn't cover the board well enough due to air pockets.

So I would not put it past them if several balls were laying inside the coils cans or on the main PCB to cause these jumps. OR the SMD caps for trimming were loose or pooly soldered into their "cubby" on the coil form.

Even the Epoxy can migrate causing issues - you've been dealing with it since day one...

Hope this helps!
:+> Andy <+:
 
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I measured the voltage at several points, but everywhere I get similar results Voltage at L26 and L27 Voltage starts about 5v and gradually drops to 0 volts. reminds me of a cap discharging, even used the scope with a 10x probe to make sure the digital VOM wasn't giving false readings (I think)
Is there a better place to measure voltage to verify voltage is not my problem?? I also measured voltage at D53, D54, D55 with similar results.
Any help or suggestions are greatly appreciated
Thanks
Plumber-G
 
Umm, you're not using the metal chassis as the ground connection, are you?

The chassis does not have a DC circuit path to the radio's black power wire. You need to use a circuit-board ground to make a DC measurement. This would be a ground point that's connected to the power cord's black wire.

The chassis is connected to the circuit board's DC ground through a bunch of ceramic-disc capacitors. They make the chassis a ground for the radio's RF signal on transmit and receive.

Only for RF, not for DC current.

The 'voltage fall off' seen on the meter is those capacitors charging up, not discharging.

But it's not a DC circuit. This was done in the past to allow the radio to work in a truck with the positive side of the battery grounded. This was done to reduce corrosion in diesel motors back when radiators were made of brass and bolted to the chassis ground.

Gasoline-powered vehicles all use negative ground AFAIK. The need to make a radio compatible with either type of vehicle ground no longer matters. Plastic truck radiators made the positive-ground setup no longer necessary. Only an antique truck will have positive ground.

A very few newer radios have the chassis connected to the negative power wire, but not this one. The design dates back to the era of positive-ground trucks. The negative lug on the inside of the DC power socket will work, if you can't find a more-convenient spot on the circuit board to use for your DC voltmeter's ground connection.

73
 
Greetings!

Man! Don't get mad at me guys but we've been going round and round over the issues around the Main PCB, we haven't even touched on the issue of bad hardware.

My guess, with all the jumps you're experiencing - there may just be an assembly or cold solder joint issue back at the Mode and Clarifier board.

If not there...

The 2.5kHz shift issue leaves me wondering is part of the Main PCB that routes part of the IF signal voltage control (those steering diodes for routing power to these modes) has failed or is showing bad. You can switch to the other two modes, they're rock steady, the third, USB is not.

That has me wondering if the clarifier voltage, or the mode switch - is faulty - one pole is flaky?

Ok, before another one jumps on me for this, the X4 section - has no varactor, just the coils - so the only things that are left are - steady voltage, leaky switch, faulty components - so since the tuning jumps quite a bit - did we check the C140 150pF disc?

Since nothing mch else can affect the tuning per-se if conventional thinking is presumed them the SHIELD of the coil needs to be removed or the whole coil itself pulled so the core can be examined,...

But what about all that other AM/USB/LSB mode switching further across the board?

Like Q24, that one is supposed to "Knock Out" AM when in SSB modes, there's no FM in it so my guess is somewhere in the "Conversion" area - where FM board would be stuffed with all it's parts in there. But is there an error in assembly, flaky component or soldering problems?

I'd start expanding the search. The biggest clue is the 2.5kHz, so if you needed to shift something - I've seen radios use that with the 10.695(X4) section and start to look there now, because the PLL may see something not right and "Bombs" out at a mixed product that shows up instead. FL3? L23, L24 L25? What's staying on when it should be off?

It looks to me that the AM mode wants to stay up...

Yes, you are checking the D5x series in the region, but you may need to expand the search. If any coils are flaky, they show results by just tapping on them. So if not the X4, or if the X4 - what else goes with USB mode to cause the shift? L24, if ok, try C140 150pF - so the coil if not the problem - shows you can't touch it so I'm thinking open passthru into the coil or even coil internally is loose, or winds are not stable - or ANYTHING dealing with USB , or crosses into the AM line can do this...

