Oscilloscope fuse keeps blowing

[gkmaia]

The scope works fine for a couple weeks then blows the fuse (F501).

It slowly blows it as I can hear a hiss and the CRT signal starts getting blurry and weak till it shuts off. The vertical and sweep circuit seems to be fine as the CRT goes away as the wave is still visible till it goes away.

The fuse looks foggy and burnt on the inside.

May be a coincidence but both times it blew I was measuring signals from my signal generator. A cheap chinese one.

The components marked with the little orange "v" have been tested and are all fine. Caps are 20 years old but still have less ESR then new chinese ones.

I don't want to remove every component and test it as the board and I don't want to be heating the PCB too much.

Based on the diagnostic I gave above is there any other things I should be checking apart from the suggestion from the manual?

Model is TRIO CS1352 -> https://www.manualscenter.com/manuals/kenwood/cs1352-service-manual.html#.XHIAsZMzaL4

[alsetalokin4017]

Are we allowed to know what kind of oscilloscope it is? Make and model number?


Regardless of that, your symptom may point to the transformer (as the troubleshooting flowchart says.) The filament of the CRT is elevated to high voltage, and this can leak through the filament winding into the rest of the transformer and cause the unit to draw more current than it should, blowing the fuse.

You can check this by powering the CRT filament by an external transformer instead of the scope's transformer, being aware and careful of the HV elevation. If the scope works and doesn't blow the fuse, you can then permanently install the extra filament transformer inside the case, thus keeping the HV out of the main transformer.

[gkmaia]

Are we allowed to know what kind of oscilloscope it is? Make and model number?


Regardless of that, your symptom may point to the transformer (as the troubleshooting flowchart says.) The filament of the CRT is elevated to high voltage, and this can leak through the filament winding into the rest of the transformer and cause the unit to draw more current than it should, blowing the fuse.

You can check this by powering the CRT filament by an external transformer instead of the scope's transformer, being aware and careful of the HV elevation. If the scope works and doesn't blow the fuse, you can then permanently install the extra filament transformer inside the case, thus keeping the HV out of the main transformer.

It  is a TRIO CS1352 -> https://www.manualscenter.com/manuals/kenwood/cs1352-service-manual.html#.XHIAsZMzaL4

Also disconnected the transformer windings for the high voltage and tested them. Not sure if we can make much out of it...

[james_s]

If it's making a hissing sound I would first look inside it in a dark room and see if you can see any signs of arcing, also sniff around for the smell of ozone.

If that all looks good, I would check and/or replace the electrolytic capacitors in the power supply.

[gkmaia]

If it's making a hissing sound I would first look inside it in a dark room and see if you can see any signs of arcing, also sniff around for the smell of ozone.

If that all looks good, I would check and/or replace the electrolytic capacitors in the power supply.

Can you give some examples of how the arching may present itself visually?

[james_s]

Arcing or corona, I don't know how to describe it exactly but dim often bluish light coming from areas of the HV circuitry or wiring. Like a very very tiny arc welder.

[Boyd1303]

Hello

Is there a VDR (varactor) on there?

Maybe if you cut it of or replace it the fuse doesn't blow up any more...

Kind regards,
Boyd.

[caulktel]

Take your muti meter set to AC volts and check between the BNC connector of your signal gen to ground, If you find any high voltage on it that would be why your scope fuse keeps blowing. Had the same problem with my HP frequency counter and a cheap Chinese signal generator. Bad filtering in the switch mode power supply of the Sig Gen.

[jonpaul]

Old CRT scopes usually develop bad HV transformers or HV multiplier, as the insulation ages and fails under HV, HF stress.

Fuse and other parts are the side effect not the root cause.

j

[wasedadoc]

The fuse that is blowing is on the 25 volt dc power line into the regulator and power transistors feeding the transformer (not the mains transformer) that generates all the rails for the scope except the battery charging circuit.  Something would appear to be overloading that transformer/those transistors.  Can't see how any fault on the Chinese generator could do that.

[slbender]

I think TRIO is the same as Kenwood, I have a “Audio Scope” from Kenwood that looks like the same “Yikes” type of circuit board I guess anything is suspect in there.  Usually the caps (if oil type) and electrolytic types are the first suspects.  Sometimes caps can bulge or hiss too!  After that maybe some transistors have lost their “beta” (gain) and are messing up the voltages.  First try to find a schematic and service manual.  Then pull/test parts.  I once fixed a set where a small power transistor that activated a solenoid, should have had a beta ~100-200 well, it measured a solid “2”. From the symptoms it looked like a logic problem- but wasn’t.

Steven