how to turn down power output on QYT KT-980 Plus

Discussion in 'General Technical Questions and Answers' started by KN4WVY, Oct 16, 2019.

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  1. KN4WVY

    KN4WVY Ham Member QRZ Page

    I'm considering buying a QYT KT-980 Plus from a fellow ham and while it seems to meet my needs and wants for a simple FM dual band radio giving me an option for higher power if needed, I'd like to be able to turn down the power some to avoid the radio's fan turning on, or reduce the fan's use at least.

    It has a 2" square fan in the back that sounds like a swarm of angry mosquitos when it runs up to full speed, which it does after several moderately long QSOs / exchanges. I looked up the specs on the fan (model HD-3010H12S) and no wonder it makes noise, it can run at speeds up to 12,000 RPM. One might expect it to sound noisy if it gets up close to full speed, huh? :)

    I think the radio is operating normally, it is being used out in the open on a desk so it has good air circulation, it has a near 1:1 SWR so there is minimal reflected power heating up the radio, and the entire external heat sink that takes up the entire underside of the radio seems to be heating up evenly and significantly with this sort of use so I know the transistor has a good thermal connection to the heat sink.

    It sounds like thermal issues are common with so-called CCRs (cheap Chinese radios). I don't think it's necessarily a poor design if one is aware and prepared to work with it. A smaller, lower-power-rated unit might struggle more to reach its rated power or even put out on low power for extended QSOs, I've read of many cheap, smaller CCRs that burn out finals due to overheating for various reasons and I think I'll be less likely to encounter that with a bigger radio that I will usually run at low power.

    I may add a larger fan below the unit at some point but I'd rather for now just turn the power down some, as it seems to be able to hit all of the repeaters in my area fairly easily even at low power. We measured with 13.8 volts in at low power the radio is putting out about 40 watts VHF and 20 watts UHF, a little less than advertised but it's said that's common too with CCRs.

    I wrote the manufacturer and while they've been helpful answering some general technical questions they said they can't send me a schematic due to "policy." I know it may contain proprietary information and so I don't think that's unreasonable, it just makes doing any sort of modding more difficult.

    The QRZ user VK4GAP created a post about how he replaced a blown final transistor and remarked he might want to adjust or turn down his power later. I don't know how to message a user or "tag" them in a post so if someone could tell me I'd appreciate it but others here may be able to help as well.

    I know this would be quite a hassle to go through when most users would just keep their QSOs short or rig up a bigger fan. I may play with lowering the power supply voltage slightly, like maybe to 12 volts or a little less. That would likely reduce power but I don't know if it would be good for the radio otherwise to be run undervolted like that, or if doing so would affect the sound quality or something else negatively.

    I'll include below a hopefully clear hi-res photo of the top of the circuit board. Of course the components I'd need to change may be on the underside of the board, which I can get to but it'd be a hassle and I should only do that after buying the radio.

    I'd appreciate any suggestions for how I could best trace or infer specifically what components may be involved in lowering output power when the power is set to "low". I'd accept doing something to reduce power in both the high and low power modes as well (if there is one change I could make that would affect both modes, that is), but I'd prefer to just have a lower power low power mode, if that's practical to do as a mod.

    I suppose there is some sort of voltage regulator that cuts the voltage to the final (or maybe to the final's driver, I don't know) and changing a resistor or something in that circuit may reduce the power. The resistors and other smaller components though are mostly tiny surface mount devices which I'll have to be very careful handling.


    QYT-980Plus-innards.jpg
     
    Last edited: Oct 16, 2019
  2. KN4WVY

    KN4WVY Ham Member QRZ Page

    I posted the above question several day ago and it's now on page three here with no comments, hope it's ok to bump this now that it's the weekend and maybe more people will be on here who can give some information on what I might do here.

    The main reason I want to turn the power down on my radio to maybe about half of what it is putting out now to prevent the unit from heating up so fast. Given what I had heard about CCRs (cheap radios, etc) I was concerned the heat sinking may be inadequate but as the heat sink overall heats up evenly and at about the same temp as the final transistor I don't think poor heat sinking is an issue. I may install an external fan at some point but even if I do I'll be better off running less power if possible, no reason to use power I don't need and it can only help to run a radio designed for higher power at a lower power if possible for lower heat and stress all over. Also I may want to use an external amp soon, especially on 2 meters for maybe 150 watts for DX and I'll definitely need lower power going into the amp for that.

