Valve Equivalents

[HowardB]

I am restoring a Murphy A242 and hoped to improve the VHF reception by replacing the first VHF valve, a 6L34 with a Mullard M8099.

The new valve gives disappointing results. It works, but nowhere near as good as the 6L34. According to the valve museum, the 2 valves should be equivalent. The 8099 was sold as NOS, never been used, AVO tested. I have no reason to doubt this, it certainly looks nice and clean.

Am I missing something here - is there a small difference between the two that requires a circuit change?

Thanks
HowardB

[Cobaltblue]

Just a quick though did you re-align the set after replacing the valve?

I am not sure how tolerant the Murphy is but the manufacturers service say to re-align if any of the RF valves changed.

Especially as its possibly a different type/manufacturer it may have thrown out the alignment.

Cheers

Mike T

[Herald1360]

Originally Posted by HowardB

.......

The 8099 was sold as NOS, never been used, AVO tested. I have no reason to doubt this, it certainly looks nice and clean.

.......

Thanks
HowardB

Someone commented recently that "NOS, never been used, AVO tested." is in itself meaningless noise unless actual test figures obtained and presumably quoted do indeed reflect the results for a good, unused valve and are warranted.

[HowardB]

Thanks Mike. Yes, the service sheet says VHF circuits must be re-aligned if V1-V5 are changed.
Unfortunately, that's above my skill level at the moment.

[Radio Wrangler]

OK, time to bring your skill level up. If you have a signal generator that covers the band, there are people on here who can guide you.

The main selectivity of the receiver is in its IF, so the frequency of the local oscillator dominates the setting of where the receiver is listening. So you first of all accept the poor sensitivity and adjust the local oscillator tuned circuit to make the tuning fit the scale markings.

Nowadays, with the VHF/FM band fairly fully populated, you don't need a sig gen, you can adjust the set to get one station near the bottom of the band and one station near the top of the band appearing on the right frequency markings of the scale.

The next task is to adjust the RF tuned circuit to agree with what the LO and IF are doing. This is where the sensitivity should improve. What you do is tune to a station (or your sig gen) near the bottom of the band and peak the tuned circuit, then tune to one near the top and peak again.

So you can do a lot even without the sig gen, if there are stations in your area spread across the band.

Usually inductors are tweaked at the low frequency end, capacitors at the top. By repeating these adjustments, they should converge on a good compromise.

Now, because you've only replaced a valve, and the set was better before, it's likely that only the capacitance has changed, and the inductance isn't out.

So a quick and dirty short cut is to find the LO trimmer capacitor, tune to a station near the top of the band and twiddle that trimmer to plant it on the right spot on the dial.

Then find the RF tuned circuit trimmer capacitor and twiddle it for sstrongest signal.

At this point, you really need someone familiar with that set, which I'm not.

Phantomless twiddling!

David

[HowardB]

Hi David,

Thanks for your detailed instructions. I only have an audio signal generator so your method of adjustment to reposition and maximise broadcast stations is the only way open to me at present. I think I've found the caps you mention - LO trimmer C18/C21 and the RF tuned circuit trimmer C6/C7 in the anode circuit of V1. I think these are the ones, but there are so many variable components it's very confusing to a beginner.

I do have a modern oscilloscope and I've experimented with the FFT function to view the spectrum at various points in the circuit. It's starting to make sense as I'm seeing peaks at the right frequencies. I did wonder if this could be used for alignment?

Anyway, I'm not going to do anything rash at this point. I will have to do lots more reading before I pluck up the courage to start twiddling anything, as I would hate to break what I've got, having spent ages replacing most of the caps and resistors and getting the set working reasonably well.

HowardB

[Radio Wrangler]

If you've replaced any of the RF caps in that front end, you could have stirred things up quite a lot.

Usually the RF small value capacitors survive just fine and can be left alone to avoid re-alignment, but in VHF/UHF stuff, just a new valve is enough to stir things up.

