Some of the items listed have been repaired without any circuit diagrams available. A point that may be of use is that when servicing equipment that uses IC's and you have no circuit, get the IC data off the internet. Generally, the circuits around the IC will be very close, if not identical, to the examples provided in the data.
HP 1702 LCD monitor stays on for a few secs after power up then goes blank. LED stays green but no image. Shine a torch into the screen to verify it's just the backlight gone off. Replace the 100uF 400V filter capacitor fed by the mains rectifier. Cap tests OK in terms of capacitance, but ESR is too high causing supply voltage to drop under load (i.e. when the backlight comes on).
On a separate issue, why do manufacturers continue to use .47uF and 1uF electros? These are notoriously unreliable, especially in switchmode power supplies. An MKT type has lower ESR, temperature stability and fits in the same space.
Dreambox 7025 satellite receiver. Power supply: 12V standby suppy working but main supply starts up for a fraction of a second. Replace all the electros on the primary side. Because of heat, they have all gone low in value. Fortunately, this doesn't seem to be a design that self destructs when this happens. Use an MKT for the 1uF replacement. To activate main supply, connect first two pins of multipin plug together with 10K resistor. 1st pin is 12V all the time (standby) & 2nd pin is the power control. Electros near heatsinks are a recipe for problems. I recommend a fan be installed for anything that has a switchmode supply that runs hot.
HP D530S desktop PC. Major
production fault in the power supply. If the computer is unplugged from
the mains for a while (~few weeks), the PSU will blow up when next plugged
in. The problem is the brown glue around the main switching FET becomes
conductive and absorbs moisture when the power supply has cooled down (i.e.
when not plugged in). Being a high impedance device, the FET doesn't
need much leakage between its gate and drain terminals to switch on, which
it happily does so upon next power up, discharging the filter electro through
itself and the control IC. The 22R also blows.
If you have one of these computers, remove all the brown glue on the power supply PCB while you still have a working PSU. Either that, or never turn off at the mains.
LG RH1777 DVD/HDD Recorder.
Theben time switch. This DIN rail mounted 240V time switch had no display. The supply for the relay is about 35V DC which is obtained from a .15uF dropper capacitor and bridge rectifier. From this 35V supply, a supercap is charged to about 3.7V to drive the clock. No 35V supply was present, but everything worked when external DC was fed into the filter cap on the DC side of the bridge rectifier. There is a .1uF surface mount cap on the AC input of the bridge. This was s/c.
Dick Smith and Jaycar Inverters.
Sanyo VTC9300 Betacord VCR.
JVC HR3660 VHS VCR (HMV HV3000). Intermittent tape loading, FF & rewind, even though heads spin etc. Gradually gets worse. The two main flat drive belts are slipping. The loading up functions and reel drive are done by the motor just to the right of the cassette housing. If you see the pulley spinning but the belt not moving, that's the problem. This belt drives another pulley which reemerges on the underside of the chassis to drive a second flat belt. This may also be slipping. Generally, mechanical things not happening are belt related in this model. Once or twice belts have come off due to a jammed tape. The reliability of these machines is incredible. These are from 1980, and out of both of mine, have only had to replace a few belts (once), a cassette lamp, and an RF amplifier IC after a storm.
JVC HR7700 VHS VCR (Telefunken VR540, Akai VS10, Rank Arena RV330).
Early VHS and U-Matic VCR's, Various Models. No tape functions. Check the cassette lamp. If this goes open circuit, the microprocessor shuts down disabling mechanical functions. The cassette lamp is used to sense the end of tape (transparent leaders). Later VCR's use IR leds instead of an incandescent bulb.
If the machine seems to randomly shut off when servicing, see that any room lighting or sunlight is not striking the optical sensors. I recall one JVC CR6060 U-Matic that would shut off in the morning when looking for a totally unrelated fault. The sun was coming through the window at a particular time, striking the opto sensor which made the machine think it had got to the end of the tape. Betamax machines detect metallic leader tape and do not exhibit this problem.
Crompton PIR sensor switch.
Fault was that although the light could be switched on by a rapid turn off and turn on of the supply, in the usual way, the PIR part was non functional. I took the LDR out of circuit so that it would think it was night and thus function with ordinary room lighting whilst working on it. Tried changing the LM324 first. Touching the back of the PCB actually made it work, so it was obviously a resistor gone high. The 100K resistor next to VR1 had risen to 245K.
Fisher & Paykel GW508 washing
machine motor controller (phase 4).
