1. General Instrument Repairs, Modifications and Teardowns
Action Status Problem Manufacturer Model Function
Repair Ongoing Ch2 stuck in CC American Reliance PPS-1202 Programmable PSU
Restore Fixed Lethal Chinese Nawzad N&H NHT-4000W Mains Variable Voltage Regulator
Repair TBD Erratic CSC 4001 Pulse Generator 5MHz
Repair Fixed Bad IEC filter Hewlett Packard HP4952A Protocol Analyser
Repair Ongoing Bad delay ctrls Hewlett Packard HP8082A 250MHz Pulse Generator #1 250MHz
Repair Fixed Noisy controls Hewlett Packard HP8082A 250MHz Pulse Generator #2 250MHz
Repair TBD Only ±1.7kV Leysop M5000 ±2.5kV EO Differential Amplifier
Repair Ongoing Corrosion Pulsetek 233 Dual Pulse Generator #1 50MHz
Repair Ongoing Bad ext i/ps Pulsetek 233 Dual Pulse Generator #2 50MHz
Repair Ongoing Bad controls Pulsetek 240 Single 50MHz Pulse Gen w. adj Tr & Tf
Repair TBD Inaccurate Sencore LC-53 LC analyser
Repair Fixed Abused by owner Texscan RA5030 0-1GHz 0-50dB Variable Step Attenuator
AMERICAN RELIANCE (AMREL) / MOTECH
Amrel PPS-1202 dual channel 0-18V 4A Programmable PSU, 3.5 digits, resolution 0.05% 5mV / 0.15% 1.5mA
Problem: Ch2 stuck in CC mode / Noisy fan
This is a nice dual channel 0-18V 4A programmable PSU that I bought without realising one channel was faulty. The seller gave me a 75% refund and allowed me to keep it.
Overview of series here:
1. Ch2 Stuck in CC mode
When the PSU boots it first identifies itself on the LCD and momentarily displays firmware build V8.02A.
Channel 1 works fine but according to the LCD (the little dot to the right of 'CC2') channel 2 is stuck in Constant Current (CC) mode and its output is 29 Volts, way beyond its supposed maximum of 18V:
My first thought was a fault with Ch2's output driver. I looked on the web for a service manual and schematics, and found a post on EEVblog about the 1-ch 18V 4A PPS-1002 that led into a discussion comparing it with a 2-ch 0-30V 3A PPS-1204, which is very similar to my 2-ch 0-18V 4A PPS-1202:
https://www.eevblog.com/forum/testgear/motech-amrel-pps-1002-diagramschematic/
Links are provided for manuals, added below, but full schematics appear unavailable; there is a block diagram in the Service manual, but little else. However the discussion describes an amazing reverse engineering of the address decode logic held in a GAL16V8D PAL on the 1002 which is probably the same in my 1202, and there is a technical description of the 1204 circuitry with a partial schematic.
Popping the lid off my 1202 unsurprisingly reveals a similar design to the 1204: a large mains toroid is mounted behind the front panel and two driver pcbs sit on heatsinks at the rear. The IEC mains inlet feeds into a pcb with rectifiers and large smoothing electrolytics. In the middle two analogue pcbs plug into a small digital motherboard mounted on the base containing a Temic TSC80C31-16CA µC, a TI TMS27C512-10 Eprom, an Atmel 93C46 EEP and a Lattice GAL16V8D-25LP PAL. Chip manf dates are circa 1999.
As with the author, I too removed my Eprom and EEP and saved the contents. Wix won't upload BIN or HEX files, so I uploaded both BIN files below as .TXT files. If you D/L them you need to rename them .BIN.
The analogue boards are identical, and each has an identical connector that leads to one of the output driver blocks. The analogue pcb nearest to the mains toroid at the front is Ch1.
Swapping their positions in the mobo revealed the fault appears to lie with analogue board 2, as the LCD CC indicator swapped from CC2 to CC1, and I was able to correctly program Ch2 and get a valid voltage out but no longer on Ch1 which is now stuck at 21V. This suggests the output driver stages are ok.
However there appear to be a couple more issues as Ch2 LCD voltage is incorrectly shown as fixed at 13.868V with over-voltage protection set, and Ch1 is shown as 'OFF' despite the presence of 21V.
The next move is to try to debug analogue pcb #2. I'll first see if it's possible to build an extender to raise analogue pcb #1 above pcb #2 to facilitate easy comparison in-situ.
