Hi All,

I have a Zytec Power Supply (22917399) died on my Tektronix DSO (Model TDS340).

Fortunely i do have adequate troubleshoot and repair skills to determine the power resistor on component R17 was open circuit.

However, the component does not have any specifications i.e. no values on the body.

Does anybody on this planet know the value? If so, please advice.

If not, can someone PLEASE suggest a sound and safeway to estimate the value?

I believe looking at the PCB, the damaged resistor was sitting as a collector load for a NPN transistor (part #- TIP50).

If more info is required, please advice.

Thanks in advance for all the advice and opinions!

 

Do you have a circuit diagram of the power supply?

Or try to post a picture with some comments on it.

 

Hi mate,

I don't have a circuit diagram and i found more info on the internet about it.
http://www.electronicspoint.com/tektronix-tds350-psu-t25635.html

According to the info, the value is a 3K9 5W. I may also have a shorted bridge diode.

I will follow the provided advice and see if it works.

Stay tuned!

 

According to the thread you referenced, the rectifier is an FEP16DT; 200V 16A common cathode fast rectifier in a TO220 package.
Mouser stocks them; $0.90 each if you buy 1:
http://www.mouser.com/search/ProductDetail.aspx?qs=sGAEpiMZZMtEwUVCuofpuNRuzoljhYPCOvlI9osQkkU=

Double-check the part number on yours to make sure; different suffix letters change the configuration from common cathode to common anode or series configuration. Downloading the datasheet would be a good idea (available on the above link). Having a few spares available would also be a very good idea.

 

Hi SgtWookie,

Interestingly, my bridge diode mentioned is NOT the same manufacturer. I have MURF1620 (marking was U1620) instead of FEP16DT. Similar current and voltage specs, with same switching speed at 35ns.

Its nice to know Mouser have it, but i'm from Australia. So its not much use when shipping cost $75USD.

I also measured the resistance last night and it appeared to be ok, no sign of shorting, but it does short when i measured pin 1 and pin 3 according to the datasheet, also pin 1 and pin 3 connects to the transformer.

So i pull the diode out and in process damaging the legs, and found the transformer pin was shorted but the diode seems ok, the forward voltage (Vf)) measured to 0.6V with a DMM diode function (though the specs say 0.95V). I have a feeling the transformer maybe buggered since the 0.6V reading maybe due to DMM limitation!

But i can't tell and i'm not sure about the transformer because it maybe doing a high ratio of step down. Secondly i don't have specs or info on it. (The part is EP45877-9). If the transformer is shorted, i am really buggered.

Since i'm abit desperate, i'm going to buy FEP16GT from Farnell and try.
(http://au.farnell.com/fairchild-sem...wer-rectifier-uf/dp/1651046?_requestid=298301). This diode have slower recovery time (typ 50ns) and higher Vf=1.2V.

Maybe above suggestion will not work at all but for $5AUD, its worth it, because a new power supply was quoted around $450USD. That's pretty much the price of my scope at used price.

I'll update results.

In the mean time, if anybody have info on the transformer. I'll be appreciated to know.

Thanks!

 

I would expect a much lower Vf using a DMM; the DMM can't source much current, and the datasheet gives Vf at the rated current.

Without knowing more about the circuit, it's hard to say what's going on. Does the transformer look OK itself? No signs of heating?

 

SgtWookie, what CRO do you own?

The transformer looked OK, with one minor observation. The magnetic core was wobbled from the plastic housing, it appeared the magnetic core was glued to the plastic housing and the adhesive broke off, maybe from the heating etc (who knows).

Anyway, i replaced and changed R17 and inserted new power rectified diode (FEP16GT) and it appeared to be working. So a switching speed at 35ns by the power rectifier is not necessarily.

This is only an assumption, i suspect the R17 was destroyed due to high ambient temperature. During Feb highest temp was 45+C (approx 100+F)

One other important thing ,i will not leave the DSO on standby power unless i am operating it. Because it appears at standby the R17 heatup very quickly as the FAN don't power on. Once power up with fan on, the R17 felt ok.

Oh well, at the end i saved many hundreds of dollars!!!

I'm happy!!

 

SgtWookie, what CRO do you own?

I have an old Tektronix 2215 and an old Hitachi V660, both 60MHz dual trace.

The transformer looked OK, with one minor observation. The magnetic core was wobbled from the plastic housing, it appeared the magnetic core was glued to the plastic housing and the adhesive broke off, maybe from the heating etc (who knows).

Anyway, i replaced and changed R17 and inserted new power rectified diode (FEP16GT) and it appeared to be working. So a switching speed at 35ns by the power rectifier is not necessarily.

Time will tell. Keep an eye on the thermal rise. You did use heat sink compound, right?

This is only an assumption, i suspect the R17 was destroyed due to high ambient temperature. During Feb highest temp was 45+C (approx 100+F)

One other important thing ,i will not leave the DSO on standby power unless i am operating it. Because it appears at standby the R17 heatup very quickly as the FAN don't power on. Once power up with fan on, the R17 felt ok.

Sounds like standby mode is a very likely killer of the supply; very good that you discovered the conditions causing the heating.

I leave my 'scopes off unless I'm actually using them; as there may be days or weeks between usage.

 

I too have a 60MHz Philip PM3055 (an analog and used to owned and calibrated by the Australian defence force) and also this one, Tektronix TDS340 at 100MHz. The first item I purchased on ebay, both the seller and I were very nervous at the time as i require him to ship to Aust from US.

In fact the PM3055 is very heavy and solidly built and NEVER fail. Where as the TDS340 do felt abit empty. However function wise, the TDS was a real treat, it has waveform capture to a floppy, frequency measurement and pulse trigger. I'm sure its nothing new to the technology these days but for the price i got was a bargain. One thing i really didn't like though is the slow sample rate (this makes trace rendering very slow), i think fastest sample rate allow on specs is 500Ms/s, where as most new DSO allowed 1Gs/s or 2Gs/s

You did use heat sink compound, right?

Nope, instead i used the heat sink PAD. I think they both do the job as long as they can provided enough contact with the heatsink.

I think there is closure to this thread, so i see you around!!