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Old 10th Aug 2021, 9:58 pm   #1
Steve_G4HTZ
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Default AR88 BFO problem.

Recently aquired a AR88 at a local auction house for £65 …got it home not expecting much ….works really well ..probably needs a tweak here and there but receives well up to 6MHz at least.

One nagging problem….on switch on the BFO works perfectly….but over a period of about an hour the BFO eventually stops working completely….any clues ? ..searched this forum but can’t find anything.

Main interest is using it on AM anyway on 160 and 80m with a Johnson Viking 2 AM tx I have restored ……amazingly it works as well ….hoping to get the AR88 upto scratch as well ….although I’m learning as I go with this valve gear .

Steve G4HTZ
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Old 10th Aug 2021, 10:55 pm   #2
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Default Re: AR88 BFO problem.

Couple of possibilities..... have you tried another 6J5?

I've also heard of this sort of problem with bad capacitors. Look around for those damnable 'Micamolds' despite the name they are paper jobs and they are crap!
Any dampness?

David
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Old 10th Aug 2021, 11:22 pm   #3
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Default Re: AR88 BFO problem.

Hi David
Thanks for the suggestions ….not actually done anything yet !!

Will have a delve inside…..replaced lots of caps and out of spec resistors on the Viking 2 overhaul …so will do a section at a time on the 88 .

Regards
Steve
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Old 11th Aug 2021, 9:08 am   #4
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Default Re: AR88 BFO problem.

Do not forget about that cap from primary sound output tx anode end to earth, put across the primary, as if it fails, it is goodbye sound tx!
Also the oil filled caps need re stuffing etc.
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Old 11th Aug 2021, 11:27 am   #5
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Default Re: AR88 BFO problem.

OK thanks for that information, will dig the circuit out on VMARS and see what’s involved.

Regards, Steve G4HTZ.
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Old 16th Aug 2021, 7:04 am   #6
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Default Re: AR88 BFO problem.

Hi Steve, you probably know this already, but I'll mention it in any case. The BFO injection level is very low ( via stray capacitance coupling) so it is necessary to turn down the HF gain in order to resolve SSB. As suggested try another 6J5 or else check and replace the capacitors within the BFO circuit. Also worth setting up the BFO frequency per service manual. Cheers, Jerry
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Old 16th Aug 2021, 10:24 am   #7
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Default Re: AR88 BFO problem.

Isn't there a capacitor inside the BFO inductor can... long time since I had one apart.

David
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Old 16th Aug 2021, 10:13 pm   #8
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Default Re: AR88 BFO problem.

Quote:
Originally Posted by cathoderay57 View Post
Hi Steve, you probably know this already, but I'll mention it in any case. The BFO injection level is very low ( via stray capacitance coupling) so it is necessary to turn down the HF gain in order to resolve SSB. As suggested try another 6J5 or else check and replace the capacitors within the BFO circuit. Also worth setting up the BFO frequency per service manual. Cheers, Jerry
Hi Jerry
It works fine when switched on …and have tried the settings you suggested …but still dies after a while.

Haven’t had time to look at it yet ….not sure if the BFO valve is a metal or glass version …maybe both fit ok

Thanks for all reply’s so far
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Old 17th Aug 2021, 3:48 am   #9
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Default Re: AR88 BFO problem.

I've used both glass and metal 6J5 without problems.

I like to set mine up with the capacitor half-meshed and the beak on the knob horizontal tweaking the inductor so the BFO is on the centre freq of the crystal filter.

This way I can tell at a glance whether my BFO is offset for USB or LSB operation. EG beak at 10 O'clock has BFO above IF (LSB demod), 8 O'clock has BFO below (USB demod).

It just takes a bit of guesswork out of things.

David
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Old 17th Aug 2021, 9:23 am   #10
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Default Re: AR88 BFO problem.

Back in the late 60's to very early 70's lots of NOS military stock was being sold off. AR88's were in that class. My first oscilloscope came from that sell-off, a mind numbingly heavy boat anchor Hartley 13A.

Performance was not its strong point, but the build quality was superb - in the same way as the AR88, but at least that has stood the test of time and is a thing of beauty.

Craig
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Old 17th Aug 2021, 10:23 am   #11
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Default Re: AR88 BFO problem.

Quote:
Originally Posted by Steve_G4HTZ View Post
One nagging problem….on switch on the BFO works perfectly….but over a period of about an hour the BFO eventually stops working completely….any clues ? ..searched this forum but can’t find anything.
Voltages?

