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Old 12th May 2022, 11:17 am   #1
Merlin25
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Default Pye Mozart HFT108 Tuner

Hello
I am trying to reactivate an HFT108 tuner (which is in a sorry state) but have come across a bit of a puzzle that I'm hoping someone may help with.
I have a circuit diagram, but the valve lineup on the service sheet differs from my chassis. The service sheet shows V1 to V5 as EF80, ECF80, EF89, EF80 and EF80 respectively. My chassis only has provision for 4 valves, namely ECC85, EF89, EF80 and EF80.
Please could someone enlighten me as to whether this was a later (or earlier ?) modification, the thinking behind this, and ideally provide a copy of the circuit diagram relating to this ?
I am also lacking the mains power transformer, but this is a separate issue !
Thankyou
Mike
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Old 12th May 2022, 4:24 pm   #2
Edward Huggins
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Default Re: Pye Mozart HFT108 Tuner

This may be the later ECC85 tuner front-end modified version. Can you post a pic of both sides of the chassis.
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Old 12th May 2022, 10:47 pm   #3
Synchrodyne
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Default Re: Pye Mozart HFT108 Tuner

As far as I know, the later version was the HFT109, with valve line-up ECC85, EF80, EF89, EF184.

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It was evidently derived from the HFT113 FM-AM unit, which preceded it.

Basis the HFYB listings, the Pye Hi-fi tuner sequence of the period was:

HFT111 FM, LF, MF 1956 – 1960
HFT108 FM only 1959 – 1962
HFT113 FM, MF 1961 – 1963
HFT109 FM only 1963, 1964

In 1965, only the solid-state HFT300 was listed, FM, LF, MF. Assuming that it was all-bipolar, perhaps all-germanium, then to paraphrase the well-known phrase, its RF performance was probably “inadequate”.

I have not seen the schematic for the HFT109, bit I imagine that it is similar to the FM pathway of the HFT113, other than having an EF80 1st IF in place of the ECH81.

It would appear that Pye took progressive (regressive?) downward steps in respect of its FM front end circuitry.

The HFT111 used an EF80, ECF80 combination with three gangs but no AFC. The HFT108 also used the same valve combination, but with two gangs and with the ECF80 configured as an autodyne pentode with the triode doing AFC. Then the HFT113 stepped down to an ECC85, two gang permeability tuned, and with crystal diode AFC. I imagine that the HFT109 was the same. Back in 1955, the ECC85 was considered adequate for the FenMan I, but something better was considered necessary for the FenMan II and its derived HFT111 tuner. It looks as if the bean counters eventually beat out the engineers in this case


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Old 13th May 2022, 7:38 am   #4
Edward Huggins
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Default Re: Pye Mozart HFT108 Tuner

Thanks for this detailed insight. The Mozart Tuner and the Fenmans were all developed by the same team headed by Laurie Green at the Pye Labs. They were given a free hand at first, but then the "bean counters" did have their way. Emphasis was on the FM performance with the AM circuitry being fairly conventional by comparisom.
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Old 13th May 2022, 10:40 am   #5
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Default Re: Pye Mozart HFT108 Tuner

I have an HFT108 somewhere bought at a carboot years ago as it looks so good. I did an initial test replacing the dead selenium rectifier and discriminator electrolytic. Observation at the time was the case is far too small for the content.
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Old 14th May 2022, 1:14 pm   #6
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Default Re: Pye Mozart HFT108 Tuner

Thankyou, variously, for your swift and informative replies. It's interesting from a historical perspective as well as technically how these things evolve.
Hopefully attached are some photos of the chassis as requested by Edward. Those familiar with this chassis may observe the rather large hole where the power transformer should be - the comment about compactness of design is very true ! I'm going to attempt to power it up further today with a separate transformer (cautiously, with capacitor reforming in mind as well) and see what happens. So far got to about 100V HT without drama (though a circuit diagram would still be useful if anyone could locate one ...)
Cheers

Mike
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Old 14th May 2022, 10:57 pm   #7
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Default Re: Pye Mozart HFT108 Tuner

Interesting! That confirms what Edward said, essentially that Pye made a “silent change” to the HFT108, replacing the original two-valve front end with a single-valve type, but retaining the established IF strip, EF89 and 2 x EF80.

The unit at interest has a “diplex” (multiplex) output, which as far as I know was not present on the original. That might (or might not) have indicated a production breakpoint at which the front end change was also made.

Is the ECC85-based front end capacitor or inductance tuned? It is hard to tell from the pictures. If capacitor tuned, then it was not the as that fitted to the HFT113 (and probably the HFT109), which, at least according to the service data, was inductance tuned.

