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 Hum and Hiss
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 Return to top of page · Post #: 1 · Written at 5:18:01 PM on 16 May 2019.
Dgnelson's Gravatar
 Location: Perth, WA
 Member since 15 April 2012
 Member #: 1133
 Postcount: 39

I've just replaced all the electrolytics and paper caps in a 1939 AWA Radiola model 200 console. I've powered it up and re-aligned it with a signal generator and an oscilloscope.

The radio works, but the sound quality isn't very good, a lot of hum and hiss. The antenna is just a short piece of wire of a few metres.

I'm not sure where to look next. Should I start replacing mica caps? Valves? Is my antenna too short? Any suggestions would be helpful.

Thanks.

Dan


 
 Return to top of page · Post #: 2 · Written at 7:38:58 PM on 16 May 2019.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1074

Use your oscilloscope to find the source of these things. Do they vary by using the volume control?

Disconnect the antenna and see if the hum & hiss changes.

Hiss could be from anywhere, such as old resistors, bad joints, etc.

Hum is most likely from the power supply or unshielded wires.


 
 Return to top of page · Post #: 3 · Written at 7:39:50 PM on 16 May 2019.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 320

Sounds like you need a much longer aerial wire to start with.
However this is exactly why it’s not good practice to just change all the caps.
And the situation will probably get worse if you change the mica caps as well.
These old radios should be appraised and repaired first to proper working condition.
Then in the interests of safety and reliability, aging parts replaced.
That would include electros, leaky paper caps, and for the point I am trying to make, the dial cord.
Get them going properly first!
JJ


 
 Return to top of page · Post #: 4 · Written at 10:15:13 PM on 16 May 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3701

The first thing never to do is power it if it has been sitting for a long period. You cannot power it with old electrolytic caps and yes one appraises it before touching it. Wiring is not always safe, rubber especially. Be warned that it was probably abandoned as it broke down. E.g. If its a Kriesler they have an attrition rate with speaker transformers. So some testing is done before touching anything with power, or a soldering iron

Dial cord does not always fail, however, some of it is a nylon which goes hard & rigid a process accelerated by getting circuit board cleaner on it.

Our mica caps I do not worry about. If one is out of circuit OK I will leakage test it. Failures are rare, but one with HV on them are the likeliest to fail. Cracking & hissing are their normal fail mode. A paper cap that does not leak is a very rare find: I have one standard for fixing commercial, or not: If it leaks its a dud. I also make a point of checking resistors as I rid the set of paper caps & old electrolytic caps.

That normally has a better than 90% chance of the set running. Then you get the bugs like dodgy volume controls etc. I normally hook up at that point for an IF calibration. That will tell me if its going properly from Pentagrid to speaker. Too much signal where there is AGC will wreck calibration if it cuts in.

At this point with this set going snap crackle & pop aerial is the least problem. The circuit comes up as 1937. This is "back biased" I am hoping the filter caps are 500v (specified) to 600V. If the first one is negative to chassis you will get hum. It has to be negative to centre tap.

You could have dirty switch contacts and valve pins, terminals on fly leads shorting, debris and bad connections, or a bad valve? A chopstick is good for probing. Turn the volume down & if the noise disappears then the issue is in the RF, if it persists its in the audio.

Check that lot & report back.


 
 Return to top of page · Post #: 5 · Written at 11:17:40 PM on 16 May 2019.
Dgnelson's Gravatar
 Location: Perth, WA
 Member since 15 April 2012
 Member #: 1133
 Postcount: 39

Thanks for the replies.

The filter cap is indeed negative to centre tap and 600v. Would something like a .001 μF in parallel with it help?

I tested all the resistors when I replaced the caps. All were good except a 1.75 M in the output stage that I replaced.

I replaced the power cable with a new one and soldered the earth lead to the chassis.

The components are all pretty tidy, I made sure they were when I replaced the caps. I hate messy circuits.

I'll poke around with a chopstick when I get a chance. That's a good suggestion. I'm used to poking my finger around low voltage circuits but that obviously won't work here!


