Zastone MP380 Antipolarity P6KE18A Protection Diode Shorted
My old times good friend and old Offset work colleague often brings very interesting devices that are defect in the hope they still can be fixed. Like above Zastone MP380 Mobile Radio this repair is about. And these devices like SDR projects are what he mostly is interested in. It was dumped at a second hand/recycling shop where he partially works as a volunteer. And because it was completely dead it could be bought for a very affordable price.
In the past he also had brought a dead about 30 year old used Tektronix 2440 Memory oscilloscope that was defect because it completely short circuited when it was plugged into our 230V AC mains power line. After I had found the cause and had fixed the problem by replacing all defect components that luckily after 30 years still were available on eBay, I had bought the 2440 oscilloscope from him for a fair price. And my 2440 oscilloscope is completely re-calibrated internally and externally and also upgraded with new FM FerroRam memory chips. Also my Tektronix 2465A oscilloscope got this new FerroRam upgrade. This preserves the stored calibration settings of both scopes even after the original lithium SRam backup batteries are drained. Those articles also were shared on Jestine’s blog in the past. Also a GPIB 2440 screen dump tool was recently added. Now back to the Zastone Mobile Radio. Below Zastone’s label sticker.
Previous photo on the right showed the front with display. This device also already was short circuited because the 7A glass 5x20mm fuse was defect and the short circuit was measured on both 12V input wires and also on the RF transmitter power mosfet with smd marking A5M06.
Like often no service manual or schematic could be found online. But there was a video on replacing a defect RF output transistor in this Zastone model. The link to that video is : https://www.youtube.com/watch?v=k42aVSmCelQ
Next photos show how the unit was disassembled before it further could be examined. Four screws were removed to open the metal cover plate. Next photo shows the board with all connections.
Before the board could be removed out of the metal housing first the RF connector was removed.
Above photo shows the DC12V 0.13A fan model HD-2510H-12S that cools the RF output transistor.
After watching the video it at first looked like here also the RF output transistor was the culprit. So I already ordered one. But soon after this I noticed that the RF transistor AFT05MS006N (with smd code marking A5M06) was not the culprit because another component was the reason. Next photos show more about the Zastone MP380 board that was removed from its metal case after all screws and connections were removed.
The real culprit here was the special safety diode in above photo on the right next to the red and black 12V DC input wires. It was a P6KE18A diode. That diode intentionally was placed in parallel with the 12V input to prevent any wrong polarity damaging the electronic circuits in this device. Because as soon the +12V and GND input voltage was connected wrong this diode immediately short circuits and blows the 7A fuse with it. This way even when this safety diode accidentally afterwards would burn open, the open fuse makes sure the intended safety keeps on working. So also a set of 20 P6KE18A diodes, and a set of 7A fuses were ordered too.
Previous photos showed more of this board’s component- and solder side.
To remove this board from the metal case we also remove the front display with the push buttons. After all wires plus the flatcable and also the speaker wires were desoldered, the display front was easily taken of after unscrewing the 2 screws that still hold the display front on the other side of the case. Followed by a last photo of the speaker with its black wires and the fan’s red and black wiring.
So previously the short circuit could be measured on the RF output and the RF transistor and also on the 12V red and black DC input wires. Why I at first assumed that the RF output transistor was defect after seeing mentioned video.
But after the board was free of the case and front, the actual culprit was noticed. Immediately after the special antipolarity protection diode type P6KE18A was removed the short circuit was completely gone. So this means that someone likely had placed a wrong polarity on the 12V DC input. So the black wire was mistakenly used for the +12V and the ground for the red wire power input.
And if nothing else was done wrong to this device after that, this diode saved all components apart from the 7A glass fuse from damaging!
After the safety diode and the fuse are replaced, the antipolarity protection will be restored back to new. And this Zastone Mobile MP380 Radio set will work again without any problem as long as the polarity of the 12V DC input voltage is correctly connected.
My friend will be very happy to see this quite expensive device fixed as new. And as often at little cost since only a new P6KE18A and a new 7A fuse were needed. With enough new ordered spare parts left in stock for future repairs.
Albert van Bemmelen, Weert, The Netherlands.
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Note: You can read his previous article on Fixing A Quite Annoying Tristar PD-8781 Air Fryer Problem