Resistor Breakdown in FY6800 Signal Generator

[robot.golem]

Hello:

I am repairing my FY6800 signal generator, and discovered that one of the SMD 49R9 resistors resistors literally broke in half (see 1-st photo).  On the second photo there's peculiar yellowish residue at the soldering point (photo 2).  What have cause the resistor to break?  -- It does not look like a thermal breakdown, or is it?

Thanks!

[pcwrangler]

Looks like a faulty resistor resulting in catastrophic failure. You can see the burns on top and it had to get hot enough to flow the solder at least partially. Interesting break though. Lucky it didn't crater your pcb.

[bob91343]

It appears that a strong signal was put into this device.  Making a 50 Ohm resistor that hot probably took a Watt or more.

[pcwrangler]

Were you testing a transceiver and accidently hit the PTT?

[robot.golem]

I think it's a 1/2 W resistor, 1206 type.  I will try to replace it, but it remains to be seen if the op. amp's output circuit is not fried as well.  It would greatly help to have schematics for the board, but I could not find it, not for this model.

[robot.golem]

What is a PTT?  This is not a final stage of the amplifier.  Could it be affected at all by what I connect to the instrument's outputs?

[bob91343]

PTT means push to talk, the transmit switch on a transceiver.  The query was whether a transceiver was being tested and the transmit switch inadvertently operated, thus frying the output section of the generator.

I did that to my HP 8657B.  Its protection circuit wasn't enough to protect it and I damaged two ICs.  Fortunately they were cheap and easy to replace and now the generator is functional again.

[bobbydazzler]

Looks like it got hot enough to melt it's own solder and soft explode in the middle, fingerprint of a massive overload.