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Old 26th Jul 2022, 8:56 pm   #41
Steve_G4HTZ
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Default Re: AR88D gone low audio output

Hi graham

Thanks for reply

Yes it is the correctly marked , metal , valve ….will get a new one to try .

So many helpful people on here prepared to try and help newcomers.

Will let everyone know if a new valve does the trick

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Old 26th Jul 2022, 9:03 pm   #42
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Default Re: AR88D gone low audio output

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Hi Graham

Thanks for reply

Yes it is the correctly marked , metal , valve ….will get a new one to try
Might be worth measuring the resistance between the valve pins.

Is R38 OK?

You could try monitoring the valve's anode voltage from the time power is applied to see if it drops instantly or when the valve warms up.
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Old 26th Jul 2022, 9:24 pm   #43
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Default Re: AR88D gone low audio output

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Hi graham

Thanks for reply

Yes it is the correctly marked , metal , valve ….will get a new one to try .

So many helpful people on here prepared to try and help newcomers.

Will let everyone know if a new valve does the trick

Regards
Steve G4HTZ
Sorry forgot to say R38 reads 1.65M so slightly high
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Old 26th Jul 2022, 9:26 pm   #44
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Default Re: AR88D gone low audio output

That won't be the problem then.
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Old 27th Jul 2022, 6:24 pm   #45
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Default Re: AR88D gone low audio output

Might be also worth checking/computing V10's g1 bias voltage, less bias voltage means lower anode voltage.

Lawrence.
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Old 27th Jul 2022, 6:42 pm   #46
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Default Re: AR88D gone low audio output

I trust the cap from anode of output valve to chassis has been changed and put across primary? Forgive me if I have missed this
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Old 27th Jul 2022, 9:27 pm   #47
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Default Re: AR88D gone low audio output

Beware of RCA's original AR88 manual, they use the 'M' letter for the x1,000 multiplier and use 'meg' for the x1,000,000 multiplier.

It's caught a lot of people out, so double checking is a good idea.

David
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Old 27th Jul 2022, 9:42 pm   #48
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Default Re: AR88D gone low audio output

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Might be also worth checking/computing V10's g1 bias voltage, less bias voltage means lower anode voltage.

Lawrence.
Good idea. This set doesn't use cathode/auto bias. The negative bias voltage is derived from R43, R44 and R45. It's already been suggested that these can go high in value, but apparently they've been replaced. Are the new values correct?

Also check R36, R27. Bias voltage has to be measured between cathode and grid.
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Old 27th Jul 2022, 10:14 pm   #49
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Default Re: AR88D gone low audio output

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I trust the cap from anode of output valve to chassis has been changed and put across primary? Forgive me if I have missed this
Hi
Not sure if I have checked this cap …will do so tommorow
Thanks Steve
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Old 27th Jul 2022, 10:16 pm   #50
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Default Re: AR88D gone low audio output

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Quote:
Originally Posted by ms660 View Post
Might be also worth checking/computing V10's g1 bias voltage, less bias voltage means lower anode voltage.

Lawrence.
Good idea. This set doesn't use cathode/auto bias. The negative bias voltage is derived from R43, R44 and R45. It's already been suggested that these can go high in value, but apparently they've been replaced. Are the new values correct?

Also check R36, R27. Bias voltage has to be measured between cathode and grid.
Hi graham
Yes resistors have been changed previously and values are correct

Will check the bias voltage as you suggest

Thanks
Steve
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Old 27th Jul 2022, 10:18 pm   #51
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Default Re: AR88D gone low audio output

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Beware of RCA's original AR88 manual, they use the 'M' letter for the x1,000 multiplier and use 'meg' for the x1,000,000 multiplier.

It's caught a lot of people out, so double checking is a good idea.

David
Hi David
Yes did confuse me initially….but that’s not hard too do …

But gradually working my way through the set with the help of everyone on here

Regards
Steve
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Old 28th Jul 2022, 10:51 am   #52
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Default Re: AR88D gone low audio output

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But the HT is also similarly treated to a link in the plug fitted to the back panel socket. This is early enough in the HT circuit that the audio output valve and the rest of the receiver get power.

Worth mentioning in case someone wonders where the HT shoots off to in the harness (Or forgets that the back panel dummy plug isn't fitted, don't ask!)
Which two pins of the plug is the HT link connected between ? I can't see it in the schematics I have, they just show the heater link and the mains link.

Lawrence.
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Old 28th Jul 2022, 12:00 pm   #53
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Default Re: AR88D gone low audio output

That's interesting.

I have an early AR88 from before the suffix letters. It has a PVC wiring harness which is very professionally fitted and seems to date back to a refurb in the early fifties. I think it was at REME. There is a label inside, I remember.

It's been messed with a fair amount, but I came across the HT link when I was building an HT inverter for it, about 1970. It wasn't a successful project. I needed a 5v car battery for the heaters and a 12v one for the inverter. Efficiency was too low trying from 6v and I wasn't about to rewire all the heaters to do 12v/6v selectable.

I've come across a number of discrepancies between it and the AR88D manual diagram both connections and component values. It looks like I've just stumbled across another one.

When I next have it apart, I'll have a look around. I think it's a link in the HT path, but it could be HT back inside for something else.

The audio output transformer and wiring is the very early arrangement with various output impedances and the PVC loom is for two headphone jacks.

At some time there had been a product detector bodged in under the chassis. At a guess from a Shortwave Mag article knowing the reading habits of the former owner and his friends.

The worst mod was the tuning scale. The former owner (a pharmacist) had tried cleaning the main tuning scales and succeeded in cleaning some of the scale off. He replaced it by hand in blue ballpoint!

At a G-QRP Rochdale doo in the late 80s, I found a brand new tuning scale on a rally stall. Double-bagged AND it was the all yellow early type, not the alternating yellow/black/yellow that came soon after.

OK, so mine's rather atypical. I never noticed the difference in links.

David
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Old 28th Jul 2022, 12:19 pm   #54
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Default Re: AR88D gone low audio output

Well new valve arrived a new old stock …plugged it in and you guessed it still the same .

Unfortunately no time to delve any further as we now have a moving date so started boxing the shack up …..amazing how much rubbish you keep from years ago that you think will come in handy one day but never has

Thanks for all the help will try and get back on the AR88 in a few months

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Steve G4HTZ
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Old 28th Jul 2022, 12:57 pm   #55
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Default Re: AR88D gone low audio output

That cap I mentioned by the way, needs to be across the o/put tx primary. If it fails, goodbye tx.
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Old 28th Jul 2022, 1:43 pm   #56
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Default Re: AR88D gone low audio output

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That cap I mentioned by the way, needs to be across the o/put tx primary. If it fails, goodbye tx.
Ok cheers
Will look at it next time round
See it currently goes from anode to earth 3n cap
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Old 28th Jul 2022, 4:25 pm   #57
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Default Re: AR88D gone low audio output

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I trust the cap from anode of output valve to chassis has been changed and put across primary? Forgive me if I have missed this
I simply removed the capacitor from my AR88 output circuit. Its removal lets through a useful bit of ‘pentode treble boost’ which helps brightness and readability on severely band limited transmissions. The treble cut tone control can always be used if it’s too much.

Martin
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Old 28th Jul 2022, 4:58 pm   #58
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Default Re: AR88D gone low audio output

Thank you Martin.
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