AR88D gone low audio output

[Steve_G4HTZ]

Thanks David for information.

Yes it looks like all bathtub have been replaced along with many caps and resistors….haven’t looked inside rf box section yet .

Regards
Steve G4HTZ

[Radio Wrangler]

The AR88 is cursed with a number of "Micamold' capacitors, some in the RF and oscillator boxes. The micamold name was a triumph of the marketing art, skilfully designed to be read by people and to make them feel all happy.... mica capacitors are generally considered to be reliable. Not these beggers. They're fakes. Paper! and all the reliability that's known for. They're just made in smaller values overlapping what real micas can do. Change on sight.

There are also some high value resistors in the RF areas. They were high values when new, they'll be a good bit higher by now. Screen resistors as have been mentioned, also HT droppers are worth a look.

Some of these are not in easy locations.

David

[vosperd]

Steve
Not sure which manual you have but if you haven't found it already there is more info on the VMARS Manuals site. No639 is a good start with a more conventional circuit diagram.
Don m5aky

[ms660]

Originally Posted by Steve_G4HTZ

Has anyone got a link to a more in depth service manual …if one exists !!

I found that the EMER E772 manual to be more in depth (VMARS Archive)

Lawrence.

[Steve_G4HTZ]

Thanks don and Lawrence for links ….I have seen those manuals …but thought there might be a more in depth one regarding expected voltages etc apart from the brief list …guess that’s a good starting point

[DMcMahon]

Originally Posted by Station X

It's true that the "bathtub" capacitors fail, as do others in these sets, but replacing them all won't necessarily cure your problem.

What are bathtub capacitors ?

David

[Jon_G4MDC]

I hope this photo explains.

[DMcMahon]

Thank you, yes it does if it is the triple 0.1uF metal cased capacitor.

David

[ms660]

Bathtubs at the end of the bathtub curve.

Lawrence.

[Aub]

When I had an AR88, those bathtubs had leaked their suspected cancer causing red liquid over the bottom cover. I disconnected them and used modern capacitors mounted outside of the cans. Some folks restuff the cans.

Cheers

Aub

[Steve_G4HTZ]

Tested with a digital multi meter on output transformer …3 to 4 shows 400 ohms ..not sure if that’s normal

Also the audio transformer in mine is marked 1 to 7 …the circuit diagram I have shows 1 to 5 …maybe a earlier or later model ?

Hope to do some voltage checks this weekend

[Steve_G4HTZ]

Ok
Done a voltage checks around audio stage

Output valve
Pin 3 290v max rf gain 305 min rf gain
Pin 4 260v max rf gain 290 min rf gain

1st AF valve
Pin 6 38v
Pin 8 23v ….manual says 83v ?

Only weak audio from headphones or speaker

Can’t track down the 2x 4700 caps …probably hidden away somewhere

Haven’t extracted the chassis from the case yet as wanted to some basic checks where possible

[Steve_G4HTZ]

Originally Posted by Steve_G4HTZ

Ok
Done a voltage checks around audio stage

Output valve
Pin 3 290v max rf gain 305 min rf gain
Pin 4 260v max rf gain 290 min rf gain

1st AF valve
Pin 6 38v
Pin 8 23v ….manual says 83v ?

Only weak audio from headphones or speaker

Can’t track down the 2x 4700 caps …probably hidden away somewhere

Haven’t extracted the chassis from the case yet as wanted to some basic checks where possible

Found the caps someone had combined them into a 10n single cap …not shorted

[Keith]

Hi Steve,


23v on pin 6 (anode) of the 1st audio valve (6SJ7) is very low. Maybe R40 and/or R41 have gone high.


As already commented the wire-wound bias resistors (R43/44/45) can go o/c due to high-ish dissipation. This can result in excessive AGC voltage making the set appear deaf. The attached is an anotated sketch showing this area.

[Steve_G4HTZ]

Originally Posted by Keith

Hi Steve,


23v on pin 6 (anode) of the 1st audio valve (6SJ7) is very low. Maybe R40 and/or R41 have gone high.


As already commented the wire-wound bias resistors (R43/44/45) can go o/c due to high-ish dissipation. This can result in excessive AGC voltage making the set appear deaf. The attached is an anotated sketch showing this area.

Hi Keith

Thanks for the suggestions…will,check those resistors next opportunity.

Thanks for picture of wire wound resistors section …I did check the values in situ and all seemed ok …they have been replaced by a previous owner .

It was a lot easier working on the Viking 2 transmitter !!!

If anyone else has any suggestions please fire away

Regards

Steve G4HTZ

[Radio Wrangler]

Originally Posted by Keith

The attached is an anotated sketch showing this area.

Good handy diagram.

I notice it shows the 6.3v heater feed taking a trip to the octal socket on the back apron and back via a link in the plug (which has to be fitted for mains use) -This socket is used when an external power supply is plugged in for mobile use.

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!)

David

[ms660]

Originally Posted by Steve_G4HTZ

Also the audio transformer in mine is marked 1 to 7 …the circuit diagram I have shows 1 to 5 …maybe a earlier or later model ?

The AR88 had, so far as I know, a 5 pin transformer, the schematic for the AR88-D that I'm looking at shows a 7 pin transformer:

http://www.vmarsmanuals.co.uk/archiv...8-D_Manual.pdf

The EMER manual also shows a 7 pin transformer for the AR88-D.

Lawrence.

[Steve_G4HTZ]

Originally Posted by ms660

The AR88 had, so far as I know, a 5 pin transformer, the schematic for the AR88-D that I'm looking at shows a 7 pin transformer:

http://www.vmarsmanuals.co.uk/archiv...8-D_Manual.pdf

The EMER manual also shows a 7 pin transformer for the AR88-D.

Lawrence.

Thanks Lawrence ….does help when you find the D circuit

[Steve_G4HTZ]

Originally Posted by Keith

Hi Steve,


23v on pin 6 (anode) of the 1st audio valve (6SJ7) is very low. Maybe R40 and/or R41 have gone high.


As already commented the wire-wound bias resistors (R43/44/45) can go o/c due to high-ish dissipation. This can result in excessive AGC voltage making the set appear deaf. The attached is an anotated sketch showing this area.

R40 measures 274k …meant to be 270k
R41 measures 99k ….meant to be 100k

I measured R54 meant to be 2.7k goes to pin 6 on audio transformer ..
In circuit measures 107 ohm …lift resistor leg at pin 6 shows 3.4k …..obviously higher in value ……would this have much effect .??

On the wire wound resistor as indicated on drawing
I get readings of -27v - 18v -1.5v ….shown as -23 -14 -1.5

I also took out 1st AF valve and pin 8 went to 290v …23v valve in
Pin 6 went to 270v …38v with valve in

All other resistors seem to be in spec having been replaced by previous owner who also replaced bath tubs with fixed capacitors

Am I missing something obvious ….or possibly looking in wrong area .
Radio tunes ok but very weak audio / signals .

Sorry for questions this is my 1st attempt on a vintage RX

[Station X]

When in circuit R54 is shunted by R39 (100R) so hence the low reading. As you say R54 has gone high, so could do with replacing, but it won't make much difference to performance.

The fact that the anode and screen voltages of the 6SJ7 drop significantly when it's plugged in show that the valve is drawing excess current. I would suspect a fault with the valve like an inter-electrode short. Make sure it is a 6SJ7. Is R38 1.5M OK?

If the fault were in an external component, such as a leaky capacitor, around the valve the voltage would be low without the valve being plugged in.