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 HMW Little Nipper C13D was working, now dead
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 Return to top of page · Post #: 1 · Written at 11:15:18 AM on 9 October 2017.
AndyM's Gravatar
 Location: Bacchus Marsh, VIC
 Member since 19 July 2017
 Member #: 2140
 Postcount: 15

Good morning,
Sometimes you just should leave things alone.My technical experience with valve radios is limited and mostly not successful despite being involved with electronics for 40 plus years(now non practicing was never very good technician). When I was at trade school in the early 80's valve theory was given as a history lesson.So with this being said it's really my first attempt at a valve radio.

I thought I'd have a look at this radio and replace the caps and redo the dial cord and clean the case.
What started as a working radio yesterday is now a dead unit.
I replaced the electros one at a time,checked the wiring and turned on.All good so far,radio still working.
Next to replace C21,the 0.005μF cap across the tone control VR2.
Switch on,radio works for about 5 seconds,then nothing.
No obvious signs of a mistake,as the cap is easy to get to and I can't see any shorts or wiring errors.
I unplug radio from mains and turn upside down and notice R11 an R12 are very hot.
I wait till cool then try again. Nothing working,just hot resistors.Some other resistors look worse for wear.
So far I can say the dial lamps are still working and filaments on valves are glowing.
Is there anything I should check first?

I better not go any further without some help.

Thanks in advance.
AndyM


 
 Return to top of page · Post #: 2 · Written at 1:15:06 PM on 9 October 2017.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 5175

R11 and R12 are (or should be) 1 Watt each. Two watts of heat will be too hot to touch.

Re your method: are you saying that you tested the radio after each and every capacitor replacement? If so, then there would appear to be a problem with or around the C21 step, which you could reverse and see if it makes a difference.

Otherwise, it does sound like an error -- perhaps an electrolytic wired in backwards, or a solder bridge causing a short. (This assumes that the insulation of the existing hook-up wire is in good condition, too.)

Are you being careful to use the correct voltage ratings with the replacement caps?

It's wise to check the values of resistors during a recap, and that involves lifting one end of them so as not to be influenced by their position in the circuit.


 
 Return to top of page · Post #: 3 · Written at 6:56:57 PM on 9 October 2017.
AndyM's Gravatar
 Location: Bacchus Marsh, VIC
 Member since 19 July 2017
 Member #: 2140
 Postcount: 15

Thanks for the info GTC, All the electro caps are rated 500V and the metal film caps to replace the paper ones are 630V.
R11 and R12 are hot!
I put the original cap back for c21- no change.
I checked for shorts,solder blobs and reverse polarity caps. Couldn't see anything obvious.
Whatever happened the damage is done.
Is it possible there is a fault with the secondary winding of the power transformer?
The filaments and dial lamps still work,but I have a feeling there is a problem with the other windings for the rest of the circuit.
What is a typical voltage on the secondary?


 
 Return to top of page · Post #: 4 · Written at 7:20:39 PM on 9 October 2017.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 5175

Your nose can usually tell when a power transformer bites the dust, not to mention the smoke. They don't usually give up easily unless they have developed a fault or have been working into a dead short situation for some time.

The 6X4 rectifier typically has in the vicinity of 300-325 volts AC per plate (i.e. between plate and ground). Maybe lower in your set but not by much.

As a quick test, pull out the rectifier and measure voltages between Pin 1 and ground and Pin 6 and ground.

Be careful as you go. You are dealing with lethal voltages. Also be aware that if you get your meter across the secondary windings, the total voltage may exceed your meter's and/or probes insulation rating.

Also, what is the output valve type in your set: 6AQ5 or 6M5? The schematic I have contains a note about connection of C18 depending on output valve type.


 
 Return to top of page · Post #: 5 · Written at 9:25:10 AM on 14 October 2017.
AndyM's Gravatar
 Location: Bacchus Marsh, VIC
 Member since 19 July 2017
 Member #: 2140
 Postcount: 15

Hi GTC,apologies for not replying sooner.

My work on the radio has ground to a halt as I have some other things to attend to at present but never fear I will get back into it.
The output valve on my version is a 6M5
Yes high voltages scare me,was never keen on pre switchmode TV repairs and EHT zaps.
On closer inspection I had made a few mistakes C23 was put in reverse polarity (stupid mistake)
When I was looking at replacing other caps I moved out C18 of the way to look at something and I suspect it shorted out near other pins of the 6AV6 (not paying attention am I?)

