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Old 17th Sep 2023, 9:20 pm   #1
jammy dodger
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Default RF gain control on AR88 not working

Hi All.
I acquired an AR88D recently in nice condition and good working order. I have replaced a couple of the bathtub caps but nothing else. I have been listening to SSB on the amateur bands and VOLMET signals almost daily, today I tried to do the same after tuning from an AM broadcast station. I turned the RFgain down but nothing happened, audio level stayed the same regardless of the control setting. I switched off and restarted it later with the same result. It has been working fine until today. Just wondered if anyone can help with a diagnosis. Many thanks.
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Old 17th Sep 2023, 10:19 pm   #2
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Default Re: RF gain control on AR88 not working

RF gain control and agc relies on a negative (WRT chassis) bias voltage achieved by connecting the centre tap of the HT winding on the transformer to the chassis via a set of dropper resistors and bypass capacitors.

These resistors are run hot and aren't terribly reliable, and paper capacitors of this era go leaky and if you've been changing capacitors, an error could also short the negative bias arrangement. It';s also used to create negative bias for the audio output valve and too much current here can burn out output transformers (another known weakness)

Have a look around these areas., Check the negative bias supplies, check the resistors adn check -ve bias at the 6V6/6K6 grid.

Check blocking capacitors (2 n parallel for reduced reliability) feeding 6V6 grid (can drive grid positive!) There are also tone correction capacotprs from 6V6 anode to ground. Replace these as a precaution... they also destroy output transformers. Wire new capacitor not to ground but directly across transformer primary. It does the same job, better in fact and avoids disaster if it ever breaks down.

David
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Old 17th Sep 2023, 10:34 pm   #3
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Default Re: RF gain control on AR88 not working

Many thanks David, I will take a look around and do checks as you suggest.
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Old 18th Sep 2023, 12:26 am   #4
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Default Re: RF gain control on AR88 not working

The negative bias developed across R43, R44 & R45 (in series) is approximately 23 Volts.

The full negative bias goes to one end of the RF gain control potentiometer.

Smaller bias voltages go to the audio stages.

The negative end of C96 and C97 should not be connected to chassis.
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Old 18th Sep 2023, 8:45 am   #5
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Default Re: RF gain control on AR88 not working

Thanks Silicon. I will check out the readings you gave me.
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Old 18th Sep 2023, 7:21 pm   #6
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Default Re: RF gain control on AR88 not working

Hi Guys.
I have done some tests today and found 37 volts at the lead out from R43 on the resistor board, 27 volts at the junction of R43 and R44 and 23 volts on the , grid of the 6K8 but zero volts on pin 1 of the RF gain pot. Where has the 37 volt loss occurred Could I bridge from R43 to the pot or are there other connections going off that line somewhere. I dont know.
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Old 18th Sep 2023, 7:30 pm   #7
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Default Re: RF gain control on AR88 not working

Hi Guys.
I have done some testing today and found 37 volts at the output of R43 on the resistor board, 27 volts at the juction of R43 and R44, 27 volts on the grid of the 6K8 and 0 volts on pin 1 of the RF pot. Where has 37 volts been lost between R43 and the pot Could I bridge from R43 to the pot and see what happens, would I need to disconnect the wires at both ends first or are there any other leads going somewhere off that line.?
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Old 19th Sep 2023, 8:44 am   #8
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Default Re: RF gain control on AR88 not working

Are there any components or wires on pin 1 of the RF Gain pot? If so verify which components they are and trace any wires connected there. Does one go to R43/C97?

Passing thought, is yours the RAF version of the AR88 which had pin 3 of the RF Gain control earthed via R55 and a shorting plug?

https://www.vintage-radio.net/forum/...ad.php?t=50017
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Old 19th Sep 2023, 9:47 am   #9
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Default Re: RF gain control on AR88 not working

Someone else had the same problem last month, see the thread linked to below, in Post#4 there are links to the wiring diagram for the RCA CR88 which is more or less the same as the RCA AR88, in Post#5 I gave a worded account of how tag 1 of the RF gain control gets its -ve voltage in terms of the connections/route in the receiver.

Maybe some use, maybe not:

https://www.vintage-radio.net/forum/...d.php?t=202756

Lawrence.
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Old 19th Sep 2023, 11:06 am   #10
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Default Re: RF gain control on AR88 not working

As the receiver was working properly one day and not the next, we can probably discount the possibility that the shorting plug got pulled out.

A lot of AR88Ds were used in racks of three at a time, set up for diversity reception with their AGC systems interlinked. These sets had no S meters fitted, the rack had a panel with all three together for easier comparison. The AR88 had an added small bakelite 2-pin connector mounred on its side to the outside face of the back-drop of the chassis. It should have a shorting plug to allow solo use of the set. The shorting plugs had a bit of string to stop them getting lost.

(There was a wartime shortage of jewelled bearings needed for making good quality meters, and this added to the number of AR88 that wound up on the surplus market with the blanking plate instead of the meter.)

All three AR88 in a diversity rack were tuned to the same signal, otherwise it wouldn't make sense.

This was a bit of a stop-gap. Later receiver models were more dedicated to diversity reception allowing sharing of VFOs like the RA17/RA117 series.

