A "miracle" problem at my HP 8116A function generator (ser# 2124G02272)
At start-up it showed E21 - thus I started to investigate...
test 1: measure the supplies (all boards connected)
result: +24V (after F1/W15, fuse) and -24V (after F2/W16) are 0.00V - both fuses are blown
all other voltage levels are within given tolerances (correct)
note: change #64 applies: fuses instead of Zero-Ohm-Bridges, F1 replaces W15, F2 replaces W16
mmhmm.. shall be 125V, 5A fuses - 0.5A may probably correct value
test 2: visible check for suspicious parts on the board (dark resistors, leaking caps etc.)
result: nothing suspicios found
test 3: disconnect all boards, test main board A1 only, measure current with ammeter instead of fuses.
result: main board sink ~0.6A @ +24V, ~0.8A @-24V - full low impedance short
note: max. current limited by the +/-24V regulation circuit
- solder two 12k resistors over the burned fuses, to be able to measure in the circuit without excessive current.
voltage drops down to ~30mV behind the resistors => short impedance less than 25 Ohms
- analyse the schematics for potential locations creating the low impedance shorts
(transistros, diodes capacitors, theoutput amplifieer is normally the first candidate)
schematic 1B is power supply (until fuses W15 and W16)
schematics 1C, 1E and 1F do not use +/-24V
schematic 1D use +24V, but not -24V
+24V at U200A (pin 4) OpAmp positive supply, GND is negative supply => may cause at +24V, but not at -24V
+24V at R211 (1 MOhm) and R218 (1 MOhm) to Ramp Current Source - can't sink such high current (1 MOhm)
schematic 1G use +/-24V (ouput amplifier, based on tranisistor, would expect the problem there)
+24V at R516 (11k) - can't sink such high current
+24V at R558 (75R) to capacitor - can't sink such high current, would kill resistor (>10W)
+24V at R524 (100R) - can't sink such high current, would kill the resistor (>10W)
+24V at R533 (19.8R) - can't sink such high current, would kill the resistor (>10W)
+24V at CR503 - potential defekt, may have current via Q503, Q505, L500, Q507, L501, Q508, Q506, Q504, CR504 to -24V
would be a very complex (unrealistic) current path, but need to be tested
+24V at L504 - can't sink such high current via Q510 and/or Q512, would kill R543 and/or R545 (>10W)
verified by cutting connection - no change
-24V at L505 - potential defekt, current via Q511
verified by cutting connection - no change
-24V at R527 (100R) - can't sink such high current, would kill the resistor (>10W)
-24V at CR504 - can't sink such high current, would kill the resistor R537 (>10W)
SO WHAT NEXT?
Are there some know issues like Tantal capacitors?
Any tip or help is highly welcome.
PeLuLe