2. Noisy fan
The fan is very loud and is on all the time regardless of load. Maybe fit a quieter pwm fan? Noctua?
It's 80mm square x 23mm thick and has only 2 leads. The fan label says: AVE AV825H12B ZP 12V 180mA.
I assume the H means high speed/power as on the net I found a 120mA M model and 90mA L model.
I assume B means it has better Ball bearings than the more common S model which I'm guessing means it has cheaper/noisier/shorter life Sleeve bearings. I can find nothing more about it or the manufacturer. Noctua explains fan bearing differences here: https://noctua.at/en/sso-bearing
This video shows a simple mod to an Amrel PPS-2322 that only turns the fan on when a heatsink is hot:
https://www.youtube.com/watch?v=XUs7ZQLJke0
I'd rather have one that runs it slowly instead of flat out.
The same guy above also has a teardown of an Amrel PPS-2322:
http://www.kerrywong.com/2015/05/06/amrel-pps-2322-programmable-power-supply-teardown/
CHINESE MAINS REGULATOR/DIMMER
Nawzad N&H NHT-4000W 'thyristor electric voltage regulator AC0-220V'
Problem: lethal design
The price of variacs see [Variac Repair & Add Meters] has risen considerably over the years, regardless of being old, original workhouse British types or new Chinese variants now with high shipping (2024).
In 2023 I noticed eBay China selling semiconductor based mains regulators for £16 and bought one. Having already observed underhand practices with the Chinese 75W 12Vdc to 110Vac 60Hz inverter I bought in the past see [Projects: 20Vac 60Hz PSU], I did expect some issues, but nothing like the many I discovered.
Original auction/ad:
220V 4000W Variable Voltage Regulator Speed Motor Fan Control Controller UK Plug
The inductive load is 1/3 of the resistive load.
Do not use with air pump, electric/electromagnetic machinery or LED lamps as these will be damaged.
This product is a metal case and must be grounded safely.
Specification:
Input Voltage: AC 220V
Output Voltage: 0~220V (Adjustable)
Adjustable Voltage Range: 0~220V
Max Power: 4000W (resistive load)
Rated Current: 9A
Max. Current: 18A
It's only specified up to 220Vac so I opened it to see if it would withstand UK mains which is specified as 230Vac nominal -6% +10% so can reach 253Vrms (x √2 = 357Vpk). The electronics seemed fine but the mechanics revealed it to be a lethal nightmare. The issues are not expensive to resolve and can only be put down to ignorance. Perhaps tellingly, it didn't even have the infamous 'Chinese Equipment' CE mark.
The first thing to note is the voltage dial, which has graduations from 0 to 22. Given the specified operating voltage of 220Vac, the average consumer would likely assume this to mean the output voltage will vary from 0 to 220Vac. Of course it doesn't, and the word thyristor in the description is the giveaway, although the part is actually a BTA41-600 triac. A thyristor performs the same function but only conducts in one direction whereas a triac conducts in both directions.
The circuitry is very simple. Triacs are used as switches for ac signals, and in this instance all that is happening is a portion of each full mains sinewave cycle is being switched out. You get less average voltage, but you still get the full on mains voltage. However its sinewave is heavily distorted with vertically chopped sections, which is why the ad warns: 'Do not use with air pump, electric/electromagnetic machinery or LED lamps as these will be damaged.'
LETHAL
Voltage dictates insulation distances for bare wires that must be maintained to meet safety regulations. There are multiple instances where this is not the case. There are no fuses (not even in the plug) and the mains inlet earth lead goes directly to the output socket, as a result the metal box is not earthed.
'This product is a metal case and must be grounded safely.' Clearly they expect me to add this feature!
Unlike UK mains sockets that have a spring-loaded cover, the output mains socket has bare contacts for live and neutral, although I saw these before I bought it, intending it for a permanent connection.
Internal connections are not insulated. Unsurprisingly the 'overload protection switch' is just a single pole mains inlet switch; the mains lead isn't retained, and ventilation holes in the box are so large you could get a screwdriver into it which would instantly kill you when it touched the exposed mains.
In the original layout the centre terminal of the pot, at mains potential, was only 2mm from the case. The inside of the case is painted but this is no remedy. The case isn't even earthed! Utterly lethal.
Ordinarily I'd chuck it in the bin but having paid for it I decided to pick up the challenge to make it safe, or at least safer, see description and photos below:
IMPROVEMENTS
At least the triac is in an insulated package so the heatsink is too. A quick remedy was to rotate the heatsink and pcb 180° so there is now plenty of clearance between the pot leads and case. I also sprayed the back of the pcb with an insulating conformal coating for good measure.
