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sBitx #sBitx PA MOD Plan
#sBitx
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I am waiting to hear from Ashhar that he has the hot switching diode issue solved.
I assume that there may be a kit of parts and a board made available for those of us with a junk box + 30 years old or who operate in a parts desert,? Some visually challenged my need to purchase a finished new mainboard. My sBitx sits on the shelf until changes are confirmed. J TI4JWC |
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The T/R switching has survived 15 days of FT8 and an ssb contest. The t/r add on board is being assembled and tested. They will be posted this week. I will post the PA mods in the next 24 hours. On Sun, Nov 6, 2022, 8:17 PM John via <n0ure=[email protected]> wrote: I am waiting to hear from Ashhar that he has the hot switching diode issue solved. |
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The TR as I've built (similar) it is running well in another radio at 50W
and also in the SBITX. I'd expect few issues with it. The layout and modification for the TX as WP3DN need a fpit ure or two, schematic for that section and a few words for those that are not following closely. FYI the diode heating of the LPF switching, check the schematic used? as you may have a error there.? I believe but unconfirmed the diodes that are heating are the active (selected ones). But since there are two (input and output side) which one or both. Allison ------------------ Please use the forum, offline and private will go to bit bucket. |
开云体育Yours is on the shelf.? So is mine Gerry Sherman Sent by the Linux Thunderbird On 2022-11-06 14:47, John via groups.io
wrote:
I am waiting to hear from Ashhar that he has the hot switching diode issue solved. |
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Allison , Hans cracked it for me. They were 1n4001s relabelled as 1n4007s. Now, the LPFs run very cool. On Sun, Nov 6, 2022, 8:54 PM Gerald Sherman <ve4gks@...> wrote:
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I built a similar diode switched LPF for testing and?
using FT817 and 100W amp at 60W using WSJTx ft8 as test signal at 13.99Mhz Into the dummy load results after 3 days 72 hrs sustained the diodes get warm.? Warm is less than 40C measured using IR thermometer. For 80M they get hotter 50C as as that's getting near the lower limit for them.? Still an acceptable temp.? 160M, forget that.? Diode switching below 2MHZ requires exotic stuff as recombination time has to be very long. Measured loss suggests about .5 db per pair depends on band but its hair splitting. The SBITX still has its relays for band selection with wetting current added (8ma) and they have not caused a moments grief but the only time the are active is band change.? I'm mixed as to the need? change. Allison ------------------ Please use the forum, offline and private will go to bit bucket. |
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1n4001 though 1n4005 will not work, they have a pin structure but
thinner! Only the 800 and 1000PIV parts have that thicker PIN structure. That great news.? Fire the supplier or at least let them know those were frauds! You can see from my testing I found little issue.? The temps I got were at measured 60W sustained to a load back off to 40W then cool a fair amount.? But 50C is not too hot. Allison ------------------ Please use the forum, offline and private will go to bit bucket. |
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I'm curious, how do you know they are 1n4001's?
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Scrape the paint off and saw 1n4001? Have you determined they go into zener or avalanche mode due to excessive reverse voltage? Or have you determined they don't have persistent charge carriers like a PIN diode? The spec for the 1n400x series differentiates between the various flavors only? by max reverse voltage.?? It's possible to build a 1n4007 that complies with the spec but doesn't work as a PIN diode. Here's a quote from a guy who says spec compliant 1n4007's that don't work as PIN diodes could be quite common: ? ??/g/BITX20/message/32099 So get your 1n4007's from a reputable source, and stick with one manufacturer. Jerry, KE7ER On Sun, Nov 6, 2022 at 07:41 AM, Ashhar Farhan wrote:
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Jerry,
The prime characteristic is the 1n400x series are controlled recovery diodes and that is to help with parasitic effect for power supplies.? Most common datasheets usually for get Trr for rectifiers but include it for so called fast diodes like 1n4148.? ?My old Moto book (1988) lists Trr at 2us, really slow. The acid test is try to use a 1n4001-7 as a rectifier at greater than 1mhz. Also 1N4001 is a 50V diode so fobbing it off as 1000V part would cause any AC supply at 100V or more to emit smoke.? As the voltage exceeds its breakdown.? IT can be seen non destructively? on a curve tracer that has a voltage source high enough.? The reverse current will be really low (nanoamps) until you hit the reverse avalanche where it goes sharply higher. So if it fails there its not a 1n4007 pin or not. Allison ------------------ Please use the forum, offline and private will go to bit bucket. |
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Trr is easily tested, Cj is usually specified.
What's funny is any diode in glass package may be a "1n4007" but it never behaves as its light sensitive. The complete spec is hard to find but Trr is a real thing. YOu drive some forward current though the diode and then switch it off (very fast fall time) to a reverse voltage,? watch the forward current into a resistive load with a really fast scope.? The current continues to flow in the reverse direction. Why is it important... for a power supply at 60hz or 400Hz and you need say 3000V at milliamps sides that do not switch (stop conducting) predictably have to be loaded with R and C to keep the voltage across them.? Especially with really sluggish diodes for higher currents. Look at any decently engineered supply for any of the big tube amps.? Allison ------------------ Please use the forum, offline and private will go to bit bucket. |
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As I was suggesting, it probably meets spec as a 1n4007
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but happens to fail as a PIN diode.? However, I can easily believe somebody out there is selling a 50V part as a 1n4007. Jerry On Sun, Nov 6, 2022 at 09:11 AM, ajparent1/kb1gmx wrote: The prime characteristic is the 1n400x series are controlled recovery diodes |
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Jerry,
Therein rests the problem, LIES.? ITs a fraud and some get away with it. That diode would fail in a 300V supply with likely a resounding BANG! Some however the markings are really indecipherable. Its also a significant problem when a part has a known spec does not meet it.? Industry plague. Forensic engineering. Keep in mind the circuit used puts greater than 90V reverse bias on the diode if it avalanches the 470K resistor is a current limit but in the avalanched state is essentially on to some degree and that's bad.? High losses and heat.? If all else was OK the 50V diode never had a chance. Allison ------------------ Please use the forum, offline and private will go to bit bucket. |
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On Nov 6, 2022, at 12:53, ajparent1/kb1gmx <kb1gmx@...> wrote:
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Actually its easier than that.
A basic curve tracer that hits more than 600V will cull low voltage diodes, That and knowing your source. One you have the signature for the real thing its easy to spot a fraud. FYI a 1n4001 works IF the voltages are less that 50V peak.? That would be around 22W.? But it also must be above 5mhz.? To do that when a higher voltage part exists is kinda nuts. A PIN diode for this power class and for lower HF use if you can find them is likely over 20$ each.? For VHF and UHF use there are plenty of parts and cheap but they do not work at all well at HF.? I use them at 6M and up? and the usual parts available are a 1$ each for the .25W and less range and up to maybe 10W under 3$.? Parts for 300W, well a good coaxial relay was more appealing (and not cheap). Allison ------------------ Please use the forum, offline and private will go to bit bucket. |
开云体育Put quite a few megohms in series with a tube transmitter’s power supply and just measure the back voltage across the diode with a few micro-amp or two of reverse currentOn Nov 6, 2022, at 15:28, ajparent1/kb1gmx <kb1gmx@...> wrote:
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