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AnyTone AT588 Receive self oscillation


I am using this AnyTone 144MHz 10/25/50W FM mobile device last years for local  144MHz traffic.
A lot of power from a very small VHF mobile rig 
Though it will get very hot in longer chats at the 25 or 50W level
The RX-standby power is  0.25A
But sometimes it happens that sudden the squelch seem to open and a loud noise comes out.
The RX signal level showed 5 bars even with disconnected  antenna.
The signal happens to be generated internal.
The noise/signal disappears when just touching the power cord or AnyThing(...) external at the device
Ghostly ...
Found internal a beautiful well build device with gold plated PCB.
I have no service document or diagram. Hard to believe that this professional build device would suffer from a self oscillating pre-amp stage, but i can't draw an other conclusion.
I could not see any malfunctioning parts.  A wide-band design no tuning parts.
Suspect is the first RX FET to have a constant parasitic self-oscillation.
Hard to detect with my RF detector probe , the level is low and oscillation stops or changes when placing the probe at the receiver pre-amp fet. Close by at PCB left is the local oscillator that of course does show rf level.
I could move the oscillation frequency by hand effect near the RX input stage. It is a very weak oscillation , strange by nature an oscillator will work at maximum or not.
See Youtube recording below.
Having a test signal at 144.550 i could clearly hear an interfering floating signal passing by when approaching the PCB near i guessed it must be the RX pre-amp stage.
The UHF/SHF trick using a piece of anti static foam did not work.
Oscillation stopped with an 1/8W 1k5 resistor from ground direct to the drain of the FET
The receiving of the test signal did not seem to suffer  by this rather rough approach.
Possible someone knows a better way to stop the parasitic oscillation but the parts are all SMD and i did not want to mess up the very clean PCB layout.
A few days standby now the annoying noise burst did not happen again yet.
Inside the AT588 .
  Right of the thin shielded enclosure upper x-tal is the local oscillator FET
 right of that is the RX pre-amp part
The 1k5  blue resistor is soldered from drain and ground and oscillation stopped.
The drain voltage about 7.5V did not drop much by this still above 7V
 I placed a video of this noise problem on YouTube
https://youtu.be/YIvC4IdI0l4

one response  i got
Dear Jaap,

I read your article about receive self Oszillation. M TYT TH-9000 (which seems to be the same radio or maybe a clone) did show exactly the same problem. As far as I remember I bought the TYT in 2012. It was impossible to find anything about the issue and so therefore I´m astonished to get your hint today and no other reports.
You are using your AT-588 for a year. Did you buy it as a new rig or used? Could be that Anytone/TYT (and probably Alinco too) still do have the malfunction, because obviously  the boards do have the designs a long time.

Best regards
Martin (DL3IAD) nr the PA-Border in JO31IS



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Propagation 80m a bit restored last 24 hour   sorry enough i had to remove my west Beverage due farmers activity's  it was again cut by mowing machines.
TimestampCallMHzSNRDriftGridPwrReporterRGridkmaz# Spots
 2019-05-16 19:32  PA0O  3.570094  -21  0  JO33hg  5  VK2DDI  QF55hf  16535  68  1 
 2019-05-16 21:00  PA0O  3.570101  -20  0  JO33hg  5  VK3KHZ  QF22pe  16434  78  1 
 2019-05-16 21:00  PA0O  3.570103  -23  0  JO33hg  5  VK3MI  QF22ne  16423  78  1 
 2019-05-17 02:42  A7XGB  3.570064  -19  0  QH83  0.05  PA0O  JO33hg  15185  335  1 
 2019-05-16 21:00  PA0O  3.570100  -15  0  JO33hg  5  DP0GVN  IB59uh  13835  186  1 
 2019-05-16 22:20  M0SBY  3.570056  -18  0  NL82  0.5  PA0O  JO33hg  8045  321  9 
 2019-05-17 02:48  WD4DDA  3.570094  -25  0  EM42  5  PA0O  JO33hg  7640  40  6 
 2019-05-17 02:38  K1RKE  3.570100  -17  0  EL88wv  10  PA0O  JO33hg  7395  41  3 
 2019-05-17 01:42  WA9WTK  3.570063  -21  0  FN42fk  10  PA0O  JO33hg  5654  49  4 
 2019-05-17 01:42  EA8DBU  3.570061  -25  0  IL18oe  5  PA0O  JO33hg  3383  28  2 
 2019-05-16 19:42  PA0O  3.570099  -14  0  JO33hg  5  EA8BFK  IL38bo  3213  220  2 
 2019-05-16 19:38  TA4/G8SCU  3.570099  -11  0  KM56ov  5  PA0O  JO33hg  2629  322  1 
 2019-05-16 21:00  PA0O  3.570100  +6  0  JO33hg  5  TF4M  HP85fp  2154  321  1 
 2019-05-16 19:42  PA0O  3.570101  -5  0  JO33hg  5  UA3245SWL  KO85qw  1986  69  2 
 2019-05-16 21:00  PA0O  3.594107  +2  0  JO33hg  5  UB1AKX  KO59cs  1605  54  1
 

     
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