I haven't checked voltages with rectifier valve removed yet.
Once corrected,still damage may be done.I need to order a few more caps as I missed a few on my list to continue work so once I have that done I will get back into it.

Thanks.


 
 Return to top of page · Post #: 6 · Written at 10:36:46 AM on 14 October 2017.
Tallar Carl's avatar
 Location: Latham, ACT
 Member since 21 February 2015
 Member #: 1705
 Postcount: 1352

AndyM what caps do you need ?


 
 Return to top of page · Post #: 7 · Written at 12:39:14 PM on 14 October 2017.
AndyM's Gravatar
 Location: Bacchus Marsh, VIC
 Member since 19 July 2017
 Member #: 2140
 Postcount: 15

I'll make up my list this afternoon.Is your email contact the one listed on your profile?
If so I will get in touch with you that way.

Cheers


 
 Return to top of page · Post #: 8 · Written at 12:42:53 PM on 14 October 2017.
Tallar Carl's avatar
 Location: Latham, ACT
 Member since 21 February 2015
 Member #: 1705
 Postcount: 1352

Yes thats correct.


 
 Return to top of page · Post #: 9 · Written at 3:00:15 PM on 11 June 2019.
AndyM's Gravatar
 Location: Bacchus Marsh, VIC
 Member since 19 July 2017
 Member #: 2140
 Postcount: 15

I know this is an old thread. I have this radio working now. Thanks to everyone for their assistance.
It certainly helped to have another radio with the same chassis to check things.
I have learnt much from this experience.

Cheers.


 
 Return to top of page · Post #: 10 · Written at 11:41:00 PM on 11 June 2019.
Tallar Carl's avatar
 Location: Latham, ACT
 Member since 21 February 2015
 Member #: 1705
 Postcount: 1352

So dont keep us guessing. What was the problem Smile


 
 Return to top of page · Post #: 11 · Written at 5:06:02 AM on 12 June 2019.
AndyM's Gravatar
 Location: Bacchus Marsh, VIC
 Member since 19 July 2017
 Member #: 2140
 Postcount: 15

Hi Carl,
Quite a few problems,most caused by my haste and inexperience with valve gear.
C18 and C20 were the wrong values,I had transposed the numbers incorrectly.Probably not a huge deal but best to adhere to the right spec.
I went to check voltages on the 6M5. Pin 7 was around 250V and Pin 1 was 0V
I checked my other radio with the same chassis and those voltages were correct so it took me a while to figure it out.
On closer inspection of my wiring I discovered the + side of C19 and the 2 10K resistors weren't connected to Pin 1 of the 6M5.
Perhaps removed at some point when I had replaced C19 and C23.
I connected a wire to remedy that and when I switched the radio on I got howling through the speaker.
I had previously replaced audio transformer T2 (open circuit primary) with a 100V line transformer from Jaycar.
When I swapped the primary wiring around,the howling stopped and I could tune in a radio station,so all good.
Also replaced original 2 core mains lead with a shiny new 3 pin plug with earthed chassis.
All good now and currently restringing dial cord then putting all back together.
What did I learn?
Patience is required, and make lots of notes,and takes lots of photos.

Thanks


 
 Return to top of page · Post #: 12 · Written at 5:17:06 PM on 12 June 2019.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 5175

Patience is required

When I was a trainee, I was often told by the older guys that "patience is a technician".


 
 Return to top of page · Post #: 13 · Written at 7:28:30 PM on 12 June 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3615

One of the first things to do before attacking a set, is to ensure that you have good detailed photos: Even if you know its wrong.

This allows for interruptions & senior moments that happen after you have removed something & can't remember where you got it from.

It is also good policy to not get too far ahead. There are times where several bits just have to come off one node. Like its not uncommon to remove the end of a cap, & find out that all, or some of the resistors associated with it are all duds.

Circuit board cleaner removes Texta & most of the paint from paint pens. So one can dot the point you took something from, sort it clean it & move to the next item.

"More haste: Less speed" applies. This is why I consider changing caps and not checking resistors as you go, time wasting. Invariably there will be dud resistors and the time wasting occurs & risk of damage increases when you have to work out where the fault is and go back & fix it. If you had checked the resistors in the first place, there would likely be (from my experience) a better that 95% chance it will fire up & you are then just faced with annoyances like bad pots, mandatory alignment and perhaps a dud valve if you were unable to test them.

Marc


 
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