FM Limiter as AM Demodulator

[Murphyv310]

Hi.
The 6/30L2 is pin compatible with the ECC85, they in the main work but are on the edge, I tried one many years ago with no success, then a few years later tried another in a different set and it worked OK. I found out then that some did some didn't and others only at the low end of the band.
Emma's Dynatron Marlow has an ECC88 fitted and its really sensitive, works so much better than it did with the ECC85/
.

[Murphyv310]

Hi.
Regarding the TV sets with FM, Murphy used dual conversion on the sound IF on the V430 & V530. My V530 was really good on FM, it also had AFC. I've never looked at the V330 circuitry but as its rh next set on my list for an overhaul we will see.

[Murphyv310]

Hi.
the following three scans are:
1, The Chapman circuit
2. Murphy V530 sound stage
3, Murphy V530 sound circuit description.

Thought they my be of interest.
   
   
   

[Synchrodyne]

Thanks for those.

The Murphy circuit certainly does have some unexpected features.

The use of a 6C12 (ECH81) for the second conversion is as expected, although I had wondered whether one might find a 6C9.  Murphy had hung on to the latter – understood to be quite robust – until quite late in its radio receiver circuits, after all of the other signal valves had been changed to novals.

But the use of a second 6C12 to provide the second sound IF gain stage and the first audio stage was initially a surprise.  It does make sense, though.  Murphy required a remote cutoff valve for the 2nd IF stage, and an additional triode for the AFC DC amplifier.  So a double valve was a logical.  And whilst another triode-pentode might be the first port of call, such types with remote cutoff pentode sections would have been very scarce when the circuit was designed.  On the other hand, the 6C12 heptode was remote cutoff, and was designed for use as a gain-controlled IF amplifier as well as a gain-controlled mixer, so it was in fact a good fit.  And the 6C12 triode was designed to be used as an AF amplifier as well as an oscillator, so it too was a good fit.  In respect of the disposition of the two triode section, that in the 30PL1 had a separate cathode, and so was a better fit for the DC amplifier role, where the cathode operated with divider bias.  

The 6C12 heptode as operated as an amplifier with AGC bias on TV sound (AM) and as a delayed limiter with standing bias on FM.  As the anode and screen potentials were not switched between the two modes, presumably the AGC bias was arranged to keep the valve away from limiting on AM.  The AGC bias was derived directly from the demodulated AM signal, and so was not delayed.  It was also applied to the mixer grid on AM only.  On FM, AGC bias for the RF amplifier was obtained from the limiter grid.

Given that the 6C12 was operated as a limiter on FM, grid detection for AM would have been possible (and was used as a source of RF AGC bias on FM), but it is clear that as two gain stages were required on AM, this option was precluded.

That a DC gain stage was required for the AFC bias possibly reflects the fact that a ratio detector was used for FM.  Had a Foster Seeley discriminator been used, then maybe there would have been enough AFC bias without a DC amplifier.  But that almost certainly would have required an additional IF stage, to give two gain stages and a full limiter, meaning an extra valve envelope.  In that case, though, AM grid detection would have been possible.

The AFC system was unusual in that it was a long loop, including the second frequency changer stage.  Presumably the second oscillator was sufficiently stable that a situation where the AFC was correcting for second oscillator drift by pulling the first oscillator out of correct tune did not arise in a material way.

A neat feature was the use of the AM noise limiter diode to provide sound channel muting during warm-up.

In the main AGC system, the IF bias was clamped at the point where the RF bias started.  This was different to the usual Mullard curves where the IF bias continued to rise after the RF bias started, with the curves crossing at the points of respective maximum bias.  Apparently, it was preferable to ensure that the IF bias did not take the valve(s) to the bend in the curve (with resultant severe non-linearity) before the RF amplifier got there, the more so if sharp cutoff valves were used in the IF strip.  Evidently Murphy ensured that condition by simply clamping the IF AGC bias at the appropriate point.

That Murphy circuit was interesting enough that in retrospect, it’s surprising that Murphy did not “plant” a full article in Wireless World at the time.  But both the use of dual-conversion and the subsequent addition of AFC were noted in brief items:

           


Cheers,

Steve

[Synchrodyne]

The Chapman S6BS/FM Mk II tuner had separate AM and FM sections (except for a shared tuning indicator and a common power supply), which was unusual in British practice, although reasonably common in American tuners.

The AM side looks to be the regular S6BS circuit, which was Chapman’s piece-de-resistance, the original version dating from early 1953.

When the original S6BS/FM was introduced, c.1958-59, it was said to be a combination of the S6BS and FM91 units.  It does look as if with the S6BS/FM Mk II, there was some improvement.  (The FM91 itself was discontinued at about the time that the Mk II version was introduced, so probably never received the same improvement.

As best I can determine, Chapman initially used the ECC85 front end in its FM81 tuner of 1953, but switched to pentodes, including a pentode self-oscillating mixer with the FM81 Mk II in 1956, and stayed with them thereafter.  Evidently it thought that two pentodes were better than a single ECC85.  AFC was used on the FM80 switched tuner of c.1957, and adopted on the variable-tuned model with the FM91 iteration in 1958.  This required a triode-pentode in place of the pentode, still self-oscillating, with the triode as reactance modulator.  Several British makers retroactively added AFC to their variable FM tuners, probably having discovered that however stable they thought their oscillators were, it wasn’t quite enough.  This group included Armstrong, Chapman, Jason, Lowther, Pye and Quad.  Leak used AFC from the start with its Troughline series.  In some cases, such as Quad, the AFC action was relatively mild, intended simply to correct minor drift, and not to be a substitute for initial correct tuning.

Two-gang front ends, with an aperiodic input to the RF amplifier, were the norm in British valved FM tuners, although there were some exceptions where three gangs with tuned inputs were used.  Leak moved up to three gangs (and a cascode RF amplifier) with the Troughline II of 1959.  Chapman had hitherto followed the norm, using two-gangs.  But then came the previously mentioned improvement to the FM section of the S6BS/FM Mk II.  It had three gangs, but the third gang was not used to provide a tuned input, but rather a bandpass interstage.  One may only guess at the reasoning behind this.  Presumably the aperiodic input was not seen to be a problem; the RF stage was not AGC’d and likely an aerial input attenuator would be used in excessively strong signal conditions.  The tuned interstage would provide greater RF selectivity than a tuned input stage anyway, so using the third gang to provide a bandpass interstage would have provided the biggest benefit in terms of pre-mixer RF selectivity.

The two-stage IF strip and ratio detector was characteristic of the Chapman FM tuners throughout, although some of the lesser AM-FM models with combined IF strips did have a third stage, probably simply because it was there.  A feature of all of Chapman’s valved AM and AM-FM tuners, going back to the original S4 of 1951, was that the AM section included an EBF80 in an IF side-chain to provide amplified and delayed AGC.  This approach avoided differential distortion and would also have minimized modulation rise in the final IF stage.


Cheers,

Steve

[Murphyv310]

Hi Steve.
Yet another great write up.
I passed on my V530 last year to Bobs privately run museum but I still have a V330 that needs some attention. The V330 doesn't have AFC but Murphy tuners are exceptionally drift free and don't even have a fine tuner. Certainly there is never any drift on the V330 and the FM radio can be on for hours with no issues. A belt and braces approach was then adopted in the V430 & V530 with the addition of AFC.