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QCX40 stopped working: Replace 74ACT00 with 74HCT00?

 

Thanks Alan,

I think your theory is right, I need to replace IC3. I note that it appears easier to obtain a 74HCT00 rather than a 74ACT00 as originally supplied, do you or anyone else have any thoughts on whether the HCT version should work?

?

73 Carl
 

Hi Carl

A 74HCT00 is fine as far as the input voltage thresholds are concerned (it will convert the 3.3V output of the Si5351A nicely to 5V). But the 74ACT00 is a faster device and will do better particularly on the higher bands. How much of a difference there is, I couldn't say.

But I note that Arrow has the 74ACT00 at $0.20?and with free international shipping, with no minimum order. At the moment.

73 Hans G0UPL?
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On Thu, Jul 26, 2018, 21:12 <m0icr@...> wrote:

Thanks Alan,

I think your theory is right, I need to replace IC3. I note that it appears easier to obtain a 74HCT00 rather than a 74ACT00 as originally supplied, do you or anyone else have any thoughts on whether the HCT version should work?

?

73 Carl
 

I think your theory is right, I need to replace IC3. I note that it appears easier to obtain a 74HCT00 rather than a 74ACT00 as originally supplied, do you or anyone else have any thoughts on whether the HCT version should work?
Carl

Remove IC3, check the 5V line and regulator is OK.
Then you will possibly have to replace all items I mentioned.
I do not know for sure without looking it up but maybe the ACT is faster, probably make no difference for use on 40m.

73 Alan G4ZFQ
 

Alan,

Many thanks.

Progress report:

I have removed IC3 and checked the 5V line which reads correctly at 5V off the regulator. I will need to get a replacement for IC3 but I do have a small stock of BS170 to replace the 3 x PA transistors and Q5.?

With IC3 removed the LCD display is now displaying correctly and the buttons and encoder are also working correctly ... but the regulator continues to get too hot to touch after just 10 seconds (despite have a small heatsink fitted). I am hoping that there isn't an issue with IC4 but for now the next activity will be to replace the potentially damaged BS170 to see there is a short somewhere causing the overheating of the 7805.

73 Carl M0ICR

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Hi Hans,

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Thanks I have just ordered a few of the correct devices and a bunch of other useful components for the parts bin. Excellent prices given the free international postage.

Now to try and work out what is causing the 7805 regulator to get so very hot, next job to replace Q1-3 and Q5?

73 de Carl, M0ICR

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¿ªÔÆÌåÓý

Carl:
Should be OK. Some variance with manufacturer Texas Instruments slightly faster than On Semiconductor.
HCT fine to about 30 MHz, ACT to 50 MHz.
73
Fred
WD9HNU


On 7/26/2018 2:12 PM, m0icr@... wrote:

Thanks Alan,

I think your theory is right, I need to replace IC3. I note that it appears easier to obtain a 74HCT00 rather than a 74ACT00 as originally supplied, do you or anyone else have any thoughts on whether the HCT version should work?

?

73 Carl
 

So ... I have removed IC3, Q1, Q2, Q3, Q5 and Q6 and the 7805 regulator is still getting too hot to touch.

Any thoughts please?

73 Carl M0ICR
 

Do you have access to a thermal imaging camera ?

If you cant borrow one, or justify ?100+ to buy, build your own

On 26/07/18 22:12, m0icr@... wrote:
So ... I have removed IC3, Q1, Q2, Q3, Q5 and Q6 and the 7805 regulator is still getting too hot to touch.

Any thoughts please?

73 Carl M0ICR
Chris Smith
 

I would thoroughly visually inspect all parts and solder joints top and bottom if this has transitioned from working to non working.

Worth checking the input current to the 7805 and output voltage of the 7805 as well. That will define the load characteristic and outline some clues as to what is happening on the output side of the regulator or the regulator itself.

Also run your finger around the board and see if you can find any warm bits other than the 7805.

