Hi,

Sadly my adventure into valve radio has taken a set back.

As per my "Hi and Radilao 573MA" thread in the intro section, I ordered parts to replace the caps and valves from Evacto. I was pretty excited when they turned up, and using the documents and circuits from Kevin Chants web site I proceeded to fix the radio.

Anyway, the set has gone from very faint and unable to pick up any stations below 873kHz to absolute silence.

As I have never done any work on valves radios I really dont know what its problem is now. The cct diagram shows a pair of 22μF electro caps, but there were none in my set. I replaced all the caps I could with new tubular ones and was left with these two .1μF paper caps. So, I replaced them with the 22μF electros, and measured that this is indeed where they should go. Replaced all valves, but nothing.

The 6X4 and 6AQ5 valve get very hot and the others are warm. No crackling, no heat or issues from the caps I replaced. But not all bad news, the 2 new dial light bulbs now work

Anyway, dont know what to do now. Pretty disappointed, but I also had a win with an old National T-100 transistor radio that was broken and I fixed that, and it's a beauty... so mixed feelings on the days workshop experience.

So, I can listen to the cricket with the old tranny, but I won't be able to do it with the Radiola.

River

I replaced all the caps I could with new tubular ones and was left with these two .1μF paper caps. So, I replaced them with the 22μF electros, and measured that this is indeed where they should go. Replaced all valves, but nothing.

I don't recommend going that high with condenser capacities. If the manufacturer specified 0.1μF then that's what should be there. The valves that are getting very hot may be reacting to the larger condensers but as I'm not in front of the radio I can't be certain. I'd be leaving the radio off for the moment and obtaining some 0.1μF condesners before doing anything else. Once these are in the circuit, double check the whole circuit against a circuit diagram for that model, also checking for short circuits.

In some cases, valves do not like large condensers near them. The larger electrolytic types are most often only used for mains filtering and a 22μF type should only be used where it is replacing one of that value or very close to it.

If after doing all this the weak signals return it may be that the radio will also require realignment. That involves operating the grub screws on the top and bottom of the coil cans but this has to be done with care. With some experience this can be done by ear but many collectors use a signal generator and other equipment to achieve an optimum result. It's not just a simple case of tightening these screws and when this happens it is possible to damage the mica condensers and/or ferrite slugs that these adjustment screws are fitted to.

Hi River.
The 2 X 24μF 350V electros (C23 & C25) in your radio are chassis mount types. They can be left in place but must be disconnected when replacing.
The 22μF replacements should be small enough to mount underneath the chassis. The negative lead on C23 goes directly to the chassis but C25's negative goes to R17 & the centre tap of HT section of the power transformer (marked black on T2)

Hi again.
Just having a closer look at the circuit.
Put the C10 (0.1uf) back. That goes between the screen (pin 7) of the 6BE6 and chassis. The other 0.1μF (C17) runs parallel with one of the 24μF electros (C23). The chassis mount electros will have their lugs underneath the chassis. These are to be disconnected and not to be used.

Hi,

Ahh, chassis mounts!

Bugger! I'll get two .1μF tube caps to replace the two that I removed, as the existing ones were leaking badly and almost fell apart. Will disconnect chassis electros.

I'll get back to you. Again, thanks.

River

Hi,

I can either order two more 0.1μF tube caps from Evacto, which means they won't arrive until late next week, or can I use 0.1μF Greencaps, whcih I could get from Jaycar tomorrow? Are the Greencaps ok to use?

Here is the link to them...
http://www.jaycar.com.au/productView.asp?ID=RG5210

In either case, I'll put in the 0.1μF (either the tube types or Greencaps, depending on your advice), disconnect the chassis electros, and re-check the new 22μF electro are in the proper place, and then give it another whirl.

River

PS: The Greencaps are metallised polyesters.

You need to note the working voltage of the smaller condensers. If the working voltage is too small the new caps will just go bang. Jaycar does sell some high voltage ones but you'd need to check this on either the old caps or the circuit diagram before purchase.

Jaycar greencaps (they are actually red!) 2J types are 630v working and will do just fine. Yeah, and I bet you those two 0.1μF caps were those horrible MSP black pitch types. Yes, they do tend to fall apart when you try to remove them.

