Avian’s Blog

Some time ago I found a box full of old vacuum tubes in my grandparent's cellar. Today I finally got to photograph and catalog them.

It looks like a loose collection of tubes gathered from TV (there are quite a lot PL 36 for example) and radio receivers. I'm told some of them were also used by my dad for high school and university projects.

I have not yet managed to identify all of them. Some markings on the glass became very fragile and can be easily destroyed by touching them so it's not surprising that a lot of them have disappeared with time. I can also only guess at their age and state. Some look like they haven't been used yet and at least one (RV2 P800 has "Wehrmacht" printed on it) appears to be from the second world war.

I would be very grateful if anyone can provide information that is missing below the pictures. Please drop me a mail. Thanks.

Here's some assorted information about Sony RDR-HX710 DVD/hard disk recorder (because I like spoiling business for people that are selling trivial information like this on various forums for 10$ apiece):

• To get into the service menu, turn the recorder off, simultaneously press REC, REC PAUSE and REC STOP on the front panel and while they are pressed turn the recorder back on with the ON/OFF button. You can then use the remote to navigate the service menus.
• MIP3E7MY integrated circuit in the power supply can be replaced with MIP3E3SMY without any obvious side-effects. This is however only based on experimentation. I couldn't find datasheets for the original chip, so this replacement may not work to specifications. YMMV (by the way, while this looks like a transistor it really contains complete power supply control electronics and a high-voltage power MOSFET).
• When removing the power supply from the case, be very careful and only touch it along the edges. When it's out, take a resistor and discharge all the big capacitors on it (even if you think those capacitors are already empty). Unless of course you would like to sport a couple of new burn marks on your hands. Fun fact: there are places on the circuit board for bleeder resistors, but they are empty. I guess marketing department won an argument there (this reminds me of a certain domestic computer).
• Hard disk is a normal 160 GB Seagate with IDE interface. No partitions and no easily recognizable video streams.

Yesterday I found these transistors manufactured in former Yugoslavia while repairing an old EPROM programator:

BC212 equivalent in TO-92 package, from Ei Niš.

2N2907 equivalent in TO-18 package, from Radio industrija Zagreb.

Nothing special actually, except that I didn't know that they were making transistors at all.

A capacitor in such a sad state can still hold more than enough charge to burn out an ohm-meter.

Coming up next: repairing a Metex digital multimeter...

On a second thought, if I wouldn't destroy the instrument I would probably at some point touch the terminals with my hand and that could be a bit more serious.

Here's the final form of my ultrasonic downconverter:

It features 30 - 100 kHz input frequency range and 15 kHz bandwidth. It's a fully analog design that uses heterodyning for frequency shifting (the left knob controls the frequency of a local oscillator which is then mixed with the signal from the sensor). Output port can be used with headphones or a small speaker.

It was completely designed with free software tools from the gEDA project (and OpenOffice.org for the stickers).

The piezoelectric ultrasonic sensor is detachable so different kinds can be used (for different sensitivity and directivity).

It works surprisingly well considering my problems with noise on the protoboard. The grounded metal case really makes a big difference with such a sensitive circuit.

After a couple of walks around the house I found that the most loud things in the ultrasound spectrum are compact fluorescent lamps and motion detectors in car alarms.

Here is an interesting circuit I came across today. It's from an older "Standard Charger" for Ericsson mobile phones (in other words, it can't be used on recent models). The circuit itself is a pretty standard flyback switching power supply (240V AC input, 5V DC output at 450mA), however the PCB layout has two interesting features:

• Designer used a "ground mecca". Connections from all elements to the ground on both high voltage (left) and low voltage (right) sides radiate from one point that is near a large electrolytic capacitor. This significantly reduces noise problems in the control circuitry (I had to use the same technique when I designed a switcher last year).
• There's a feature resembling a spark gap between the high and low voltage sides. I'm guessing it is some kind of a safety feature. On both sides it is connected to ground, so perhaps it makes sure that if a spark makes a galvanic connection between the high and low voltage sides it first connects both grounds. However I'm not sure what good that would be, since the "ground" on the high voltage side may be at 380V in reference to the earth (or the unsuspecting user) depending on how the charger was plugged in.

