GBS Elektronik MCA 166 - Manual (Page 54)

_______________________________________________________________________
54
5 Troubleshooting
General Remark:
Before changing anything concerning the hardware, plugging or pulling cables etc. shut
down the high voltage and switch off the MCA.
Soldering or manipulating with the SMD boards is strongly not recommended for users.
We have specialists for that. If you have a problem which cannot be solved by the table
below, contact us. If it is really a hardware fault, we can repair this for a fair price.
Software tells "MCA not connected!"
• Switch on the MCA.
• Check cable between MCA and Computer. Serial cable at HP200LX may be loose or connected upside
down. Connect cable correctly
• Check MCA battery. Charge MCA battery
The MCA is not recognized (“MCA not connected”) running MCA DOS Software in a DOS-Box on
Notebooks
• Disable Power Save Mode for COM1 Port
MCA lamp does not flash regardless if the switch is on or off
• MCA battery empty. Charge MCA battery.
• The MCA firmware programming switches are in a wrong position. The firmware programming switches
must be all in “off” position. Do not play around with them!
MCA lamp flashes irregularly, Computer tells "MCA power failure" or beeps
•
MCA battery very low. Charge MCA battery.
• MCA internal or communication error. Switch off and on MCA, restart software.
Power Supply Error: Goto Diagnostics, Measurement is not possible
• MCA battery very low. Charge MCA battery and switch on HV and preamplifier supplies again.
• Go to diagnostics, blinking power supply currents indicate overload.
• Short circuit in detector? Check if detector is defective. Disconnect detector, try again. (Do not pull or
plug connectors when MCA is on!) Change detector.
• Too power-hungry preamplifier. A fully charged MCA battery may help sometimes. Otherwise you have
get a powerpack supply with increased current capability from GBS, or you have to exchange the
preamplifier.
• If the diagnostics menu shows irregular high currents or even overload although there is no detector
connected to the MCA and the battery voltage is ok, there may be a fault in the internal power supply.
Contact GBS for repair.
The threshold of the spectrum seems to be much higher than expected.
• The input polarity may be wrong. Does the threshold depend on radioactive source and count rate? Try
some other input polarity.
• P/Z may not be set correctly. Please check if P/Z is correctly adjusted.
• If the lower cutoff rises by itself, this is most probable caused by the autothreshold circuit which is
responsible for detecting the noise level and adjusting the threshold to it. This autothreshold circuit uses
the negative part of the amplifier output pulses to estimate noise. If now pole zero is severely
misadjusted with strong undershoot, the undershoot is misinterpreted as noise and the threshold is
increased, especially at medium and high count rates.
• Please check the detector preamplifier waveform. The decay half time of the signal should be optimum
40 us, but never shorter than 25 us. If it is shorter (as experienced with some NaI's) then it is not
possible to adjust P/Z correctly, the time constant of the preamp has to be adjusted instead.
The high energy part of the spectrum is reduced or even missing