upgrading/improving mobile final thermal paste and heat sink performance

Discussion in 'General Technical Questions and Answers' started by KN4WVY, Sep 11, 2019 at 12:08 PM.

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  1. KN4WVY

    KN4WVY Ham Member QRZ Page

    I'll be buying a working QYT KT-980 Plus from a friend for my first mobile radio and I've heard that there are issues with many CCRs (cheap Chinese radios) with poor thermal issues with the finals. In many cases I've read the issue may be little or no thermal paste between the final and the heatsink, and in other cases it may be an underrated transistor that is being overdriven to give a higher output. The QYT KT-980 Plus doesn't so much have the latter issue as the transistor seems correctly sized for 75 VHF/50 UHF watts or so, but I wonder about the thermal paste and heat sink issues.

    I want to better avoid this issue with my unit and I thought I would take the final transistor off to add some high quality thermal paste underneath. I don't think it would hurt, assuming I don't damage the final or board in the process.

    I want to phrase this as a general question here and get an idea on if this would be a good idea to try and if anyone else has done this already because I think this may be an issue that a lot of CCRs have. I suppose these radios are made on mostly automated assembly lines and little attention or testing is done to evaluate how well the heat sink works under a heavier duty cycle of use.

    I hope I don't sound like I'm overthinking here, there isn't anything obviously wrong now with the unit I'm considering buying but I was told (and saw) that the heat sink wasn't heating up at all with a moderate use test I watched. That may not indicate anything wrong but at least I'd like to touch the final transistor (after turning off the power) after a QSO test I'll run to see if it heats up at all and if the heat sink material adjacent to it heats up along with it.
     
  2. W1TRY

    W1TRY XML Subscriber QRZ Page

    My opinion is if it's isn't broken, don't fix it.
    However, it may be wise to examine the radio and get a view of the layout in the event there is a problem.
    If there is a documented issue with the final and thermal paste, depending on cost, you could prepare yourself with the necessary spares in the event of a failure.
    You may have one of the radios that was built properly! Good luck.
     
  3. KP4SX

    KP4SX XML Subscriber QRZ Page

    If you're handy and comfortable with working on these things I guess its worth a look see. I recently worked on a little solar charge controller and the thermal paste application was a joke.
    Let us know how you find it - good or bad.
     
  4. KJ4YEV

    KJ4YEV Premium Subscriber QRZ Page

    It doesn't take a lot of heat sink compound to do the job. A thin film between the device and the heatsink is all that's required. I've seen equipment where it was grossly overused either at the factory or by a repair tech. It makes for messy repairs the next time around.
     
  5. W9KEY

    W9KEY XML Subscriber QRZ Page

    Seems like a pretty easy thing to check - which I'd sure do before ripping apart a completely good radio.

    See if the specs list a published duty cycle - transmit ON time, versus OFF time - to cool down.
    Hook up a properly sized dummy load, or set up a local simplex contact with a friend and have a "long" chat at full power - while monitoring the heat sink and possibly final output transistors with a non-contact IR temperature meter. If you can get close enough (perhaps with cover off), you might be able to note the temperature difference between the heat sink and transistor body. If properly assembled, I'd think those two temperatures would be fairly close. Others might be able to suggest an actual upper (acceptable) temperature limit - I have no practical experience in that regard, although you might consider comparing it against a similar test with a "higher quality" radio.

    However, I know that my cheap Baofeng HT (high power version) gets "warm" enough on a long winded 2 meter net to get uncomfortably hot to hold! But that requires transmit times (duty cycle) far in excess of those recommended in the spec sheet.
     
  6. KN4WVY

    KN4WVY Ham Member QRZ Page

    Thanks for the quick and helpful replies! I will post an update on what I find out once I get into the unit.

    I plan to check the temperature of the final transistor and the adjacent heat sink material after operating the radio for a short while (a short conversation, maybe five minutes) to see if the final is heating up considerably while the heat sink remains cool. The greater the "delta" the more likely there is a thermal problem, maybe thermal paste, maybe a mechanical issue like a warped interface between the transistor and the heat sink, etc. I don't have a thermistor to quantify the difference, I think if I can touch and feel (with the power off) a very significant difference there's probably something to be concerned about, and if not I probably won't try to fix what's not obviously broken.

