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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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11th Jul 2022, 12:44 pm | #1 |
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AR88D gone low audio output
I’ve had a AR88D a while now ..picked up cheap from a local auction house .
It’s been working fine for months and months ..calibration a bit off on higher bands but still received ok Turned it on the other day after about a couple of months and only got low audio out of the speaker or headphones . Still tunes ok and picks up strong stations …although weaker than before it would seem ….and audio at very low level Set seems in good condition..even has the trimming tools in place Any ideas what to look for initially ….it’s never had any components replaced by the look of it ….appreciate it’s probably a question with multiple answers . In the middle of a long drawn out house move so may have a bit of time to look at possibilities for repair Thanks Steve G4HTZ
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11th Jul 2022, 1:16 pm | #2 |
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Re: AR88D gone low audio output
Voltage readings?
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11th Jul 2022, 2:11 pm | #3 |
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Re: AR88D gone low audio output
Oil filled caps leaky, if not already changed or rebuilt?
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11th Jul 2022, 2:18 pm | #4 |
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Re: AR88D gone low audio output
Hi
Nothing tested as yet …need To pull up a manual at some point …appreciate it’s a question with many possible answers
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11th Jul 2022, 2:19 pm | #5 |
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Re: AR88D gone low audio output
Not changed by me …but don’t look like they have been
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11th Jul 2022, 2:28 pm | #6 |
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Re: AR88D gone low audio output
I'll just warn you that these sets are notorious for destroying their output transformers. They're impossible to rewind and replacements don't come cheap assuming you can even find one.
I strongly suggest that you take voltage readings around the output stage. It's true that the "bathtub" capacitors fail, as do others in these sets, but replacing them all won't necessarily cure your problem.
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11th Jul 2022, 2:39 pm | #7 | |
Triode
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Re: AR88D gone low audio output
Quote:
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11th Jul 2022, 3:16 pm | #8 |
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Re: AR88D gone low audio output
So don't put power on it again until the planned refurb has happened.
The audio amp stage anode (6SJ7) to audio output stage g1 (6V6) capacitor is known to cause trouble... it can destroy the output transformer and sometimes the mains transformer. The tone correction capacitor is from 6V6 anode to ground. When these fail, you guessed it, they overstress the transformers. The negative centre tap of the HT winding doesn't go directly to chassis, instead there is a resistor network voltage dropper to make negative voltage for helping with manual RF gain control and AGC. It's also used in biasing the 6V6. Problems in here can AGC-down the RF and IF gains, making a quiet set. The resistors are wirewound and usually well-cooked. There's a lot to go through, but start with checking 6V6 g1 voltage. Fit a new coupling capacitor as a matter of routine. Fit a new tone correction capacitor, but connect across the primary of the output transformer so it goes from 6V6 anode to HT. Not to chassis. David
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11th Jul 2022, 3:30 pm | #9 | |
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Re: AR88D gone low audio output
Quote:
Yes wise words I think ….just been going through the manual on line and seen the table of expected volts on the valves …..will start buying up some caps and resistors …..hopefully not everything will need changing !!! Thanks Steve
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11th Jul 2022, 5:53 pm | #10 |
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Re: AR88D gone low audio output
Sorry probably being a bit thick here
The audio output valve says 6K6GT on circuit diagram not 6V6 maybe it’s a equivalent
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11th Jul 2022, 6:09 pm | #11 |
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Re: AR88D gone low audio output
Early 88s were fitted with a 6K8 or similar pentode; later ones used the 6V6 beam-tetrode. They're essentially interchangeable.
First thing I would check is the voltage-drops across the three wirewound resistors in the negative HT-line; these provide the various bias-voltages needed [remember that the AR88 did away with cathode R-C bias networks for its RF and IF valves in order to minimise 'degenerative feedback'] and instead used a resistor-network in the negative HT line - if these resistors haver gone intermittent/high you could have a _lot_ higher bias-voltage than the designers intended. Equally, anything causing an excess HT current [such as a screen- or anode-decoupling capacitor leaking HT-to-chassis] will wind-up the bias and cause all sorts of issues.
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11th Jul 2022, 7:21 pm | #12 |
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Re: AR88D gone low audio output
Whoops...I think that's supposed to say 6K6!! I don't think a 6K8 frequency changer will work very well as an output valve.....
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11th Jul 2022, 7:27 pm | #13 |
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Re: AR88D gone low audio output
Yes, fingerfumble...
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11th Jul 2022, 7:29 pm | #14 | |
Triode
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Re: AR88D gone low audio output
Quote:
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18th Jul 2022, 4:03 pm | #15 |
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Re: AR88D gone low audio output
Ok …I’ve had chance to remove the bottom cover …looks like a previous owner has replaced the bath tubs and a lot of the resistors from a 1st glance ..have to start working my way through it …
Will try and upload the bottom of the set pictures
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18th Jul 2022, 8:13 pm | #16 |
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Re: AR88D gone low audio output
It's easier if you slide the whole set out of the cabinet and you can get to the components on the side drops of the chassis. Less weight to wrestle as well.
David
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18th Jul 2022, 8:26 pm | #17 |
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Re: AR88D gone low audio output
Just a point, the screen feed reses can go high on these if not already said.
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18th Jul 2022, 8:49 pm | #18 | |
Triode
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Re: AR88D gone low audio output
Quote:
Wasn’t really planning to start this yet …as house move should be hopefully happening … Will look and see how easy it is to get chassis out
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18th Jul 2022, 8:51 pm | #19 | |
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Re: AR88D gone low audio output
Quote:
Has anyone got a link to a more in depth service manual …if one exists !!
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18th Jul 2022, 9:41 pm | #20 |
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Re: AR88D gone low audio output
It's easier than taking the bottom cover off.
With the set right way up, make sure there's slack mains cable so it doesn't get hung-up. Remove the four big screws on the dress strips at the sides of the front panel. Pull the chassis and panel out as one unit. Do it this way up because the case has slide channels for the chassis to run in. Insert this way up when you want to put it back together. The case has a couple of lugs that engage in holes in the back drop of the chassis. The AR88 is ******* heavy. With the case off the chassis is merely ****** heavy. With large masses and sharp edges, look out for your fingers. It looks like it's been discombathtuberated, but if it hadn't you'd need the chassis out to get at bathtub mounting screws. Oh, look on the outside of the panel/chassis gusset plates. There may be a UK mod record or info label. David
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