Whe have an old HP4890A GC with FID.

We have a trouble with de detector signal. Sometimes the detector signal is 0, but the flame is burning. We think that the problem could be caused by the electronics, perhaps it's a bad internal electrical connection.
Has anyone had this problem and if so, could I repair it by myself or we must call the technical sevice.
Any help much appreciated

Don't know if you really meant HP4890 or if you type like I do and meant HP 5890. On the 5890, there is a spring which connects the detector board to the collector body. You might check to make sure it is touching the collector, if it isn't, you will not get a signal. If needed, you can pull the spring slightly toward the collector to make contact. Be careful that it doesn't touch anywhere else or the signal will max out. After you have checked for proper contact of this spring, you might try gently pushing down on the spring (with the ignitor castle removed) until it touches the FID body to see if you get a maximum signal.