David

[HowardB]

It's ok, I didn't replace any caps in the front end, couldn't reach them even if I wanted to, as they are all tightly enclosed in the metal box.

I'll leave the original valve in place for now and put some serious effort into understanding alignment!

HowardB

[Radio Wrangler]

Alignment is actually quite simple.

A mixer is fed with a wanted signal and a local oscillator. It gives outputs on two frequencies, the sum of the frequencies of those two inputs, and the difference of those two frequencies.

In a receiver, we are usually interested in reducing the frequency of the incoming signal because it is easier to amplify and to filter at lower frequencies.

So if your tuner is set to receive 100MHz, the local oscillator must be tuned 10.7 MHz different to that, so that the difference frequency coming out of the mixer hits the 10.7MHz IF.

Notice that there is ambiguity here. The LO (local oscillator) could be either 110.7 MHz or it could be 89.3MHz. Both would work. The designer has a free choice, but other factors have influence.

Putting the LO on the high side means that the ratio of frequency change is less and this makes the oscillator easier to design. But having the oscillator on the high side means that if the radio is tuned into the upper parts of the band, the LO is tuned to frequencies reserved for aircraft navigational aids - the instrument landing system and auto-land systems. Of all the things to interfere with, this could be the worst-ever choice, but some radios do. This is the reason behind the ban on having radios turned on in passenger aircraft cabins during landing and take off.

The IF is much more selective than the tuned circuits used at the incoming RF frequency, and the LO sets what RF frequency is converted to the IF frequency. So the LO frequency is the strongest determinant of what frequency you are trying to listen to.

The normal routine is to first align the IF. This needs a signal generator tuned accurately to the intended IF frequency and IF transformers are just peaked for best gain in an AM set. In an FM set it may be better to sweep the signal generator and display the response on a scope so it can be adjusted to a wider, flat-topped shape. The sweep generator was traditionally called a wobbulator (Handy for scrabble, but sounds quite rude to the uninitiated) You've only replaced the front end valve, so your IF should be OK unless you replaced any small tuning capacitors on the FM IF transformers? IF alignment is the hardest bit.

If you trust the IF is OK, then the LO tuning range is the next priority. You alternate between testing at the top of the band and at the bottom. At the top you twiddle a capacitive trimmer, at the bottom you adjust an inductor core. For VHF coils, you might have to stretch or squeeze them a little as there may be no adjustable cores.

The set should now tune pretty close to the figures on the dial.

Once you're happy with the LO, Only the RF stages remain.

As with the LO, alternate between the top and bottom of the band several times. Adjust inductors for best sensitivity at the bottom, capacitors at the top.

The inductors at the bottom, capacitors at the top thing is important because the adjustments interact, but this routine makes you converge on a good set of settings. Do it the other way round and it diverges and you get lost.

For a basic domestic FM radio, just peaking the IF with a sig gen at 10.7 MHz will do. Swept adjustment of the IF is needed by hifi grade receivers. The discriminator coil can be adjusted by ear for minimum distortion on a basic receiver, but do it with the swept alignment on any serious receiver.

There is the legendary phantom twiddler, the set owner ho goes inside, thinks the adjusters are loose screws and 'tightens them all'. The above procedures will get even such a set back on track.

Until you've done one, it seems difficult. Afterwards, it's just a routine task, no rocket scientists need be harmed.

Anyway, for just a front-end valve swap you only need to do the easy parts... get the LO to agree with the tuning scale, and peak the RF stages.

David

[crackle]

Originally Posted by Radio Wrangler

If you've replaced any of the RF caps in that front end, you could have stirred things up quite a lot.

Usually the RF small value capacitors survive just fine and can be left alone to avoid re-alignment, but in VHF/UHF stuff, just a new valve is enough to stir things up.

