Understanding Retro Electronics

[tjohnson]

My scope is a Tenma 72-8225A which is a rebranded Uni-T UTD2052CL Digital Storage Oscilloscope with 50Mhz bandwidth.

I got it from CPC Farnell and they now sell it for £270 (https://cpc.farnell.com/tenma/72-8225a/oscilloscope-2-ch-50mhz-500msps/dp/IN07409?ost=72-8225a&ddkey=https%3Aen-CPC%2FCPC_United_Kingdom%2Fsearch).  It is ok and I got it brand new for £170 but I think I would spend more now if I was buying again and wouldn't spend £270 on it, it claims to support Video triggering but doesn't appear to be in the firmware for mine and no upgrades are available.  The price rise in a couple of years perhaps shows rather worryingly how much real inflation is going on or maybe I bought it on sale I don't exactly remember.

I've never used it for the CPC, maybe I should give it a try to see what results I get, can you suggest some tests Bryce and I can post some output from it which might give people an idea of what to expect?

[Bryce]

The highest bandwidth in a CPC is pin 4 of the 74HCU04. This is the 16MHz clock signal. Test this on your scope and you'll have a good idea of the limits of the scope.

Bryce.

Edit: Make sure you have the channel set to DC coupling and the 20MHz bandwidth limit turned off.

[Bryce]

I haven't posted anything here in a while, so I thought I'd discuss a topic that although it's not really a problem on the CPC, I have seen it regularly pop up on other retro computer discussions, especially the Amiga scene (and many of us here don't limit our retro collection to just Amstrads)...

Capacitor replacement.

We've all heard that electrolytic capacitors go bad over time (even faster if they are in a hot environment). They burst open, they go short, they develop internal resistance, they leak acidic gunk, they cause computers to become unreliable etc. and eventually need to be replaced. The CPC has luckily just one electrolytic capacitor on the power rail and due to the fact that it's on a well regulated rail it has a long life expectancy. Other computers, especially the post A500 Amigas aren't so lucky.

I've noticed a growing trend of Amiga owners replacing the electrolytic capacitors in their machines with Tantalum caps. Although this has a few advantages such as the fact that they can't leak, are slightly easier to solder than SMD electrolytic capacitors, need less space and have very low ESR (internal resistance) values, they do have a few dis-advantages that need to be taken into consideration, so I thought I'd mention a few things for those who are considering going to tantalum when re-capping.

- Voltage Specification: Electrolytic capacitors work pretty much right up to the voltage specified on the capacitor, so using a 16V capacitor on a 12V line is fine and has (almost) no effect on the performance or lifetime of the part. With Tantalum caps it's a completely different story. Make sure that the parts you choose are specified for at least double the line voltage. I usually use 16V parts for 5V circuits. If you choose parts speced too close to the line voltage they will work ok at first, but have a relatively short lifetime and it won't end good.

- High transient spikes: Generally tantalum caps are good for removing transient spikes in the voltage, but only as long as they are within the parts specs. Whereas an electrolytic capacitor can survive (and smooth) larger spikes above the capacitors rating, tantalums don't do this very well. Due to their very low ESR, they can in some circumstances actually amplify the spike which could end up damaging your machine. This is due to the very thin oxide layer in these caps, which the voltage manages to jump across. Each time this happens, the performance of the cap deteriorates slightly and eventually the part will fail.

- End of life - Going out with a bang... The problems mentioned above can eventually cause a short-circuit inside the cap. Due to the power available in a modern PSU, the capacitor will actually explode or burn and if you are really unlucky, take some of the PCB track with it or even burn a hole in the PCB. There's no pre-warning or mild leakage like you have with an electrolytic.

So, are tantalums a better choice than electrolytics for your prized computer? My recommendation would be to use properly rated (at least 16V parts) tantalum capacitors for the smaller 5V line capacitors, but stick with electrolytic capacitors for the larger caps, especially those directly at the power inputs. Make sure you've chosen parts with voltage specs well above the line voltage and you should have the best solution all round. Modern electrolytic capacitors have a decent lifetime, so even using electrolytics everywhere isn't really a bad idea. A regular check for leakage should only be needed after about 10 years.

Bryce.

For those who underestimate the damage a failed tantalum capacitor can cause, I'd suggest you take a look at the pictures here: https://www.hobbyelectronics.net/electronics-blog/archives/06-2013