    Any hints on how to find the adjustments or components that may be involved? I suppose there is some power regulator that controls power to the final or the final's driver that I could adjust, if only by (**carefully**) adding resistance or something to the circuit. As I noted above in my first post, I wrote the manufacturer and while they've been helpful answering some general technical questions they said they can't send me a schematic due to "policy." I know it may contain proprietary information and so I don't think that's unreasonable, it just makes doing any sort of modding more difficult.
     
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    One reason for no answers may be because no one wants to help you smoke your radio.

    The power level is set in firmware, And if you have the proper software you can change the settings.

    Lowering the input voltage may be the easy way.

    Good Luck.
     
    KN4WVY likes this.
  4. KN4WVY

    KN4WVY Ham Member QRZ Page

    Thanks, I don't want to smoke/bake/sautée the radio either. It's also not mine yet to cook at all, I am considering buying it but whether or I do not the current owner also wants to know how to turn the power down. I could certainly reduce the input voltage to the radio but that won't reliably, consistently turn the power down as much as I'd like to maybe half of what its putting out now.

    I haven't been able to get the manufactuer's software working that can change things like that but I've seen screenshots of what it has and it doesn't allow for incremental adjustments to the output power, only a "high / low" toggle which both CHIRP and the front panel/microphone programming can also change.

    There was another QRZ user here who posted about replacing the final output transistor on this model and he mentioned he may later want to turn the power down. He did not though say how he thought he could do that. I've messaged him and am waiting for a reply.

    As far as control through firmware goes, I'm sure firmware is involved in something between the unit's controls on the front panel and microphone, etc, and the components I'm interested in that are being controlled. That firmware must though control something in the signal or power supply path to the final transistor or its driver to switch between high and low power. I thought it might be a couple of resistors or diodes I might be able to swap or modify, and of course any components like that may be inside an IC which I can't get into or they may be in surface mount components which may be difficult or impractical to swap or modify. I've looked at the circuit board to try to see where the final transistor leads go and it's not very clear what the power supply path is to the final nor where a driver transistor may be. If someone knows what the driver transistor or related components may look like, knowing that would be helpful.

    I don't know how or if this could be done on this particular model but I believe that mods or service adjustments for this are done routinely in CBs and other mobile radios to adjust the output power, either to comply with legal limits (like in CB) or simply to adjust the output power to stay within component limits (like when a final is replaced, like the QRZ user I mentioned above did) or to lower the power to appropriately drive a power amplifier.

    That's why I thought there really ought to be some way to manually adjust the output power, driving current, etc, if only because it's a common adjustment done in repair and the use of a mobile radio with power amplifiers.

    That may not be something that could be done with this particular model but I thought there might at least be general suggestions for things to look for to try to trace what components are involved.
     
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    Have you looked at those pots on the board ? What circuit they are in ?

    One looks like deviation level, Not sure what the other is.

    The radio settings are normally based on a 1-127 or 1 to 254 binary level. I do not think chirp can modify those.

    Did you get a computer disk with the radio ? I do not have that radio, But the disk I have lets you modify things chirp will not.

    Maybe putting a fan that blows on the heat sink, Would be a good thing to do.

    Good Luck.
     
    Last edited: Oct 19, 2019
    KN4WVY likes this.
  6. KN4WVY

    KN4WVY Ham Member QRZ Page

    Thanks, well, I've looked at the two devices that have a phillips-head-looking adjustment and I didn't know if they were adjustments or not. One might suppose they were but I didn't know if the screws went down to the board level for mounting or if they were indeed adjustments.

    I've added numbers to the photo below for the four components that I thought might have some related function. Maybe you or someone else could comment on this.

    (1) and (2) may be adjustments, I can't tell though if they are resistors or capacitors. If anyone could comment on what they may be that may be helpful. If you recognize one is likely modulation, the other may be power, and I could focus on that.

    The adjustment device (1) appears to be closer to some sort of power device, labeled (3) at the lower left-hand side of the photo. I haven't been able to make out the numbers or letters on it yet but I'll try again with light at different angles, etc. It has five leads, and I thought it might either be a regulator with multiple outputs or an audio amplifier used for the speaker or for modulation (the speakers leads connect to the board right above the (3) label. I could put a voltmeter probe on the leads to see if there is a DC voltage but I wanted to ask for comments before I start poking anything.