The 15.5V power supply had blown up; more specifically, the TOP224Y switchmode IC, the P6K200E transient suppressor diode, and what appear to be the zener diodes for the error control. Of course, the 4A fuse had blown also. For those not au fait with these machines, F&P use a three phase stepper motor rather than a conventional induction motor and gearbox. This allows an extremely high spin speed, etc. The stepper motor is switched by MOSFETS fed from 340V DC (rectified and filtered 240V AC). The 340V DC also feeds the switchmode power supply that provides 15.5V for the control circuit and solenoid valves. The pump is conventional and is switched by a Triac from the 240V AC supply. So, it can be imagined that any spikes on the mains can easily destroy the motor controller. A novel feature is that the MOSFET heatsink is water cooled. Incoming water to fill the tub is circulated through a length of aluminium tubing to which the power semiconductors are clamped.
First point of call was to feed 15.5V from a bench supply into the controller - it actually looked very promising with LED's lighting up and the buzzer doing its start up sound. None of the motor switching MOSFET's tested short circuit. Given the time taken to get the parts to repair the 15.5V supply, and that it would still be a typical vulnerable SMPS, the decision was to make an external linear supply to provide 15V. The damaged parts were removed and also the switchmode transformer. A 15V 1.2A transformer, rectifier, fuse, 3300uF cap and 7815 regulator assembled in a plastic box did the trick. Wires were run out of the motor controller to the new black box secured to the back of the machine. Complete success.
Note that the 15.5V circuit is all live at mains potential. Therefore the external power supply must be completely insulated and connecting wires rated for 240V AC.
I replaced the 4A PCB mount fuse with an M205 fuse. It just so happens the PCB allows the use of an M205 fuseholder. Also note the PCB is covered in a plastic layer - I used 3M clear protective coating to restore this. With 340V DC circulating, it won't take much leakage to destroy something.
While these F&P machines give an excellent wash and have a fantastic fault diagnosis system, they are unreliable and the internet has many stories of problems. This particular machine has over the last few years had the pump, lid switch, hot solenoid valve, and balance switch replaced. Luckily these parts are cheap on ebay. It was only a matter of time before something went wrong with the yellow box...
I would advise these machines to be unplugged from the mains when not in use - the SMPS operates and 340V DC is present while ever the mains supply is on. A surge protected power board is also a wise idea.
Motorola Razr V6.
This phone would not recognize the SIM card. A close look revealed what looked like dry joints on at least one of the SIM card contacts. Resoldering these fixed the fault. The service manual is available on the internet which describes how to dismantle the phone.
AWA Line Output Transformers.
The type referred to was used by AWA from the mid 1960's in monochrome
TV sets until the last P1's in 1973. These have an additional winding for
the horizontal AFC circuit. This AFC winding can short to the main primary
winding, and since one side of the winding is earthed, the 6AU4 plate glows
red and excess B+ current is drawn. The repair is first to disconnect the
AFC winding and let it float at the primary voltage. At this stage, the
EHT and line output should be restored to normal.
Next, wind on a new AFC winding one one side of the transformer core to produce 350Vp-p. See the service manuals on where to measure this. It will need to be phased correctly or the line oscillator will not lock. I used 0.3mm enamelled copper wire. Wind a layer of tape around the core first for insulation. These transformers operate with about 7Vp-p per turn, so 50 turns are required for 350Vp-p.
This repair done to a P9Z chassis has lasted 25 years at the time of writing. As an interesting thought, it should be possible to simply do away with the AFC circuit and directly inject the sync pulses into the line oscillator. Since there are no off air transmissions now, there does not need to be the noise immunity provided by the AFC circuit.
New winding under the white tape.
Sony SL-C7 Beta VCR. (September 2021)
KRK Rokit 10-3 Powered Monitor Speaker. (September 2021)
Blown fuse. Switching FET S/C. (No surprises there). Carbonised insect found between the gate and drain pins. The driver IC was unknown and a likely type was chosen for replacement. It required modification to the circuit to make it work. Note that the main filter electro remains charged at 340V even when not powered up from the mains and is a hazard when servicing it. There is nothing to discharge it. Use a light bulb to discharge it before working on it. See the video here for more details: https://youtu.be/hMdYD7gMOPA
AVO VCM163 Valve Tester.
Failure of the gm meter to show any reading. The 500R pot RV1 was O/C. This is on the vertical pot panel. During the repair it was noticed that R35 (2.4K) was burned and had doubled in value. This resistor is mounted on the ma/V switch. Use a 22K//2.7K to get 2.4K.