 
 Return to top of page · Post #: 6 · Written at 4:45:04 PM on 17 May 2019.
Trobbins's avatar
 Location: Melbourne, VIC
 Member since 11 July 2012
 Member #: 1179
 Postcount: 9

How did the volume control test go? You may need to link the wiper to chassis to confirm.


 
 Return to top of page · Post #: 7 · Written at 11:09:31 PM on 17 May 2019.
Dgnelson's Gravatar
 Location: Perth, WA
 Member since 15 April 2012
 Member #: 1133
 Postcount: 39

Haven't had a chance to look at it again. Maybe tomorrow. I know that there's some scratchiness in the volume control, something is not quite right there. But need to examine further.


 
 Return to top of page · Post #: 8 · Written at 2:48:40 PM on 18 May 2019.
Dgnelson's Gravatar
 Location: Perth, WA
 Member since 15 April 2012
 Member #: 1133
 Postcount: 39

Thanks again to everyone for your suggestions.

I put a probe on the HT and saw 50 mV of AC ripple, which is nothing. So I started looking at the signals beginning at the input. Started around the mixer and swapped out the valve and all came good. So I'm quite happy.

Now on to the cabinet!

Dan


 
 Return to top of page · Post #: 9 · Written at 3:32:59 PM on 18 May 2019.
Dgnelson's Gravatar
 Location: Perth, WA
 Member since 15 April 2012
 Member #: 1133
 Postcount: 39

Spoke too soon. It was working beautifully. I turned it off and turned it on again half hour later. Didn't move, touch anything. It's gone back to how it was. Sad


 
 Return to top of page · Post #: 10 · Written at 5:06:55 PM on 18 May 2019.
Redxm's avatar
 Location: Tamworth, NSW
 Member since 6 April 2012
 Member #: 1126
 Postcount: 435

Valve sockets or mica caps


 
 Return to top of page · Post #: 11 · Written at 9:25:46 PM on 18 May 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3701

One does not put an NP cap across the first cap. unless there were paper filter caps, there is normally anything from 0.05 to 0.25mfd across the second filter cap.


 
 Return to top of page · Post #: 12 · Written at 11:42:20 PM on 18 May 2019.
Tallar Carl's avatar
 Location: Latham, ACT
 Member since 21 February 2015
 Member #: 1705
 Postcount: 1403

Get some pipe cleaners and soak in brasso then give the sockets a good clean then rinse with contact cleaner. Then get underneath and squeeze the valve sockets with longnose pliers. I also use a electric toothbrush soaked in brasso on the bandchange switch contacts. Your radio sounds like it needs this.


 
 Return to top of page · Post #: 13 · Written at 12:42:22 AM on 19 May 2019.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1074

I've seen so many radios with faulty band-switches, it's like a disease. You know you have the problem when things suddenly go bad, and wiggling the switch fixes it, then later it goes bad again by itself.


 
 Return to top of page · Post #: 14 · Written at 11:02:46 AM on 19 May 2019.
Dgnelson's Gravatar
 Location: Perth, WA
 Member since 15 April 2012
 Member #: 1133
 Postcount: 39

Got it going.

Once again, thanks for all the suggestions.

I cleaned up the valve socket contacts but that didn't help. I had also flipped the band select switch back and forth to see if I could get a better contact but that didn't do anything. I'll give them a clean, but I don't think that was the problem.

When I used a signal generator to do the i.f. alignment I could see a good signal passing down the valve sequence. Originally thought that maybe there was some ac coupling somewhere, but when the radio temporarily came good I realised that I was lacking signal. Had a look at the oscillator and was getting a good waveform there. That lead me to think that the r.f. front end was the problem, either the aerial coupling capacitor or the coil and capacitor tuned circuit. The coil showed a few ohms and looked okay. Luckily I had a 4.7 pF cap on hand and replaced the aerial coupling cap. And that did the trick, working nicely now. Couldn't get some of the weaker stations before but now I can. And the stronger stations are loud and clear.


 
 Return to top of page · Post #: 15 · Written at 11:24:06 AM on 19 May 2019.
Dgnelson's Gravatar
 Location: Perth, WA
 Member since 15 April 2012
 Member #: 1133
 Postcount: 39

Spoke too soon, again.
Sad


 
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