Double fused battery input, replace or bypass ground blown fuse?

[videobruce]

The equipment is a Tektronix RFM151 SLM (around 20 years old). Long story short, the battery input has two SM fuses (2.5a) one on each side. The ground side blew due to a wrong polarity of a battery connector. The meter works off a AC adapter fine.

I never saw a fuse used in both sides before and it's the ground side that blew. Being a very odd (to me) fuse and the fact it's on the ground side with the same fuse on the positive side, can just bypassing that blown fuse be ok? There doesn't appear to be any diode protection as one would think.

See attachment, it's the SM left side fuse (F1 now removed) in the upper left corner of the pic.

[georges80]

Fusing + and - is common in some situations, such as for example HAM radios used in a vehicle where the +/- cables of the radio connect directly to the battery.

The reasoning is, that if the radio is mounted so its chassis is grounded say in the cab and then a main battery cable is disconnected, the radio ground wire now becomes the ground path for the vehicle between the body (via the radio chassis and radio's ground wire) and the battery and other loads (like the starter motor...) will push high current through that path.

With your tek, the dual fusing may be for a similar situation or combination of possible mis-wires (or reverse polarity connection of your battery...)

cheers,
george.

[videobruce]

I wouldn't/couldn't think this was were intended to be used in a vehicle. Surely not as an entertainment device since it sold for between $2k & $2.5 k in the late nineties when it came out. 

Ok, but this didn't affect the coaxial jack input from a AC supply, just the battery. If it did, that wouldn't still power up the meter. Correct? 

[schmitt trigger]

What Georges80 said

The Tektronix engineers must have applied a DFMEA analysis, during which they identified a particular failure mechanism which could trigger an excessive battery negative terminal current.
The simplest mitigation for that fault may have been a fuse.

Case in point: the fuse blew because an incorrect connection.
I would not bypass the fuse.

[georges80]

Yeah, didn't imply it would be used in a vehicle

Just pointing out that fusing both +/- is important where you have cables that carry +/- and that there can be unintended currents that can flow through a -ve wire due to losing a ground connection somewhere else when there is a common chassis (in the tek situation the common chassis is the metal case). In that situation the fuse is to protect the wire from burning out in a high current situation versus protecting the actual electronics etc.

No idea in your Tek, you would need to see whether a scenario as above is possible. Maybe there were just being safe and following company policy when running such cabling...

cheers,
george.

[videobruce]

Could a diode be substituted for that fuse?

[georges80]

You could just use a nail 

cheers,
george.

[videobruce]

I mean to protect against reverse polarity. 

[georges80]

Add the diode and you've added a voltage drop (say 0.5V), what will the charger think is going on with the battery?

Put a fuse back in there.

cheers,
george.

[schmitt trigger]

The battery is a rechargeable one, correct? If so....
How are you going to connect the diode?  anode or the cathode to the battery?

Think your answer for a second.

[videobruce]

I did forget about the voltage drop.
I went ahead and wired a small glass fuse in place (same amperage) secured it to the ribbon cable header right next to those fuses. Trying to order those 'odd' fuses would of been a pain, then waiting a week wasn't worth it.

[tonyalbus]

Will be an owner of a RFM151, does anyone has the CSS151 software that he can share?

[videobruce]

PM me with your e-mail address.

[tonyalbus]

Thank you @videobruce i reveived it!

[videobruce]

I was wondering happened to you.
Glad to do it, it's a nice unit in spite of it's age especially considering the original street price of around $2k.