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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 5th May 2012, 10:51 pm   #1
Pete M0PKH
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Default Ar88d

I will be picking up one of these on Tuesday. I can get my hands on a variac to give the radio a bit more of a gentle wake up, but could anyone reccomend a small list of the first things to check before I dive in?

Many thanks

Pete
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Old 5th May 2012, 11:13 pm   #2
onewatt
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Default Re: Ar88d

Hi Pete,

If you do a search on the forum you'll get a good idea of what to do.

There's also a stack of info available on the main vintage radio web site link at the bottom of the forum page.

AR88's have a delicate o/p tx so as with all valve sets, it's recommended to change the grid coupling cap on the output bottle.

The mains smoothing cap is worth checking as well.

A search will find a number of ar88 projects that have been or are being worked on.

Enjoy.

Onewatt
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Old 5th May 2012, 11:26 pm   #3
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Arrow Re: Ar88d

Like any piece of old kit, give it a thorough visual inspection first.

► What state is the mains cable in? Is it chafed? If a 13-amp plug is fitted, check the wiring inside it and the fuse: a 3-amp. would be a sensible value. If a 3-core mains lead is fitted (and it should be a 3-core), check that the supply earth is connected to the receiver chassis. Has there been any obvious modifications to the mains input wiring inside the receiver?
► Are all the valves present - and if you have the data, are they the correct types?
► Are there any signs of modifications that you can see? Signs of any repairs? And if there are, do they look well implemented, or do they look badly done?
►Run the tuning knob from end end of the travel to the other - but gently. Check that there is a firm 'stop' at the end of the travel. (A common defect is for a 'soft' stop to be felt: usually caused by over-enthusiastic operators banging the tuning against the stops. A strip down of the tuning mechanics is required to fix this). Although it can be difficult readily to determine small amounts of backlash in the tuning without a signal tuned in, significant amounts can usually be discovered by rocking the tuning knob back and forth whilst looking at the main tuning scale. (Again, gearbox strip down is required to fix excessive backlash). And whilst you're there, check that the tuning scale has all of its frequency markings: careless cleaning can cause some of them to disappear!

There are many other things that could - and possibly should - be done prior to applying power for the first time; I'm sure others here will pick up on any I've overlooked.

Al.
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Old 6th May 2012, 8:13 am   #4
Chris Parry
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Default Re: Ar88d

These sets can suffer from rotten wiring, especially above the chassis where the high-mounted pots and dial bulbs are located in the hottest location.

Look for messy oil leaks from the large capacitor block next to the rectifier. These capacitors run at some 500V when the radio is in standby mode.

Recommend changing all pink lozenge capacitors as these go leaky. Ordinary w/e 400V polyesters are fine as substitutes.

Strongly agree about fragility of output transformers due to leaky grid coupling caps. The fact that the 6K6GT is a dodgey type of valve does not help. It's a good plan to fit a 6V6GT instead.

Gearboxes are badly designed. There is no positive endstop, and teeth frequently get severely worn or even broken. This problem is especially common on late radios fitted with the inferior Mk2 gearbox design. Fortunately, most AR88s in the UK were early types which were a little less vulnerable to damage.

Excellent radios, of which there are many survivors.
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Old 6th May 2012, 8:55 am   #5
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Default Re: Ar88d

Pete, Have you ever driven an AR88 before?

If you're not familiar with them, there are a few points:

1) Never never ever leave the mode knob in 'Transmit' this disconnects the HT from the receiver, and the voltage on the reservoir capacitors soars. During a rebuild it could be worth linking the HT as always on. If you want to run with a transmitter, you need to inject negative volts onto the AGC line to mute the receiver.

2) There's enough BFO injection to work CW, but not enough for SSB and also the BFO voltage is measured by the AGC detector. For SSB wind down the RF gain, wind up the AF gain close to maximum, turn off the AGC, then advance the RF gain to taste. This scales down the signal voltage at the detector so that the BFO is relatively stronger. Early AR88 used stray pick-up for BFO injection, later receivers had injection through an actual capacitor component.

3) Develop safecracker's fingers. The tuning is a bit quick on the higher bands, hence the comments about gearbox backlash.

4) Cancel the gym membership, you won't need it now

5) Enjoy. An AR88 in original condition sounds fabulous.

I've lived with an AR88 since the middle 60's. It's been stored for long enough that I intend replacing all the 'Micamold' capacitors. Micamold is a trademark, they have no connection whatsoever with mica the mineral. Then all the bathtubs, then the DC block to the audio output grid, only then might I think of applying a little power.

There's lots of stuff on the web about these great receivers, but the amazing thing is that nearly all the advice agrees!

David
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Old 6th May 2012, 9:21 am   #6
Pete M0PKH
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Default Re: Ar88d

Thanks for all the advice I will report back when I have more findings. Looking at the picture it looks like someone has added a switch and possibly extra socket on the front. Pity as I like things to be original (at least on the outside!).

Pete
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Old 6th May 2012, 9:31 am   #7
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Default Re: Ar88d

A toggle type OFF/ON switch, to the left of the rotary switch, was a feature of the RAF version.
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Old 7th May 2012, 1:15 am   #8
Keith
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Default Re: Ar88d

I believe there's a spare fuseholder on the back. Well worth using for an HT fuse (having personaly smoked one AR88 mains transformer!). Also "tone correction" capacitor is between o/p valve anode and ground which could be an o/p transformer killer. Safer to re-wire across o/p transformer primary. If the RX goes deaf intermittently, probably front end screen resistors (bu**er to get at, though). Enjoy.
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