The available data on the HFT108 all seems to refer to the original data. Perhaps data for the later version is lurking somewhere, unknown to its holder(s) that it is in fact different to that readily available.

Re the HFT109, a possible rationale for its existence is that having rearranged what was to start with a tightly-packed unit to arrive at the HFT113 FM-AM tuner, it made sense from a production and cost viewpoint to derive therefrom a minimum-change FM-only unit instead of maintaining the somewhat different HFT108.

An unusual feature (in FM tuner/receiver terms) of the HFT113 (and the HFT109) was the use of the EF184 frame grid valve as the limiter. A possible rationale was that as the 2nd IF stage was not configured as a limiter as such on FM (although without agc bias, it might limit on very large signals), something better was required for the following limiter stage. (And the EF184 was used in that role as single-stage intercarrier sound IF in TV receivers, so it had the cedentials.)


Cheers,
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Old 15th May 2022, 12:01 am   #8
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Default Re: Pye Mozart HFT108 Tuner

In post 6 picture 3, it looks as if there is a horizontally-mounted permeability tuning assembly (under the ECC85), with operating peg on the left under the pulley working against a tensioning spring attached to the chassis post on the right.
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Old 15th May 2022, 2:31 am   #9
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Default Re: Pye Mozart HFT108 Tuner

The dog that didn't bark in the night seems to be the lack of any visible ganged tuning capacitor.

I love the label warning to not mount the tuner above a Mozart amplifier...

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Old 15th May 2022, 5:15 am   #10
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Default Re: Pye Mozart HFT108 Tuner

Thanks for that clarification.

So the ECC85-based front end of the later version of the HFT108 could have been the same as was used for the FM side of the HFT113.

Perhaps if the tuner had been named "Beethoven", it would have been OK to place it on top of the Mozart amplifier.....

(makes quick exit stage left)


Cheers,
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Old 15th May 2022, 11:52 am   #11
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Default Re: Pye Mozart HFT108 Tuner

Or put it on it's side and call it a Liszt ...

Anyhoo, for completeness as this has sparked some interest (well spotted, Turretslug) - more photos of the area around the ECC85.
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Old 15th May 2022, 12:56 pm   #12
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Default Re: Pye Mozart HFT108 Tuner

I do not remember this type of sub-assembly for the front end at all. And the ECC85 is un-screened and with no provision for it on the ceramic valve holder.
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Old 15th May 2022, 4:31 pm   #13
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Default Re: Pye Mozart HFT108 Tuner

Never knew Pye had had Rowland Emett working for them...

David
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Old 17th May 2022, 5:08 am   #14
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Default Re: Pye Mozart HFT108 Tuner

Although the general direction of front end circuit development for the Pye units at interest was downwards, apparently for cost reasons, there is one transition that might have had some technical basis, namely that from the HFT111 to the original HFT108.

Both used an EF80, ECF80 combination. In the HFT108, the ECF80 was employed as an autodyne mixer to free up the triode for use as an AFC reactance valve. Perhaps there was some performance drop-off that was considered acceptable to avoid the need for another valve envelope in what was a crowded unit anyway. (Although perhaps the use of an ECF80 rather than an EF80 for the 2nd IF would have been one way to provide an extra triode without an extra envelope.) But more significant was the change from three-gang to two-gang tuning. On its face, that looks like a cost reduction exercise, but there may have been an element of technical tradeoff between front end selectivity and noise, with the aperiodic input of the HFT108 being chosen for some improvement in the noise factor.

Certainly, aperiodic inputs, with two-gang tuning, were the norm in British FM practice where pentode RF amplifiers were used. For example, Leak did it that way with the original Troughline. But with the Troughline II, it went to a tuned input and three gangs, but with a cascode RF amplifier. Presumably the quieter nature of the latter somewhat offset the extra noise of the tuned input. I think the same sensitivity was claimed for both versions, although I am not aware that actual quieting curves were published.


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Old 17th May 2022, 5:44 am   #15
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Default Re: Pye Mozart HFT108 Tuner

If you factor in that it was usual in Britain to only cover the bottom end of the HF/FM band then a broadband stage is easier than trying to cover the whole 88-108 MHz.

Trying to make a narrow, tuneable front end with plenty of image rejection means loss in the resonators and a noise figure disadvantage, while trying to make an aperiodic front-end for >20% bandwidth starts to become difficult from matching reasons.

The UK, with just a few BBC stations all giving good signal strength in their service areas, and none getting even near each other's image frequencies made life relatively easy for radio designers. Maybe too easy.

