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Author Topic: National NCX-A Question  (Read 2082 times)
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W1TTL
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« on: July 01, 2022, 04:45:57 PM »

Hi everyone,

I currently have a National NCX-5 and NCX-A on the bench that I'm starting to restore. 

I just finished updating the capacitors and diodes on the NCX-A and I am getting DC voltages that are way over what they should be.  That and the transformer is buzzing like crazy.  Even if I put the NCX-A on a variac and bring it up to 100VAC, the DC voltages are still way over.

Does anyone have the wire color coding for the NCX-A's transformer?  And the resistance values for the different windings?  It would be nice to test out the power transformer to make sure it's good... unfortunately, the NCX-A's schematics do not have the transformer wire colors or the winding resistance values.

Thanks and 73,
Tony W1TTL
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Pete, WA2CWA
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« Reply #1 on: July 01, 2022, 08:40:02 PM »

Almost sounds like the power supply is the NCX AC 500. The NCX-A and the NCX AC 500 look almost identical externally but the AC 500 HV (from memory without pulling the folder) is somewhere between 900 and 1000 volts. There are circuit and part differences between the two. The NCX AC 500 was only mated with the NCX-500.
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
W1TTL
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« Reply #2 on: July 01, 2022, 08:59:28 PM »

Pete,

Thanks for the tip.  Yes, the HV was putting out somewhere around 900V.  However, I can’t find any markings on the chassis that says it’s an NCX-A or NCX AC 500.  Also, it doesn’t have the 230/117V that the NCX AC 500 is supposed to have (when comparing schematics). 

Maybe the transformer in this power supply was meant for the AC 500?

73,
Tony

Almost sounds like the power supply is the NCX AC 500. The NCX-A and the NCX AC 500 look almost identical externally but the AC 500 HV (from memory without pulling the folder) is somewhere between 900 and 1000 volts. There are circuit and part differences between the two. The NCX AC 500 was only mated with the NCX-500.
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ab1mn
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« Reply #3 on: July 02, 2022, 05:43:20 PM »

Hi,

Here are some notes I made a few years back:

NATIONAL NCX-A VERSUS. AC-500 POWER SUPPLIES

The Power Transformer is same P/N in both supplies.

The AC-500 H.V. supply is a capacitor input vs. a choke input in the NCX-A. This is how the higher B+ voltage is obtained. It looks like the NCX-A H.V. filter choke (L1) is moved to the AC-500 L.V. supply in order to reduce the L.V.

The AC-500 did not include a case or speaker. The power cord is 7-conductor instead of 9 conductor as there are no speaker wires in the cable. The AC-500 chassis is different and does not fit the NCX-A case.

High Voltage supply (1100v)
AC-500 removed the swinging filter choke (L1) from the H.V. supply (moved to L.V. supply as L1?)
AC-500 has 3x 80uf in series, each shunted with 2x7500 10W in series (15k @ 20watts). NCX-A only has 2x 80uf in series, each shunted with 2x7500 10W in series (15k @ 20watts).
AC-500 uses a single bridge rectifier module, NCX-A uses discrete diodes.

Low Voltage supply (280v)
L1 Swinging Filter Choke added to L.V. supply

Bias supply (-100v)

Bob,  K7DYB
Bias supply: NCX-A has 1800 ohm resistor after diodes and before filter cap. AC-500 does not.
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W1TTL
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« Reply #4 on: July 02, 2022, 06:01:04 PM »

Bob,

Thanks for your notes!

Well, it looks like a false alarm on my part.  The HV, LV, and negative bias voltages were all high apparently because I didn't have the NCX-5 attached to the NCX-A.  Once I connected the NCX-5 and brought the NCX-A up on the variac, I saw pretty much the correct voltages (except for the HV... I may have an issue in the NCX-5 because I'm not hearing any audio).  I'll have to post a follow-up once I fix that.  At any rate, here's what I'm seeing in case someone down the road has the same questions.  I'm including the voltages I'm seeing along with what's specified for 117VAC in parentheses.  Also, I'm including the resistance readings for the windings since I haven't found that documented anywhere else.

T1 Transformer

HV
No load: +876 VDC
Load: +850 VDC (should be +700 VDC per schematic)
~27 Ohms

LV
No load: +450 VDC
Load: +280 VDC (should be +280 VDC per schematic)
From one end to center tap: ~13.5 Ohms

Negative Bias
No load: -99 VDC
Load: -80 VDC (should be -80 VDC per schematic)
~127 Ohms

Filament Voltage (12.6 VAC)
~0.2 Ohms

Primary Winding
~0.7 Ohms

And for those of you wanting to replace the HV bridge rectifier IN2359 diodes, use EM513 diodes.  For the negative bias IN534 diodes, use 1N4007.  Both replacement diodes have acceptable peak inverse voltage ratings (which are 1600V and 500V, respectively).

73,
Tony W1TTL
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MikeKE0ZUinkcmo
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« Reply #5 on: July 06, 2022, 11:05:01 PM »

The most obvious differences between the two is that in the "500" supply, they removed the choke in the High B+, and added a third, cap in the filter string, and, they simply added the HV choke in series with the existing choke in the Low B+ line.



...and the NCX-5 power supply.



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Mike KE0ZU

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