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I need help please, with the great kit (QCX 40) and super instructions I built the kit and everything worked the first time. But then I got nosey and was checking to see why my BS 170’s we’re running hot ....?

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I finished assembly and setup, everything seemed to be working. I was receiving and appeared to be transmitting, ( I could receive on my FT-450D). The bs 170’s were running quite hot but otherwise no problems. I tried to make a measurement on the PA and don’t know what happened, it sounded like a short then the screen went blank and a squeal in the headphones, I couldn’t transmit. (Not even a side tone). The final 170’s were now cold, must have fried them.

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-The LED has no data display but R-47 does run 0 to 4.9 V I do get two rows of? ?

?white boxes.

? -With a 13.8 Vdc power supply I read 14.1v,

-The 7580 reads 4.9v out.

?-IC3 pin 3 reads 13.7 v ??

-IC3 pin 1 & 2 read 0

-I have a squeal in the headphones after a short warm up, the processor is slightly warm and if I blow gently on it the squeal stops ?? (I know, it sounds silly)

-IC3 pins 1,2,6,8,12,and 13 all read 0, should be 5v

-the 20 MHz crystal is working, I couldn’t hear the 27 MHz.

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There’s a lot of other measurements that aren’t right as well. I must have really screwed things up.

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If anyone can make heads or tails of this I would appreciate any help you can give me.

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Jack

The bs 170’s were running quite hot but otherwise no problems. I tried to make a measurement on the PA and don’t know what happened, it sounded like a short then the screen went blank and a squeal in the headphones

-The LED has no data display but R-47 does run 0 to 4.9 V I do get two rows of white boxes.
?-IC3 pin 3 reads 13.7 v ??
-IC3 pin 1 & 2 read 0
-IC3 pins 1,2,6,8,12,and 13 all read 0, should be 5v

Jack,

This does seem serious.
You seem to have the PA problem others have reported, often by going into TX with no load.
Remove Q1, 2, 3, 6. Also I think Q3 must be removed.
Then apply power and see if the display shows anything.
You could do the test after Q1, 2, 3, 6 are removed but it seems that is also faulty.


If no display then maybe the Si5351 has gone as well..

73 Alan G4ZFQ

Hi Jack,

It looks like the BS170s failed with at least one having a drain to gate short which would explain +13.7V on pin 3 of IC3.? My approach would be to replace IC3 and the BS170s as a starting point.? I am concerned that IC3 failed in such a way that a high voltage was applied to pin 6 of IC1 (SI5351A).? A damaged IC1 would explain the lack of a 27 MHz signal.? Removing and replacing IC1 is extremely difficult without the proper equipment.? I was working on a friend's QCX and I needed IC1 replaced.? I took it to a local electronics contract manufacturer and I was quoted $50 to replace it.? That, plus the cost of the SI5351A, was more than the QCX cost.? We repaired his QCX by building a new one!

Keep in mind that integrated circuits almost always have a parasitic diode between each pin and Vdd (and Vss for that matter).? Those diodes are reversed biased in normal operation.? That means the +13.7V applied to pin 3 of IC3 appeared on pin 14 (minus a diode forward drop), at least until it opened up due to excessive current.? All of the semiconductors powered from +5V would have seen it, including IC1, so there may be more damage.? There's no way to be sure until IC3 and the BS170s are replaced.

Sorry about my cheery note.? Good luck with it!
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73,
Jim W7EED

My approach would be to replace IC3 and the BS170s as a starting point.

Jim, Jack,

I'd do the first tests with them removed. No need for those components to be fitted to see if the main chip works.

IC3 failed in such a way that a high voltage was applied to pin 6 of IC1 (SI5351A).? A damaged IC1 would explain the lack of a 27 MHz signal. Removing and replacing IC1 is extremely difficult

Not too difficult, I'd do it with a standard soldering iron and some solder wick.
It's thinking about it that makes it difficult!

Keep in mind that integrated circuits almost always have a parasitic diode between each pin and Vdd (and Vss for that matter).? Those diodes are reversed biased in normal operation.? That means the +13.7V applied to pin 3 of IC3 appeared on pin 14 (minus a diode forward drop), at least until it opened up due to excessive current.? All of the semiconductors powered from +5V would have seen it, including IC1,

Possibly, a similar failure recently seems to have hit the Si5351 but not IC2, but it might not be easy to discover exactly what has been damaged.
I'm thinking that isolating IC3 pin 3 with a capacitor would prevent this possible 12V feedback. (And a resistor to ground at the PA gates.)

73 Alan G4ZFQ

Alan ,

I agree with your solution to isolate the IC3 pin3 .

But as time is important at this point I don't believe in a RC-combination . Rather than this I would isolate with a Schottky diode and of course a resistor from gates to ground .

Willy ON5KN



Op 8/08/2018 om 20:53 schreef Alan G4ZFQ:

Hi all,

I agree that a lot of the circuitry can be checked without installing new BS170s and the 74ACT00 and that's a good idea.? I would install a good quality socket for the 74ACT00 rather than soldering it directly to the board.? I prefer machined contact sockets like the DigiKey ED3014-ND at just 72? each.? If it has to be replaced again, the pads almost certainly won't be able to take it and will lift.

Concerning the comment, "Not too difficult, I'd do it with a standard soldering iron and some solder wick.? It's thinking about it that makes it difficult!"? I would tend to agree only if the SI5351A were not surrounded by much taller components.? The proper way to remove it is with a hot air gun with a dam around the SI5351A to keep the adjacent parts cool.? A vacuum pick is used to lift the SI5351A.? Although, I suppose, if it needs to be replaced you have nothing to lose so give it a go.? When you replace it, use a lot of flux.? I use an MG Chemicals (p/n 835-P) rosin flux pen and Sn63/Pb37 solder.? It cleans up nicely with 99% isopropyl alcohol.? OK, I know it's not RoHS compliant!? :o))

Good luck with it ...
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73
Jim W7EED

I removed the BS170’s and the lC3 and powered up but didn’t get any data in the display. It looks like there’s more problems. Looking at IC1 it looks like more than my ham fist should handle so I think ordering a new QCX would be the best way for me to go. .......... sorry this posted by accident I hadn’t finished. Thanks Alan,Jim & Willy for you expert help, I will be adding the PA protection in the next one.

Jack K9VLX