Dedicated to Vintage Hewlett-Packard Documentation
HP3577 Cursor Knob Repair – by Paul Grohe
Here is Paul’s photo collection and procedure for repairing the cursor knob optical encoder assembly on the HP3577A (and other similar instruments). Photos are “click to enlarge” links
The HP3577 suffers from a failure mode where the cursor knob stops working. The cursor can be moved with the arrow keys and the analyzer otherwise functions normally with no error codes. This failure is caused by a burnt out incandescent lamp light source inside the optical rotary encoder assembly. Inside the encoder is a 5 Volt, 60mA, T1 size (3mm) wire lead “grain-o-wheat” bulb (Chicago Miniature #683 or equiv). Simply replacing the bulb restores full operation, but getting to the bulb is the fun part. For the HP3577, the front panel needs to be loosened enough so that it can be raised up enough to gain access to the encoder. The self-contained encoder module is mounted to the front panel by just the shaft mounting nut and simply drops out. It is not necessary to remove any major PC boards or modules. This type of encoder is also used on several mid-80’s vintage instruments which may suffer from a similar failure mode.
This procedure is to restore operation of an inoperable cursor knob on a HP3577A Network analyzer
Remove cursor knob. There are two hex set screws. DO NOT remove the shaft nut yet
Remove top and bottom panels
Remove top and bottom plastic filler strip (top and bottom) if present.
Remove side adhesive foam inserts if present (both sides). It should easily peel off
Remove carrying handles (both sides, front and rear). Note that the front fastener has fingers that slip under the panel bezel and the rear fastener wraps around the edge.
Remove perforated side covers. Slide towards the rear and lift up
Place instrument on it’s rear panel (with front panel straight up) and remove ALL the screws around the perimeter of the front panel bezel. These are the recessed flat-head screws along the edge of the bezel that were covered by the strips. Note that the two shorter flathead screws are used on the CRT. On the sides, remove the four silver screws, the two recessed flat-headed screws and the two flat heads on the angled frame edges closest to the front panel.
Flip unit bottom-side up and disconnect the four front panel semi-rigid cables on the bottom. Only loosen the top nut where the coax enters the connector – DO NOT loosen the lower nut. Gently tuck the cables in towards the chassis to keep them from catching on outside items or the table. The center pins are VERY delicate.
Flip the unit back topside-up – but be VERY careful not to crush the loosened semi-rigid cables underneath. Remove all the small screws from the large shield plate covering the CRT module.
Remove the rear CRT shield plate.
Remove the long processor board shield.
Disconnect CRT module ribbon cable from the processor card.
Disconnect reference coax connector.
Remove both processor boards and store in a safe, anti-static place.
Remove large screw in the upper side right corner inside the processor board well.
Remove large screw on the upper left outside corner (Second from the rear inside the “channel”).
Remove smaller, long screw on the upper right top corner.
Remove Large screw on the upper left top corner.
Remove long screw in the lower-rear of the CRT module.
The CRT module should now be able to slide forward. You may need to apply a little force. The module only needs to slide out about 1-2″. Be careful, the forward travel is limited by two large trashcan capacitors in the rear of the module.
Carefully place instrument on it’s rear panel (with front panel straight up) and start wiggling the entire front panel upwards to break it free.
Start lifting the right-side of the panel. Use something to prop the panel up high enough to get your hand underneath (An Altoids candy box is used in the picture).
Once the panel is raised up enough, the encoder should be visible.
Unplug the encoder cable and remove the encoder shaft mounting nut. The encoder is mounted only by the shaft.
Be careful, there are five spacer washers on the shaft under the panel. Hold the bottom of the encoder while removing the nut so that it does not
suddenly drop down and “spill” the washers into the instrument.
Remove encoder module from the instrument.
The encoder module plastic body snaps apart. Gently pull apart the two top tabs while pulling the rear cover off.
The lamp should now be visible. It is friction-fit onto the light chamber and held by the metal reflector. Simply pull the lamp out by the leads.
Unsolder and replace the lamp with a new 3mm (T-1), 5V, ~60mA “grain-o-wheat” wire lead bulb. The base of the bulb should be flush with the case. Carefully route the leads making sure they will not short together of against the components around the lamp solder pads.
At this this point, you may want to bench test the encoder to make sure it works. You will need a 5V, 100mA power supply and an oscilloscope, logic probe or DMM to monitor the outputs. The outputs are standard TTL levels. Turning the shaft should create pulses on both outputs proportional to the speed.Encoder Pin Out:
1. GND Black
2. OUT Phase 1
3. OUT Phase 2
5. +5V WHT/RED
Re-install encoder and reassemble the instrument in the reverse order. Be careful with the semi-rigid coax. Do not over tighten.