Chris
M0XTE


On Thu, 26 Jul 2018, at 22:12, m0icr@... wrote:

So ... I have removed IC3, Q1, Q2, Q3, Q5 and Q6 and the 7805 regulator is still getting too hot to touch.

Any thoughts please?

73 Carl M0ICR
 

A quick check of other components running very very hot include the Si5351 (oh dear!) and the D1 and D2.

73 Carl M0ICR

?
 

A quick check of other components running very very hot include the Si5351 (oh dear!) and the D1 and D2.
Carl,

Continuing with guesswork it seems that when Q6 failed it sent 12V through one of the BS170s (that had also shorted) to IC3. This then seems to have somehow sent the 12V through to the the Si5351. Maybe it again shorted internally and sent 12V back through the Clock 2 output.

As IC2 is still working this MAY be all. The Si5351 is not difficult to replace but with the other components around it not so easy if you have never done any SM work.

If I'm right - sorry.

73 Alan G4ZFQ
Chris Smith
 

That sounds like the Si5351 is dead or heavily loaded. It's worth checking what it is connected to first before replacement. If it is drawing lots of current that might be because one of the IO / output pins is being pulled down or is shorted to ground or Vdd. Have a look at the schematic and trace all lines off and look for shorts etc.

New ones aren't terribly expensive or difficult to replace despite looking evil however. RS sell 5 off for ?5 approx with next day delivery. To get it off, just flood both sides of the IC with solder alternately and lift off with tweezers, then clean up the pads with some chemwick braid. Soldering a new one on, I would refer to many of the tutorials on YouTube.

I've got to say one thing though and that is that I'm not actually that happy with the dual diode dropper implementation on the Si5351A power supply. The part is rated to max 3.6V and I measured the loaded voltage at 3.8V. I consequentially replaced the two diodes with an MCP1700-3302 low dropout regulator (bodged in) and changed C2 for a 1uF ceramic 0805 cap when I was constructing it. That keeps it solidly at 3.3V.?

Thing here is that the MCP1700 will keep the max current into the Si5351A at a manageable 200mA if something goes wrong. 7805 will throw an entire amp and a half into it.

Chris
M0XTE



On Fri, 27 Jul 2018, at 00:45, m0icr@... wrote:

A quick check of other components running very very hot include the Si5351 (oh dear!) and the D1 and D2.

73 Carl M0ICR

?
Clint Sharp
 

The Si5351 is apparently a little fragile, they die if you overload the outputs apparently...



On 27 July 2018 at 09:59, Chris Smith <me@...> wrote:
That sounds like the Si5351 is dead or heavily loaded. It's worth checking what it is connected to first before replacement. If it is drawing lots of current that might be because one of the IO / output pins is being pulled down or is shorted to ground or Vdd. Have a look at the schematic and trace all lines off and look for shorts etc.

New ones aren't terribly expensive or difficult to replace despite looking evil however. RS sell 5 off for ?5 approx with next day delivery. To get it off, just flood both sides of the IC with solder alternately and lift off with tweezers, then clean up the pads with some chemwick braid. Soldering a new one on, I would refer to many of the tutorials on YouTube.

I've got to say one thing though and that is that I'm not actually that happy with the dual diode dropper implementation on the Si5351A power supply. The part is rated to max 3.6V and I measured the loaded voltage at 3.8V. I consequentially replaced the two diodes with an MCP1700-3302 low dropout regulator (bodged in) and changed C2 for a 1uF ceramic 0805 cap when I was constructing it. That keeps it solidly at 3.3V.?

Thing here is that the MCP1700 will keep the max current into the Si5351A at a manageable 200mA if something goes wrong. 7805 will throw an entire amp and a half into it.

Chris
M0XTE



On Fri, 27 Jul 2018, at 00:45, m0icr@... wrote:

A quick check of other components running very very hot include the Si5351 (oh dear!) and the D1 and D2.

73 Carl M0ICR

?







