Randys Bench

A repository for projects past and present

A Timex Sinclair Retro revisit

Way back I was tasked with filming my Wife’s family gatherings (and a few of my own), summer picnics, Christmas festivities, Thanksgiving dinners, Birthdays and countless other occasions. The primary video camera I used then (’80’s – ’90’s) was a full size VHF tape affair that sat on one’s shoulder (heavy). Over the years a lot of VHF tapes (40 or so) were produced and gathered dust afterwards. With that I decided, maybe 7 years ago or so I would see about transferring the video to a medium less prone to breakage. Since I no longer either had the camera or a VCR transport at my disposal It was time to look and see what was available to carry this plan out. I found a rather nice solution in ION’s VCRtoPC product. Easy to use, pop in the tape fire up the software and press play…that’s about it. In the end you end up with a burned DVD or a stored video file ready for further processing. The software has a nice video preview screen AND the box has composite video inputs as well.


Once the job was completed the ION unit was boxed and forgotten about up until quite recently. I decided to revisit the composite video out mod I has done years ago on one the two TS1000 early ’80’s computers I had acquired long ago. I worked, just not very well. the mod was simply tapping off the signal ahead of the incorporated RF modulator (using coax) to a rear panel mounted RCA connector. Easy but the results were a bit dark on a modern TV. Moreover lugging the computer around the house to hook it up was a pain. I needed another solution for testing that didn’t involve buying another set, we have enough!

Well as it turns out I had a reasonable solution all along once I thought about it. Getting the ION box software working with a bit more modern PC (Win7/64) turned out to be a bit problematic. Software updates didn’t help much and I ended up reviving an older XP machine and it worked a treat. The benefit of using this (although the preview screen size is a bit small..oh well) is that I can record the resultant video along with easy screen shot capabilities as well as the convenience of having it locally at my bench.

So now with an at hand solution it’s back to the issue I have with this old computer’s video output. After a bit of research on retro computer sites I’ve come up with, I think, a reasonable solution to the lack of acceptable composite video output:

I used a low noise 2N5088 NPN here but could be a variety of transistors including the ubiquitous ‘3904.
Here’s what the hardware implementation looks like:

I ended up gutting out the RF modulator since it’s circuity is no longer used.

Shown below are before and after screen shots of the direct ULA pin 16 connection and that of the circuit implementation shown above:

Overall a nice improvement for the old gal. Considerably better than the former quick & easy composite output method. So to wrap it up here’s a few additional screenshots of the TS1000 running some simple basic programs:




Can we Talk?

I‘ve spent a little time recently re-acquainting myself with the General Instruments SP0256-AL2 Speech chip from the ’80s. Originally I built it up to interface with a TS1000 (ZX81) computer but I quickly found that computer became quite worrisome to use. I ran across a web site (here) that could control (and test) the aging chip with an Arduino…perfect!

Here’s what I put together…

I proceeded to add a few additional allophones to the sketch and give it a go…worked well out of the gate except I missed wired the volume trimpot…it’s always something I guess. Next a put the scope on a few interesting pins shown below:

The lower most trace shows the direct digital output before the 5KHz lowpass filtering from the SP0256 IC. The center trace is as well an output termed SBY (standby) and indicates when an address (data) is loaded and the chip is active the output will go to a logic low. A logic high occurs when the IC is inactive. The top trace, (ALD) pin 20 shows when a negative pulse is input the 8 address bits (6 in this case) into the input port. The chip begins to pulse-width modulate the digital output..can’t really see that here at this timebase setting.

The next plot is taken at a bit faster timebase and, in this case, the lower trace is captured a the output of the lowpass filter feeding the audio amp (LM386). Here the audio is a bit more apparent…

I put together a short gif animation of the above signals:

So all is well in speech land. Incidentally, Radio Shack sold this chip along with it’s companion, the CTS256-AL2 text to speech IC in the early ’80s which was a bit after I myself worked there as a counter salesperson back in 1972 at the Berkeley Ca. store. No computers being sold then, still had a tube tester in the rear of the store, and sold a lot of stereo’s and turntable’s etc. We also were required to wear ties…great
And last for today I made a rather quick video of the state of the Z80 simple computer (really it’s just a light show). Near the end is a demo of the SP0256 doing some talking…

Z80 Microprocessor Part2 – SP0256-AL2 Speech chip


The MC3356 SA and restarting a neglected Project

I‘ve decided that during the MC3356 SA boxing (rather slow it would seem) I’d finish up the remaining circuit tweaking before I get too far along. The plot below is pretty much what I had span-wise before alteration: It shows a span of a bit under 90MHz. The harmonic blip on the far right is 80MHz. I used a 10MHz square wave source here…

After a bit of L2 coil adjustments and a few re-cal’s I was able to nudge the span up to 100MHz (wahoo!) Below find the updated plot using the same source…The blip at the right is our 100MHz harmonic.

