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Old 17th Nov 2021, 12:47 pm   #41
Gabe001
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

How about aiming for a greater (more negative) agc voltage by altering the r24 to r26 ratio?. That should cause more attenuation of stronger stations and leave the weaker ones largely unaffected

(Apologies David your comprehensive reply wasn't visible to me when I was typing this)

Last edited by Gabe001; 17th Nov 2021 at 12:55 pm.
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Old 17th Nov 2021, 3:06 pm   #42
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

Thanks very much David and Gabriel. While the set is now readily useable by reducing aerial input on strong FM signals as a workaround, after spending so long on this it would be a shame to compromise at the final hurdle. Owing to the changes from the original circuit it looks as though the designer had some trouble getting an optimum performance. Re: short circuit of AFC output, on mine I found the AFC signal was decoupled by a 0.1uF brown Wima humbug (I mean visually looks like a humbug, not its function) cap, leaking and since replaced with a new yellow poly. I can easily disconnect the AFC link and see what happens to bass quality. Cheers, Jerry

Last edited by cathoderay57; 17th Nov 2021 at 3:12 pm.
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Old 18th Nov 2021, 2:14 pm   #43
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

Hello again. Yesterday, I did some component replacements. I fitted new diode load resistors R25 2.7k, R26 27k, R27 27k. R26 and R27 are different values to those quoted in the service sheet (2.7k) but are the values that I found fitted and looked original. I replaced the 2 diode anode/cathode electrolytics again (as-found: 2 x 25uF originally, 22uF replacements used) with brand new components. Unfortunately this had no effect. The voltage across both capacitors was 5.0v. Next I copied the circuit using 2.7k for R26 & R27 and a single 4.7uF electrolytic as per circuit. The voltage across the electrolytic was now 2.1v; the audio was slightly reduced but the bass note distortion was still there. I refitted the 27k resistors for R26 & R27 along with the 2x 22uF electrolytics. Next I put a short to chassis from the junction of R11 & R9 to inhibit AFC. The tuned frequency shifted up the dial by about 1 MHz but the distortion was the same. I then tried an experiment with AGC. Using a battery with a high resistance potential divider I injected more negative volts into the AGC line. Without the battery, on zero signal the AGC measured -0.3v; with strong signal -2.7v. When I connected the variable dc supply to the AGC line I could alter the volume of the signal but the distortion was still there . So at least I can conclude that the distortion is not being caused by overloading in the FM IF stages and is not an AGC fault. I tried David's experiment using the AM sig gen (without modulation). I coupled the signal to V1 using a ring of copper wire round the valve as per alignment instructions. I set the receiver to 95 MHz and put the meter probes between the junction of R23 and C45 (AFC output). When the sig gen was above 95 MHz the AFC voltage was -0.5v. As I reduced the sig gen frequency, the AFC voltage went down to -1.7v, up to 0v, up to +2.5v and then lock was lost so dropped back to -0.5v. So as things stand the only way I can get rid of the distortion is to use a stub of wire in the aerial socket a few centimetres long. Therefore it seems the front end could be overloading? I have been unable to find how much power the local station is pumping out (I thought they are limited to 25W) and as far as I know the transmitter aerial is between 1 and 2 miles away. The station comes in clear on a transistor portable without overloading and I have no other valve sets that cover up to that frequency (105.1 MHz). Cheers, Jerry
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Old 18th Nov 2021, 2:56 pm   #44
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

These sets were designed before stereo transmission and after bass/treble boost, compression, and adding the 19KHz pilot the signal hammers against the bandwidth limits. The IF bandwidth will have been set for mono so possibly the pilot tone is being lost on weaker stations.
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Old 18th Nov 2021, 5:14 pm   #45
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

Thanks PJL. You prompted me to re-do the FM alignment and this time there is a definite improvement in sound quality. I think it is now about as good as it is going to be. Thanks to all for your time and patience. It's a really good performer so unless something dire happens I won't bump this thread any more until I get the cabinet done. Time now to start on a Bush VHF61 for a bit of light relief hopefully! Cheers, Jerry
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Old 18th Nov 2021, 5:21 pm   #46
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

I wonder if the discriminator is out of adjustment?

