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Azur Electronics

REPAIR HP 8175A DIGITAL
SIGNAL GENERATOR
July 2014

The HP 8175A Digital Signal Generator was dusty on the inside and there were missing screws and loose coax cables, it was completely stripped down and all the parts thoroughly cleaned up.

Top Cover and Rear Connector Plate removed
The top plate is for safety, covering the switched mode power supply.

Top Plate removed
Warning!: lethal power in the A10 Power Supply, A3 Display System and CRT Assembly.

Removing the Boards from the Motherboard and Chassis is a bit difficult as they are a tight fit. The extraction tool is made of plastic so not much help. Take care not to damage the Board!

A10 Power Supply
A20 Microprocessor Board
The A20 Microprocessor Board is normally 08715-66520 but this unit has 08715-66524 fitted.

The rechageable battery had leaked so was unsoldered and the Board cleaned up.

Battery NICAD, HP 1420-0251, GE 2.4V 110mAh, K02A113AA-GT1 8621, 14.5mm diameter, 35mm long plus solder tags.

I need to find a replacement - see below.

A30 Clock Board
This has 3 SMB coax cables which have to be removed. Left to right they are: Ext Clock (Clear Sleeve) to Rear Panel EXT CLOCK; Trig (Yellow Sleeve) to Front Panel EXT INPUT; Ext Ref (Black Sleeve) to Rear Panel EXT REF.

A30 Clock Board
The A30 Clock Board is 08715-66530.

Confusingly, the other 4 SMB coax cables are not internally connected. These are from the Front Panel: TRIG OUTPUT (Red Sleeve); OUTPUT A (Blue Sleeve); OUTPUT B (White Sleeve). Plus from the Rear Panel: TRIG OUTPUT B (Yellow Sleeve). These unconnected cables made me think the 8175A was incomplete. However they were fitted for a future Option 002 Waveform Generator. This is the A70 Dual Arbitary Waveform Generator 08175-66570 which is intended to fit at the bottom of the A1 Motherboard. This option cost $3,456.

A40 Data Board
The A40 Data Board is 08715-66540.

A50 Fine Timing Board
The A50 Fine Timing Board is normally 08715-66552 but this unit has 08715-66523 fitted. If the Option 001 is not fitted, then a Buffer Board 08715-66550 is fitted instead.

Chassis with Boards removed
The rest of the 8175A can now be dismantled.

A3 Display System
Danger!: before disconnecting the CRT anode, discharge it to ground first.

The A3 Display System is an OEM unit from Hitachi 0950-0894 and is the same as in the HP 1630D/G Logic Analyzer.

Front Panel
The 4 coax cables have to be unsoldered from the BNC connectors.

The CRT is made by Matsushita M22JPT3GH/M
The A2 Key Board is 08715-66502.
Rear Panel with Fan and Side Plate with Fan
Both Fans are 115Vac operation.
Chassis with inner board frame
The inner board frame can now be removed with the A1 Mother Board.

Inner board frame with A1 Mother Board
The A1 Mother Board is 08715-66501.
The A1 Mother Board has the DIN41612 connectors to the plug-in boards as well as maintenance connectors at the top to allow any board to be tested out of the frame to provide easy access to both sides of the board.

All re-assembled complete with replacements for the missing screws and the coax cables tyrapped to the inner board frame. Still need a replacement battery. Powered up on a Variac with just the Power Supply, then one Board added at a time. Powering-up slowly on a Variac allows you to monitor the mains voltage and current increasing, but does cause various errors in the Power Supply so the red LEDs will turn on. Switching the power on/off at mains voltage and the green LED should be on with all the red LEDs off.

The 8175A now powers-up and passes all its self-test routines. Have a display with "System Page" screen obtained (green text on black background).

February 2015

Sourced a replacement battery from www.smallbattery.company.org.uk It is a NiMH (instead of NICAD) 2.4V 2 x 1/3 AA 350mAh with tags. 2.4V, 350mAh up to 1000 cycles, weight 15g, diameter 14.5 mm, height 34.0 mm. Need to check out the charging circuit before fitting the battery.


March 2015

Without a battery, VBAT is +4.980V with mains power on. Fitted a new battery and VBAT is +2.499V with mains power off. With mains power on VBAT now climbs to +2.696V indicating charging.

NOTE: NiMH batteries do not like being trickle charged when they reach full charge. As this equipment will probably be used only intermittently then this should not be a problem. However the equipment does need to be powered up occasionally so that the battery does get a charge!