Since I seem to be the local go-to for any dead electrical equipment, this brand-new Silverline polisher has landed on my desk. Purchased cheap from an auction this was dead on arrival. Checking the fuse revealed nothing suspect, so a quick teardown to find the fault was required.
Above is a photo of the commutator with the brush holder removed, and the source of the issue. The connection onto the field winding of the universal motor has been left unsecured, as a result it’s managed to move into contact with the commutator.
This has done a pretty good job of chewing it’s way through the wire entirely. There is some minor damage to the commutator segments, but it’s still smooth, and shouldn’t damage the brushes.
Chewed Wire
A quick pull on what’s left of the wire reveals the extent of the problem. It’s entirely burned through! Unfortunately the stator assembly with the field windings is pressed into the plastic housing, so it’s not removable. An in-place solder joint was required to the very short remains of the wire inside the housing. Once this was done the polisher sprang to life immediately, with no other damage.
This unit probably ended up at an auction as a factory reject, or a customer return to a retail outlet. If the latter, I would seriously question the quality control procedures of Silverline tools. 😉
After having a couple of the cheap Chinese PSUs fail on me in a rather spectacular fashion, I decided to splash on a more expensive name-brand PSU, since constantly replacing PSUs at £15 a piece is going to get old pretty fast. This is the 30A model from Mercury, which seems to be pretty well built. It’s also significantly more expensive at £80. Power output is via the beefy binding posts on the front panel. There isn’t any metering on board, this is something I’ll probably change once I’ve ascertained it’s reliability. This is also a fixed voltage supply, at 13.8v.
Rear Panel
Not much on the rear panel, just the fuse & cooling fan. This isn’t temperature controlled, but it’s not loud. No IEC power socket here, the mains cable is hard wired.
Main Board
Removing some spanner-type security screws reveals the power supply board itself. Everything on here is enormous to handle the 30A output current at 13.8v. The main primary side switching transistors are on the large silver heatsink in the centre of the board, feeding the huge ferrite transformer on the right.
Transformer
The transformer’s low voltage output tap comes straight out instead of being on pins, due to the size of the winding cores. Four massive diodes are mounted on the black heatsinks for output rectification.
SMPS Controller
The supply is controlled via the jelly bean TL494 PWM controller IC. The multi-turn potentiometer doesn’t adjust the output voltage, more likely it adjusts the current limit.
Standby Supply
Power to initially start the supply is provided by a small SMPS circuit, with a VIPer22A Low Power Primary Switcher & small transformer on the lower right. The transformer upper left is the base drive transformer for the main high power supply.
The other day at the local canal-side waterpoint, this TV was dumped for recycling, along with another later model Colour TV. This is a 1970’s Black & White mains/battery portable made by Thorn. It’s based on a common British Radio Corporation 1590 chassis. Having received a soaking from rain, I didn’t expect this one to work very well.
Tuner
Being so old, there is no electronic control of the tuner in this TV, and only has the capability to mechanically store 4 different channels. The tuner itself is a cast box with a plastic cover.
Tuning Lever
The mechanical buttons on the front of the TV push on this steel bar, by different amounts depending on the channel setting. This bar is connected to the tuning capacitor inside the tuner.
Tuner Compartments
Unclipping the plastic cover, with it’s lining of aluminium foil for shielding reveals the innards of the tuner module.
Tuner Input Stage
Here’s the tuner front end RF transistor, which has it’s can soldered into the frame, this is an AF239 germanium UHF transistor, rated at up to 900MHz.
Tuner IF Mixer Stage
As the signal propagates through the compartments of the tuner, another transistor does the oscillator / IF mixing, an AF139 germanium, rated to 860MHz.
Tuning Capacitor
As the buttons on the front of the set are pushed, moving the lever on the outside, the tuning capacitor plates intermesh, changing the frequency that is filtered through the tuner. The outer blades of the moving plates are slotted to allow for fine tuning of the capacitance, and therefore transmitted frequency by bending them slightly.
Mains Transformer
Being a dual supply TV that can operate on either 12v battery power or mains, this one has a large centre tapped mains transformer that generates the low voltage when on AC power. Full wave rectification is on the main PCB. The fuse of this transformer has clearly been blown in the past, as it’s been wound with a fine fuse wire around the outside to repair, instead of just replacing the fuse itself.
Chassis Rear
The back of the set has all the picture controls on the bottom edge, with the power input & antenna connections on the left just out of shot. The CRT in this model is an A31-120W 12″ tube, with a really wide deflection angle of 110°, which allows the TV to be smaller.
Main PCB
The bottom of the mainboard has all the silkscreen markings for the components above which certainly makes servicing easier 😉 This board’s copper tracks would have been laid out with tape, obviously before the era of PCB design software.
