Sefram 7835: Power circuit with problem, need to identify component

[Bicurico]

Hello,

One of my two recently purchased Sefram 7835 field meters has a problem related to the power management circuit. This circuit deals with running the device on the battery or PSU, as well as, charging of the battery.

The symptoms are:

- Sometimes the device behaves normal
- Sometimes it just switches off: sometimes I can switch it one again, sometimes it won't switch on again!
- When it does not switch on again, it helps to leave it charging (=heating up, the battery has charge and runs fine on the other device). After a while it will switch on by itself or at least tries to switch on (screen goes on/off and pressing the ON button will eventually turn the device on)
- When left alone, even without PSU connected, it sometimes goes on/off by itself

I have looked at the board and found that the capacitor next to a MOSFET has leaked. I don't think it is any glue. Take a look at the attached picture.

This same section has been changed (manually, I guess, in a previous repair) on the other device - so I think it is a problematic region of the PCB. It was by comparing both PCB's that I noticed that the other device has had the MOSFET replaced with another type, an extra wire added and one of the cap's replaced. Probably some traces had been corroded.

On this device, things don't look so bad.

I did clean the board a bit and the device started to behave normally, but today it is erratic again.

I think I need to replace the SANYO 68uF 20V 105ºC cap, but I fear that the MOSFET is not doing so well, either.

Now the question: what exact type is this MOSFET? I can read: 418 20P ON 03HL

418 --> Type?
ON --> Manufacturer?
20P --> ?
03HL --> Production date?

Can anyone tell me a replacement for this IC?

Also, from the pictures, would you agree that the orange stuff is leaked electrolyte from the CAP - very visible next to the Q100 print?

Is this electrolyte conductive? Could it be causing the erratic behaviour?

Can I replace this CAP (seems to be aluminium foil from what I read) with a generic no-brand 64uF/25V cap?

Thanks!

Regards,
Vitor

[Bicurico]

I attached the two pictures showing the same region on both the devices.

Picture 1 is the faulty device.
Picture 2 is the working device, which seems to have been repaired in the past.

[Bicurico]

For your convinience, a new picture with #2 next to #1.

[Bicurico]

After scratching the dried electrolyte from the MOS, the device worked fine.

I do need to replace the cap, eventually.

Anyway, wrote a test report on the Sefram 7835 for those interested in TV/CATV/SAT field meter:

https://vma-satellite.blogspot.com/2022/09/sefram-7835-test-report-how-useful-can.html

Cheers,
Vitor

[fzabkar]

MTD20P03HDL, ON Semiconductor, Power MOSFET 20 Amps, 30 Volts, P-ch, Logic level, D-PAK:
http://www.bitsavers.org/components/motorola/_dataBooks/2001_DL135-D_ON_Power_MOSFETs.pdf

[Bicurico]

Thanks!

That was what I was looking for!

[Bicurico]

The problem continues on BOTH devices: my observation is that with low ambient temperature (summer is over and it is getting colder), the devices don't switch on when pressing the Power softkey.

Because one of the devices had work done in the power circuit and because I found what seemed to be dried electrolyte on the MOS, I removed the cap sitting next to the MOS, only to find out that it was clean on the underside. Also, I measured the cap. It is rated 68uF and I measured 86uF. I would say the cap is in working order, so I soldered it back, instead of replacing it with a new no-name one with slightly different specs.

Now the current observation is: when both devices are cold, they won't switch on. Leaving them 10 minutes up to 1 hour connected to the power supply will cause them to switch on - by themselfes.

After that, I can switch them on/off for the largest part of the day. But one will randomly switch on by itself.

What kind of component (cap, MOS, transistor, resistor, etc.) could be causing this?

These devices are a bit cumbersome to disassemble, but when I get the time, I will selectevly heat the caps around the power circuit to see if that highlights the problematic one.

Any ideas?

Oh, and another question: how does a Power softkey work in principle? They keyboard (which is OK) is just a membrane with a flat cable connecting to the main board. Is there any MCU controlling if someone pressed the power button and if so generate the voltage for the MOS to switch on? Could the problem therefore be somewhere near this keyboard controller rather than the power circuit itself?

Thanks,
Vitor

[Haenk]

Honestly, it *does* sound like a cap problem. Hot air testing certainly is a good idea.
However I would not trust the Sanyo ones. (Though a big brand manufacturer, I had quite a few bad ones so far...)

[Bicurico]

Yes, you are probably right. The symptoms are getting worse on a daily basis.