So the RF gain control/AGC/Bias system is an area where post manufacture modifications are likely, but on a set which just stopped working, you're likely looking for a resistor/capacitor failure. Given that the resistors were run overstressed, going through the whole area and replacing anything that's bad and using beefier resistors is a good idea.

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Old 19th Sep 2023, 11:11 am   #11
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Default Re: RF gain control on AR88 not working

Almost all of the HT current taken by this receiver passes through the 'bias string' (R43, R44 & R45).

Assuming R43, R44 and R45 have the correct resistance value, the current passing through them in Your receiver appears to be 37/175 = 134mA.

This may be too high, and it may not be good for the mains transformer, rectifier and other components.
Until you find the fault, try to power it up for a short time only.

A high HT current could be the result of leakage through several HT bypass capacitors.
It could also be the result of the failure of the RF gain control circuit.

Alternatively, the resistors may have changed value and the negative bias voltage is now too high.
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Old 19th Sep 2023, 11:24 am   #12
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Default Re: RF gain control on AR88 not working

I can't find the official HT current consumption in the manual.

It does state that the alternative battery supply needs to be rated at 90mA.

The bias voltage of 23V that I found online suggests a HT current of 84mA.
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Old 19th Sep 2023, 12:58 pm   #13
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Default Re: RF gain control on AR88 not working

134mA looks like either those resistors have gone high or something is taking significantly too much current. Or some of both.

Investigation is a priority.

David
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Old 19th Sep 2023, 1:07 pm   #14
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Default Re: RF gain control on AR88 not working

Quote:
Originally Posted by jammy dodger View Post
Hi Guys.
I have done some tests today and found 37 volts at the lead out from R43 on the resistor board, 27 volts at the junction of R43 and R44 and 23 volts on the , grid of the 6K8 but zero volts on pin 1 of the RF gain pot. Where has the 37 volt loss occurred Could I bridge from R43 to the pot or are there other connections going off that line somewhere. I don't know.
Shouldn't the output valve be a 6K6?
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Old 19th Sep 2023, 1:57 pm   #15
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Default Re: RF gain control on AR88 not working

The missing -ve voltage on tag 1 of the RF gain control means that the bias voltage on the 1st and 2nd IF valves will be lower and the bias voltage on the 1st and 2nd RF valves will probably be almost zero, the effect of that would be to increase the HT current through the bias resistor chain (R43, R44 & R45) and the voltage across it somewhat.

If the OP's observations are as said then the missing -ve voltage on tag 1 of the RF gain control can only be due to a disconnect and not a resistor or capacitor component failure so far as I can see.

Lawrence.

Last edited by ms660; 19th Sep 2023 at 2:16 pm. Reason: extra info
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Old 19th Sep 2023, 2:49 pm   #16
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Default Re: RF gain control on AR88 not working

It'll be interesting to learn what the cause of this fault is. It's unlikely that a piece of wire would go open circuit except at the terminations.

Perhaps pin 1 of the RF Gain pot has become grounded which would give a reading of zero volts, the same as a disconnection?
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Old 19th Sep 2023, 3:33 pm   #17
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Default Re: RF gain control on AR88 not working

The snippet below is according to the OP's reported voltage measurements -37 Volts, -27 Volts, 0 Volts.

Lawrence.
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Old 19th Sep 2023, 4:09 pm   #18
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Default Re: RF gain control on AR88 not working

0v at one end of a wire, -37v at the other? There must at least be a disconnection. It was said earlier that the bathtubs had been replaced, so a bad connection is a possibility.

Still, it's an AR88 and I haven't yet seen any confirmation that the DC blocks and tone correction capacitors at the audio output stage have been done.

David
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Old 19th Sep 2023, 5:03 pm   #19
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Default Re: RF gain control on AR88 not working

https://www.vintage-radio.net/forum/...88&postcount=5

In conjunction with the attachment below.

Red/Yellow wire No. 47 connects R43 to the secondary center tap on T1

Red/Yellow wire No. 53 connects the secondary center tap on T1 to tag 6 of the octal socket J1

Red/Yellow wire No. 68 connects tag 6 of the octal socket J1 to the common connection of C96 & C97

Red/Yellow wire No. 69 connects the common connection of C96 & C97 to tag1 of R46 the RF Gain control.

Guilty as charged for any errors.

Lawrence.
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Old 20th Sep 2023, 11:52 am   #20
jammy dodger
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Default Re: AR88 wiring loom query

Thank you all for replying to my post. I now have the RF gain working. It looks like the pot had been changed in the past and R56 was very close to the connections on the RF gain pot. I decided to unsolder R56 to get a better look at the connections and to give my meter probes more room. On putting one probe on pin 1 still no continuity but applying more pressure I got a beep on the test meter. Looking closely at the solder joints on the replacement they looked poor, so I removed all three, cleaned the wires as best I could and re soldered them and lo and behold I got continuity to the rest of the components. Switched on and waited, then as the receiver came alive turned the RF gain and Bingo!!! It worked. So it was a disconnection problem not a failed component. A big THANK YOU to all who responded. I know where to come in future when a problem arises.
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