Components
The components on this board are laid out everywhere, not just in square grids. The resistors used are the carbon composition type, and at ~46 years old, they’re starting to drift a bit. After measuring a 10K resistor at 10.7K, all of these would need replacing I have no doubt. Incedentally, this TV could be converted to take a video input without the tuner, by lifting the ferrite beaded end of L9 & injecting a signal there.
Flyback Primary Windings
The flyback (Line Output Transformer) is of the old AC type, with the rectifier stack on top in the blue tube, as opposed to more modern versions that have everything potted into the same casing. The primary windings are on the other leg of the ferrite core, making these transformers much more easily repairable. This transformer generates the 12kV required for the CRT final anode, along with a few other voltages used in the TV, for focussing, etc.
Rectifier Stack
The main EHT rectifier stack looks like a huge fuse, inside the ceramic tube will be a stack of silicon diodes in series, to withstand the high voltage present.
Horizontal Output Transistor
This is the main switching transistor that drives the flyback, the HOT. This is an AU113, another germanium type, rated at 250v 4A. The large diode next to the transistor is the damper.
I’ve managed to find all the service information for this set online, link below!
[download id=”5616″]
More to come if I manage to get this TV working!
Well it’s time for a new DMM. After the last pair of eBay El-Cheapo Chinese meters just didn’t last very well, I decided a proper meter was required. This one is a Tenma 72-10405, stocked by Farnell for under £60. Not quite as many festures as the cheapo Chinese meters, but I expect this one to be a bit more reliable.
PCB Rear
Since I can’t have anything without seeing how it’s put together, here’s the inside of the DMM. (Fuse access is only possible by taking the back cover off as well. The 9v PP3 battery has a seperate cover).
PCB Rear Bottom
He’s the input section of the meter, with the 10A HRC fuse & current shunt for the high-amps range. The other fuse above is for the mA/µA ranges. The back cover has a wide lip around the edge, that slots into a recess in the front cover, presumably for blast protection if the meter should meet a sticky end. The HRC fuses are a definite improvement over the cheap DMMs, they only have 15mm glass fuses, and no blast protection built into the casing.
There are some MOVs for input protection on the volts/ohms jack, the jacks themselves are nothing more than stampings though.
PCB Rear Top
Not much at the other side of the board, there’s the IR LED for the RS232 interface & the beeper.
PCB Front
Most of the other components are on the other side of the PCB under the LCD display. The range switch is in the centre, while the main chipset is on the left.
DMM Chipset
The chipset of this meter is a FS9922-DMM3 from Fortune Semiconductor, this is a dedicated DMM chipset with built in ADCs & microcontroller.
I almost forgot about this bit of kit, that came with one of my LED torches as a Lithium Ion charger. As I never plug in anything that comes from China via eBay, here’s the teardown & analysis.
Another Lethal Charger?
Here’s the unit itself. It’s very light, and is clearly intended for American NEMA power points.
Specs
Claimed specifications are 100-240v AC input, making it universal, and 4.2v DC out ±0.5v at 500mA.
Considering the size of the output wire, if this can actually output rated voltage at rated current I’ll be surprised.
Opened
Here’s the adaptor opened up. There’s no mains wiring to speak of, the mains pins simply push into tags on the PCB.
PCB Top
Top of the SMPS PCB. As usual with Chinese gear, it’s very simple, very cheap and likely very dangerous. There’s no real fusing on the mains input, only half-wave rectification & no EMI filtering.
PCB Bottom
Here’s the bottom of the PCB. At least there’s a fairly sized gap between the mains & the output for isolation. The wiggly bit of track next to one of the mains input tags is supposed to be a fuse – I somehow doubt that it has the required breaking characteristics to actually pass any safety standards. Obviously a proper fuse or fusible resistor was far too expensive for these.
The output wiring on the left is thinner than hair, I’d say at least 28AWG, and probably can’t carry 500mA without suffering extreme volt drop.
Here’s an odd & sudden failure, the power adaptor for a Samsung device. It’s been working for months & on being plugged into the mains today the magic blue smoke escaped.
Samsung Charger
It’s one of their 2A models, for charging bigger devices like tablets.
Flash Burn
Strangely for one of these chargers, no glue is used to hold it together – just clips. This made disassembly for inspection much easier. Evidence of a rather violent component failure is visible inside the back casing.
PCB
Here’s the charger PCB removed from the casing. As to be expected from Samsung, it’s a high quality unit, with all the features of a well designed SMPS.
PCB Reverse
However, on turning the board over, the blown component is easily visible. It’s the main SMPS controller IC, with a massive hole blown in the top. The on board fuse has also blown open, but it obviously didn’t operate fast enough to save the circuit from further damage!
Here is an AVR powered optical tachometer design, that I adapted from the schematic found here.
