05-08-2012, 10:09 PM
Things are looking on the up with this item - at last.
The 10 kHz section seems to be working O.K. Which makes sense, now, of course, since if the 100 kHz section isn't doing its stuff, the 10 kHz section can't do its job either!
So as far as the calibrator functionality is concerned, I reckon I'm 99% there. Why only 99%? Because when the 100 kHz cal. is selected, there are beats where there shouldn't be, i.e. between 100 kHz points. Most of these are substantially weaker than the genuine 100 kHz ones, and there is a disproportionately strong beat at 500 kHz intervals. Looking at the O/P waveform from the 100 kHz stage, although it is a pulse, it is by no means 'clean' as regards rise and fall times - so some sub-harmonics are in there.
The questions thus arising now are:
(a) Was it always like this? - i.e. when it was new and left the factory? To which I suspect that there is a reasonable chance the answer is "yes". And I say that since to my eyes and understanding, the mixer circuit design is a rather crude unbalanced affair. It seems strangely out of context when one considers the exorbitant and extensive lengths Marconi have gone to in many other areas of the design of this generator. Or am I being unkind / unreasonable / mistaken here?
(b1) Do I simply live with it? After all, it isn't a major feature of the generator and if I ever do need to know the O/P freq. accurately, it's easy to connect a freq. counter to the O/P: this generator even has a dedicated BNC O/P on the front panel for that very purpose.
(b2) And thirdly, as far as a 100 kHz cal. 'pip' is concerned, this is mainly only going to be useful of on the lower freq. ranges anyway: this generator tunes from 10 kHz to 88 MHz: above about a few MHz, the 100 kHz 'pips' will be very close.
And the same argument applies to the 10 kHz cal. 'pips' too ~ perhaps more so.
So I intend to spend a little time investigating purity of waveform - mainly for peace of mind before I 'blow the whistle' on this calibrator repair job.
Having said all that, and on the topic of repairs, when I first inspected this generator, I did discover no R.F. O/P on the lowest but one freq. range - but very soon after that, that condition / fault mysteriously disappeared.
But now that fault it has come back.
The 'good news' is that because this particular fault is restricted to one tuning range only and the unit is designed with separate osc. modules for each band, the 'search area' for the fault is thereby substantially narrowed. (Least, that's the theory & hope - at present. )
So enough for today: bed-time beckons.
Today I have achieved more than I anticipated: I found I had some unexpected time available: it was put to good use.
More to come later: don't g'way now!
Al.
The 10 kHz section seems to be working O.K. Which makes sense, now, of course, since if the 100 kHz section isn't doing its stuff, the 10 kHz section can't do its job either!
So as far as the calibrator functionality is concerned, I reckon I'm 99% there. Why only 99%? Because when the 100 kHz cal. is selected, there are beats where there shouldn't be, i.e. between 100 kHz points. Most of these are substantially weaker than the genuine 100 kHz ones, and there is a disproportionately strong beat at 500 kHz intervals. Looking at the O/P waveform from the 100 kHz stage, although it is a pulse, it is by no means 'clean' as regards rise and fall times - so some sub-harmonics are in there.
The questions thus arising now are:
(a) Was it always like this? - i.e. when it was new and left the factory? To which I suspect that there is a reasonable chance the answer is "yes". And I say that since to my eyes and understanding, the mixer circuit design is a rather crude unbalanced affair. It seems strangely out of context when one considers the exorbitant and extensive lengths Marconi have gone to in many other areas of the design of this generator. Or am I being unkind / unreasonable / mistaken here?
(b1) Do I simply live with it? After all, it isn't a major feature of the generator and if I ever do need to know the O/P freq. accurately, it's easy to connect a freq. counter to the O/P: this generator even has a dedicated BNC O/P on the front panel for that very purpose.
(b2) And thirdly, as far as a 100 kHz cal. 'pip' is concerned, this is mainly only going to be useful of on the lower freq. ranges anyway: this generator tunes from 10 kHz to 88 MHz: above about a few MHz, the 100 kHz 'pips' will be very close.
And the same argument applies to the 10 kHz cal. 'pips' too ~ perhaps more so.
So I intend to spend a little time investigating purity of waveform - mainly for peace of mind before I 'blow the whistle' on this calibrator repair job.
Having said all that, and on the topic of repairs, when I first inspected this generator, I did discover no R.F. O/P on the lowest but one freq. range - but very soon after that, that condition / fault mysteriously disappeared.
But now that fault it has come back. The 'good news' is that because this particular fault is restricted to one tuning range only and the unit is designed with separate osc. modules for each band, the 'search area' for the fault is thereby substantially narrowed. (Least, that's the theory & hope - at present. )So enough for today: bed-time beckons.
Today I have achieved more than I anticipated: I found I had some unexpected time available: it was put to good use.More to come later: don't g'way now!
Al.