'scope?

Lawrence.
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Old 17th Aug 2021, 11:22 am   #12
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Default Re: AR88 BFO problem.

Steve,
There aren't that many components in the bfo circuit.
A good start would be to check the anode voltage to the 6J5.
As with a lot of the valve gear of this vintage the resistors are prone to drifting high in value and capacitors fail. The AR88 used a brand of caps called Micamold which are worth replacing on sight.
The problem I have on my AR88 bfo is a dodgy contact on the wafer of the mains switch which I must get round to replacing when I can find one.
Don m5aky

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Old 17th Aug 2021, 12:21 pm   #13
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Default Re: AR88 BFO problem.

Don,

Is this what you are looking for. It's never been used. I've taken care of it for 40 years and my days of lifting AR88s, even rolling them over on the bench are done.

Jim
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Old 17th Aug 2021, 1:42 pm   #14
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Default Re: AR88 BFO problem.

Quote:
Originally Posted by G4XWDJim View Post
Don,

Is this what you are looking for. It's never been used. I've taken care of it for 40 years and my days of lifting AR88s, even rolling them over on the bench are done.

Jim
Yes indeed, I was about to place a wanted add in a new thread.
I will contact you via PM.
Don m5aky
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Old 17th Aug 2021, 2:31 pm   #15
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Default Re: AR88 BFO problem.

Quote:
Originally Posted by Radio Wrangler View Post
Isn't there a capacitor inside the BFO inductor can... long time since I had one apart.

David
It's slightly unusual -

From the army EMER:

Beat Frequency Oscillator.

V12 is a shunt-fed Colpitts oscillator operating about the intermediate frequency. C86 (BFO adjust) varies the frequency hence the beat note.
The BFO excitation frequency is fed to the 3rd I.F. stage V7, stray capacitive coupling being used.
This is achieved by means of a lead connecting an unused pin (4) on V12 valve base to an unused terminal (A) on TR9.
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Old 17th Aug 2021, 2:50 pm   #16
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Default Re: AR88 BFO problem.

The BFO is decoupled from the 150V stabilised supply by a 47k resistor and 4.7nF Micamold (economy Bakelite-cased paper capacitor, and truly infamous for going leaky)- it's quite conceivable that this capacitor starts off a bit leaky and the leakage snowballs with heating of the capacitor itself and the set overall, starving the BFO of its already constricted supply. It could even go dead short without the rest of the set being aware.

The BFO coupling to the final IF amp grid is described as "electrostatic", i.e. very light proximity effect, OK for CW sidetone with appropriate control settings but making SSB reception a bit finicky. The post-war SC88 version got a 10pF BFO coupling capacitor to improve things, but this sort of thing is a fine line between adequate carrier injection and excessively stimulating the AGC.
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Old 17th Aug 2021, 2:54 pm   #17
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Default Re: AR88 BFO problem.

Quote:
Originally Posted by G4XWDJim View Post
Don,

Is this what you are looking for. It's never been used. I've taken care of it for 40 years and my days of lifting AR88s, even rolling them over on the bench are done.

Jim
Interested to note that that appears to have a double-pole mains switch in it! Quite a few UK military-use sets had a separate mains toggle added, I wonder if the integral switch was considered/recognised as a weak point.
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Old 17th Aug 2021, 3:12 pm   #18
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Default Re: AR88 BFO problem.

My AR88 wes rebuilt in the early fifties by some arm of the MOD. It has a purpose made PVC insulated wiring harness which can't be original. The harness is definitely made for the separate toggle switch, and not the mains switch on the rotary mode selector switch bank.

So I'm fairly sure the toggle was 'official' the second headphone jack seems more 'creative'

David
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Old 17th Aug 2021, 3:49 pm   #19
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Default Re: AR88 BFO problem.

There are 4 of those mica mold caps inside the bfo can, change them for the values marked on them as one of them is a different value to the cct diag. The bfo will not tune corectly if you use the cct values, took me a while to figure this out on my AR88 when I switched on.
The mains switch is well known for failing, so I fitted a mains relay to switch the tx on so that the mains switch only operated the relay coil.
I have the same problem with the bfo failing after about 30 minutes. The only part that I have not changed yet is the 6j5, but if I switch off and on again the bfo works. One day I will try another valve. The 6j5 tests perfectly on the valve tester.
John.
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Old 17th Aug 2021, 5:25 pm   #20
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Default Re: AR88 BFO problem.

Identifying the values of those caps can be a bit of a challenge.
Don m5aky
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