I upped the frequency source to 50MHz and the result follows:

And then up to 100MHz (My source is a bit wonky at this frequency):

In the top plot taken with the 10MHz source notice the non-linearity setting in around 70MHz. This is due to the varactor based tuning of the VCO and having but one sweep breakpoint adjustment. All is well and I’m overall quite pleased with this project!

With this SA now basically in hand I’m going to travel a bit back in time to a micro project I had begun but left in the wake of another. In October of 2017 I posted about a Z80-based project using two D/A converters to enable writing to a scope in X-Y mode. This project was taken from an old laboratory manual which accompanied the 1981 edition of the classic text book, The Art of Electronics. Not a complex project but takes up (at least for me) quite a few solderless breadboards.

I’ve finally begun working on the schematics for the initial D/A goal and I’ll post as they get completed. The 2nd goal will be getting an SP0256-AL2 speech chip I’ve had since the 80’s integrated into this simple Z80 project. Stay tuned…

All for now…Randy

A Side of Python – Addendum

I thought I’d add some additional information on the PyDSA python-based spectrum analyzer discussed in the previous post. I thought it might be interesting to bring out the double conversion SW receiver I had boxed up to see how well the receiver’s VFO frequency (metering) matched up with PyDSA’s readings as well as the MC3356 SA’s display. Below shows the (still not enclosed ) receiver below:

For this experiment I’ll be using a DIY magnetic near field probe as well as a 3T pickup coil for the measurements which are then coupled to the Rigol DSO / MC3356 SA. These will be used near the VFO and the 2nd IF coils, but not so close as to pull the frequency too much. The initial tests picking up the VFO oscillations showed a rather limited effect as witnessed by the frequency counter readings. The DIY probes are shown below. Pretty simple builds using bits and pieces from the bins:

So first I looked at the receiver’s VFO coil region with the pickup loop. The VFO was set to a frequency of 20MHz: Here’s what the Rigol DS1054z showed:

VFO coil: 10MHz / Div


Detail: 20MHz VFO setting – 400KHz / Div


2nd IF Stage coil: 10MHz / Div


2nd IF Stage coil: 5MHz / Div Mag Field Probe

Next, using the PyDSA program and the Rigol DSO as the RF front end I re-visited similar measurements of the receiver’s HF coil’s…

SW VFO low end 16.3MHz input


SW VFO high end 23.8MHz input


SW 2nd Mixer coil low end 16.3MHz VFO freq / 10.7MHz IF frequency

Note: 1st spike is the 10.7MHz IF


SW 2nd Mixer coil high end 23.8MHz VFO freq / 10.7MHz IF frequency

The results indicate descent correlation between methods used for the experiments attempted. I plan at some point to expand the testing at a future point as well as get the receiver boxed up…we’ll see how that goes!

73 Randy

The MC3356 Spectrum Analyzer…and a side of Python

Well I’d like to report I’ve made significant progress on enclosing the MC3356-based Spectrum analyzer but in fact I’ve accomplished little to this point. I managed to get a suitable module arrangement that will work with the chosen box (it’s really a bit small but, it’s what I have).


Secondly I made a quick pass on a front panel template (below).

But my attention wondered a bit when I ran across a post concerning a Python-based spectrum analyzer (PyDSA) which uses a Rigol DSO as a digitizing front end which feeds a data acquisition software based system. The program code was originally written for use with a Rigol DS1102 model but revisited in order to accommodate a DS1054Z scopes programming requirements.
Original Python project link:

You’ll find this PyDSA.py file in the distribution download located here:

My familiarity with Python was pretty much zero (and still is) when I began to setup the python environment on my Win7 x64 machine. Fortunately the process was straight forward enough to get things going after a little on-line reading. But, there was definitely some glitches along the way. Getting one of the program’s dependencies installed proved to be a challenge that lasted for a few days of frustration. PyVisa 1.4 simply would not install no matter what I tried. I was ready to give up on this till I made a slight change in the programs .cfg file which amazingly worked…don’t know why.