I replaced the uninsulated soldered connections with insulated crimps and secured them with RTV, which I also added between the heatsink and case. I drilled holes for earth leads to both case sections, first scraping the paint off both sides, and verified a low resistance connection throughout.
MORE TO BE DONE
The switch should be double pole but there isn't room and there should be fuses on both the input and output, and which I should fit, but haven't. For now I have simply chopped off the original unfused plug and replaced it with a fused one. I also added a Ty-rap to the mains lead just inside the box so it can't be pulled out like the original that relied solely on its rubber grommet.
The lethal output socket should be replaced with a UK safety socket but again there isn't room so I've added warning labels. The case holes are a major problem and a simple workaround to this is a plastic case around the entire box. In fact the best solution overall would be to mount it in a sealed box with the necessary switch, mains socket and separate input and output fuses on that instead.
I dread to think how many people who bought this on eBay are dicing with death every time they use it.
FIND REVERSE-ENGINEERED CCT FROM MY LOGBOOK
Left, original eBay photos; Right, there was only 2mm between the case and the pot centre connection.
The controller is simply a triac with a pot to vary cycle width. Conformal coat added
HS rotated 90° is safer Crimps & RTV added Fused plug instead Paint removed for earth
Earths added to all of case RTV secures pcb & HS Case holes covered Dial vs Vout
Tek P5102 HV probe Tek THS720A on Vout Output is steady over time
As can be seen below, it varies the average mains voltage by chopping out a portion of the leading edge of the positive and negative ac cycles whilst maintaining the rest of the peak to peak mains voltage on the output. Even long after the cycle peaks are eaten away, the overall pk-pk voltage is still dangerous. The leading edge is never less than ~90ns. I didn't see any spurious spikes. The LED voltage indicator is meaningless but like the dial it gives a non-linear relative value, see my graph above.
Tek THS720A 5ms, 500mV/div Tek THS720A 5ms, 100V/div
Tek THS720A 5ms, 100V/div Tek THS720A 5ms, 100V/div
CONTINENTAL SPECIALITIES CORP
CSC-4001 0.5Hz - 5MHz pulse generator
Problems: 1. No output / 2. Added power LED / 3. Erratic
New text box
HEWLETT PACKARD
HP4952A Protocol Analyser
Problem: IEC filter blew
When one of these appeared for £50 I bought for fun it as a memento of the era I worked in. Given the freely available PC-based apps now doing largely the same thing much more easily, there is otherwise little point having one. As if to prove the point, the one time I did try to use it, it let me down.
Whilst attempting to log temperatures on the Tek 109, see [Projects: Tek 109 1-Shot & LF Mod] and discovering my PC serial port refusing to communicate with my Pico TC-08 temperature datalogger, I thought I could have a bit of fun with the 4952A. This was short-lived when a few minutes after I powered it up, I was met with a puff of smoke and a ghastly stench that took several days to vent out of the lab. Weirdly the CRT was still working. Of course I shut it down immediately. Perhaps it was mocking me for trying to use it for what was a trivial loss of a signal later found almost instantly with a scope probe.
When the 109 mod was complete, I returned to investigate the 4952A. Inside it's crammed full of pcbs, see below. Nothing was immediately amiss, so I removed the top 2 pcbs and on the bottom pcb I spotted black gunk over a couple of MOSFETs located close to the main IEC connector on the rear panel.
My immediate thought was the MOSFETs were dead. It wasn't possible to test them in-circuit, so I unsoldered them but they both checked out fine and once I'd cleared the gunk off the pcb, there was no sign of damage there either.
Peering in more closely at the filter on the back of the IEC mains inlet, I noticed traces of black gunk on the metalwork below, and a tiny hint of it on the bottom of the filter itself. It dawned on me a capacitor inside it must have blown, melting and ejecting the black potting compound outward with considerable force (and smoke) to cover the nearest components, which were of course the MOSFETs.
No-one stocked the original IEC, fuses, switch and Schaffner filter combo but I did find a slightly different filter in the same arrangement. I mused any filter was better than none since the 4952 would rarely be used; I just wanted it working again, and indeed once retrofitted, it came back to life.
Pondering why it had exploded in the first place, I worried I might not be so lucky with other older instruments that were in use, convincing me to look at stabilising the lab mains voltage close to its nominal 230Vac instead of the 250Vac it now seems to run at, see [Variac Repair & Add Meters].