There was a slow response from the gm meter when the tester was first switched on, taking around a minute for the pointed to come up to the "cal" marking. Replacing all the electrolytics, .01uF, and two 0.47uF's on the amplifier board fixed that.
To get access to everything, remove both side panels and the bottom panel. To get the bottom panel out, loosen the screws holding the transformer chassis to allow the side frame to be spread slightly. Don't undo the bolts surrounded by rubber.
Tektronix TAS475 CRO. (10/11/21)
Burning smell. There is a metalised paper .047uF 300VAC X1 capacitor connected across the mains input before the switch. Therefore, mains voltage is applied to it while ever the CRO is connected to a live power point, even though it is switched off.
Remove the power supply and look for the cap across the IEC input socket.
Tektronix 2712 Spectrum Analyser. The digital and sweep PCB's are loaded with surface mount capacitors which have started leaking and eat into the adjacent tracks. Unfortunately, replacing them has not fixed the fault.
Noisy 6M5 Valves. If your
6M5 equipped apparatus is making a scratching sound and has low ouput,
check what kind of 6M5 is installed. The original Philips production is
prone to silver migration between pins 1 and 2; these being the screen
grid and control grid pins.
These valves are easily identified because they have the sharp silver plated pins, and the flat glass base is made separately to the valve's envelope. Scrape and clean the silver migration off the glass and all will be good again.
Rank Arena Tripler Replacement. 1970's era Rank Arena colour TV's use an expensive and unique tripler. Typical of set models are C2201, C2601, C2209, etc.
Above is the original tripler circuit. EHT is connected to the CRT in the usual way, but also to a bleed circuit which also supplies the focus voltage. This is a large tubular 132M resistor, along with a 28M resistor and 10M focus pot. As well as supplying the focus voltage, the bleed circuit provides a degree of EHT regulation.
An inexpensive and widely available Philips tripler can be used as a replacement. Instead of paying $80 for the original (not that you'd get one in the present day), a Philips tripler, or a clone thereof, can be obtained for under $20. However, some alterations are required:
The Philips tripler has a focus voltage terminal, labelled Uf. The Rank Arena focus circuit is modified to use this, as it would be in other sets using this kind of tripler. The existing 10M focus pot is retained, but two high voltage focus resistors are added as shown. Do not use ordinary resistors. They will simply not last with 8kV applied. If by chance, it is not possible to peak up the focus, swap the position of the two resistors. At this point, EHT and focus voltages will be obtained and the set will work. But, there is a problem in that EHT regulation is poor, and the picture width will vary with brightness - more so with 26" sets. One other thing has to be done:
A clamping circuit is connected to terminal D of the tripler. This clamps the negative pulse from the line output transformer, and restores regulation by lowering the effective output resistance. The capacitors are 630V rated. Alternatively, a 1000pF 1kV capacitor can be used. The resistors are ordinary 470k 1W types. This extra circuit can be built on a tagstrip and mounted near the tripler.
I have used this modification in many Rank Arena sets since the late 1980's and it has been completely reliable.
GW GPS-3020 Power Supply. (26/11/21) Voltage adjustment working OK, but current control does nothing - current is always at max setting. DZ7 is a 5.6V zener and was short circuit. The current control does not use that which is already in the 723. Instead, it uses a two transistor differential amplifier, which operates on the 723 voltage control. One transistor base is fed from the current shunt (actually the 2N3055 emitter resistor) and the other transistor base is fed from the current control pot. The common emitter resistor is 3.3k and is fed from DZ7.
Cotek S150-224 Inverter.
(29/11/21). This 24V to 240V 150W inverter would run for 2 seconds and
shut down. With the inverter fed from 12V it would stay on. Surprisingly
it was still putting out 240V, but the waveform was more like a square
wave instead of the sine wave specified. It appeared that the under/over
voltage protection was set up for a 12V inverter, because it would stay
on from about 10V to 15V. With microprocessor controlled inverters, one
of the pins of the micro usually samples the supply voltage and shuts down
the inverter according to whatever voltage has been programmed into the
firmware. It seems that pin 18 of the PIC16F819 is used for this. If the
pin voltage rises to 2.17V, it shuts down. However, looking at the voltage
divider 56K + 10K, the voltage at this pin should be 3.63V with a 24V supply.
In order to make the inverter stay on, the pin 18 voltage could be reduced
by paralleling an 8.2K resistor across the 10K. Now, the inverter would
work at 24V and shut down at 29V. So far so good, except the under voltage
protection wasn't working. This seems to be detected somewhere else besides
pin 18, since pin 18 has no effect on this.
