Lamp Limiter / Dim Bulb Tester

One piece of test gear I've never had is a lamp limiter or dim bulb tester. 

They're ideal for s-l-o-w-l-y awakening our vintage electronics up from years of slumber, without too much drama. 

I've been helping my neighbour, Steve, build a pub in his back garden, and while we were testing out some of the electrics, he donated a box of three incandescent 60w lamps to the cause. Ideal. You can't use LED lamps, and halogen lamps are far from ideal.

Here's the schematic.

It's a simple enough circuit. The live side of the mains is supplied via two 60w lamps (in series), each lamp can be switched out as required. This will limit the current and give us a (very) visible sign if there's any issue! 

I purchased a cheap energy monitor from eBay. With both lamps switched out it'll give similar functionality to the Hopi or Killawatt testers. 

I got it from eBay from eBay seller topwoodfly, and it arrived promptly. It has it's own current transformer, as you can see in the picture. 

https://www.ebay.co.uk/itm/394197388411

It's mounted in a box, along with the lamp holders and switches. 

I've put a change over switch on the input to the energy monitor, so we can measure the output voltage. I expect if the output voltage falls too low the energy monitor will stop functioning. I'm not expecting any miracles of accuracy here!

All wired up in a jolly shade of yellow, just to make fault finding on the unit as difficult as possible.

The neutral wire from the mains is passed straight from the input to the output, via the centre of the
current transformer. 

Here's the finished box, running off the bench variac at about 138V

There's a short circuit across the output, just for demonstration purposes!

I must say I'm actually quite impressed by the accuracy of the energy monitor!

Quite a useful addition to the workshop :)

62256 SRAM Tester

If you've been my youTube channels lately, and following the VIC-20 story, you'll know I built a Hyper-expander. You may have also noticed that it didn't really get used much in the repair video. There's a reason for this.... 

It didn't work.. 

Even once the VIC-20 was repaired, nothing I did would get the VIC to recognise more than the 3K extension. The EPROM wouldn't load at all. 

Investigation required. 

The cartridge was pulled apart , the EPROM checked, and the two 74LS148 gates. I wish I'd socketed everything! The EPROM and the two decoders proved innocent.  

That leaves the 62256 RAM IC's. 

So, how to check them?

I could pull the ZX81 apart, and test them in there, but we can't access the full 32K.

SO, inspired by the 2114 tester I constructed for the 2114 SRAM during the VIC repair (https://github.com/skjerk/Arduino-2114-SRAM-tester) I decided to have a look for an Arduino based tester, whilst there is a version for an arduino Mega, I don't have one to hand. The issue is, a normal Uno (or any ATMega328 based Arduino) doesn't quite have enough pins. We need 15 outputs for the address bus, and 8 for the Data bus. Two 74HC590 counters are fielded in to drive the address bus from just two pins (clock and reset).

Here's the theory of operation. The Arduino resets the two counter IC's to 0, by pulsing D3. A pattern of data is generated sequentially between 0b00000000 to 0b11111111 and written to the first address. It's then read back, and compared with the written pattern. If all is well, then the address is incremented by pulsing D2 and the procedure is repeated. If the pattern is not read back correctly, the fail LED is lit, and a message with the error written to the serial interface, but the test will run until all addresses have had all patterns written and read back. It takes quite a while to run.

While the test is running, both PASS and FAIL LEDs are illuminated. The green LED I added to pin 2 of U2 (via a 1K resistor) to provide some indication that the test is running, if no terminal is connected. 

Normal running looks like this on the serial interface.

62256 SRAM Tester by Doz.    

Running test pattern 00000000

Running test pattern 00000001

Running test pattern 00000010

Running test pattern 00000011

Running test pattern 00000100

Running test pattern 00000101

Running test pattern 00000110

Running test pattern 00000111

...

Running test pattern 11111110

Running test pattern 11111111

All tests complete 

PASS

Hit reset to re-run

a failure looks like this:

ERROR at 0x5A64 - Got: 01100001 Expected: 01100010

The code can be found, as usual, on my github page.  https://github.com/andydoswell/platformio/tree/main/Projects/62256%20test 

Please note this is a PlatformIO framework project. You can probably clone the git, and pull main.cpp out of the SRC folder, and rename it .ino , and it'll probably run on the Arduino IDE, although I haven't tried it. 

To cut a long story short the RAMs were innocent, the fault was back in the VIC-20 itself, but at least we have a device to test them now!

Video rack repairs and new video multiplexer upgrade!

5 years ago, I built a rack to house the standards converter and video source equipment in the workshop (you can see that post here)... it's time for a few repairs and a refresh.