HP8082A 250MHz Pulse Generator #1
Problem: Ext I/P N/S
(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
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:
HP8082A DIY Input Voltage Limiter
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:
Add photo 701jH25 of 1" on 8082A
(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.
HP8082A 250MHz Pulse Generator #2
Problem:
text
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Leysop M5000 OE High Voltage Differential Amplifier
Problem: Outputs won't go higher than ±1.7kV.
INITIAL INSPECTION 24/06/19 LIBS5 P.56
Leysop ignored my request for a service manual but in 2022 I found
their website now has a link to a user manual complete with
circuit diagrams, see below.
http://www.leysop.com/m5000.html
The ±2.5kV Outputs are fused at 50mA max output current.
Original photos from the eBay auction:
Pulsetek 233 Dual 50MHz Pulse Generator #1
Problem: The EXT TRIG / EXT GATE input, LED and controls don't work.
text
Pulsetek 233 Dual 50MHz Pulse Generator #2
Problem: Corrosion.
text
Pulsetek 240 Single 50MHz Pulse Generator with adjustable rise and fall time
Problem: Many controls are erratic.
Switch cleaner fixed most of them.
Sencore LC-53 Capacitor & Inductor Analyser
Problem: Inaccurate.
text
Texscan RA5030 / RF557 variable rotary attenuator
Dual knob rotary attenuator, 0-1.5GHz, 0.5W, 0-50dB in 1dB steps
Problem: Electrical short in certain positions
Repaired: 02/11/20 LIBS6 P.6
Damaged due to being opened by a previous owner unfamiliar with the assembly construction.
Attenuation can be set from 0db to 50dB in 1dB steps. A large inner knob selects 0, 10, 20, 30 or 40dB and this is added to the 0 - 10dB value selected by the small knob on the end.
Externally there is a BNC socket at each end. At some point a previous owner marked the one nearest the knobs Sk2, and the one at the rear end Sk1.
Usually when I dismantle equipment like this I first think how external connectors might affect its construction. It's tempting when noticing 3 screws at each end of its metallic tube, to think all that is needed to pull the unit apart is to undo them, with no consideration for the two BNC connectors.
Of course any connections to these must first be disconnected.
It turned out the reason why this didn't work was precisely because the last owner didn't think about this. Result: the BNCs jammed and bent the grounding/EMI fingers on the resistive sections, and the wiper blade on SK1 was bent into a '<' shape instead of '|' which depending on the knob position, was either open circuit or shorted to ground through the fingers.
CONSTRUCTION
Internally the attenuator is realised using two separate sections of resistors that rotate.
Each section has an inner and an outer layer of resistors arranged in a ring.
Each outer end of the resistor sections has a ring of round metal bumps corresponding to each dB position that make contact with a wiper contact blade on an external BNC connector.
The inner end of each section also has a ring of round metal bumps that mates with a central wiper that electrically connects the two resistor sections together.
The central twin wiper assembly mounted on the outside is isolated from the case/ground. Internally it consists of a square spacer of what looks like paxolin between contact blades joined by two rivets, forming an electrical path between the inner rings of contacts on the two resistive sections.
OPERATION
The resistive sections rotate on a concentric spindle. The outer knob/spindle rotates the bottom resistive section to achieve 10dB steps. The inner knob/spindle rotates the top resistive section to achieve 0-10dB steps. The central wiper affectively sums them.
Complete assembly Knobs removed Grey gasket is inert One BNC wiper is bent
Central dual wiper removed Closeup of central dual wiper assembly SK2 0-9dB R section pulls out
SK2 R section removed Sk2 R section outer resistor ring Sk1 x0db range R section
SK1 R section & bent EMI/GND fingers Wipers wrt assy BNC wiper straightended Test setup
Above far right, DAS-45 dc source 1.0000V into Texscan SK2 & top DMM HP3468A; SK1 to bottom DMM Hp3478A.
After
straightening the bent BNC wiper and re-aligning all resistor section fingers, the texscan now functions as intended, see test results to right, together with my initial sketches:
Refer to table [T14] for the exact resistor values corresponding to the selected attenuation in dB.
text
µ Ω ± ° ⌠ ⌡ ∫ │ ─ √ φ θ Θ ∂ δ ζ ξ ς λ ψ ω τ µ Ω ∆ Δ ∑ ∏ π Ξ ○ ≠ ³ ² ±