John,
I heated my QMX up doing FT8 until it dropped power (radio out of the case). Tested in situ and no change in the drive voltage, but a drop in Tx ma. From what I understand since no drive change then all before the finals hasn't changed and the issue must be beyond that. Went over the board again and touched up couple solder points, but no change. Replaced all the BS170s to get a matched pair. No change in power. FWIW all bands are down, not just one band. Today, once it dropped in power it hasn't gone back up so I'll see over night if something cools down and the power comes back up.?
The output transformer did not get hot today. Just comfortable warm. Any ideas how to test the transformer without pulling it? Since it's all bands I don't see any common component except the transformer T501 except the SWR transformer, T507.
Details:
Initial: After hot: Power ~4.0 watts 2.3 W Tx Current 785ma 700ma Drive V on both sets of finals 2.510v 2.510v
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John,
Like you say, not likely, but "maybe".
I just ordered a can of freeze spray for testing.
I'll try to heat the QMX up by usage and when the power drops try to check the drive to the finals. That should be a big clue. If the drive is the same then I'll start thinking about redoing the transformer. If the drive has dropped then the issue is back the other way.
Cliff,Yeah, that is a puzzle. Here is a (highly speculative) possible explanation:- Initial power drop is caused by temperature rise due to basictransistor parameters as per linked post, exacerbated by weak heatsinking.- A secondary sharp power drop comes from a nicked-wire short kickingin as the transformer heats up along with the board and things expand.That is sort of the Challenger O-Ring scenario, I know, but not implausible.JZ KJ4AOn Sat, Sep 30, 2023 at 3:38?PM Cliff <ae5zaham@...> wrote:
John,
I see what you mean, but I still wonder if there is something else lurking around also.
Initially the drop is almost unnoticeable then it drops over time about 1/2 watt. I could live with that, but at some point it all of a sudden drops another watt to watt and a half. Now I'm down to around 2 watts or a bit less after I started out at 3.5W and this is at 10v trying to be easy on the finals. Seems like something gives up which is what got me looking at the drive to the finals. I'll have to set things up so I can get it hot enough to cause the drop then test the drive on the finals.
What triggers the big drop? Cool it off and it's back up to 3.5W.
73,
Cliff, AE5ZA
On Sep 30, 2023, at 14:16, John Z <jdzbrozek@...> wrote:
Cliff,
I think this may explain your sagging output power.
/g/QRPLabs/message/111164
I'm concerned that a shorted turn in your output transformer may have
set you up for this. The transformer should not be running hot.
JZ KJ4A
On Sat, Sep 30, 2023 at 3:07?PM Cliff <ae5zaham@...> wrote:
I think you're right Paul.
Now back to the original issue of significant power drop after the unit has been used for a while.
73,
Cliff, AE5ZA
On Sep 30, 2023, at 13:59, Paul - AI7JR <paul.hanchett@...> wrote:
If you've got that 2.5 volt measurement on the gates, I'd think the driver IC is probably OK.
On 9/30/23 10:24, Cliff wrote:
Just pulled the finals planning to change them all, but decided to test the voltage on the gates with them out. The voltages are different, slightly and I noticed that one side the drive voltage increased slightly when the finals were pulled and the other side not. 2.523 vs 2.546 with the finals out.
Am I making a big deal out of nothing here? Only 0.023v different. I don't remember looking that closely before so maybe they always have been that way, but my recollection is that they were much closer.
I hate to change the driver IC for nothing, but if it'll make a difference, I have the chip to do it.
BTW, with the board cool power out is back up to normal.
73,
Cliff, AE5ZA
On Sep 29, 2023, at 20:07, Cliff <ae5zaham@...> wrote:
Ryuji,
I probably will change all the finals, but just wondered if there was some testing that could be done to help an on going problem. That idea of the foam might help also.
I was using 10V on a 12V build to see if I could get more life out of the BS170s. Transformer is the original design. Unit was build nearly 5 months ago now. Worked great for a long time until I stressed it with no antenna attached and transmitting several times. Driver IC and finals have been changed multiple times.
73,
Cliff, AE5ZA
On Sep 29, 2023, at 20:01, Ryuji Suzuki AB1WX <ab1wx@...> wrote:
I would replace all final transistors. If you routinely experience very hot finals, you might want to consider inserting a bit of thermally conductive foam between the board and the aluminum case. Good ones are about 15 W/mK if I believe the catalog spec. Those good ones are loaded with heavy minerals and it's quite heavy and cool to touch.
Which transformer configuration did you use? What's your supply voltage?
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