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 Leader SG 11 repairs
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 Return to top of page · Post #: 1 · Written at 5:40:47 PM on 11 June 2019.
Normf's Gravatar
 Location: Erowal Bay, NSW
 Member since 19 June 2018
 Member #: 2256
 Postcount: 78

This is a follow on from a previous post in Wanted and for Sale
https://vintage-radio.com.au/home.asp?f=3&th=1242
Thought I would continue it here.
As suggested by Fred and Marcc, I have used a 240 to 12.6v ct transformer, using half the secondary(6.3v) to drive the 6v winding on the original transformer.
Have some photos of how I mounted the second transformer.
Replaced all electros and oil filled caps, also replaced selenium rectifier with silicon one.
With no valves installed, measuring 110v AC out of transformer secondary to rectifier other side of rectifier measuring 150v DC.
I know I should be getting a higher DC voltage out of the silicon rectifier as would get out of selenium rectifier.
The circuit I have shows 100v AC out of transformer and 105v DC out of rectifier with 75v DC supply to valve anodes after 3K dropping resistor.
Can anyone confirm what these voltages should be.

Leader SG 11
Leader SG 11


 
 Return to top of page · Post #: 2 · Written at 10:39:38 PM on 11 June 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3708

That appears pathetically low and does not appear on the valve data sheet. Catch 22 that sounds about right. They obviously run that low to get harmonics.

What I spotted was that this voltage was right on the "Knee curve" of one tube & that can produce a weird shaped modulation pattern.


 
 Return to top of page · Post #: 3 · Written at 5:42:15 AM on 12 June 2019.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 574

Hi Norm, somewhere around 70 volt or so is correct going by the circuit arrangement.

With 100 volt AC RMS you would have about 100/0.7 volt peak. That's about 140 peak or so. If the rectifier had negligible drop and the capacitors could charge right up you would have 140 DC or so on no load.
But then you have drop and ripple as the caps are being charged up by 1/2 wave plus the drop across the 3k resistor, so if you wound up with 70 or so across C15 you would be lucky.
I'm guessing the current draw would only be in the 10ma range so that would drop about 30v across that 3k resistor. Keep in mind the voltages will have a lot of ripple being 1/2 wave so your DMM may read them differently depending on how it responds, whether RMS, average, peak or whatever.

Fred.


 
 Return to top of page · Post #: 4 · Written at 7:58:43 AM on 12 June 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3708

I have the Instruction books (as may have been noted) for both the Dick Smith TE-20D clone and the SG-11. I did have a restored SG-11 however that and dollars got swapped for one with a fried transformer.

Both have 12BH7 and 6AR5. Naturally in keeping with Australian tradition, voltages on circuits are scant. The DSE actually notes a secondary of 120VAC and heater 6.3VAC.

If this is what you have? The DSE uses 20μF 150V (Working) filters, specifies a Silicon Rectifier and between them (capacitor input filter) 2K1Watt.

I would need to open up the DSE and measure voltages if needed.


 
 Return to top of page · Post #: 5 · Written at 12:04:55 PM on 12 June 2019.
Normf's Gravatar
 Location: Erowal Bay, NSW
 Member since 19 June 2018
 Member #: 2256
 Postcount: 78

Thanks Fred and Marcc.

I was using an AWA Voltohmyst 1A56074 to measure the voltages but got a different readings using my
analogue Hioki OL-64D multimeter (it's about 50yrs old and still going well, bought it when I started my training with OTC)
and these are the voltages indicated in the post above and also those below.

I have a Tech TE 20D so I measured it's voltages.
110V AC from transformer, 135V DC after diode at 1st filter cap and 105V DC after 2K resistor at 2nd filter cap.
So it's drawing about 15 mA.

As the TE 20 D and the Leader SG 11 are similar circuits (didn't realise how similar they were till I just now compared them) excepting the SG 11 uses a 5K plate resistor on 6AR5 and
the TE 20D uses a 10K plate resistor on the 6AR5.


 
 Return to top of page · Post #: 6 · Written at 12:16:03 PM on 12 June 2019.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 574

Hi Norm, the readings in your last post look ok to me and 15ma sounds normal so no fault in the power supply to worry about.

Fred.


 
 Return to top of page · Post #: 7 · Written at 2:31:24 AM on 15 June 2019.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 5510

Photos uploaded.


