[R-390] xtal wave spikes
Dan Merz
[email protected]
Tue, 26 Nov 2002 20:23:38 -0800
Bob, all this I'm talking about is a standard 3010 B or C receiver, no mods
(except I had to add a freq. readout to the vfo on one of the sets which was
missing the dial tape and mechanism). The use of the attenuator is essential on
strong signals because the front end tube has very high mu (50,000 !!). That
tube in front of the diode ring mixer, a 7788, was designed to reduce intermod
effects ( or so I read from an article in Electric Radio) I'm just starting to
appreciate this set; it produces very clean signals on the HF bands but a
bit of a pain to tune because you have to separately tune the front end
circuit, which has it's own bandswitch for a number of designated ranges. Once
you tune it though, it doesn't take constant tweaking over a ham band, except
160 and 80 meters. I did go down and check the 3 6BL8 tubes as oscillator and
they were essentially the same as far as wave amplitude, 2 RCA's and an
Amperex. So the voltages are 0.5 v pp and less for the different xtals. So
far, I've not used attentuation except on the BC band, and the set has a BC
band high pass filter that is switched in when operating on frequencies higher
than 1.8 Mhz to tame those signals. The circuit and manual is available on the
BAMA site. thanks for your comments, Dan.
and one of these days I'll remember not to try to forward a message when I
forgot to put the R-390 list in the address on the first try - apologize for the
dud message , Dan
Bob Tetrault wrote:
> Dan,
>
> The 1N82a's are a similar RF type diode as compared to the 1N34. While the
> turn-on voltage is lower for a germanium as you pointed out, the oscillator
> voltages still seem marginal. They can still be found, though I don't know
> where. I think a suitable newer tech replacement would be the Schottky type,
> which has a similar turn-on threshold. Matching them would be easier than
> the old one; they are much more consistent in the processing, and in any
> case, the matching need only amount to a match of the forward voltage as
> measured by a DMM in the continuity mode. The Mini-Circuits (MCL) mixers are
> 50 Ohm devices which might work with a cathode follwer as the LO driver, but
> not with the IF output, which I am sure, in your case, is probably very high
> impedance as most toob circuits were designed for such levels. The MCL
> diodes are Schottkys.
>
> I take it that this is a mod? Or a production design? If it is a production
> design, it would explain the BC band performance issue. Sure as can be, that
> is a common band for such phenomena if the receiver is compromised for
> intermod.
>
> Bob
> -----Original Message-----
> From: Dan Merz [mailto:[email protected]]
> Sent: Tuesday, November 26, 2002 3:34 PM
> To: Bob Tetrault; R-390 List
> Subject: Re: [R-390] xtal wave spikes
>
> Bob, the diodes in the ring detector are germanium 1N82a's so I think the
> turn-on voltage is lower if I remember right compared to silicon type. I
> was
> down measuring some more and found lower oscill. output voltages than before
> for
> some reason and thought maybe I was reading my scope wrong before I
> checked
> the 2nd receiver I have to see if I could get what I measured before since I
> had
> both of them on the bench when I made the first set of measurements. I got
> the
> lower voltages there too so I must have made a slip between my notes and the
> scale factor, and the highest voltage is around 0.5 volt (not 1.7). And
> now I
> don't see the spikes that I saw before !!! The waveforms are now very
> smooth.
> I'm at a loss on that. I've been passing one 6BL8 tube between the two sets
> for
> the oscill/cathode follower as I'm short on that tube type. It may be
> possible
> that in exchanging tubes I used a different 6BL8 before for the oscillators;
> the sets have 2 each and at one point I only had 2 to use between the two
> sets
> and was exchanging both; now I found another one and have 3 and only
> exchange
> one of them. Maybe there's something in one of those 3 tubes that would
> account
> for the spikes. And I noticed that for one of the xtals that I thought was
> bad, I could get a 0.1 volt pp wave or maybe a bit lower at the cathode
> follower output but on the ring I didn't get the waveform showing up so I
> interpret that to mean that the output is barely there and not strong
> enough,
> near the threshold. But there may be another reason for this behavior,
> maybe
> the operation is just erratic for that particular crystal. This'll take a
> little more snooping on my part. This particular crystal was diagnosed as
> not
> working several weeks ago but was working when I checked it later and then
> was
> working again today at the particular location mentioned but was not at the
> other location. By the way, I'm wondering if there is a replacement unit
> that
> might be put here in place of the 4 diodes since these were put in back when
> this type of mixer was just coming into the picture and they might not be as
> good as later units. I'm also wondering if you would ever find a matched set
> of
> 1N82a diodes to replace the originals. Mini-Circuits makes a balanced
> mixer
> but I don't know if you could stick one of these in and I don't know what
> kind
> of diodes are in these mixers. At this point I have no reason to think I
> should
> replace the diodes but am wondering what-if.... One of these receivers
> works
> pretty good and is on a par with the 390a as far as detecting weak signals
> so I
> have no reason to think the oscillator level is really too low for the good
> xtals at least. I haven't noticed any problems with strong signals other
> than
> bc stations need about 20 db atten on the front end (built in attenuator is
> there for that) to behave nicely, a known characteristic of this radio.
