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No RF out but signal heard on nearby receiver

 

This is similar to other no-output posts but maybe different enough for a new post. ?The symptoms are no RF out on a watt meter. ?But, if modulated by voice or a 1000hz tone, it can be heard on a nearby receiver. ?In addition, Bitx receiver ?operation was unaffected. ?The problem turned out to be Q13. ?Previous posts have cautioned about exposing Q13 to strong signals or strong RF from a nearby transmitter. ?The latter may have been the culprit in this case.?
More info:
On a scope, p-p voltage at the gate of Q15 with a 1000hz tone: 40mv with no RF out and 200mv after Q13 replacement
Q13 is a BC849 and Mouser has them for 0.139 each (sku 833-BC849B-TP)
73,
Pat AA4PG
 

The big kids usually handle these but this time I am going to take a stab at it. This sounds like the final PA is not receiving the supplied DC power. Or the PA is blown, and needs replacing. Without the PA working at all, you can still hear a signal on a "nearby" receiver being produced by the driver transistor loud enough to hear. At least this was my experience.?

Clip the leads on the PA instead of desoldering, and solder you new thinned and shortened PA leads to the backs of the original leads still on the board. Because you are going to do this more than once. ?Let's see if the experts agree.
 

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Actually, that was my first thought and I did exactly as you have described. That had no effect. I didn't really know how much signal to expect at Q13 and Q14 but with the previous reports Q13 seemed more likely. That turned out to be the case
73
Pat AA4PG

Sent from my iPhone

On Aug 9, 2017, at 9:09 PM, John Smith via Groups.Io <johnlinux77@...> wrote:

The big kids usually handle these but this time I am going to take a stab at it. This sounds like the final PA is not receiving the supplied DC power. Or the PA is blown, and needs replacing. Without the PA working at all, you can still hear a signal on a "nearby" receiver being produced by the driver transistor loud enough to hear. At least this was my experience.?

Clip the leads on the PA instead of desoldering, and solder you new thinned and shortened PA leads to the backs of the original leads still on the board. Because you are going to do this more than once. ?Let's see if the experts agree.

 

You say: ?"The problem turned out to be Q13."

So you replaced Q13 and now all is well?
Or are you asking for help?

If the IRF510 is still pulling around 100ma of idle current when transmittting, then the IRF510 is probably not blown.

You say: "Q13 is a BC849 and Mouser has them for 0.139 each (sku 833-BC849B-TP)"
Q13 is an MMBT3904. ?The schematic says BC849 cuz that's the symbol Farhan happened to have handy.
The 250 mW max dissipation of the BC849 might work fine in most spots on the Bitx40, but at Q13
the surface MMBT3904 is actually exceeding its 350 mW max dissipation spec, and running quite hot.
If I were replacing Q13 I would use a leaded 2n3904 in the standard TO92 package, with 625 mW of max dissipation.?

Here's a good place to start for those debugging the transmitter: ?/g/BITX20/message/24225

Jerry, KE7ER ? ?(not particularly big, but getting kind of old)


On Wed, Aug 9, 2017 at 07:09 pm, John Smith wrote:
The big kids usually handle these but this time I am going to take a stab at it. This sounds like the final PA is not receiving the supplied DC power. Or the PA is blown, and needs replacing. Without the PA working at all, you can still hear a signal on a "nearby" receiver being produced by the driver transistor loud enough to hear. At least this was my experience.?

Clip the leads on the PA instead of desoldering, and solder you new thinned and shortened PA leads to the backs of the original leads still on the board. Because you are going to do this more than once. ?Let's see if the experts agree.
 

Pat,

The Q13 id actually MMBT3904, the schematic shows a different number for some reason.
Farhan has explained long ago.

Raj

At 10/08/2017, you wrote:

Actually, that was my first thought and I did exactly as you have described. That had no effect. I didn't really know how much signal to expect at Q13 and Q14 but with the previous reports Q13 seemed more likely. That turned out to be the case
73
Pat AA4PG

Sent from my iPhone

On Aug 9, 2017, at 9:09 PM, John Smith via Groups.Io < johnlinux77@...> wrote:

The big kids usually handle these but this time I am going to take a stab at it. This sounds like the final PA is not receiving the supplied DC power. Or the PA is blown, and needs replacing. Without the PA working at all, you can still hear a signal on a "nearby" receiver being produced by the driver transistor loud enough to hear. At least this was my experience.

Clip the leads on the PA instead of desoldering, and solder you new thinned and shortened PA leads to the backs of the original leads still on the board. Because you are going to do this more than once.? Let's see if the experts agree.

 

The gate drive is too low. I remember it being much much higher.

I suspect one of the trifilar toroids has a short and this will show up if you move them. I personally saw this issue
in a defective board Farhan gave me. I just moved the leads till the short went away and blobbed some hot glue to
keep the wires apart.

Also see that your audio drive is at least 150mv p-p or more.

Raj

On a scope, p-p voltage at the gate of Q15 with a 1000hz tone: 40mv with no RF out and 200mv after Q13 replacement
Q13 is a BC849 and Mouser has them for 0.139 each (sku 833-BC849B-TP)
73,
Pat AA4PG
 

I see now, it's a new symptom description of an overloaded, or something, Q13. Right? I first read it as a question. Well... good on ya then.?

But once I did hear with a receiver while the final had no power. And was strong too.

I am starting to forget bits like this. I am just an amateur after all.
 

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Yes,

Replacing Q13 did fix?the problem. ?But, many thanks?for the correct Q13 type. ?I simply used what was on the schematic. ?Another great thing about this list!

73,

Pat AA4PG


Pat Griffin
http://www.cahabatechnology.com

From: [email protected] <[email protected]> on behalf of Jerry Gaffke via Groups.Io <jgaffke@...>
Sent: Wednesday, August 9, 2017 8:54:47 PM
To: [email protected]
Subject: Re: [BITX20] No RF out but signal heard on nearby receiver
?
You say: ?"The problem turned out to be Q13."

So you replaced Q13 and now all is well?
Or are you asking for help?

If the IRF510 is still pulling around 100ma of idle current when transmittting, then the IRF510 is probably not blown.

You say: "Q13 is a BC849 and Mouser has them for 0.139 each (sku 833-BC849B-TP)"
Q13 is an MMBT3904. ?The schematic says BC849 cuz that's the symbol Farhan happened to have handy.
The 250 mW max dissipation of the BC849 might work fine in most spots on the Bitx40, but at Q13
the surface MMBT3904 is actually exceeding its 350 mW max dissipation spec, and running quite hot.
If I were replacing Q13 I would use a leaded 2n3904 in the standard TO92 package, with 625 mW of max dissipation.?

Here's a good place to start for those debugging the transmitter: ?/g/BITX20/message/24225

Jerry, KE7ER ? ?(not particularly big, but getting kind of old)


On Wed, Aug 9, 2017 at 07:09 pm, John Smith wrote:
The big kids usually handle these but this time I am going to take a stab at it. This sounds like the final PA is not receiving the supplied DC power. Or the PA is blown, and needs replacing. Without the PA working at all, you can still hear a signal on a "nearby" receiver being produced by the driver transistor loud enough to hear. At least this was my experience.?

Clip the leads on the PA instead of desoldering, and solder you new thinned and shortened PA leads to the backs of the original leads still on the board. Because you are going to do this more than once. ?Let's see if the experts agree.

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