Hi,

Yeah, all the old caps just about fell apart. They were those yellow waxy things. Sticky, leaky and the wires just about fell out of them.

Thanks for the confirmation on the Jaycar caps. They are 0.1μF 630V units, so that'll be cool. Looks like I got something a bit more interesting to do tomorrow, rather than mow the lawns.

I'll let you know how it goes.

River

Hi,

Okay, here's the update.

Put in the 0.1μF caps where the old ones were. Disconnected the chassis electro caps and soldered in the new ones.

Radio makes a real loud humming sound after a few seconds when powered up, even with volume switched to minimum. So, that doesn't sound good, and I turned it off.

That's it. Voltages around these old sets are a tad more dangerous than the TTL stuff I usually work on, so I'm not touching this baby anymore. It's a tad scary. Kudos to you guys, I take my hat off to you for being ballsy to play with such voltages and get these wonderful old sets running.

Not sure what to do now. I may do some checking to see if there are people who can fix it - but as I spent $100 on parts I'm not really into throwing more coin at it. Alternatively I may pack it up and shove it under the house which is my old equipment cemetary.

River

Hi River.
Sorry to hear you are not having any luck.
That hum sounds like electrolytic cap problems. Remember that electros are polarised and have to be put in the right way round. Also the chassis mount electros lugs are used as tie points for other components. These other components cant be connected to the old electros either. As also pointed out in a previous post, C25 negative side doesn't go directly to earth, but to a 470K resistor, a 150ohm 1watt resistor (R17) and H.T. centre tap. You'll note on the chassis that negative side C25 is insulated from the chassis. So you solder the negative side of the new C25 on the point underneath the old C25 can where a 470k resistor and a black wire to the H.T. centre-tap is. The positive end of the new C25 can go direct to pin 7 of the 6X4. Remove the wire between the positive lug of the old C25 and pin 7 of 6X4. One of the speaker tranny leads also goes to the positive lug of the old C25. Remove this and also place to pin 7 of 6X4. If you want I can send you a good quality scan of the AWA 573MA service manual. It has a detailed picture of both above and below the chassis, plus a numbered grid so you can identify individual components.
I hope this all makes sense.
Good Luck.
Cheers.

Hi,

Thanks Monochrome for the clear explanation. I did indeed have the -ve of C25 in the wrong place and attached it as per your text, and verified the connection with a multimeter.

It fires up now and I can hear radio again. It's still very faint and only pulls in the stations properly and gets louder if I touch the ferrite rod.

I also purchased a full set of new/replacement valves, but when I put in the original valves the radio is louder, but still need to put my hand on the ferrite rod to get any decent volume.

Still won't pull in anything at all below 1100kHz.Before I began work on it I could tune down to about 870kHz.

So, still horribly perplexed by all of this, but sort of getting there in small steps, which is good.

River

Hi,

Okey-dokey it's alive!!!!

I used my circuit cleaner solution across the tuning plates. I checked and they were dirty but not binding or touching. Made little difference.

Then I replaced each old valve with a new one, one by one, and cleaned the valve sockets out as I went.

Yahoo!!!! After that the set roared loud and strong and picks up every station and more. Plenty loud and crystal clear.

The tuning peg thing is about 1cm out to the right, so I assume you can just manually move it on the string so it's on the actual station when tuned it?

Thanks for your help. Very, very much appreciated. I needed it and I learned a lot. Now I can hear the dulcite valve sounds of the cricket this boxing day, while I look through some tube valve books I asked my wife to buy for me for Xmas.

River

Hi River,
Start looking at your resistor values + do some voltage checks. R16, which is 2 X 10K 1 watt resistors twisted together off pin 7 6X4 should read 5K. Also replace C20, 100pf mica. In fact, any resistor or mica cap around the H.T. should be considered suspect. Also, is there any aerial wire off the ferrite rod?

Cheers

Hi,

I can do that.... replace the resistors and mica caps, but is it necessary now, seeing the radio is working like a new one?

There's no loose or broken wires around the ferrite rod.

The radio kicked in perfectly after I cleaned the valve sockets, specifically the 6BE6.

River