A wobbulator/sweeper and a scope is the only easy way to check. The asymmetry in the voltage peaks on the AFC drive points that way.

ALSO if your IF is tuned such that there is a rapid change in group delay across the channel, that will translate into audio distortion. Non flat group delay means time-distortion of the different frequency components in an FM signal spectrum. FM demod converts timing into voltage..... hence distortion.

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Old 18th Nov 2021, 11:03 pm   #47
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

Thanks David. I don't have a wobbulator, only the scope and a Heathkit RF1-U Sig Gen. I may have a look round for a wobbulator and would need to bone up on using it. Cheers, Jerry
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Old 19th Nov 2021, 1:01 am   #48
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

I am curious about the impact of the switch to FM stereo as the modulation index may have been reduced to accommodate the additional signal bandwidth. Does this mean there was a reduction in volume on older mono sets or were mono transmissions not using the entire bandwidth? The de-emphasis will wipe out the high frequency pilot and L-R signal but were there problems with existing mono FM radios? I notice that the Armstrong IF alignment instructions work with 10.7MHz +/-25KHz but stereo transmissions use far more than that and is that likely to impact AFC systems? I realigned a mono FM receiver to allow me to connect a stereo decoder and really struggled to get the bandwidth wide enough and reasonably flat but it does now decode stereo very reliably although I may have compromised selectivity.

Last edited by PJL; 19th Nov 2021 at 1:07 am.
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Old 19th Nov 2021, 2:13 am   #49
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

In mono days, the FM maximum deviation was ±75 kHz. With the Zenith-GE stereo system, 10% of that was allocated to the 19 kHz pilot tone, leaving 90%, or ±67.5 kHz, for the signal itself. As (L+R) and (L-R) interleaved, each could go to ±67.5 kHz. Thus for mono reception of a stereo broadcast, there was a small (0.9 dB), probably negligible, reduction in signal level.

Stereo required a greater IF bandwidth, given that there was more energy in the outer sidebands. Thus some mono equipment probably had lower-than-desirable IF bandwidth for good conversion to stereo. Various bandwidths have been quoted, such as 180 kHz for mono, and 210 to 240 kHz for stereo, but there is no absolute answer, given the non-finite sideband structure of FM.

Note that in the USA, small amounts of the ±75 kHz maximum deviation were allocated to the SCA subcarriers where used, meaning a further reduction of that available for the main signal. In the Russia/Eastern Europe Band I FM system, the maximum deviation was ±50 kHz, and stereo was by the polar system.


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Old 19th Nov 2021, 9:24 am   #50
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

I've located the wobbulator construction article by Raymond Haigh in Radio Bygones No. 82 April/May 2003. I'm going to start a new thread on this in the homebrew section. Cheers, Jerry
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Old 20th Nov 2021, 2:21 pm   #51
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

Afternoon all. I have kindly been loaned a splendid Marconi TF995B/2 sig gen by member Boulevardier Now I seem to be getting somewhere. Let me first describe the test set-up. The sig gen is set to FM 10.7 MHz (checked on Racal 836 frequency counter) with an RF output of 16mV. The RF output is connected to grid 1 of the second IF valve (EF89, V3). The scope is set to external trigger with the trigger input (X) connected to the sync output of the sig gen, which is outputting its 1 kHz AF modulation signal. The sig gen is set to internal modulation. The Y input to the scope is connected to the dc AF output of the ratio detector at the junction between R23 and C45. The scope's timebase setting is 0.2 ms per division. The first image below shows the scope screen when the sig gen is switched to 75 kHz deviation. I think there are two cycles as a result of the timebase setting. There is an obvious double peak and the trace goes above zero to +60 mV and below zero to -100 mV. The second image shows the scope screen when the sig gen is switched to 25 kHz deviation. Above zero goes up to +16 mV and below to -30 mV. So clearly something needs tweaking. Presumably this means L23 and L24, since the sig gen is injecting a signal only into the second IF stage. Or is there something else wrong as well (why is the signal asymmetrical about the zero axis)? What shaped trace should I be tuning for, a flat top or a peak? Advice please. Cheers, Jerry
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Old 21st Nov 2021, 2:42 pm   #52
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