Below is a scan using the antenna that came with the RTL-SDR I bought a few years ago covering 0Hz to 200MHz. Evident is the FM radio band (88-108MHz) at center of plot:

Next is a scan of the 1MHz output from my square wave generator out to 50MHz:

Below is a plot from the MC3356 SA with a 500KHz square wave input:

Below is a plot from PyDSA with the same 500KHz square wave input:

If you’re not interested in loading Python and it’s dependencies below is a link to a windows executable which contains a few enhancements. Just run PyDSA exe from within the directory structure provided:


In my case there’s a caveat…I was unable to to get the auto-scan to function properly whether I tried the PyDSA .py file or the compiled version. I think it must have to do with my Rigol DS1054z DSO and it’s firmware rev…don’t know for sure. So your mileage may vary. It does however work well in single scan mode. If you have any questions leave a comment. Happy to help.

One other thing…If you’re not aware of Fran Blanche’s YouTube channel and Podcast I highly recommend you take a look. She works on many interesting projects and has a refreshing take on science and electronics…

See you soon…Randy

The MC3356 Spectrum Analyzer…time for a proper box

While our house moving is ongoing I’ve still a few things which haven’t been boxed up yet (and a few I’ve unboxed…don’t tell my Wife please!). A few of the bits & bobs I’ve uncovered recently are the MC3356 Spectrum analyzer boards and it’s intended enclosure. Time to do something about it I’m thinking.

But first, Let’s see if the SA fared well enough after being relegated to storage for a while.

Getting things hooked up again it would seem all is well but needs a calibration re-visit. Looking at the VCO ramp linearity the breakpoint adjustment was a bit off.

VCO Breakpoint final adjust


Below, once the ramp breakpoint is optimised and the sweep cal pot tweeked, we find a spectral range quite near a maximum of 80MHz…not bad:

10MHz square 10MHz/Div

Next a detail of the previous plot harmonic shape charactor:

The linearity is not quite where it needs be on above plots but I think could be coerced a bit closer with a few more iterations of pot adjustments as they tend to interact with each other. Possibly it could use a few more breakpoint additions to the circuit to compensate for the inherent nonlinearities of varactor tuning…not really an issue. It is after all, a simple spectrum Analyzer. Speaking of issues, I may have reported in an earlier post that my implementation of zero span on the sweep/video card assembly did not work at all. I re-visited that circuit a few days ago and came up with a better, working solution. I added an additional switch position on the span (MHz/Div) rotary switch with a connection to circuit common. This is illustrated below (Rev C):

Find a larger version of the schematic above HERE


Along with this change I would recommend using a multi-turn pot for the center frequency adjustment R31. If not you might want to add back in the fine frequency control which I had previously removed. Here’s an additional plot of the 10MHz sine output from a previous project, the RF sweeper…The zero spur is located at the far left (normal for a superheterodyne SA and good for a calibration point).

So beyond some additional testing It’s about ready for a good boxing….oh I’m still thinking about the audio out feature…See you next time


The Rev. George Dobbs, G3RJV Silent Key

From http://www.arrl.org:


The founder of the GQRP Club and Amateur Radio author the Rev. George Dobbs, G3RJV, of Littleborough, England, died on March 11. He was 75. Dobbs was reported to have
been in ill health for some time and had been living in a care facility, where his condition deteriorated quite rapidly over the past few days. He was the honorary
secretary of the GQRP Club (G5LOW), which he founded in 1974 to cater to those interested in low-power Amateur Radio communication. Dobbs served as the editor for the club’s quarterly, SPRAT. Dobbs was the author of QRP Basics, The International QRP Collection (co-authored with Steve Telenius-Lowe, 9M6DXX), and Making a Transistor Radio.
He was a frequent Hamvention® attendee, and in 2015, he received the Hamvention “Technical Excellence Award.”

Thanks to Lee Boulineau, KX4TT

RIP George…