AR88D no audio

[AlanG4MOM]

Hello All,
Listening on the AR88 to radio Caroline this weekend and all was well with the world.
I forgot to turn it off overnight but the shack was toasty warm in the morning. Both RF and AF gain controls were turn to minimum during its overnight run.
I turned it off and thought nothing of it.

Next listening session the dial lights come on but no audio.
I have pulled the chassis from the case and there is the usual mess from the bath tub capacitors
in the bottom of the case.
I’ve done some reading on the various threads and am now concerned that it may be the output transformer.
I’m planning to replace all the bath tub caps. I have already pulled C99,112,113 which looks to be in the poorest state.
Is there a way of checking the output transformer without unsoldering and pulling it out of circuit ?

Cheers
Alan

[Radio Wrangler]

Power off, 6V6 pulled out and an ohmmeter across its primary winding ought to tell you if it survived. You may have to disconnect that tone correction capacitor from anode to chassis (it should never have been wired there - should have been across the primary) in case it's gone short.

David

[AlanG4MOM]

David,
Thanks for the swift reply.
Pulled the output valve and cut out C119 to ground. It measured ok but it certainly looks to be original.
The resistance reading across the primary is 530 ohms. Does that seem ok ?
Someone had fitted a second phones socket to the front panel. This is fed via a capacitor
from the anode of the 6V6. I think I will remove the second socket along with the cap and blank off the hole with a grommet.
Alan

[HamishBoxer]

Those caps are always ok,until they get some voltage across them.Then,it is wave goodbye to sound out tx.

[Boater Sam]

530 ohms is about right.
Is there any noise at all from the speaker?

Time to check all the voltages around the output valve. Put them up here for help.

[AlanG4MOM]

Thanks for that.
I need to replace C119 and the 0.25uf bath tub first.
Then I can fire it up and check the speaker and voltages.
Alan

[TrevorG3VLF]

Measuring the resistance of the primary of the output transformer with an analog meter should give a click in the speaker.

[HamishBoxer]

The headphone socket can give problems through dirty contacts,perhaps worth a check too.

[AlanG4MOM]

Checked the output transformer with an analogue meter and have a healthy click from the speaker.
Also cleaned up the headphone socket contacts.

[Radio Wrangler]

That's sounding promising.

Be sure to replace the cacitor which carries audio from the 6SJ7 anode to the 6V6 grid. It might look OK on a low voltage ohmmeter, but any leakage here will cause trouble.

The original capacitor is two 'Micamold' parts in parallel. Despite the misleading name these things are just basic paper dielectric types. THere are several of these type in the AR88 and they are generally considered best replaced.

David

[AlanG4MOM]

I've checked the capacitor as you suggested.
This looks relatively new (2 red disc caps in parallel) replaced be a previous owner.
I think the micamold caps you mention are brown square beasts with coloured dots indicating the value.
Alan

[HamishBoxer]

Those are micamold with the dots.

[AlanG4MOM]

A quick update.
I changed out the bathtub caps for C99, 112 and 113. 0.25 uF
Changed C119 which was the old micamold type.
Powered up and still no audio. S meter (original type) all the way to the right.
I then moved to the next bath tub C79, C84, C92 and noticed a burnt out resistor which I believe is R22 connected to C79 which is on the output of the 1st IF valve.
I plan on changing C79,C84,C92 and of course R22 1k.
Does anyone think more damage could have been done by R22 letting go ?
Alan

[Station X]

I'd say the most likely cause of R22 failing is C79 being electrically leaky.

Originally Posted by Boater Sam

Time to check all the voltages around the output valve. Put them up here for help.

This is a very good suggestion from Sam. How about following it?

[Radio Wrangler]

The S meter is right hand zero, reading IF valve current, so your receiver is fully biassed off on the AGC line. The AGC voltage is created partially by rectified RF signal, partially by negative DC bias voltage from dropper resistors in the negative end of the HT supply.

This area needs careful checking. Resistor problems are not unusual

David

[AlanG4MOM]

I tried reading the volts on the output valve but couldn't find anything.
I flipped the receiver back over and noticed the stabiliser wasn't striking.
I've checked the power transformer and that is giving volts through to the rectifier
both HT and LT. So looks like there is a fault in the L/C circuit or the associated R's.
In retrospect I should have noticed VR150 not striking in the first place.
At least I'm finding my way around the circuit and identifying the components.
i really appreciate the help I am receiving from the group.
Thanks

[Keith]

If there's no voltage on screen grid (pin 4) of the output valve, then there's no HT, which would explain your symptoms. Is there anything on the cathode (pin 2) of the rectifier (V14)?

[Station X]

Originally Posted by AlanG4MOM

I tried reading the volts on the output valve but couldn't find anything.

No voltage at all on anode and screen grid of the O/P valve points to L50 being open circuit. Check the DC voltage on L49 and L50.

[AlanG4MOM]

On pin 2 of the rectifier there is -0.64v wrt chassis.
On both sides of L49 and L50 its the same voltage.
Across pins 4 and 6 of the rectifier there is 690v
Across pins 2 and 8 of the rectifier there is 5.8v
Maybe the rectifier has failed ?
Hello Keith and thanks for the circuit Graham.

[ms660]

If the rectifiers heater lights up make sure that there's continuity to the chassis from the transformer centre tap via R43, R44 & R45 which form the bias voltage chain.

Lawrence.