Murphy a252 vhf question

[Mike76]

I'm just waiting for my days off work to get a chance to have another look. Need to work out what I'm doing with my signal generator and have a go at aligning it. I'm hoping that is the problem with it now. Not done it before, and I don't have any test leads for it. On that subject, what kind of plug will it be to connect to the signal generator. It's an advance b4b5 and the plug is a coaxial one similar to a uhf type connector.
I see your point about not trying to align things first, I guess I am looking for a way to point towards any faults not having the experience yet to know where to look.

Mike.

[Murphyv310]

Hi.
Its often hard to advise of how to go about things as we never really know how much experience someone has. Giving basic advice to someone experienced can come across as patronising and if a newbie is given too much theory then it can be off putting.
You are probably better making up a lead yourself, again depends on your skills but it's good learning and quite simple to do.

[pwdrive]

So far as I know the tuning on the Advance B4B5 only goes up to 30MHz, you might or might not get harmonics up to VHF but if they are present the level won't be known, thought I'd mention that incase you were thinking of aligning at VHF with it or using it as a sensitivity indicator at VHF.

Lawrence.

[Mike76]

Sorry does that mean that it's not suitable for aligning the fm part of this set. Also it's just the type of connector or plug that I can't seem to identify to plug in to the generator.
Practical skills I'm fine with, the theory side of how it all works I'd say I have a fairly good, basic understanding of.

[Mike76]

[quote='Mike76' pid='72001' dateline='1510665500']
Sorry does that mean that it's not suitable for aligning the fm part of this set. Also it's just the type of connector or plug that I can't seem to identify to plug in to the generator.
Practical skills I'm fine with, the theory side of how it all works I'd say I have a fairly good, basic understanding of.

[ppppenguin]

To align the FM part of the set you need a generator that goes up to more than 100MHz. If the front end is OK then you only need up to 10.7MHz for the IF.

May we have a photo of the generator's output socket please? There are some unusual conenctors that have been used. If it's a piece of kit that works for its living (as against a restoration of the kit) then I'd fit a more common connector such as BNC. At the bodgery end of things I've soldered BNC sockets to odd connectors before now.

But first and foremost, please don't mess with the alignment until all the voltages are right and you are sure there are no faulty components. Unless Mr Twiddler has been in the set before you the alignment is likely to be reasonably OK.

[Mike76]

The alignment is mostly because one of the cores in the fm part of the set was melted through to the core so I replaced it. That and for the practice of doing it. I've also replaced all of the wax capacitors throughout the set. What is actually meant by the front end, excuse my ignorance.
I did think of fitting bnc plugs, so I think I'll do that.

Thanks again for all the advice,

Mike

[pwdrive]

Front end = RF section, not IF or AF section.

Lawrence.

[Mike76]

Presumably no harm can be done by alignment, apart from physically breaking a stuck core maybe, and at most it might be considered as a waste of time. Having never done it I really just want to go through the process to see how it's done.

[Terry]

Presumably no harm can be done by alignment, apart from physically breaking a stuck core maybe ...

It is vital that you use the correct trimming tools to fit the cores. In the first place, they mustn't be magnetic and are usually plastic, nylon or sometimes include small amounts of non-ferrous metal.

If you have cores with hexagon holes, you must use the correct hexagonal tool  - other wise the core will crack and it is a lengthy and dangerous (to the core/coil assembly) to drill it out and fit another.

Older cores sometimes have a recess which looks designed for a screwdriver - but screwdrivers are ferrous and mustn't be used! When I was in my teens I used to scrounge broken bone knitting needles from my mum and file them down but I don't think you'll find them these days!

Very small cores with a continuous slotted hole through them need a tool which is usually nylon with a long non-ferrous blade attached so the it runs through the core and applies turning pressure against all the length of the core.

Regarding your signal generator and front end alignment, your generator almost certainly produces harmonics but at an unknown level.

Harmonics are products at multiples of the design frequency so, in your case, you would need to rely on a reasonable third harmonic to produce a 90MHz signal at its maximum 30MHz setting, and so on.

However, given the plethora of FM stations found in most places these days, you should find clearly identifiable stations at both ends of the VHF band which can be used for the RF alignment - provided, of course, that the tuning scale has a range of reasonably accurate markings. Identify the stations that you want to use and make a mark for each frequency somewhere that won't be seen when the set is used normally in its cabinet. Then ignore the tuning scale and tune to these marks for accurate repetition during alignment.