It's got a few niggling faults.

1. The DVD player no longer ejects, and sometimes doesn't want to play.
2. There's a couple of channels which no longer work
3. There's quite a lot of crosstalk between channels.

Let's address a few of these...

1) The DVD ...

First off some disassembly of the rack case, which allows the DVD player to be removed.











Powering the player up, and there's nothing on it's output... until after a few minutes the unit powers up, and reports "Bad disc" on it's on-screen display...


A few screws later, and the lid's off, the tray front removed and the trapped disc released.











A known good DVD is inserted. At this point it's obvious why the thing won't eject. I suspected a failed tray belt, but that wasn't it... the tray is catching on the front escutcheon. I'll deal with that later. The disc attempts to spin up, fails miserably to get to speed, and then "Bad disc" appears once more.. On the second attempt the player seems to re-start it's firmware...

I check the outputs from the power supply with my multimeter... +5, -12 and +12.. +12 is varying between about 10.5 to 12.5, which may be acceptable. I switch to the 'scope and check the +5V again....

Woah! That's awful - 40V pk-pk of 50Hz crap riding on the output !












The power supply is removed, and a blanket change of all the electrolytic capacitors is performed, to ensure reliability.










The power supply is replaced, and on powering up, the disc spins up rapidly, and then reports "No Disc" ... hmmm ...











The laser pick up is given a gentle clean with some window cleaner (Don't use IPA on any CD or DVD, it damages the coating on the lens), and ....



Bingo, the test pattern DVD fires right up!





The guilty parties....

The sticking front tray is resolved by relieving the front a touch with a file... I suspect it's got distorted by the shelf above it in the rack for all those years...

Good, one fault fixed ... on with the next...

2 & 3 - Channels not working and crosstalk.

This is being caused by the mechanical switches failing on the front panel, there was always a bit of cross-talk (one channel seeming to "float" behind the video of another) as the switch does not provide any correct loading, or buffering between channels... 

What's needed is a proper video (and, of course audio) multiplexer.

I designed this a while ago in Eagle... thankfully Kicad reads eagle files :) KiCad, the gift that keeps giving :)

There are 8 audio and video inputs. 

Each video input is loaded by a 75 ohm resistor, and fed to two MAX4312ESE multiplexing ICs. One outputs the video to a UHF modulator, and one outputs the video to a 625/405 line standards converter. So, under the control of my favourite flavour of microcontroller, the ATMEGA328P, any of the video sources can be sent to either 625 or 405 sources.

The select line of the video multiplexer also feeds two ADG1408Y multiplexers, which switch the audio through to the relevant output device in the same manner. There's a 5532 buffer on the audio output, as there's none in the audio mux, unlike the video mux.

The audio mux requires a negative supply rail, and this is supplied from the microcontroller. A continuous squarewave is output on pin 5 of the micro, is floated by C9, and rectified by the double diode D1 (BAT54S) to provide a "-5V" supply, in reality it's more like -4V. 

The micro is controlled by a front panel , which has two push buttons, and a small OLED display.

A PCB was created... not modelled as it was Eagle, but Kicad has a go at it!

When Kicad imports an Eagle creation, it doesn't seem to cope with the ground plane or component models, but you get the idea... 

... anyway, the real thing turns up from PCBWay.

... and duly stuffed. I'll clean that flux up later ;)

The front panel was also designed. Simple.

and again, built up... not quite so simple though, as my new display has the power and ground pins reversed from the one I modelled :(

... so a minor bodge is deployed...













The software is very straight-forward. It acts conditionally on the actions of the select switches on the front panel to increment the channel address selected, and update the display. I used the U8glib library this time out, as it's got some nice fonts, and is quick, if a bit bulky.

The software can be downloaded from my git. https://github.com/andydoswell/video-rack

It will be noted that the hardware supports 8 inputs, and two outputs, but the software doesn't support that many, as I don't need them all! It's a simple task to enable more, if you wish.


Nice font...












So, back to the rack.



Out with the old...











Tidy up the RF wiring a bit. The black box in the bottom is the Aurora 405 line standards converter. The loop of co-ax you can see is a stub filter, as the output from the aurora is very harmonically rich, and the filter eliminates aome of the patterning caused by the harmonics effecting the UHF. What I really need is a low pass filter .... one day.






The UHF modulator (modified to run on DC, see here) is once again disassembled, so I can adjust the channel and sound carrier spacing via two buttons on the front panel. I've just soldered some wires on the existing switches, and made them available to the front panel.