--
Clint. M0UAW IO83

No trees were harmed in the sending of this mail. However, a large number of electrons were greatly inconvenienced.
 

Thank you to everyone for your useful tips and advice. After further testing I am now sure that the Si5351A is kaput, I am also sure that removal and replacement is beyond both my tools and ability. I guess I will have to put it down to experience and use the QCX for spare parts with an eye to purchasing a new QCX40 as 'radio funds' permit.

?

Thanks again for all the help.

73 Carl M0ICR
Arv Evans
 

Carl M0ICR

Replacing the Si5351a is not all that difficult.?
I would use a Dremel type tool to cut the legs from the original chip.? Then use a soldering
iron and wooden toothpicks to remove the leg parts that remain on the PCB.? Clean up the
pads and solder the new Si5351 in place.? Do not use an excessive amount of solder.? You
need just enough to wet the pads and pins when heated.? It is okay if your soldering iron
bridges a couple of pins.? Using a minimum amount of solder keeps it from bridging solder
between pins.?

Arv? K7HKL
_._


On Fri, Jul 27, 2018 at 8:01 AM <m0icr@...> wrote:

Thank you to everyone for your useful tips and advice. After further testing I am now sure that the Si5351A is kaput, I am also sure that removal and replacement is beyond both my tools and ability. I guess I will have to put it down to experience and use the QCX for spare parts with an eye to purchasing a new QCX40 as 'radio funds' permit.

?

Thanks again for all the help.

73 Carl M0ICR
Chris Smith
 

Not only that, if you screw it up you have nothing to lose if the unit doesn't work now anyway :)

I will detail my MCP1700 regulator mod when I get some time next week.

Chris
M0XTE


On Fri, 27 Jul 2018, at 15:59, Arv Evans wrote:

Carl M0ICR

Replacing the Si5351a is not all that difficult.?
I would use a Dremel type tool to cut the legs from the original chip.? Then use a soldering
iron and wooden toothpicks to remove the leg parts that remain on the PCB.? Clean up the
pads and solder the new Si5351 in place.? Do not use an excessive amount of solder.? You
need just enough to wet the pads and pins when heated.? It is okay if your soldering iron
bridges a couple of pins.? Using a minimum amount of solder keeps it from bridging solder
between pins.?
Arv? K7HKL
_._

On Fri, Jul 27, 2018 at 8:01 AM <m0icr@...> wrote:

Thank you to everyone for your useful tips and advice. After further testing I am now sure that the Si5351A is kaput, I am also sure that removal and replacement is beyond both my tools and ability. I guess I will have to put it down to experience and use the QCX for spare parts with an eye to purchasing a new QCX40 as 'radio funds' permit.

?

Thanks again for all the help.

73 Carl M0ICR
 

¿ªÔÆÌåÓý

Carl,

If replacing the Si5351 is going to solve your problem, as you're only about 25 miles away, either ship it (with a new Si5351A) or bring it over and we'll replace the chip.

Please contact me off list if you want to do this

Regards

Dave

G3WUN


On 27/07/2018 15:00, m0icr@... wrote:

Thank you to everyone for your useful tips and advice. After further testing I am now sure that the Si5351A is kaput, I am also sure that removal and replacement is beyond both my tools and ability. I guess I will have to put it down to experience and use the QCX for spare parts with an eye to purchasing a new QCX40 as 'radio funds' permit.

?

Thanks again for all the help.

73 Carl M0ICR
 

Good evening all,

I just wanted to write a quick note and thank all those who have offered, both in-group and via personal email, to help me get my QCX40 back on the road. Someone has kindly offered to have a look it for me and I shall be sending it up to him by Royal Mail tomorrow.

Thanks again to all those who have offered to help and offered constructive advice, it shows a true amateur radio spirit of shared endeavour and kindness.?I hope to be able to report success on this forum soon. Many good deeds done and I am very grateful.

73, de Carl M0ICR

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