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Which xmit transistor is blown?
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I have had periodic issues with the ubitx that led me to believe that my finals (IRF510) were blown, and the radio seemed to work as expected after replacement, so perhaps they were.
I have a question though...is there a good way to tell what has gone out in the xmit power chain?? I recently replaced Q90 with an mmbt2222a (smt version of the 2n2222a), and then the next two stages with 2n2222a's (in TO-18 form).? No other changes to the radio. It worked fine for a few days, though the 2n2222a's would get hot to the touch in digital mode transmit (which I think is to be expected).? Today I started to experience 'bouncing' power readings during transmit (as shown in the meter on my tuner) before the radio simply stopped putting out much power at all.? Nothing feels hot...and nothing looks burned or heat stressed to the naked eye. So...before I start replacing things, I wanted to ask how to diagnose the problem.? Is it most likely that I cooked some transistors, and how do I best till which ones? 73 Sean |
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Sean,
Its been several days since you posted likely its eben seens and everyone is going "I dunno". ? ?Not enough description of whats going on or not going on. ? ?Not sure of the mods done. ? ?Are you sure the power supply is working ok. Everyone running FT8? without big fans seems to fry the amp and yet people run FT8 and likely at max power possible.? ?While technically it ignoring spurs and harmonics can there is insufficient cooling for sustained long transmit sessions.? HINT: cooling required. FYI hot is how hot?? You get a blister?? You don't what to hold your finger there long?? You feel warmth but its not painful? Many of the devices will heat to the very warm (about 140-150F).? That is not deadly to the devices.? It will make things like bad soldering stand out and maybe other marginal components fail.? Some have noted board level issues like open vias. Allison ? |
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Sean,
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Occasionally I get power drop and fluctuation and I found it was the power connector. 1-2A is a lot for the pin contact.. pull the power plug from the board and put it back a couple of times and see.. Raj At 01/09/2018, you wrote:
I have had periodic issues with the ubitx that led me to believe that my finals (IRF510) were blown, and the radio seemed to work as expected after replacement, so perhaps they were. |
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I have just about permanently taken the top off the case the radio is mounted in, and have a cooling fan connected ... hoping that will be enough going forward.??
how hot are the transistors getting?? well, when I was doing some testing today, I found that simply being in transmit with no mic connected for a few seconds (< 10 seconds) the predriver and driver transistors get noticeably warm.? I have measured all six of them (in both stages) and dont think they are blown (all six measured .6 DCV from B to E).? Typically transmitting FT8 they get hot enough not to blister but to be seriously uncomfortable. I will be making some mods soon to apply proper bias to these 2n2222a's later this week...but with the biasing from the factory, thats what things look like right now. |
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Hi Raj
I'm pretty sure its not the power connector...I saw swinging power (past tense) and am now registering no more than about .5W max output, so something has cooked.? In the past, that typically was one or both of the IRF510s, and I'm beginning to suspect that is what has happened again, since the driver and predriver transistors dont appear to be bad.? I'll probably swap them while I'm re-biasing the driver/predriver stages and see if that resolves things. |
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ok, today's update...I replaced the finals (IRF510) and no difference - they were not bad.
Rather than shotgun why not diagnose? Likely the offending transistor has low emitter current.? So measure the voltage across the emitter resistor. also make sure all of the Q90 through drivers have 12V (more or less) n the collectors as well. Resistors R84, r89, r961 if they get cooked much cause pain too.? Someone is going to be missing a current or voltage.? Don't for get RV1 can be mis-set or flaky. Allison |
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sorry, forgot to post some earlier test results...but first a bit of background....I had already changed q90 (as an experiment) and installed an mmbt2222a in that spot when I also changed the predriver and drivers to 2n2222a.? I hadnt made any other changes.? I did not change anything else (I will be re-biasing, but didnt have parts).? Was doing about 5w flat out on 20m.
With that as background, I had been transmitting FT8 when I eventually got fluctuating output power and then dropped (permanently) to very low power.? As the finals were the only thing I hadnt changed out, and I made some measurements that had me thinking the other stages were ok (which I now realize I hadnt posted), I convinced myself I should change them...thinking that one at least might have gone bad. Measurements I made at the time (while in ptt with no mic audio): 3v across the q90 emitter resistor (r82 - 100 Ohm).? .44v across each of the predriver emitter resistors (r87/r88 - 22 Ohm) under drive (wsjt-x with 'tune' set on 20M), measured the following with an oscilloscope):?1.6v peak to peak after Q90 (measured at output of RV1); 2.8v p2p and 2.4v p2p at C90 and C91 (after the predriver), 2.8v and 3.2v p2p after the driver transformer, and 13.2v p2p at the antenna output. so...l had planned all along to make some other changes, and I'm going ahead with those.? As I was taking q90 out of the radio in favor of a better part (mmbth10, which will arrive tomorrow), I noticed that I had done a really bad job soldering in its predecessor, so I suppose that could have caused some issues...? ?As another data point, I just checked the resistors you suggested I look at (r84, r89, r961, rv1) as well as all of the other PA resistors, and they all appear to be in spec. I'll post an update after I make the first step of the planned changes (q90 replacement with re-biasing for q90).? |
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Ok, I think I have this solved.
Really short version: It appears that my major problem was the laptop that I was using for FT-8.? The laptop that I have been using is not putting out consistent volume/drive on wsjt-x with ft8 (even with a different sound card).? A different computer was tried and everything now works fine. Longer version: Ok, I'm a bit at a loss for 'why' this has happened, but here's what happened.? I spent a LOT of time tracing the transmit signal, going back through the mixer stages and into the mic amp (q6).? What I started to notice was that everything looked just great at DC levels, but when under drive ('tune' signal from wsjt-x), the signal levels and frequencies didnt seem to make sense.? I connected a commercial hf transceiver up to the same PC (which I hadnt done, as I didnt have access to one) and discovered that it was doing the SAME thing - effectively no output.? ?I tried a different sound card (usb connected) with the same problem.? When I switched to a different PC with the commercial transceiver, TWO things were discovered:? 1) suddenly I had signal coming out and 2) the transceiver was warning me of high swr. so...high swr.? It appears that my tuner was set up incorrectly (mfj-971 - has 2 internal jumpers that have to be set for qrp operation, and the swr movement was incorrectly set), and I was likely running high swr for quite a long time with my ubitx and not realizing it.? Once I got the tuner situation dealt with, the commercial rig works great with the NEW laptop, and still does not work well with the old laptop.? The ubitx ALSO works well with the NEW laptop now.? Whats really odd is that the OLD pc's soundcard still works fine with earbuds, just not driving either radio. so...what I'm left with appears to be an odd pc situation (inability to drive the radio, but no other noted malfunction), but functioning radio gear.? If anyone has ideas on that, I'm open to them, but I'm not going to expend much effort in that area. THANK YOU to those who posted and emailed me things to check.? I got into radio in order to learn more about electronics, and that is absolutely happening 73 Sean |
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Just to see whether it puts out the correct amount of power why not just keying down CW? Il 14/set/2018 20:11, "Sean W7SKD" <sean.jrdalys@...> ha scritto: Ok, I think I have this solved. |
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I mean into a dummy load Il 14/set/2018 21:44, "iz oos" <and2oosiz2@...> ha scritto:
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Raj vu2zap
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