    Near the power input leads at the upper left-hand side of the photo is label (4) and right below that is a square device with broad, heavy leads or mounting tabs. I thought that might be a driver transistor seeing how it apparently it is designed to handle power / heat and it's close to the final transistor which is obviously to its right (under the large yellow metal heat sink / RF shield cover). I would think that high and low RF output power is ultimately controlled by changing the voltage and current used by the driver or the final output transistor, and components affecting that might be nearby.

    If anyone would want to add to this to help explain the components or what may be a setting for deviation (which I won't touch) or power (which I'm looking for) I'd appreciate it.

    PS - on software, there is a program designed for another QYT model which Radiooddity and Amazon link to but I haven't been able to get that to recognize my radio at all, though the programming cable works fine with my radio and with CHIRP. Other users seem to have similar issues with getting the factory software to recognize their radios. I have though seen screenshots of the menus, and I am also able to view the blank screens inside the program when it runs on my computer and it does not have binary 0-127 or 256 or whatever settings, only high / low settings for defaults and for individual channel settings. It looks like this may not be something that the software does at all, and I don't know really if it would normally be used to adjust the power level anyway then servicing the unit, like replacing the final transistor, or if physical components would be adjusted. That info is apparently proprietary, which doesn't mean I can't do it, only that the factory doesn't have a way to tell me how.

    QYT-980Plus-innards-edit.jpg
     
    Last edited: Oct 19, 2019
  7. KA9JLM

    KA9JLM Ham Member QRZ Page

    I will make my best guess. I like guess by picture. :)

    Number 1 and 2 are 1 turn resistors. If you do get brave enough to adjust, Mark the position first and use a plastic tool to adjust. What they do is a good question. ??

    Number 3 is the audio amp. Look at the part number to verify. The output will be going to the speaker.

    Number 4 is the power control regulator, On/Off Switch. Look at the part number to verify. The input will be going to 12V DC IN.

    I think the proper terminology is, I am just spitballing here. Take it with a grain of salt. ;)

    Anything done may void the warranty. I normally void the warranty ASAP, To see what it is made of. :D

    Good Luck.
     
    Last edited: Oct 19, 2019
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  8. KN4WVY

    KN4WVY Ham Member QRZ Page

    Thanks, KA9JLM, voiding the warranty won't be an issue, it would be a used radio for me and it's out of warranty for the current owner even if I don't buy it and he makes these mods himself.

    I will try to get the part numbers off of devices (3) and (4) later today when I can get ahold of the radio. That will obviously help distinguish an medium-power audio amplifier (about 2 watts output) from a much higher-powered voltage regulator (the specs on the radio say it uses up to 20 amps at about 13 volts).

    Here's a general question about adjusting deviation -- I don't intend to adjust the deviation but if I twiddle the wrong adjustment I'll likely be adjusting deviation and not power. I will mark the original setting to be able to get back to it easier and I won't change the setting by more than a half-turn (assuming these are max one-turn adjustments) but I'll want to know how I can recognize the deviation has changed, if possible, as I don't have a deviation meter or direct way to measure deviation and can't easily borrow one.

    If I mistakenly change the deviation, may I expect to be able to hear a difference over a monitoring walkie-talkie (a Baofeng UV-5R), like an increase or decrease in volume? I don't know what exactly a deviation adjustment would change, I know that increased modulation on a FM radio increases deviation, so I suppose the deviation adjustment could simply be a modulation limiter or compressor?

    If anyone wants to comment on this I'd appreciate it. Without a schematic I maybe can't know for sure, but is that generally what a deviation adjustment affects, or what may it usually directly affect?
     
    Last edited: Oct 19, 2019
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    Get the IC number near pot # 2 also. That will be a clue for that adjustment.

    The power switch controlled voltage regulator (#4) is going to be for the low voltage control , IE control head On/Off and things like that. It may be regulated 8 Volts Out that feeds the 5V and 3.3 Volt stuff.. The number on it will tell its voltage Out. It should be a XX8X or something, The number being the voltage.

    There is no higher-powered voltage regulator in the radio, The final is connected directly in the Input supply voltage.


    Adjusting deviation may be noticeable on another radio, But looking on a SDR or proper test gear is best. ;)

    Marking the pot will work. I use Red Nail polish, Because that is what I paint my toenails with and have it on hand. :D

    What ever color you paint your nails with will work. :p

    Have Fun.
     
  10. VK4GAP

    VK4GAP Ham Member QRZ Page

    Nice pics
    1 = Deviation
    2 = Power

    Only trouble i found was turn it down on vhf , 70cm went down in a non linear fashion .

    Cheers .
     
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