To enable to under voltage sensing to work, the 8.2K needed to be removed from circuit when the supply voltage got too low. An NPN transistor is used for this, along with an 18V zener diode and 22K base resistor. When the supply drops to around 18V, the transistor switches off, disconnecting the 8.2K resistor. Because the supply voltage is still higher than 15V (the 12V shutoff voltage), the overvoltage protection shuts down the inverter instead of the undervoltage. This has the same effect - to protect the batteries from excessive discharge.
It would appear that something had corrupted the firmware of the PIC chip, causing it to think it had the instructions for the 12V model. The inverter was used in a area of frequent lightning strikes and that's all I can put it down to.
Leaky Alkaline Battery - strange
PCB faults. (10/1/22). It is now common and expected that alkaline
cells leak, particularly size AA, and particularly Duracell. One internet
comment was leakage is more likely since mercury is no longer added to
The alkaline liquid wicks along the negative wire and onto the PCB. It then soaks in and causes electrical leakage. In one instance, a Philips remote control for a 2BS TV set would operate intermittently. In another more recent case, the MK484 receiver started to suffer from very poor selectivity and low gain some time after it was discovered the AA cell had leaked. Nothing unusual was visible, but if the receiver was taken into the sun, it worked very well. Similarly, heating the area near the tuned circuit/MK484 input cured the fault.
Once let to cool down the fault was back. Closer observation showed the alkaline liquid had migrated along the entire negative track of the PCB, which ran close to the MK484 input. Since the MK484 relies on a high Q tuned circuit for selectivity, any extra resistance in parallel ruins performance. The fault was fixed by connecting the tuning condenser, aerial coil, and MK484 input pin in mid air.
IBM XT Computer (Model 5160).
(15/1/22). Memory fault at 192k during the POST. Error 30000 04 201. U78
RAM chip in bank 3 faulty. The computer still works once F1 is pressed,
but will not run programs that need the full amount of memory. Faulty RAM
chip was replaced with another of the same speed which restored operation.
Look for shorted tantalum capacitors on the motherboard if the computer won't start up.
Pye "High Fidelity Stereo Theatre"
Radiogram. (26/06/22). Made in 1966, the amplifier uses 6CA4,
12AX7, 2 x 6GW8. Tuner is solid state with three transistors and two diodes;
all germanium. Intermittent fault with volume fluctuating when first powered
up. Severe distortion evident when volume dropped along with poor sensitivity.
1st IF transistor found to be unusually thermally sensitive, but the change
in volume was not as sudden as with the fault. Nevertheless, it was substituted
with a BC328. This being silicon required the 120k bias resistor to be
reduced to 100k. Fault still evident. Supply voltage to tuner found to
be very critical at 7.5 to 9V. Above 9.5V the low volume and distortion
was evident. However, the supply voltage remained at 8.6V even with fault
present. 2.5uF AGC capacitor had excess ESR and was replaced. Original
IF transistor returned to circuit, but 120k found high. Fault still evident.
It was noted that the ceramic capacitors were all Ducon Red Top types. These have been noted for leakage in the past. Replacing all fixed the fault. All but one found to be leaky. Supply voltage no longer critical with tuner functioning normally beyond 15V. Like most intermittents, the fault could not be induced at will, and a lot of time was spent on this three transistor tuner.
Pye 12" T27 Portable TV. (26/06/22). No sound except at earphone socket. Speaker o/c. This is the third Magnavox speaker from early 1970's portable TV's which I've found o/c. The other two were in Thorn S1 chassis. Luckily, for this one, the break was in the wire running down the cone to the voice coil, and could be bridged.
AWA 11" P1 Portable TV. (26/06/22). Unstable line sync and AGC. Someone had already replaced the sync separator load resistor and other resistors in the line AFC circuit. Found video present at sync separator plate instead of clean sync. Video detector signal had lots of ripple and virtually no sync pulses. B+ checked and had considerable line ripple. 200uF in voltage doubler open circuit. The trick with this fault is there was no hum in the sound, and the raster was normal. For anyone repairing P1's, there are three paper capacitors to replace; the two .047uF boost capacitors, and the .0068uF in the frame oscillator. The high value IRC resistors are also suspect. Those remaining in this set were OK. Actually, this is one of the best P1's I've seen.
Sanyo CTP1601 Portable TV.
(26/06/22). This mains/12V portable colour set worked on 12V but not 240V.
