Welcome to Australia's only Vintage Radio and Television discussion forums. You are not logged in. Please log in below, apply for an account or retrieve your password.
Australian Vintage Radio Forums
  Home  ·  About Us  ·  Discussion Forums  ·  Glossary  ·  Outside Links  ·  Policies  ·  Services Directory  ·  Safety Warnings  ·  Tutorials

Vintage Television

Forum home - Go back to Vintage Television

 Repair help required - AWA Television
« Back · 1 · Next »
 Return to top of page · Post #: 1 · Written at 7:32:58 AM on 25 March 2020.
John116's Gravatar
 Location: Ryde, NSW
 Member since 29 October 2008
 Member #: 370
 Postcount: 92

I have a problem I can't sort out with an AWA television. It had been working fine, and now, no picture (raster present), no sound.

Diagnosis so far shows:

No signal apparent from tuner.
Voltage checks indicate 0V on pins 3,6,8 and 9 of the converter (6CQ6 on circuit, 6U8 in my TV).
The service manual indicates voltages on these pins should range from 54V to 66V.
Not sure where the voltage for this part of the circuit originates from and which component is likely to be faulty. Relevant part of circuit in link below:

http://59.102.71.73/images/awa.jpg (copy/paste this link)


 
 Return to top of page · Post #: 2 · Written at 8:13:24 AM on 25 March 2020.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 412

Have you got any HT voltage on any resistors mounted externally on feed through caps, outside of tuner.
Should use 6CQ8, similar to 6U8 and 6BL8.
JJ


 
 Return to top of page · Post #: 3 · Written at 10:46:16 AM on 25 March 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1909

R16 could be open circuit. Ceramic feed-through caps in those tuners are known to fail S/C, rare though.

What is the chassis model number, so I can take you further?


 
 Return to top of page · Post #: 4 · Written at 11:20:46 AM on 25 March 2020.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1380

I have to admit to not being familiar with that circuit. If the diagram is correct, then the valve 6CQ8/6U8 will be acting as a voltage divider, with power firstly flowing into pin1, through the triode, then through the pentode. If you pull out the valve to take measurements, there will be no voltages except for pin1.

Make sure you leave the valve in when taking measurements. If there's still nothing, then either the valve died or is not alight. Or, R15, or maybe one of the capacitors shorted out.

Ian or one of the other TV guys will know more though - I'm just guessing.


 
 Return to top of page · Post #: 5 · Written at 5:02:24 PM on 25 March 2020.
John116's Gravatar
 Location: Ryde, NSW
 Member since 29 October 2008
 Member #: 370
 Postcount: 92

Model/chassis number is 209C.

Thanks for all your advice above.


 
 Return to top of page · Post #: 6 · Written at 5:52:48 PM on 25 March 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1909

OK. Locate R1, 820 ohm on the tuner. It's connected to pin 4 of the tuner's tagstrip.

Or just measure the voltage on pin 4. It should be about 240 volts.

If that's OK, check the 820 ohm R1 and the 5.6 k R16 in the tuner.

If either look badly burned, check the feedthrough cap C18 and C5, which is a disk ceramic usually.

If you have NO 240 volts, check R423, a 150 ohm 1 watt resistor. Again, it this looks cooked, check for shorts on the 240v rail.

Hope this helps. I have done a major restoration of a 209 chassis a few years back.


 
 Return to top of page · Post #: 7 · Written at 6:25:09 PM on 25 March 2020.
John116's Gravatar
 Location: Ryde, NSW
 Member since 29 October 2008
 Member #: 370
 Postcount: 92

Many thanks - will check all of that. Will report back.


 
 Return to top of page · Post #: 8 · Written at 8:39:31 PM on 25 March 2020.
John116's Gravatar
 Location: Ryde, NSW
 Member since 29 October 2008
 Member #: 370
 Postcount: 92

Thanks all for the replies.
I should have added that all other voltages at the tuner valves measured reasonably OK.
I have 280V on Pin 1 of the 6U8. I assume it’s high because the 6U8 isn’t drawing any current.
I measure 15k ohms of resistance between Pin 9 and the chassis which doesn’t seem right as this part of the circuit has only capacitors between it and ground.
Would this indicate one of those capacitors is leaky ?