    I have to think this is a problem that may be affecting many owners of CCR's especially in the summer heat, and I'm surprised there's not more discussion on this online. There is so much effort made by computer hobbyists to cool CPUs quietly, efficiently, etc, in part of course because fast CPUs are expensive and they may run with a much more severe duty cycle than high-power transistors do in ham radios. I think if more attention like this was given to cooling higher-power finals in less expensive mobile radios there would be a lot fewer failures and complaints about these units breaking down in use.

    One thing I'll need consider is that the transistor's case is electrically connected to the heat sink, unlike with most CPUs, so I'll need to use electrically conductive thermal paste. I don't know if RF is supposed to pass through it or not, but I suppose I can find out with a schematic or maybe just looking at the traces on the board. If no RF passes through the paste and there is just a DC connection to power or ground I suppose I could use an electrically conductive paste used for CPUs, but I'll also look into if there are manufacturer recommendations for this from QYT. I found a datasheet for the final used in at least some of these units online: https://www.nxp.com/docs/en/data-sheet/AFT05MP075N.pdf but while it gives a lot of electrical specs it doesn't give specific recommendations for thermal paste. There may be more information in "application notes" mentioned in the last page of the spec sheet, I'll follow-up on that if needed.

    I wrote QYT with another question five days ago asking if there is a pot or internal adjustment for microphone gain (which is about the main reason why I might consider getting a BTECH 50x2 instead of this unit, it has a easy software setting to change mic gain). I haven't gotten an answer back from QYT on that so I'll follow-up shortly.

    In the meantime if anyone knows if there is an internal adjustment for mic gain that or has a schematic for this radio (QYT KT-980 Plus) I'd like to know, I haven't found one online (I've also looked at the manufacturer's website, https://qyt-cn.com)
     
  7. KN4WVY

    KN4WVY Ham Member QRZ Page

    PS - While the data sheet at https://www.nxp.com/docs/en/data-sheet/AFT05MP075N.pdf doesn't give a duty cycle or a typical or nominal tempature it does have some numbers that are good to know in this context.

    Page 2 lists the operating range of the case ranging up to 150C, which is far too hot to touch and if the internal components of the radio even approached half of that I'd imagine insulation or other material might start to deteriorate, and notes that the device should be "derated" (like power reduced, I suppose) above 25C -- barely above room temperature. No wonder some people report CCRs as reducing output power after 30 seconds to a minute of operation, if they are running at or above full rated power inside a little case with a marginal connection to a heat sink they might not be able to sustain more than that.

    A more realistic rating of the upper limit for operating temperature may be found under Table 2, thermal characteristics, which gives a figure for thermal resistance with a case temp of 80C with an input power of 70W. 80C is about 176F, hot enough to cause second degree burns per a Google search. I'm sure that temps like that aren't (or shouldn't) be reached for long in normal operating conditions. I'd also be surprised if the finals in these mobile radios normally would get that hot, I'd think they should reduce output power automatically long before that point, and if they didn't something might be wrong with the final's connection to the heat sink or the overtemperature protection these radios should have built into them.
     
    KA0HCP likes this.
  8. WA9SVD

    WA9SVD Ham Member QRZ Page

    It is NOT unreasonable that a Baofeng, or ANY other mini-H-T will get HOT quickly.:rolleyes: To provide 5 Watts OUT, the typical INPUT power (including other circuitry, display, etc.) is 10 Watts or more. So WHERE does the other power go? Answer: it's heat! Wrap your hand around a 5 Watt "night-light" bulb and see how long you can hang on.:rolleyes:
     
  9. AG6QR

    AG6QR Subscriber QRZ Page

    Thermal paste is not nearly as conductive as metal. But it is more conductive than air.

    The best heat transfer is from direct metal-to-metal contact. But normal metal parts have irregularities that prevent the entire surface area from making contact, instead leaving air gaps between the parts.

    The best use of thermal paste is to just fill those air gaps, while allowing direct contact between the metal where possible. If you slather on the paste like you're making a peanut butter sandwich, you'll prevent the metal parts from making direct contact, and reduce the heat transfer. Keep the layer of compound very thin.
     
    G4COE and KA0HCP like this.
  10. AI3V

    AI3V Ham Member QRZ Page

    Another thing to keep in mind is the increased current draw of the final with antenna system impedances lower than 50 ohms resistive, or with reactive loads.

    I would suggest compairing dc current draw between the dummy load and antenna.

    Also, consider setting the transmitter "time out timer" to prevent excessive "butt keying" time in case of accident :)

    Rege
     

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