I'm just curious if anyones looked further upstream....L26 is the same as L27 - but both their caps for resonance are different...

Again, staring at the tree and we still have a forest fire going on around us...

Just curious...
:+> Andy <+:
 
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Andy
Just to be current both usb and lsb are flaky I will keep at it and report back, Thanks for all the help
Plumber G
 
Both? Woof - ok that changes a lot of things. Almost back to the issue of assembly and those front panel boards with weak solder.

I'll hide - back in my cubby....

:+> Andy <+:
 
Hmmm.

"Crunchy" contacts in the mode selector?

If you hear scratchy noises in the speaker when changing modes, this can produce several kinds of unpredictable behavior. That switch feeds every circuit in the radio, almost. If one dirty contact causes a voltage drop, this can wobble frequencies all over the place.

73
 
I found the problem Junction J14 was not seated properly and causing the erratic voltages, I now can dial in the IF coils LSB is spot on but USB fluctuates about 10Hz.

Now not trying to sound as dumb as I feel, Is there some special way to measure
milliamps ? I have blown a couple of fuses in my multi-meter and am wondering if I am doing something wrong. I have the meter set to 200 ma and expected amperage is 50 ma??
While I am on this subject what is your recommended meter, Might be time to upgrade from my 12 dollar special<grin>

Thanks for the help and advise
Plumber-G
 
Always keep spares, and be thankful that the meter still works afterwards.

No matter how much you can prepare - you never know - you're lucky.

A DVM is your first line of defense,
A pawn in the game of Chess,
Protects you when it comes to the unknown player on the field,
One day it can be that day - the lethal one...
- could be current, could be voltage
- just remember to keep spares!

At least you live to fight the battle for another day...

:+> Andy <+:
 
DVMs have gotten so cheap, we keep one with the leads in the 20-Amp sockets just for setting driver/final bias.

If the meter's "mA" fuse is under 1 Amp, you can blow it just by forgetting to select a sideband mode and keying the radio on AM with the meter hooked up to read current. Got tired of taking DVMs apart to replace the fuse.

The correct reading of "00.06" from the 20-Amp scale isn't terribly precise, but this is not a critical adjustment. If the accuracy of the meter at a reading this low causes an error of 20 percent, the radio will still work and sound just fine.

But brushing one lead against a ground doesn't blow the meter.

Might not be healthy for the radio. Just another good reason to use care.

73
 
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I feel your pain...

I wish the "auto range" in meters allowed 20 A to 2uA ranges, but by the time they "autorange" you might be dead...or at least caught up in the middle of a small fire...

I've been slammed against a wall and tossed across a floor from trying to measure HV Tension leads years ago on CRTs' when they're "blanked" - believe me it's painful - and fuses do keep you alive for another day...

And thankful those days are gone and LCD is more the norm - I won't touch plasma displays from the lessons learned earlier. Even the EL backlites and Fluorescent / Nixie I stay away from - you don't know where they are grounded or shielded these days. I figure if it fires, it's lit, which means it works, so leave it alone...

And if you're inclined and you have a smartphone - use it's built in camera to take a "before" photo incase you need it to show damage and out-of-warranty works - it's cheap insurance when it comes to the You-say, They-say argument after a tedious repair. It can save you and your butt a lot of $$$...

That photo can also provide the clues as to how to "reset" cores, pots and coils back to OEM (thereabouts) to help you. Especially when it comes to using the MACRO setting on the phones camera settings - you can get nice closeups to show details - just touch where you want to focus on the phones touch screen - give it a moment and snap.

Review the area with the photo; zoom in, slide over and across - and if you're lucky, the problems show up and you're set to continue the repair knowing what's in front of you.

:+> Andy <+:
 
NoMadRadio
I think you hit the nail on the head, I bet the radio was in "AM" mode. I couldn't figure out why the fuse would blow, as I had the meter set to 200 MA and the expected measurement was 50ma. I also had checked this before and the meter worked perfectly.

Thanks Guys
 

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