On the other side of the 108MHz fence, filling the top part of the broadcast band made life hell for aviation radios. Aircraft owners were required to scrap both their communications radios (often two per plane), their navigation radios (often two per plane) and replace them with models complying with new requirements to work in the presence of strong, nearby broadcast signals. This applied to planes around the world as broadcasting filled up and the total power on the band increased.

David
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Old 17th May 2022, 6:29 am   #16
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Default Re: Pye Mozart HFT108 Tuner

It’s interesting to look at the tuning ranges:

Pye HFT111, with tuned input: 87 to 100 MHz
Pye HFT108, with aperiodic input 88 to 108 MHz

Leak Troughline, with aperiodic input 87.5 to 100 MHz
Leak Troughline II, with tuned input 88 to 108 MHz

The Pye case was the opposite of what one might expect. The HFT108 was evidently intended for export as well as domestic sales, whereas the HFT111 looks to have been a domestic market only product.

Some makers offered a choice of 87.5 to 100 MHz (domestic) and 88 to 108 MHz (export) tuning ranges on some of their models. Chapman was one such, using a pentode RF amplifier with aperiodic input in both cases.

The aircraft radio interference problem presumably came up quite early on in the USA, where the full 88 to 108 MHz FM band was used from 1945 or so, although more so as the band filled up in the 1960s. At some stage the FCC limited erp was to 100 kW, at least for new transmitters, but some early stations were up to 500 kW.


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Old 18th May 2022, 10:46 pm   #17
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Default Re: Pye Mozart HFT108 Tuner

I'm afraid that I can't add much to my learned friends circuit analysis, but to update any readers - the unit has come to life (sort of) !
I was missing a power transformer but managed to find and fit a spare one that had been knocking about, into the confined space of the chassis. That was the limiting factor as much as anything. It just about copes, though gets a bit warm, and in fact I had to add an extra HT dropping resistor in order to get down to about 115V as specified on the schematic that I had. The initial start point was about 195V, but as the smoothing electrolytic was rated at 275V I figured it should cope. I also changed a 0.25uF wax cap associated with the variable output control (pretty much short circuit), but despite also testing the valves on my tester it was only by wiggling one of the EF80s around it would produce some background hiss. I duly cleaned the offending valve base and it now tunes the local BBC stations in, albeit very very quietly, at approximately the right locations on the scale.
I don't know if anyone could comment on the sensitivity or otherwise of these receivers, but it does seem a bit deaf (also allowing for the fact we are in a bit of a dip in this location).
But to receive at least some some stations intelligibly is reassuring progress !
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Old 20th May 2022, 12:30 am   #18
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Default Re: Pye Mozart HFT108 Tuner

Some thoughts:

The HFT108 output level, 100 mV at 30% modulation, although normal for the 1950s and 1960s, is low when measured against later practice. To more-or-less match CD player outputs, something around 300 mV at 30% modulation would be required. So it probably would sound quiet when judged against more recent “line level” sources fed into the same amplifier input.

Also, the output needs to look into at least 100k. Anything materially lower would attenuate the output and change the de-emphasis.

In the data for the original version, it was noted:

“The audio output of the tuner is designed to suit the Mozart 10 watt amplifier and many other Hi-Fi amplifiers, and as such has been reduced by approx. 8 times. An increase of the tuner output can thus easily be obtained if desired by increasing the value of R22 and at the same time decreasing C24 until sufficient output is obtained.”

So there is a remedy for naturally low output.

With tuners that use heavy limiting followed by a Foster Seeley discriminator, as in this case, the audio output level usually reaches its nominal at relatively low signal inputs, maybe by or even well before 20 µV. The noise floor slopes down more gradually with increasing signal input. At a guess, for this kind of tuner, the noise floor probably flattens out by around 300 µV signal input.

Thus low signal strength would tend to produce a noisy signal, but not a reduced signal.

Low output, if not simply its natural level, without noticeable noise might indicate a fault somewhere. Perhaps not in the front end, but maybe low gain early in the IF strip?


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Old 22nd May 2022, 11:43 pm   #19
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Default Re: Pye Mozart HFT108 Tuner

Point taken ref the output level, though there is a pot and extra components on the chassis for this (as opposed to the schematic).
I've now polished the front plate, cleaned the tuning knob and tidied the wiring, so it all looks rather splendid.
Great feedback from you guys - thankyou for everything. I'm going to call it a day with this one, but may revisit it if anyone can pin down a more appropriate circuit diagram.
Cheers
Mike
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