I made a couple of changes to the circuit & designed a PCB & power supply module to be built in. The original design specified a surface mount IR LED/Photodiode pair, however my adjustment includes a larger IR reflectance sensor built onto the edge of the board, along with a Molex connector & a switch to select an externally mounted sensor instead of the onboard one.
There is also an onboard LM7805 based power supply, designed with a PCB mount PP3 battery box.
The power supply can also be protected by a 350mA polyfuse if desired. If this part isn’t fitted, then a pair of solder bridge pads are provided within the footprint for the fuse to short out the pads.
For more information on the basic design, please see the original post with the link at the top of the page.
Schematic
Here is an archive of the firmware & the Eagle CAD files for the PCB & schematic design.
Here is a cheap no frills microwave oven, which died after a few weeks of normal use.
Electronics Bay
Cover removed, showing the internals. Front of the microwave is on the left.
Timer
Closeup of the timer unit. Cheap & nasty.
Magnetron
Magnetron removed from the oven. Antenna is on the top, cooling fins visible in the center. White conector at the bottom is the filament terminals.
Magnetron Chokes
Chokes on the magnetron’s filament connections. These prevent microwave energy from feeding back into the electronics bay through the connections.
Magnetron Assembly
Magnetron cooling fins, tube & magnets removed from the frame.
Magnetron Tube
Bare magnetron tube.
Power Input Board
This PCB does some rudimentary power conditioning, power resistors are in series with the live feed to the power trasformer, to prevent huge power up surge. When the transformer energizes the relay, which is in parallel with the resistors, switches them out a fraction of a second after, providing full power to the transformer.
Standard RFI choke & capacitor at the top of the board, with the input resistor.
Transformer
Power transformer to supply the magnetron with high voltage.
Power output is ~2kV at ~0.5A. Pair of spade terminals are the low voltage filament winding.
Capacitor
HV Capacitor. This along with the diode form a voltage doubler, to provide the magnetron with ~4kV DC.
Diode
HV diode stack.
Fuse Element
Internals of the HV fuse. Rated for ~0.75A at 5kV. The fuse element is barely visible attached to the end of the spring. Connects between the transformer & the capacitor.
Cooling Fan
Cooling fan for the magnetron. Drive is cheap shaded pole motor.
Here is a cheap no brand hot laminator. This pulls the paper, inside a plastic pouch through a pair of heated rollers to seal it.
Heater
Top removed, heater assembly visible. PCB attached to the top cover holds LEDs to indicate power & ready status.
SwitchThermostat
Here is the thermostat & thermal fuse, the thermostat switching the indicator on the front panel to tell the user when the unit is up to temperature. This has a self regulating thermostat. Thermal fuse inside the heat resistant tubing is to protect against any failure of the heater.
Motor
5 RPM motor that turns the rollers through a simple gear system.
Here is a cheapo 500W rated ATX PSU that has totally borked itself, probably due to the unit NOT actually being capable of 500W. All 3 of the switching transistors were shorted, causing the ensuing carnage:
AC Input
Here is the AC input to the PCB. Note the vapourised element inside the input fuse on the left. There is no PFC/filtering built into this supply, being as cheap as it is links have been installed in place of the RFI chokes.
Input Side
Main filter capacitors & bridge rectifier diodes. PCB shows signs of excessive heating.
Filter Caps Removed
Filter capacitors have been removed from the PCB here, showing some cooked components. Resistor & diode next to the heatsink are the in the biasing network for the main switching transistors.
Heatsinks Removed
Heatsink has been removed, note the remaining pin from one of the switching transistors still attached to the PCB & not the transistor 🙂
Transformers
Output side of the PSU, with heatsink removed. Main transformer on the right, transformers centre & left are the 5vSB transformer & feedback transformer.
Output Side
Output side of the unit, filter capacitors, choke & rectifier diodes are visible here attached to their heatsink.
Comparator
Comparator IC that deals with regulation of the outputs & overvoltage protection.
This is an old USB 1.1 hub that was recently retired from service on some servers. Top of the unit visible here.
Bottom Label
Bottom label shows that this is a model F5U021 hub, a rather old unit.
PCB Front
PCB is here removed from the casing, Indicator LEDs along the bottom edge of the board, power supply is on the left. Connectors on the top edge are external power, USB host, & the 4 USB outputs. Yellow devices are polyswitch fuses for the 500mA at 5v each port must supply.
USB Hub IC
This is the USB Hub Controller IC, which is a Texas Instruments TUSB2046B device. Power filter capacitors next to the USB ports are visible here also, along with 2 of the polyswitches.
Power Supply
The power supply section of the unit, which supplies regulated 5v to the ports, while supplying regulated 3.3v to the hub controller IC. Large TO-220 IC is the 5v regulator. Smaller IC just under the power selector switch is the 3.3v regulator for the hub IC. The switch selects between Host powered or external power for the hub.
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