6 Other Pulse Generators

6. Other Pulse Generator Repairs, Modifications and Teardowns

INSTRUMENT REPAIRS & MODIFICATIONS (IN ALPHANUMERIC ORDER)

# Action Status Problem Manufacturer Model Function

1 Repair TBD Erratic CSC 4001 5MHz Pulse Generator

2 Repair Ongoing Bad delay ctrls Hewlett Packard HP8082A 250MHz Pulse Generator #1 250MHz

3 Repair Fixed Noisy controls Hewlett Packard HP8082A 250MHz Pulse Generator #2 250MHz

4 Design Done Ext Trig Vin for HP HP8082A Input voltage limit protection

5 Repair Ongoing Corrosion Pulsetek 233 Dual Pulse Generator #1 50MHz

6 Repair Ongoing Bad ext i/ps Pulsetek 233 Dual Pulse Generator #2 50MHz

7 Repair Ongoing Bad controls Pulsetek 240 Single 50MHz Pulse Gen w. adj Tr/Tf

1. CSC-4001 0.5Hz - 5MHz pulse generator

Problems: 1. No output / 2. Added power LED / 3. Erratic

New text box

2. HP8082A 250MHz Pulse Generator #1

Problem: Ext I/P N/S

706pH01 HP8082A 250MHz Pulse Generator datasheet.pdf

(a) External trigger N/S, suspect Q10/Q11 on A3 Rep Rate board, SM CCT P.92, Assy P.89, BOM P.81. LIBS4 P.109. Q10/Q11 = HP1853-0284 = SMT1114 = Si PNP 10Vce Ic30mA 225mW, NSN 5061-01-261-4649 in a microwave transistor shaped plastic package. Found just 2 on eBay, $25ea including shipping to the UK.

It seems odd to me they chose what looks like an RF part when it doesn't have to operate at very high frequencies. The package strikes me as inconvenient too, it's a surface mount device with a raised profile on both sides. Presumably to limit/match inductance, capacitance, HP flipped one of them but had to drill holes in the pcb to accept the raised surface on both sides.

Unsoldering two of their pins and testing them revealed Q11 was in fact the culprit - the DCA-75 thought it was an LED. The other one checked out fine with hFE (dc gain) 76. The two I bought had hFE 60 and 137. Given the circuit topology of a differential long tailed pair that constrains the input to the setting of the input level pot on the front panel, I thought it wise to match Hfe as close as I could to Q10, and replaced Q11 with the hFE 60 transistor.

Next, the all-important test and verification of the Ext Input. The HP8082A only goes down to 1kHz. I need to feed a 1Hz pulse from the Wavetek 154 [I40] into the Ext Trig for the HP8082A to generate a short pulse with controlled rise time in the order of nanoseconds. This pulse will be used to trigger my experimental Pockels cell avalanche driver (details to follow in the Q-switch section).

Right:

Top trace is 8082A

20ns 7.4V output.

Bottom trace is Wavetek 1.5V pulse into 8082A Ext Trig

Left:

The Wavetek (top) feeds into the 8082A Ext Trig.

Right:

Wavetek at arbitrary

255Hz proves there are no missing pulses

(b) 2022 - Many HP8082A front panel controls were erratic so I stripped the front panel and found the slide switches are actually constructed on the pcb and there is nothing to replace. Switch cleaner was the only remedy I could apply and amazingly it did fix most things, and an ultrasonic bath removed unsightly gunk from all knobs, but the delay switch and its rotary pot have limited control which I assume is down to another issue on the mainboard. I bought another 8082A for £50 as a cheap source of parts but found it actually works better, so now the original 8082A is the backup / parts supply, although both generators do work otherwise.

Bizarrely the variable rise and fall times don't appear to work correctly on either unit on the 2ns pulse width setting. I'm not sure if this is a fault or a design flaw.

3. HP8082A 250MHz Pulse Generator #2

Problem:

text

4. HP8082A DIY Input Voltage Limiter

Although Q11 blew and not Q10, it bothers me the Ext Trig input can only handle up to ±6V when both the HP8082A and the Wavetek outputs can go up to ±10V if offsets are added.

Given the rarity and high price of the SMT1114, I would not want to inadvertently blow Q10 up. I didn't want to muck around with the internal pcbs so instead built a tiny external voltage limiting box:

Initially I worked on the assumption I would allow a couple of volts into the trigger and selected a 1kΩ input resistor and a 2V0 zener diode to limit the input, forward fed by a fast diode, a known way of countering a zener diode's high shunt capacitance, but found this introduced ±400mV spikes at the transitions. Adding a 180pF capacitor took them out but seemed counter-productive.

Given the HP8082A ext trig input can work down to just ±200mV, I decided a better solution would be a couple of diodes back to back. I selected fast diodes to begin with but they also introduced noise and slower diodes were little better.

Putting the zener back in to experiment further, I inadvertently put it in back to front and found it produced a better result than ordinary diodes, introducing a 30ns rise time and 800ns fall time. The
Tek 571 curve tracer revealed its VF around 780mV at 5mA. I added a diode across it to counter negative spikes, a 1N4007 proving to have the best characteristics. This circuit worked well but the edges still overshot by about ±200mV. The HP8082A has a claimed input impedance of 50Ω on its Ext Trig although its schematic shows a 75Ω resistor to 0V, R10, in the input. Regardless, when I added a 50Ω terminator to the output of the limiter, the overshoots disappeared.

I tried it on the HP8082A and found the same result. Happy with the solution, I put it in a 2" long Pomona metal BNC box, but it stuck out from the instrument and I was worried I would catch it and damage the Ext Input BNC. As a final solution I chopped an eBay China 2" long aluminium box in two and made a 1" cube BNC box for the limiter that is short enough not to protrude excessively when in place. I can now run the Wavetek 154 full output voltage into the HP8082A Ext Input without fear of damage: