26-09-2022, 06:56 PM
Generally, the earlier models can be a bit "drifty". In my experience it's not the metering; it's the supply itself. I strongly suspect the voltage pots, but have never bothered to go deeper because the drift is usually just 10s of mV, so not really a serious issue for my applications. The wire-wound pot on the coarse control is a bit of a beast - a massive Colvern job. I happen to have one here - NOS, found in the redundant spares at work - and plan to fit it one day to see if I'm right.
The later model did away with expensive pots by using them as pots rather than rheostats. The one example I have here is pretty good in that regard, despite using the cheapest pots imaginable.
Funnily enough, a routine re-cap of a couple of my older examples did nothing. The main reservoirs are big, decent chassis-mount types that seem to last forever, and the smaller ones on the PCB lead an easy life. The PSU cap on the 5V rail probably works the hardest as it's supplying the LEDs.
Whereas the caps used on the later models are a bit more problematic. One of mine had invisibly leaked, upsetting the metering circuitry - the rest were measuring high-ish ESR. The bigger smoother caps on the other PCB were OK, but they are no longer the big jobbies - instead it's 4 standard types in parallel.
Of course, I don't know the previous history of my units, so probably best I don't generalise.
One of my earlier QMD models had some vaporised tracks on the switch PCB. Probably user error.
If anyone is curious to see the insides of a later model, see this thread: https://www.vintage-radio.net/forum/show...hp?t=50989
A lot better than the first generation! The change happened while they were still in brown cases - the later models were grey, but as far as I know, that change was just cosmetic. If considering a brown model, the damping switch is the easiest clue to look for, as mentioned in my earlier post, but also the position of the earth terminal is also a clue. If the unit is in front of you, the very different feel of the Colvern pot is also a "tell". The later ones are generally better behaved, but I do notice the multiplexed LEDs flicker. One day I'll measure the refresh rate of old and new...
Shorting bars for the sense terminals and the caps for the knobs are often missing.
They are very difficult to recalibrate - most of the presets have far too much range (rookie error!).
Some of them had RS branding, some others were Racal. Classic badge engineering...
The very earliest examples had a different LED module with much thinner digits - by comparison the later ones look like someone selected the bold font. Individual segments can fail, but they're generally pretty reliable.
I'm sure there's more, but that's all that comes to mind right now...
The later model did away with expensive pots by using them as pots rather than rheostats. The one example I have here is pretty good in that regard, despite using the cheapest pots imaginable.
Funnily enough, a routine re-cap of a couple of my older examples did nothing. The main reservoirs are big, decent chassis-mount types that seem to last forever, and the smaller ones on the PCB lead an easy life. The PSU cap on the 5V rail probably works the hardest as it's supplying the LEDs.
Whereas the caps used on the later models are a bit more problematic. One of mine had invisibly leaked, upsetting the metering circuitry - the rest were measuring high-ish ESR. The bigger smoother caps on the other PCB were OK, but they are no longer the big jobbies - instead it's 4 standard types in parallel.
Of course, I don't know the previous history of my units, so probably best I don't generalise.
One of my earlier QMD models had some vaporised tracks on the switch PCB. Probably user error.
If anyone is curious to see the insides of a later model, see this thread: https://www.vintage-radio.net/forum/show...hp?t=50989
A lot better than the first generation! The change happened while they were still in brown cases - the later models were grey, but as far as I know, that change was just cosmetic. If considering a brown model, the damping switch is the easiest clue to look for, as mentioned in my earlier post, but also the position of the earth terminal is also a clue. If the unit is in front of you, the very different feel of the Colvern pot is also a "tell". The later ones are generally better behaved, but I do notice the multiplexed LEDs flicker. One day I'll measure the refresh rate of old and new...
Shorting bars for the sense terminals and the caps for the knobs are often missing.
They are very difficult to recalibrate - most of the presets have far too much range (rookie error!).
Some of them had RS branding, some others were Racal. Classic badge engineering...
The very earliest examples had a different LED module with much thinner digits - by comparison the later ones look like someone selected the bold font. Individual segments can fail, but they're generally pretty reliable.
I'm sure there's more, but that's all that comes to mind right now...
