AR-88 Off/On Switch

[Andrewausfa]

I think the one on my AR-88 has broken. The Off/On bit works but it doesn't seem to want to switch to REC MOD or REC CW. It was working so I suspect one of the contacts or the *'wiper' is duff.

Does anyone have a spare or perhaps even easier some close-up photos of one so I can check the position of the 'wiper'?

Thanks -Andrew

* Wiper; I couldn't remember what the name of this part is called!

[Skywave]

First, is it actually the switch which is the culprit? A check for voltages on the switch wiring or checking with an ohm-meter will tell you that: refer to drawing attachment below. If it is the switch, then presumably, that rotary switch will mechanically click into the 4 positions, and that, in turn, implies an open-circuit contact(s) on that switch. I would remove the switch ass'y. from the receiver and have a good look at the wiper and its contacts. You may well find a fixed contact has 'opened out', so not making contact with the wiper. Or there might be mechanical damage and one (or more) fixed contacts have been displaced and pushed to one side. Most important of all is to try to establish if the cause was mechanical failure or an electrical burn-out of a switch contact. If the latter, then obviously the cause of that must be put to rights first.

I've attached a drawing of this switch function - which I hope helps.

[attachment=3817]

Unfortunately, this switch is a 'special' - as that drawing discloses. Apart from the mechanically-coupled switch for the a.c. mains supply (SW24), an analysis of the rest of the switching function (SW23) shows it could be replaced by a ganged 3-pole 4-way switch. However, if you really do conclude that a genuine replacement switch is required, all I can suggest is to generate a 'search-and-tell-me' on e-bay. Parts for AR88s are occasionally to be found there.

Al.

[Andrewausfa]

Thanks for taking the time to post this Al. It's mechanical I'm sure as it was working fine until the incident. The AR-88 is stored under my desk and I tripped and kicked the knob by mistake - well, obviously it wasn't on purpose. This bent one of the lugs on the cam switch at the back and rotated the switch past the indents so the pointer was pointing almost at the headphone socket.

I'll remove it completely and have a look at the underneath. Your remark about a fixed contact being pushed aside might be the actuality.

Thanks _Andrew

[Bushbaby]

Hi Andy
Could you post a picture of the switch?
Regards
Rob

[Andrewausfa]

Hi Rob,

Sure. I hope to have the set on my workbench over the weekend so I'll take some photos then. With my working area arrangements this is not a set I leave in bits, I have to put it all away and back under my desk every time I work on it.

Andrew

[Andrewausfa]

I asked Paul to move this out of the Sales section as the outcome of my problems was not what it seemed at first. I've had a few hours this morning to take another look at it.

When I got this set (an AR-88LF) it was working but only on the Long Wave ranges. I changed the capacitors and two of the screen resistors in the coil box and things improved tremendously. If you've not seen the inside of an AR-88 coil box then I've attached a piccie below. Not the easiest things to work within.

The mains cable was too short a length to safely reach my sockets to I changed this and checked it was working, which it was. The set went back under my bench until I had a trip and kicked the four position main power switch. The switch goes from Off to a position called Trans which, amongst other things, allows power to the valve heaters and the dial lamps but the radio sections do not work. Obviously as the set was working when it went under the bench I thought a broken switch was why the set would no longer work. If you look at the photos you can see the switch shaft is slightly bent where I kicked it.

I acquired a new switch and fitted it today. Alas, turning on gave the same problem, only heaters and dial lamps came on Checking around the mains lead connections certainly power was getting there but nothing out of the rectifier. Luckily I'd bought a new 5Y3GT to replace a U50 in a GEC domestic set so I whipped that out, stuck it in the AR88 and switched on. Within seconds the voltage stabiliser valve starting glowing violet and the speaker started to hiss. Result!

How odd is that though? A rectifier working and then not.

Andrew

[attachment=3946][attachment=3943][attachment=3944][attachment=3945]

[Skywave]

I'm glad you fixed it - as I expect you are!

Of course, what was really throwing you off-course was the apparent co-incidence of having an accident with that switch and - presumably - sometime prior to that, the failure of the rect. valve. Unfortunately, rare as such co-incidences are, they do indeed happen - and for those of us with a logical mind, geared towards cause and effect, that sort of thing drives us crazy!
Incidentally, did you carry out any checks to determine if the valve died of old age or was it killed? If the latter, then what I could say is obvious . . . so I won't!

Al.

[Andrewausfa]

Yes very thanks! The thing is the set was working fine when put away so illogical it should stop working. I've had a look at the old valve and other than it being rather loose in it's base, the filaments are open circuit. Hopefully it's old age rather than the Big T

Andrew

[Skywave]

I'm always very suspicious of 'co-incidences': perhaps it's the engineer-cum-mathematician in me. I suppose that it is just about a conceivable possibility that when you kicked that switch, and although the power to the AR88 was off, the filament might have just about been fragile enough (considering its age) to have been broken by sheer mechanical impact / vibration force. Having said all that, since an AR88 is one helluva heavy radio, that glib theory only holds water if you gave it one enormous thwack! Were you wearing steel-capped shoes at the time?

OTOH, when you last switched the AR88 off, I wonder if you moved it to its 'home' under the desk whilst the filament was still warm - albeit only a bit - and the final 'plonk' on the floor did the damage?

I joined the wrong profession: post-mortems always fascinate me!

Al.

[Andrewausfa]

Onwards and upwards with my AR-88, well electrically anyway as there is a lot of cleaning and repainting to do on the chassis.

I've been doing some more component changing over the last few days and the last hours work has brought me to this - Shanwick ATC on 5616 USB - look at the dial When I got it pretty much only LW worked.

[attachment=3984]

Andrew