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Old 18th Sep 2022, 5:23 pm   #1
Scotch24A
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Default RCA AR88 Comms Receiver. Audio output problems.

My AR 88 will not adequately power a 2.7 ohm speaker, but does give satisfactory
volume with headphones- does this mean the output transformer is blown or should I look elsewhere?
I have recently changed the c119 capacitor and moved the connection from chassis to terminal 3 of the output transformer but this has not made any difference.
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Old 18th Sep 2022, 5:34 pm   #2
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Default Re: RCA AR88 Comms Receiver. Audio output problems.

The easy way to find out is to conduct continuity tests on the output transformer windings.

If it's OK then take voltage readings from the output stage.
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Old 18th Sep 2022, 6:37 pm   #3
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Default Re: RCA AR88 Comms Receiver. Audio output problems.

Not convinced the transformer is at fault, do not normally fail like this.
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Old 18th Sep 2022, 6:44 pm   #4
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Default Re: RCA AR88 Comms Receiver. Audio output problems.

On some of the AR88's the loudspeaker is connected to the output transformer via a set of contacts on the headphone jack socket.

Lawrence.
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Old 18th Sep 2022, 6:44 pm   #5
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Default Re: RCA AR88 Comms Receiver. Audio output problems.

A bad 6K6GT.
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Old 18th Sep 2022, 6:45 pm   #6
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Default Re: RCA AR88 Comms Receiver. Audio output problems.

It is most often the primary which fails open circuit, and then you lose everything.

Primary suspect is therefore the speaker muting contacts on the headphone socket.

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Old 18th Sep 2022, 6:46 pm   #7
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Default Re: RCA AR88 Comms Receiver. Audio output problems.

Almost perfect synchronism...

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Old 19th Sep 2022, 9:49 am   #8
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Default Re: RCA AR88 Comms Receiver. Audio output problems.

What impedance speaker are you using? I've seen several AR88s where the output-impedance of the 'speaker' terminals was designed for 500/600-Ohm wired-network speakers; hooking a 3-Ohm speaker to one such wired will give very feeble audio.

[I was caught myself with this issue on a Collins TCS12 receiver I bought as a kid; eventuially I figured out what the issue was and replaced the output trANSFORMER WITH ONE DESIGNED FOR MORE-NORMAL SPEAKER IMPEDANCES]
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Old 19th Sep 2022, 5:03 pm   #9
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Default Re: RCA AR88 Comms Receiver. Audio output problems.

Thanks for all your suggestions.
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Old 20th Sep 2022, 8:43 am   #10
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Default Re: RCA AR88 Comms Receiver. Audio output problems.

I have the matching AR88 speaker. These days I use it with my RA17, but when it arrived it contained a 600/4R transformer (now bypassed).
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Old 28th Sep 2022, 9:29 pm   #11
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Default Re: RCA AR88 Comms Receiver. Audio output problems.

I’ve tried plugging in a “good “ 6y6gt with no improvement.
There is continuity across the output stage to T2(terminals 1,2-2,6-1,6). but not across terminals 3,4;although I can detect a resistance of 650 ohms across 3,4.
I haven’t found a problem with the headphone socket.
How do I check the voltage from the output stage and what do the results signify.?
Anything else to be considered.
Thanks
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Old 29th Sep 2022, 10:06 am   #12
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Default Re: RCA AR88 Comms Receiver. Audio output problems.

For "continuity" do a resistance measurement....less confusion.

The continuity function on most meters is only good up to a certain circuit resistance value, anything above that value won't indicate circuit continuity.

As far as I can make out from other commentators, the primary resistance of the original transformer was between 500 and 600 Ohms.

Lawrence.

Last edited by ms660; 29th Sep 2022 at 10:34 am. Reason: extra info
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Old 29th Sep 2022, 11:18 am   #13
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Default Re: RCA AR88 Comms Receiver. Audio output problems.

Must say, I have never known an output transformer cause low sound in over 60 years. Has the headphone socket been cleaned, as they can tarnish and will cause problems?
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Old 29th Sep 2022, 4:34 pm   #14
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Default Re: RCA AR88 Comms Receiver. Audio output problems.

Thanks everyone;so it’s possible that the output transformer is not at fault?
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Old 29th Sep 2022, 5:49 pm   #15
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Default Re: RCA AR88 Comms Receiver. Audio output problems.

Check that the noise limiter is switched out, just in case this circuit is in some way faulty. There are quite a few things that could cause low level audio, but this is a quick and easy step in the elimination sequence.
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