It does look a little odd but it is the overall response that is important. The service data alignment instructions use a meter to maximise the average amplitude so try the recommended alignment instructions then have a look with the scope.
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Old 21st Nov 2021, 4:00 pm   #53
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

Thanks PJL. First I adjusted the cores of L23 and L24 (FM IFT3) using a 10.7 MHz input to the 2nd IF valve using 25 kHz deviation. It peaked up to a clean sinewave shape, with 50mV pk-pk output from the ratio detector as shown in image #1. Applying a 10.7 MHz signal using 75 kHz deviation the waveform flattened somewhat (image #2) with peaks at +100mV and -150mV. Therefore I re-peaked the same 2 cores again and re-gained a symmetrical response as shown in image #3. I then injected the signal into the 1st IF and repeated the process adjusting L15 and L16 (FM IFT #2). There was only slight asymmetry and so only a minor tweak was needed. I then tried out the set on an aerial wire and hey presto, beautiful sound with crystal clear bass punch. I didn't adjust L7/L8 (FM IFT #1) since the sound was good enough. I think the message for anybody who acquires one of these sets is clear - don't mess with the FM alignment unless you have to, and don't try to do it without a visual representation of the alignment curve. Peaking dc output on a meter isn't precise enough for this set. Thanks once again for the help and advice. As I said earlier, I'll post a picture when I've got the cabinet fettled. Cheers, Jerry
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Old 3rd Dec 2021, 6:40 pm   #54
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

Here's the almost finished result. Just got to fit a coupling capacitor for the tweeter, and add a small piece of aluminium sheet to the fibre board back panel to deflect heat from the output valves and rectifier. Not too shabby, and a big improvement over the original box. Jerry
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Old 3rd Dec 2021, 7:00 pm   #55
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

Cracking job Jerry, both electronically and cabinet-wise. A huge improvement over the original.
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Old 4th Dec 2021, 3:25 pm   #56
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

I'm fitting an internal FM aerial inside the cabinet. The circuit is designed to take a 75 Ohm coax feeder. Clearly my internal dipole wires are going to be electrically short, a half wavelength dipole at 100 MHz being 1.5m long. I note that the Bush VHF61 uses a balun (see image) to improve FM aerial matching but I'm not sure if this is on account of the electrically short dipole, or because the receiver's input coil is centre-tapped to earth, or both. The input to the Armstrong circuit is different, as can be seen from the attached clip. Question is, should I wind a similar balun or connect the short dipole wires direct to the feeder on the Armstrong? Cheers, Jerry
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Old 4th Dec 2021, 5:13 pm   #57
Leon Crampin
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

Just buttng in here - if you're dealing with a Bush VHF61/2 as the next job, remember there's an error in the Trader sheet for this set.

Here, the locations of the primary and secondary slugs of the final FM IF transformer (ratio detector) are shown reversed. The Bush service data is correct.

Paul (Stenning) - is it possible to àppend a note to the Trader sheet when accessed fom this site?

Leon.
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Old 4th Dec 2021, 9:46 pm   #58
cathoderay57
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

Hi Leon, thanks for the tip. Jerry
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Old 5th Dec 2021, 10:58 am   #59
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

A very interesting choice of speakers! I had assumed you might opt for a 10" x 6" full range unit.
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Old 5th Dec 2021, 4:17 pm   #60
cathoderay57
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Default Re: Armstrong Jubilee Mark 2 tuner/amp - general advice

Thanks for the comments. Re: speakers, I used what I had available. Although only 4in the Audioline mid-range speaker is rated at 10W RMS and sounds pretty good. I'm still waiting for the tweeter coupling cap. The speaker terminals are easily accessible with the back cover fitted so I can still use it with one of my Infinity speakers (see attachment) that are normally connected to the Quad IIs. On the Infinity I admit it does sound superior. Before it gets lost, please can anybody offer guidance on my FM aerial question (Post #56? Cheers, Jerry
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