The Cyclone media player, and HDMI converter are mounted on the shelf, and the multiplexer PCB behind that.


The power supply is a salvage 12V 5A supply. This feeds the multiplexer, and the aurora. It also feeds the modulator via a 42 ohm resistor (to give about 8V) , and an eBay 5V buck regulator, which feeds the Cyclone and HDMI converter.











The front panel is marked out...












... and the holes chewed out with my teeth. I do wish my metalwork was better, anyway, it's only for my own consumption!










And it's all wired up ... what a rat's nest!


















The back's fitted, and it's all powered up and tested - No cross talk, all the inputs work - oh bliss!










Now, there's a rake of telly's here need fixing ....

IC tester - 555 4013 4066 and Quad op-amp.

Sometimes you just need a small test jig to test something quickly.

I built this up for quickly testing 555 timers, 4013 dual flip-flops, 4066 analogue switches and almost any quad op-amp a few years ago, I thought I'd share it here...

The 555 timers is set up as an astable. It's output blinks D2 at about 4 Hz. These pulses are used to test all the other chips, so a working 555 must always be inserted into the board.
To test the 4013, the clock pulses enter the Clock input of the first flip-flop at pin 3, Feeding the notQ output on pin 2 back to the data pin 5, results is the frequency being divided by 2. This is shown on D3. These f/2 pulses are then sent to the second flip-flop, which is configured in the same manner, the resultant output is displayed on D1, and is f/4.

In testing the 4066 switch, the pulses are fed to the switch input of all 4 quad switches. The inputs and outputs can be reversed, by reversing the polarity of the input and output between 5V. The practical upshot is the one set of LEDs should flash in time with the pulses from the 55. Flicking the switch, reverses the in's and out's, and the other set of LED will flash.

The op-amp is set up as a comparator, and all LEDs flash in time with pulses from the 555, although slightly dimmer than the other LEDs.

Here it is in action.

Surface mount component storage tip.

If, like me, you're always making circuits up or repairing stuff, and minimum ordering quantities are always more than you need, you'll end up with more parts than you need.

With through hole components, it's not so much of an issue. Some of those compartmentalised boxes, and a few hours sorting out the parts makes an easy storage solution. Not so much with tiny SMD components. The parts are sometimes not fully marked, especially MLC capacitors, so it's best to keep them in their respective bags, or write the value on the tape. Even so it's all a bit unweildy.

I'm a big fan of these Chinese sample books.

They're about A5 in size, and great for storing a reasonable amount of stock in a small space.










No amount of trawling the internet reveals a supplier :(

So, after a bit of thought, and I order some of these Photo albums for very little money from eBay.

(looks like they're out of stock now... but the seller seems good, arriving promptly and well packaged)





Compact in size...


















Lots of 6"x 4" (15.24cm x 10.16cm) pouches, that I can put my parts in, and label each pouch with a sharpie.









Excellent. Put an Index on the first page, and you'll easily find your parts!

Lindos LA102 test set repair.

Morning ... coffee ... switch the power to the bench on and contemplate the day's work..

... hang on, the ever-faithful Lindos LA102 Audio measuring test set fails to start up.

It's dead, nothing. Thankfully I have a spare.

A cursory check round shows the mains transformer to have failed open circuit...

Fair enough, it's been in there since the 80's...











A suitable replacement looks like RS 121-3839, and is duly ordered...









Now it physically fits in the hole, we'll need to switch the phases on one of the primary windings, and one of the secondaries.. The original transformer is wired 0-120 0-120, and 0-6 0-6, the replacement is 0-115 115-0 and 0-6 6-0.  I'm not worried about the 5V difference in the primary.

So let's deal with the secondary first ... This is a picture of beneath the transformer.

Cut the tracks as shown.


















And connect two insulated wires to cross the tracks over...

You can now fit the transformer.

Next to deal with the primary. Cut the original link, and carefully remove the trace completely. Note I'm working on a 240V UK unit here... if you're in that funny bit of the world where you use 110 or 115V, you'll need to work it out so both primaries are in parallel.















Then add a link wire in to couple put the windings in series, and in the correct phase...

















... and reconnect the mains supply.


















Switching on, and it's clear something is not right. The transformer is humming and getting warm rapidly!  (It's never easy, is it??)

Turns out the bridge rectifier is short circuit as well... poor thing is cracked too...

It's replaced, and now the unit powers up, but it's still not right, as the power supply is still under some considerable  load...













... and the final piece of the puzzle is the ancient nicad battery is almost short circuit. I disconnect it, as I haven't ever needed it.
















Now, let's get on with some work!