 
 Return to top of page · Post #: 9 · Written at 12:16:46 PM on 26 March 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1909

Unlikely, they are NPO ceramics. And you'd need at least 2 of them to be bad. Vanishingly unlikely!

More likely would be a leak to ground somewhere in the two switch wafers. Look for a lug close to a mounting screw. There are many switch lugs that are DC connected to Pin 9. You may possibly have a silver migration leak on a switch wafer.

That 15k you are measuring might go a lot lower when it breaks down under voltage.

What do you get with the bottle unplugged? It pays to check the easy things first before cutting wires to narrow down a fault like this.

DO NOT cut or move those coils on the wafers!!!! You have been warned!


 
 Return to top of page · Post #: 10 · Written at 3:51:57 PM on 26 March 2020.
John116's Gravatar
 Location: Ryde, NSW
 Member since 29 October 2008
 Member #: 370
 Postcount: 92

The 15k ohms fault is present with the valve in and out.
I electrically isolated the front switch wafer (S1) by disconnecting it from Pin 9, C25, C28 and C29. I still had 15k ohms to the chassis.
At some point after this while fiddling around with the switch shaft, it went open circuit.
I connected it back up and powered up the set, but the voltage on Pin 9 was still 0V and the 15k ohms to ground had returned.
I attempted to clean the front switch wafer with contact cleaner, however, now the resistance to ground is worse at 250 ohms.
It looks as though I need to extract the front switch wafer and clean it properly.
How do you disassemble the tuner ? It looks as though the side rails need to be unscrewed and unsoldered, as well as the shaft withdrawn. Does the fine tuning assembly need to come off to get the shaft out ?


 
 Return to top of page · Post #: 11 · Written at 4:43:10 PM on 26 March 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1909

Pulling those rotten AWA incremental tuners apart is not a job for the faint hearted! The whole indexing and fine tuning mechanism on mine was completely seized, probably because at one stage the TV was stored in a garage which got flooded in a storm. So you can imagine....

The cabinet resto was a challenge too. Surprisingly, the speakers came through unscathed!

Yes you need to pull the FT and indexing assy off and extract the shaft. I think then you can extract individual wafers by removing one of the side rails.

Take lots of pictures, don't touch the coils and put everything back the way you found it!

It does sound like the switch rotor has broken down to the shaft, so removing the shaft first might confirm this.

Good luck!

For what it's worth, if all else fails and you are prepared to go non-authentic, I have a same-period Philips turret tuner you can have!

I also still have the silicone moulds I made to make replicas of the channel and FT knobs in epoxy resin.


 
 Return to top of page · Post #: 12 · Written at 6:07:08 PM on 28 March 2020.
John116's Gravatar
 Location: Ryde, NSW
 Member since 29 October 2008
 Member #: 370
 Postcount: 92

Thanks so much for the advice. I went ahead and disassembled the fine tuning mechanism, then released the two internal springs, removed the three screws in the front, and withdrew the indexing mechanism and tuner shaft as an assembly. I disconnected all the electrical connections to the front switch wafter, unscrewed and unsoldered one of the side rails and extracted the switch wafer.

I gave the wafer and tuner shaft a really good clean with a soft paint brush and methylated spirits. I reassembled it all, testing periodically for the abnormal resistance to ground I had measured before. All was well. I powered up the chassis on the bench and, to great relief, measured a decent voltage on what had previously been the dead part of the 6U8 circuit.

I returned the chassis to the television, powered the set up, and was rewarded with a good picture. I’m very satisfied to have successfully disassembled, fixed and reassembled a TV tuner. Thanks everyone for the input.


 
 Return to top of page · Post #: 13 · Written at 6:13:56 PM on 28 March 2020.
John116's Gravatar
 Location: Ryde, NSW
 Member since 29 October 2008
 Member #: 370
 Postcount: 92

Picture of working TV via URL below, excuse reflection on safety glass.

AWA 209c


 
 Return to top of page · Post #: 14 · Written at 8:03:19 PM on 28 March 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1909

Well done!!


 
« Back · 1 · Next »
 You need to be a member to post comments on this forum.

Sign In

Username:
Password:
 Keep me logged in.
Do not tick box on a computer with public access.