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

 
 Return to top of page · Post #: 8 · Written at 5:31:51 AM on 15 June 2019.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 574

Looked at photos, nicely done Norm almost looks original fitting!

Does the unit work ok now?

Fred.


 
 Return to top of page · Post #: 9 · Written at 6:33:36 PM on 15 June 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3708

Looks nice but! I see an original brown jacketed oil filled cap: To me those look really nice decorating a bin. Whilst that is low voltage, I have found many oil filled caps to be as bad as Waxed paper. I tested a couple of that sort of cap in the unit that was here for leakage. Invariably on not ever seeing that type before: They failed leakage. So I tested the lot and they all failed.

I will not tolerate leaking caps in test gear.

The SG-11 seems to have been built in more than one factory. Amendment LSG - 11


 
 Return to top of page · Post #: 10 · Written at 9:10:49 PM on 20 June 2019.
Normf's Gravatar
 Location: Erowal Bay, NSW
 Member since 19 June 2018
 Member #: 2256
 Postcount: 78

Marcc I replaced the oil filled cap as you pointed out, it actually measured OK but replaced it anyway
.
When I plugged in the valves the 6.3V dropped down to just under 5V and the 110V from the secondary of the main transformer dropped to about 50V. Realized I hadn't accounted for the 1A draw by the heaters and had only used a 500mA transformer for the 6.3V input
.
Replaced the transformer with a 1A transformer I had. Now under no load reading 110V from secondary winding giving 150V DC out of rectifier, but under load now 6.3V input from transformer drops to 5.4V and AC from secondary drops to 80V giving only 80V DC from rectifier.

I'm using full wave rectifier now instead of half wave, but makes no difference in output DC voltage.

Wondering now if the original transformer secondary could also be faulty.

Found a 240V to 110V 10W transformer on Ebay for $18, of the same physical size as the original transformer, may need to get this to supply B+ volts.


 
 Return to top of page · Post #: 11 · Written at 2:36:22 PM on 25 June 2019.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1291

Those voltages are easily close enough that it should work as is.


 
 Return to top of page · Post #: 12 · Written at 2:10:59 PM on 6 August 2019.
Normf's Gravatar
 Location: Erowal Bay, NSW
 Member since 19 June 2018
 Member #: 2256
 Postcount: 78

After a lot of fiddling around, decided the original power transformer secondary could also be faulty
I purchased the 220v to 11ov transformer mentioned above, I also rewired the heater of the 12BH7 for 12.6v as the transformer I am using to supply the filaments had a 12.6v tap. Replaced rectifier with a bridge rectifier and increased the first filter cap to 16uf.
Now getting 120V AC (10% more as 240V into the primary winding rated at 220V) out of transformer for HT, 150V DC at first filter cap and 100v DC after 3k resistor at second filter cap.
Unit now seems to working OK and able to finish the alignment on the radio I was originally working on when it failed.


 
 Return to top of page · Post #: 13 · Written at 6:56:28 PM on 6 August 2019.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 574

Persistence wins out, well done.
Fred.


 
 Return to top of page · Post #: 14 · Written at 8:07:49 PM on 6 August 2019.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 5262

Good outcome. My LSG-11 died a while back and is sittting in the round tuit queue.


 
 Return to top of page · Post #: 15 · Written at 9:05:50 PM on 6 August 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3708

Beware, at the risk of sucking eggs: There are two tests that relate to capacitors.

One is leakage and with waxed paper, the smaller size oil filled non-polarised types, these tend to be the most prolific at leaking. I normally use an insulation tester for the HV ones. The idea is to test as close to the working voltage, or rated voltage. There was a quote for anything under 50M being a fail on screens & bypass and under 200M for coupling being a fail. My fail point is it leaks and a new one does not. Waxed papers can get below 10K. One can use a meter to check the value, many have big issues with low pF. If it leaks the cap can read correct value on the meter, however, if it leaks that test won't happen. Discharge the cap before testing.

Electrolytics are another matter. They are supposed to leak, but there is a limit. Left for ages unused, they can present as a short.

I normally use around 100 -200pF as a series cap for alignment with the TE-20D. I have a separate attenuator box made up which sends audio to the CRO sync, attenuated DC blocked signal to the set and signal to a frequency counter.

Do not feed more than around 50uV into the pentagrid: The AGC (where fitted) is not your friend, don't let it cut in. Do not calibrate the thing with a counter with the tone turned on.


 
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