Rigol DM3051 repair need diode ID

[R-1125F]

I am working on a Rigol DM3051 DMM.  The symptoms are that it shuts down after being on a few minutes.  It works normally until it shuts down. 

I found a shorted diode that I can't identify.  It is connected directly an output of the power transformer.  The diode has power in standby.  The part is marked 1B69 with BG below that.  There is no directional marking so it may be bidirectional.  It is shown as D602 in the photo. 

Could someone help me determine the part number?

[fzabkar]

It's probably a TVS diode.

Here is one possibility:

https://www.bourns.com/docs/Product-Datasheets/SMBJ.pdf

BG = SMBJ13CA (13V bidirectional)

Different manufacturers may use the same marking code for a different standoff voltage, so take care.

[R-1125F]

Thanks for the reply.  I thought that is what it might be.  I took a closer look at it and it does have a line to indicate direction.  I did find another BG code and it is an 8.2V zener.  The diode is connected directly to the 9VAC winding on the transformer.   I am a bit stuck on what voltage the diode should be. 

[fzabkar]

Thanks for the reply.  I thought that is what it might be.  I took a closer look at it and it does have a line to indicate direction.  I did find another BG code and it is an 8.2V zener.  The diode is connected directly to the 9VAC winding on the transformer.   I am a bit stuck on what voltage the diode should be.

It must be bidirectional if it's connected across AC. The forward voltage drop for a unidirectional diode would be 0.6V - 1V ...

[R-1125F]

I think you are correct.  I removed the diode but it tests good as unidirectional with a .6V drop.   It does appear that the circuit has this diode directly across the 9V winding of the transformer.   If this is a bidirectional TVS diode, and I think it is, it may be possible that one of the internal diodes shorted.  That would explain why it tests good as a unidirectional diode. 

If I operate the meter without this diode just for testing, do you think this will do any damage?

[Kleinstein]

Damage from mains spikes is possible though not very likely. In most cases the filter capacitors are large enough to catch the common spike from transformer turn off with inductive kick back.
The diode would likely not have failed if there were no transients that made it work hard - so there are few that could other wise do damage.
A main reason to have such protection is with really poor power quality, e.g. running of an UPS or generator with higher tolerance.


13 V bidirectional would be already at the lower end for use with 9 V AC.  The Peak voltage is regular at some 9 V * 1.4 and in many areas 10% higher than normal mains is also allowed.
The transformer as some series resistance and this limits to current on peaks when the protection kicks in. I would still go a little higher and if in doubt with a higher current rating to still not get a much higher clamping voltage.

If one direction is still working as a zener, this could give a clue on the voltage (maybe add 0.6 V for the broken diode). The voltage depends on the test current - so one has to check the data sheed for the replacement whick voltage this one likely was.

[R-1125F]

Thank you for the reply.   I think fzabkar has the correct part identified.  Everything makes sense.   I did try it without the diode and everything is working without shutting down.   The SMBJ13CA or similar should work fine.