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

Tech Talk

Forum home - Go back to Tech talk

 Royaltone
« Back · 1 · 2 · 3 · 4 · Next »
 Return to top of page · Post #: 46 · Written at 7:13:00 PM on 9 October 2021.
Brad's avatar
 Administrator
 Location: Naremburn, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6851

If replacing the valve doesn't give a result, try a clean up and test of the valve socket. If the radio was poorly stored before you got it, chances may be that the valve is fine and the socket has a dicky contact.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 47 · Written at 8:24:24 PM on 9 October 2021.
Scotty's Gravatar
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 387

Thanks Brad, what’s your thoughts on eliminating the squeal. Only occurs when treble is up or volume is up.


 
 Return to top of page · Post #: 48 · Written at 8:30:14 PM on 9 October 2021.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4746

Putting it bluntly, speculation and shot gunning is the road to ruin and potentially wastes considerable time. that is V1 and you will not fix it unless the power supply and all of the other valves are working. This has not been proven.

There is 99% chance that after long storage all of the electrolytic caps will have lost form and require binning. That also applies to the Waxed paper caps where you are lucky to find one in a thousand that do actually make spec for leakage at the running voltage of the set.

You can get pin corrosion and as some valves had silvered pins it migrating & shorting is not unheard of.

It is strongly recommended to get the power supply sorted first, in conjunction with tossing out the electrolytic & Paper caps and checking for dud resistors, in most cases that will get is going. Where fuses have blown, on always looks for a cause. Once I am happy with it, I will power it whilst monitoring the voltages.

If it does not fire up and voltages are correct. Then you start fault finding. It is also at this point where a signal Generator can be handy.


 
 Return to top of page · Post #: 49 · Written at 10:00:40 PM on 9 October 2021.
Scotty's Gravatar
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 387

Thanks Marc,
You probably didn’t see my post a few posts back.
All caps have been replaced now so that’s all good. Power supply is working well. On phono there is plenty of gain, radio dead as mentioned, and the squeal when turn up (treble or volume)
Cheers


 
 Return to top of page · Post #: 50 · Written at 10:05:58 PM on 9 October 2021.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2109

"
As for the squeal, it’s still happening but does reduce significantly when I move the B+ lead to the OP transformer. The OP transformer isn’t original. I’m wondering if I re rout this wire if it will help, or would I be better to add a cap in the circuit somewhere that will eliminate the squeal.
"

The fact that the output transformer isn't original also brings in the possibility that the dressing of the 6BV7 plate lead isn't either.

The 6BV7 is a high gain valve and so is prone to instability if layout isn't perfect. In fact the 7C7 is also a high gain valve. So the audio amplifier will be rather "hot" in terms of gain. It's no wonder it's critical!

To make it less critical, add 100pf from the 7C7 plate to ground. That's Pin 2. If that doesn't quite do it, take it up to 470pf.


 
 Return to top of page · Post #: 51 · Written at 10:09:39 PM on 9 October 2021.
Scotty's Gravatar
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 387

Thanks Ian
Much appreciatedSmile


 
 Return to top of page · Post #: 52 · Written at 2:27:24 AM on 10 October 2021.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4746

Just generalising on what needs to happen for a swift repair.

Modern caps do not have "outside foil" and lead dress and shielding is often critical. Squealing is normally more prevalent at low frequency settings as that's where an oscillator radiates the strongest signal. Many output valves will amplify RF and make things worse.

With modern caps its never unusual to have these issues around the second detector in the audio section and you may need a chopstick to move wires to see if the frequency of the whistle changes. That point will be part of or all of of the problem.

Marcc


 
 Return to top of page · Post #: 53 · Written at 10:50:46 AM on 10 October 2021.
Scotty's Gravatar
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 387

Thanks Marc,
Moving the wires on the OP transformer did make a difference. The B+ red wire being moved reduced the squealing but didn’t eliminate it entirely
Cheers


 
 Return to top of page · Post #: 54 · Written at 4:13:07 PM on 10 October 2021.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4746

Philips loved to take feedback from the secondary of the Output transformer. If its like that most of them oscillate, if the primary or secondary wires get reversed.

It looks like an induction radiation issue. One reason I calibrate with an oscilloscope is I can normally ping where it starts. Bad design has to be taken into account. It's not uncommon with the modern cap, for it to induct. Take stock of wires crossing & away from the chassis & even some running parallel will induct. Wires crossing ideally should do that at ninety degrees to reduce induction. Some valves have shields within ensure that pin is actually wired or grounded as it should be.

I quite often find the long runs of wire too & from tone & volume controls without shielding have to be rewired with shielded wire.

Marc


 
 Return to top of page · Post #: 55 · Written at 6:15:43 PM on 10 October 2021.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2109

I suspect the output transformer was originally on the speaker. That would have meant a very short wire to the socket on the back apron.

Moving the transformer onto the chassis (to the only available space) meant routing the 6BV7 plate lead right past the 7C7 1st audio circuitry. There's your problem.


 
 Return to top of page · Post #: 56 · Written at 10:47:08 PM on 10 October 2021.
Scotty's Gravatar
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 387

Thanks Ian,
So the solution is to relocate the transformer?


 
 Return to top of page · Post #: 57 · Written at 11:36:41 PM on 10 October 2021.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1736

Not necessarily.

There's several solutions you can try.

1. Run the red wire across the top of the chassis to a hole right next to the 6BV7, so that the wire length underneath is as short as possible.

2. If it's not there already, add a 0.01μF cap from the 6BV7 anode to ground.

3. If it's not there already, add a 0.1μF cap from the HT line to ground.

These things all help with stability.


 
 Return to top of page · Post #: 58 · Written at 10:31:09 AM on 13 October 2021.
BurntOutElectronics's Gravatar
 Location: Alexandra, VIC
 Member since 2 October 2019
 Member #: 2392
 Postcount: 250

On another note, make sure to clean the pins on those Loctal valves. they have narrower pins than their octal counterparts and are known for poor connection


 
 Return to top of page · Post #: 59 · Written at 5:41:10 PM on 13 October 2021.
Scotty's Gravatar
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 387

Thanks ~ yes I’ll definitely clean the pins too


 
 Return to top of page · Post #: 60 · Written at 11:39:48 PM on 13 October 2021.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4746

I would point out that lead dress is often overlooked when things become unstable. I have only seen a couple of factory stuffups with lead dress that caused issues one was in an STC autodyne & was spotted on the Oscilloscope at the mixer whilst using the shortcut method of finding where the signal got lost.

One US set had wire to the tone control past an unshielded part of the Faraday cage & another wire carrying RF meandered through the audio. Not surprising to see the two push pull drivers ramming RF into the the other four in push pull output.

Many early sets had physically big "paper" filter caps, often these got replaced with Electrolytic types. Electrolytic types do not decouple RF well. Ensure that there is a cap from 0.047μF to often 0.22μF across "B", if there is none and place at the end of the run, in the RF section.

It is amazing on some sets how much RF is present on the plate of the output valve. The Paper filters did not need the decoupling cap most times.

Marc


 
« Back · 1 · 2 · 3 · 4 · 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.