David

At VHF frequencies it is not just changing the capacitors that can cause havoc to the alignment. If any of the wiring including other resistors and obviously the open coils in the VHF tuner section are disturbed the mutual reactance between these components can be altered enough to put the set out of alignment.
You need to be extra careful not to disturb the layout of the components and wiring too much.
As David has said, it is unusual for components in the VHF tuner to be stressed and need replacing, you will not normally find wax paper capacitors in the critical parts of the VHF circuit, they may be used for decoupling which is not so critical, but if replacing them do it carefully.
I have found in the past that one wire pushed out of the way can stop the circuit working. So I try at all costs to NOT make any component changes in the VHF tuner section.

Mike

[PJL]

The Murphy service sheet available from the link above has a comprehensive explanation of the design and some useful alignment instructions that do not require an FM signal generator.

[Malcolm G6ANZ]

I have a Cossor 524 which was very poor on FM. I replaced any out of spec resistors and caps in the set. I admit I didn't touch any of the caps in the front end as they were all types that are known not to go out of spec. After advice from here I replaced the front end and the detector valves. The set then burst int life. I had no need to do any realigning of the IFs in fact the only tweak required was to the FM discriminator coil. I did this by injecting an FM modulated 10.7MHz to the IF, only a small adjustment required to bring the set up to scratch.
Its used with an indoor dipole stuck to the wall.

Malcolm

[HowardB]

Thanks to everyone that replied, especially David for the interesting note about superhet principles and alignment, much appreciated, I'm learning lots from this forum.

To recap (no pun intended), I replaced all the waxy paper, electrolytics and out of tolerance resistors. I have a spreadsheet with the details if anybody is interested. The FM tuner within the metal box was left untouched. [I also replaced the mains lead, rectifier, magic eye, 2 overheating resistors, and a mod to improve tuning indicator sensitivity]

The A242 is now rocking on MLS. VHF is pretty good, all the stations are there and quite strong, just not as loud as AM. Maybe this is as good as it gets on my little ribbon aerial.

I bought the M8099 on eBay with the idea that it would just plug in and instantly double the volume. I didn't for one minute consider re-alignment.

Having read all your comments several times and the alignment instructions on the service sheet, I'm getting a little more confident to tackle this. I just don't want to screw up what I have already. Once I start twiddling, I'll never get it back to how it is now. Having said that, I will probably try the new valve with David's method of adjusting the LO and RF stage. Also looking at buying an FY6800 60MHz Signal Generator, around £75 on eBay.

HowardB

[HowardB]

Would the FY6800 60MHz Signal Generator be a good buy?
Is the 60MHz limit an issue?

HowardB

[FIXITNOW]

I have this little note I cut from a post on here somewhere that I needed when I did mine Apologies to the original poster for not recording it came from you

If changing tubes doesn't help, you might want to check C4. I have recently completed the restoration of a Murphy A242 and A252. The A242 suffered from exactly the same problem as yours, where the A252's FM gain was hot -- too hot. After about a week of listening to the A252 in my office, it stopped working with nothing but howls on the FM band. The solution for both the A242 (poor FM gain) and the A252 (to much gain and eventually oscillation) turned out to be the same component (even though their designs are different, they are similar). I just had a look at the schematic of the Bush VHF61 and it's similar to the Murphy's. I would have a look at C4 (540 pF?, I can't quite read the value on the schematic I'm looking). In the Murphy's, this capacitor is critical to the gain of VHF; too low and it breaks into oscillation; too high and gain is killed. In the Murphy's this capacitor was a Hunts. After I pulled out the capacitors, I measured them and sure enough in the A252 the capacitance had dropped by about 150 pF where in the A242 it had gone up.

[HowardB]

That's interesting but there doesn't seem to be a C4 in the circuit diagram or the parts list of the A242!
However, there is a note on page-3 of the Service Sheet about C8 being critical.
I checked and it was about right.
Thanks anyway.

[PaulR]

What a useful thread. I think it should be re-named to something like Simple FM Alignment the it can be found again