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Author Topic: HT-37 possible power transformer failure  (Read 1900 times)
N6AF
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Posts: 54




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« on: September 09, 2017, 07:15:18 AM »

After many years of great service my HT-37 may have suffered what I understand to be a fairly common failure with its power transformer. I tried bringing it up slowly with a variac to see what voltages were present but have to stop increasing the AC input voltage at about 60 VAC when it starts making that dreaded "power supply overloaded" deep buzzing/humming sound. I have the 6146's removed and the 6.3 V lamp voltage, -49 V bias and 750 volt plate voltages seem to be headed in the right direction as I ramp up the variac but there is definitely no output on the 150 / 300 volt output. I recall seeing an article about a 5 volt winding shorting to the high voltage winding inside the power transformer. Can anyone tell me the method to confirm whether or not this transformer is failed?  Note that when the original failure occurred there was a small amount of smoke coming out of the top of the cabinet so hoping this was a simple single component failure I first looked for an obvious roasted component and not seeing any I went through and ohmed out virtually every single capacitor and resistor. As mentioned the finals bias voltage seems OK and the selenium rectifier appears to be OK.  So this leaves me thinking it's the main transformer. Since the 150 / 300 volt output isn't ramping up as I bring up the variac I'm suspecting the transformer. Is there a relatively easy way to check the transformer for its infamous internal winding short while leaving the transformer installed?  As always, thanks for any and all suggestions so I can get this vintage beauty back on the air.
73 N6AF Chuck

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KH6AQ
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« Reply #1 on: September 09, 2017, 08:12:56 AM »

I had the HV short to the 5V winding in my HT-37 (around 1973). If I recall correctly, the test was to remove the rectifier tube. The fix was to replace the tube with two silicon diodes. I removed the tube from its base and soldered the diodes into the tube base.
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AC5UP
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« Reply #2 on: September 09, 2017, 08:25:54 AM »

The Q&D test requires an ohmmeter and something to scribble with............

Per the schematic, draw a replica of the transformer windings then ID the primary and secondary leads.  Check for continuity across all windings then check for continuity between all windings and all windings to the laminated core.  With a little (bad) luck you'll find a hard short that answers your question about where the smoke came from.  I've had good luck with drawing out test results in schematic form as it's easier to analyze a scribble than to think your way through the chassis...  Less clutter, less visual noise.
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K8AXW
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« Reply #3 on: September 09, 2017, 09:31:40 AM »

One other thing that can be done is to remove all of the secondary connections and bring the transformer up again with the Variac.  Monitor the voltages and listen for the "loading hum."

Be sure to draw a map of where each wire came from.

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N6AF
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Posts: 54




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« Reply #4 on: September 09, 2017, 09:35:52 AM »

I replaced the 5V4G with a pair of silicon diodes per the suggestion and now can turn on the rig and see nearly 300 volts so we're almost there. Something else is wrong so still snooping around. Hey at least it powers up without growling now!
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N6AF
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Posts: 54




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« Reply #5 on: September 09, 2017, 09:50:17 PM »

It does indeed appear that the transformer winding that is connected to the V18 rectifier tube's cathode was failed, shorted perhaps to another winding. Per the excellent suggestion, I pulled the tube and substituted a pair of 800 PIV hefty diodes. But L3, the 300 V choke was still nearly melting and I found that the tiny C44 air variable trimmer was all gummed up and its plates appeared to be nearly touching. So I carefully degummed the plates and straightened them up and then wound a new 2.5 mH choke for L3 (69 turns on a T50-43 toroid). Yahoo the rig is alive and well. Thanks for steering me in the right direction. Without the core suggestion about the diode substitution I cannot even guesstimate how long it would have taken to home in on that one transformer winding.
73 N6AF Chuck
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K1ZJH
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« Reply #6 on: September 10, 2017, 01:54:35 PM »

Two silicon diodes will not replace the tube rectifier in the HV section...  the diodes have to withstand the full charge voltage on the filter caps, plus the PEAK inverse voltage secondary winding.  A i kV PRV diode will not handle it.

Pete
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K4PF
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Posts: 6




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« Reply #7 on: September 10, 2017, 09:47:10 PM »

Hi,
The 5V4G is rated at 1400V rms PIV per plate (or about 2KV peak inverse voltage),
with a peak current of 525mA per plate.  Two each 1N4007 diodes in series substituting
for each of the two diode halves of the original tube envelope would be a minimal equivalent.

It may be wise to add a series power resistor to duplicate the original voltage drop
of the 5V4 (rated 25V drop at 175mA), to avoid stressing the tubes downstream.
A 150 Ohm 10W resistor sounds good.

I'm glad your HT-37 is in good hands!

73,
Ed Knobloch
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K1ZJH
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Posts: 3299




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« Reply #8 on: September 11, 2017, 01:35:14 PM »

Two 1 kV piv diodes in series, for each leg will work for the low B plus supply.  They would be marginal for the 5R4.

I replaced both tubes in my HT-32, and the B+ does run high, especially before the 6146 tubes warm up and draw idle current. They serve a load even in standby.

Pete
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KM1H
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Posts: 2465




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« Reply #9 on: September 11, 2017, 05:11:00 PM »

Once I read about the HT-32/37 failure mode several decades ago I went with a pair of 1N5408's, I had plenty on hand, and removed the 5V leads from the 5R4 socked and taped up. The HT-32A and 32B have been working fine ever since.

Most all my other gear still uses hollow state rectifiers but 866A's are all replaced with 3B28's or 1N2637's which are a plug in swap.

I love phasing rigs SSB and AM audio and sold my HT-37 back in 1965 and bought a used CE-100V which I still have.

Carl
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