AR88D - help needed

[turretslug]

You could try a portable VHF/FM radio (i.e. one without inter-station muting) with telescopic aerial near the LO section (probably best with oscillator box screen removed and the AR88 on its side). On Band 6, there's enough 3rd harmonic to put an FM radio into quieting. 28.9MHz on the AR88 dial corresponds to just above 88MHz, 32MHz indication will produce quiet around the 97.3 to 97.4 mark.

[gezza123]

Hi Alan
You don't say if you tuned in the generator at frequencies across the band, or was you only using 455Khz to test alignment, and have you cleaned the band switch.

LO-Oscillator test
1 have you got a frequency counter
2 have you got an oscilloscope.
Remove the metal can off the main tuning capacitors, and with the counter as (Tanuki) said dangle the input lead from the counter/Oscilloscope over the tuning gang at about the centre of the front two tuning capacitors.You will or should pick up the Local Oscillator plus the input station frequencies, the counter is the best for this as all you pick up on the Oscilloscope is a sine wave signal and will not show the frequencies, unless you have an expensive scope.

Example if you put your AR88 on band (3) at 10mHz you will show 10455 Khz, if not then try switching the band switch up and down to test the other bands to give you the Idea, you should be able to do this at any frequency.

If on bands(3) and (6) you don't get any frequencies then the fault may be your band switch or in your front end tuning coils/Osc IE green spot, unless the fantom twiddler has been at it..
Check (L53 Osc coil / L6 antenna coil) band (3) and (L56 Osc coil / L12 antenna coil) band (6)

Hope this helps Gezza123

[Daventry 5XX]

Hi Gezza123

Sorry for the delay,in answer to your questions,I have tried tuning the S.G.across the bands,everything seems to be OK.

I can pick up the signal from across the whole width of both bands.

All wafer switches have been given a good spray with contact cleaner.

I dont have a frequency counter or an oscilloscope,but I am considering buying a scope.

The problem is that what I know about oscilloscopes isn't worth knowing.

What should I be looking for in a scope for general purpose fault finding on valve radios and telecomms receivers?

Alan

[gezza123]

Hi Alan
I use two scopes
No.1 is an hp180a 50mHz bandwith, for my bench it can be a bit on the heavy side if you are going to be moving it around.
No.2 is an hp 1742a 100mHz bandwith, this is not heavy and I can move it about.

There is articles on hear about scopes, just type in the top search bar (buying an oscilloscope) there are lots of preferences but I like the hp.

Back to your problem!!
Sound like your IF alignment is ok, but still not sure about the Oscillator.
It does sound like you may have got a problem with the input circuits, as I said above if someone has twiddled then you need to find what coils are broken or just been twiddled so it may only need peaking up.

Without test equipment it is very hard to test correctly any such twiddeling, I can only suggest that you could try the following.
First disconect the aerial, and connect your generator in its place, and then set it at 11.5 mHz at as lower setting as you can just hear it above any noise.Set Radio Gain full up and volume half way then set the radio to 11.5 mHz on band three.
put a mark on the capacitor rods first, so you can put them back if you need to.
Carefully adjust (C39) and (C62) for maximum output on your S-Meter, note! you will find there are dead spots as you adjust these capacitors up and down, and the band goes quite so tune for maxmum signal and (only) at this frequency, I suggest you do (C39) first and then try an aerial and then try to tune in a station if that goes ok then tune for maximum with (C62) again (only) at that frequency.

The reason why I did this was because like you I did not realise I was puting in two strong a signal from the generator, you will also find that two strong a signal on these sets seem to push through the IFs with no bother when these coils incorrectly set.
I can now tune in the ham bands on a piece off 1ft wire 7.122 mHz.

I will catch you tomorrow as got to have early night for an early
appointment.good luck Gezza123

[Radio Wrangler]

Alan,
Nobody was born knowing much about scopes, or anything else for that matter. They are very useful bits of kit and there is a magic moment where you stop looking at the thing as intimidating, and see it as a help with a lot of problems.

A lot of people see the multimeter as their prime window into the otherwise invisible world of electricity, but you'll find that a lot of other people go for the scope first, and only reach for the meter when there is some need for precision in voltage measurement.

There's a sort of series of stepping stones: Multimeter, scope, spectrum analyser, network analyser.

Each is seen as being more intimidating that the previous ones, each can show things the others cannot and various people have made different amounts of progress along the series. Each has a number of people scared of it. Some would rather gnaw their own leg off than have to use one. None is particularly difficult, they're just different.

Without a doubt, the scope is the most versatile. It won't take you long to use it at a basic level, and fancier tricks aren't far behind. You'll wonder how you managed without one.

I operate with a scope and a spectrum analyser running on the bench. The multimeter spends most of its life switched to "Ohms"

David