Update: after some research it looks like this spark gap may in fact be used to protect the circuit from user not vice versa. For example if someone touches one of the low voltage terminals of the charger (which I guess may be connected to a metal phone case), it may bring the low voltage side a couple of kilovolts higher than the high voltage side (which is connected to the earth through the mains). The resulting ESD might damage the transformer or the optocoupler. The spark gap ensures that the discharge instead happens harmlessly in the air.

This is the printed circuit board for the ultrasonic down-converter I wrote about in January. Silk screen made with toner transfer came out particularly nice on this board.

I left the circuit board (with the paper stuck to it after ironing) in water over night. After peeling off the paper and drying the board there were still some white paper fibers visible. I found out that they can be easily removed with a soft pencil eraser. The result isn't perfect though and I still would trust this technique for etching, but it is more than good enough for the silk screen.

Savica was an old solid state AM and FM radio receiver made by Iskra some 40 years ago. Originally it had only a mono receiver for middle and short wave and VHF bands. Interestingly this one (from my uncle) was modified by my father to also receive stereo FM broadcasts. I hear that this modification was quite popular at the time when first Slovenian radio stations began to transmit in stereo and more modern receivers either weren't available on the market or were very expensive.

Savica was particularly suitable for such a modification because the original FM detector had enough bandwidth to receive the entire stereo signal (which is encoded above the normal 15kHz of the audio signal). This meant that the original circuit could be left mostly untouched, only the original mono audio amplifier was replaced by a stereo decoder and a pair of amplifiers.

Names of public middle wave broadcasts are permanently written on the front plate. Who needs RDS?

Most of the original audio-frequency electronics is at the back of this picture. The audio amplifier is built with discrete silicon and germanium transistors. Power supply consists of a couple of diodes (probably a half-wave rectifier) and a large silver capacitor on the left.

This is the custom-built stereo decoder using the RCA CA1310 chip. The original FM detector is in the metal box in the lower left corner.

Update: the metal box actually contains only the the radio-frequency part of the FM receiver. The detector is on the main circuit board on the first picture.

And here are the two audio amplifiers, one for each channel. The two large capacitors on the left died and had to be replaced. This was the only repair needed to bring this old machine back into service.

Taking a bit of a break from old computers...

Here's an interesting step-down switching power supply I found in a car charger for one of the older Ericsson mobile phones.

It really has a bare minimum of components. If you take out the dual op-amp, which is there only for enhancing user experience (it changes the color of the LED if the charger is in use), you end up only with two transistors, a simple shunt regulator AN1431, a coil and a couple of other passive elements.

AN1431 is used as a comparator here. When the voltage between the REF and A pins drops below 2.5V, no current flows into K and IC1 switch is open. If on the other hand voltage on REF raises above 2.5V, this chip acts as a short circuit and IC1 closes.

So basically the output voltage is determined with the voltage divider R7, R8, R9 and R10 (8.4V in this case). R6 and R5 form a positive feed-back that gives some hysteresis to this comparator so that the switching frequency doesn't get too high.

In analog electronics it's in fact mostly noise. Doubly so if there's a protoboard involved.

This the first working prototype of an ultrasonic to audio range heterodyne down-converter I was working on last week (I had some time for playing with this since I was waiting for the Galaksija motherboard to be manufactured).

I just hope that all the white noise I'm getting from it is in fact coming from bad connections between components and the protoboard and not from the opamps themselves.

Here are some photos of a Commodore 128 external power supply from Cyberpipe's computer museum.

This is an interesting design (schematics), because it first uses a transformer and RC rectifier to get cca. 20V DC and then a switching regulator to provide 5V DC for the computer.

From my experience most home computers at that time used a cheap linear regulator IC (e. g. 7805) instead of a switching regulator. Why exactly they chose a more expensive design isn't clear. With 2A of rated maximum current at 5V it would be possible to get the same result with a linear regulator and a large heat sink. Perhaps designers didn't want Commodore to heat up as much as their competition.

This power supply had a heavily damaged printed circuit board because the over-voltage protection shunt obviously overheated and caught fire. I don't know how exactly this could have happened, because the power supply has both a fuse and an over-current protection circuit.

A seriously bad capacitor. Probably damaged by the heat of the fire nearby (note the shape of rubber seal at the bottom).

This is the circuit with new capacitor and shunt installed.