A 500mA fuse was found blown, but replacing it did not restore operation.
Tracing the mains circuit, it was found that the 6 pin 12V input socket
should have a dummy plug inserted to link the 240V connections. This was
of course missing. A replacement was fabricated from cutting up a couple
of 4 pin speaker plugs, mounting them in a plastic bottle top and filling
that with car body filler. While it could have been possible to simply
link the pins at the back of the socket, this would be unsafe since there
is no isolation between the 12V and 240V supplies. Thus, if the 12V cable
was plugged in at the same time as the set was working on 240V, the cigarette
lighter plug would be live at 240V.
It is a strange set up with a separate 240V input socket. Other sets I have seen like this use the one multi-pin socket for 12V and 240V, obviating the need for the easily lost dummy plug. It would have been just as easy for Sanyo to provide a 4 pin male input socket, with 4 pin female plugs wired for 12 and 240V.
Unless they have been in regular use, most rotary UHF tuners have frozen up by now. This set was no exception. The way these tuners work, the shaft is concentric with the knob turning the steel shaft in the centre. By means of a vernier reduction inside the tuner, the outer shaft rotates at a slower speed, and it is this which drives the variable capacitor inside the tuner, and the dial via plastic sleeve coupled to the outer shaft. When the tuner freezes up, the plastic sleeve bonds to the steel shaft. It is necessary to remove the plastic sleeve from the shaft. Remove the C-clip and squirt CRC down between the sleeve and the shaft. It takes some time to free it up, but once the plastic sleeve can be removed, it can be properly cleaned. Note that the dial drive inside the tuner is secured only with a couple of soldered joints - so don't force it.
Rohde &Schwarz SMS Signal Generator.
(4/08/22). RF output permanently off, with LED always on. Switch non responsive.
Check the overload protection PCB. This is in the line to the output socket
and in mounted next to the fan. Unplugging the 4 pin connector should restore
switch operation. If so, check the board for excess dust blown in by the
fan. It appears this can become conductive. The signal output from this
PCB should be 0V under normal conditions. In the faulty unit it was found
to be +3V.
The RF output on/off switching uses the attenuator relays.
Philips MM2 "Philadelphia" Transistor Mantel Radio. (24/01/23). No sound except when left off for a long time. For something made in Australia, the construction of this set is a disappointment. It looks like it's made to a cost, and the assembly looks rather fragile. From the late 60's/early 70's, the design is based around two locally designed modules; a radio tuner, and an amplifier. Both of which found application in numerous Philips products of the time. Audio was present from the tuner, but no output from the speaker. The amplifier is a typical four transistor design, and output uses an AC188/AC187 complementary symmetry pair. The junction of the emitters should be half the supply voltage, but when faulty was around 12V (the supply voltage). Direct coupled amplifiers are a pain to service, because anything anywhere can upset the whole circuit. Also, in this instance, the wires were too short to allow the module to be worked on in situ. It has to be disconnected from the set and serviced externally. To make service more difficult, the components are crammed together with the resistors and capacitors stood on end. It was quicker simply to remove all components in one go and test individually. One 125uF electro was found to be leaky, a few resistors were high, but the main fault turned out to be the AC128 driver. It tested as a PNP transistor, but had no gain. Upon replacing it, the radio worked perfectly every time it was switched on. Aside from that, one of the 12V 100mA dial lamps needed replacement.
Philips MT5 Transistor Mantel Radio.
(27/01/23). This battery operated set looks like it was based on something
mains operated. There's a cord entry slot under the cabinet. There's no
proper battery holder, with the battery (276P) simply secured between the
cabinet back and the speaker frame. This set had a very low output. Blue
Philips electrolytics are alway suspect, and one was found to have excessive
ESR. However, replacing all electros did not make a huge difference. Ducon
red topped ceramic capacitors are another source of problems in that they
can have excess leakage. The ones in this set tested OK. The audio from
the diode detector was weak, which indicated the problem was before the
detector itself. The diode tested OK. Measuring the RF to the diode input
found that too was weak.
Probing with a signal tracer, it was found the signal at the collector of the 2nd IF transistor (i.e. IF transformer primary), was much stronger than on the secondary side feeding the detector. Temporarily substituting another IFT (from a DSE L2060 coil kit) brought up the volume by a noticeable amount. The volume was still inadequate on anything but the powerful ABC stations, so the first IFT was also substituted. The gain was considerably increased, and the performance was now acceptable. Since the replacement IFT's do not have the same pin configuration, holes had to be drilled into the PCB and tracks cut to install them.