> And
> the other receiver also is comparable but has some other dial repairs and
> intermittents that I'm trying to remedy Dan.
>
> Bob Tetrault wrote:
>
> > Dan,
> >
> > The more amplitude the better, subject to the limitations of maximum diode
> > current at the higher amplitudes. It does concern me that your minimal
> > amplitude approaches the turn-on voltage of a silicon diode; those bands
> > will be more likely to perform poorly in the presence of strong signals,
> as
> > well as suffering from increased conversion loss. OTOH, if it means that
> > you'll have to replace all the crystals and find a hot tube and re-bias
> for
> > increased output, etc, etc, one can reach a point of decision...
> >
> > Good luck
> >
> > -----Original Message-----
> > From: [email protected] [mailto:[email protected]]On
> > Behalf Of Dan Merz
> > Sent: Monday, November 25, 2002 1:17 PM
> > To: Bob Tetrault; R-390 List
> > Subject: Re: [R-390] xtal wave spikes
> >
> > Bob, the wave amplitude ranges from about 1 to 1.7 volts pp output from
> the
> > cathode follower that feeds to the single-balanced diode ring detector,
> > with a
> > couple of the xtals producing 0.8 volts pp. I don't think there is gross
> > distortion other than what I described but I'm just looking at it with a
> > scope.
> > I'll order the dead crystals after trying a couple of different tubes in
> the
> > oscillator but I don't expect a miracle at this point with tube
> > substitution.
> > Your comments about the square wave brought back some faint memories of
> > fourier
> > series representation of square waves but my memory bank need refreshing
> > about
> > that and what a mixer really does besides adding and subtracting the
> > frequencies of waves put into it. I do remember that a square wave is
> > generously
> > endowed with waves of many frequencies, and hence its utility in
> testing.
> > thanks for the comments, Dan
> >
> > Bob Tetrault wrote:
> >
> > > Dan,
> > >
> > > The presence of irregularities in a waveform is not immediate evidence
> of
> > a
> > > failing crystal. Either they don't work or they begin to lose output and
> > > ultimately don't work. Unless the oscillator is tuned, at the output,
> for
> > > the crystal frequency, and unless the oscillator is biased explicitly
> for
> > > that particular crystal and mode of operation, there is very great
> > > likelihood that some harmonic energy is being generated by the
> oscillator.
> > > Also, some oscillator circuits are better at harmonic generation than
> > > others. You would need a spectrum analyzer or a VHF/UHF receiver to
> > > investigate this, and the fact that some of your crystals do not exhibit
> > the
> > > spike on a scope doesn't mean that significant energy isn't being
> produced
> > > at *their* harmonics. The fact that this energy is being injected into a
> > > mixer mitigates the presence of any harmonics, as the mixer itself is a
> > > harmonic generator. They are sometimes referred to as a comb generator
> in
> > > the older literature. In other words, don't worry. Is there sufficient
> > > amplitude? Is it grossly distorted?
> > >
> > > This begins to get off-topic, but there is ample and long standing
> > evidence
> > > that a square wave with very fast edges is the best waveform to drive a
> > > mixer. Such a waveform has quite high energy in the harmonics. It is the
> > > relatively slow transition in the interval between either fully
> saturated
> > ON
> > > or fully cut OFF that promotes susceptibility to intermod. Thus a fast
> > > square wave minimizes the susceptible time.
> > >
> > > Bob
> > >
> > > -----Original Message-----
> > > From: [email protected] [mailto:[email protected]]On
> > > Behalf Of Dan Merz
> > > Sent: Monday, November 25, 2002 9:22 AM
> > > To: R-390 List
> > > Subject: [R-390] xtal wave spikes
> > >
> > > Hi, in looking at the output from the front end high freq. oscillator
> > > (40 to 60 Mhz xtals) on the Mackay 3010, I noted double peaks on some
> > > of the waveforms, though the nominal frequency was ok. My first
> > > reaction was that this could be symptomatic of a failing crystal though
> > > the output works ok and the right signals are received in the radio on
> > > the band in use. The spike occurs on the downside of the wave, kind
> > > of a minor glitch as the wave is decreasing. Does anyone know what this
> > > means or what is causing it? Most of the xtals don't exhibit this
> > > characteristic, but a couple are dead. I'm going to replace the dead
> > > ones - MH Electronics says they will provide replacements for $15 ea if
> > > I'll send them a good crystal as example of spec's. Is this a
> > > reasonable price compared to other possible providers? thanks, Dan.
> > >
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