Hammond Pwr Trans 374BX Diagnosis - diyAudio
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Old 26th July 2012, 01:31 AM   #1
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Default Hammond Pwr Trans 374BX Diagnosis

Hi Guys,
Need your help to check if my Hammond Power Transformer 374BX used in Simple SE is still in good condition?

Reason for asking is because my last power up I heard a loud hum after a month of "hibernation". Upon power off, the 374Bx is very hot. Afraid that it might be dead/fried I had performed continuity/resistance measurement via a multimeter and the result as shown below. How I wish I do have another power transformer for the same comparison but is the 374BX still good/fit for use?

Appreciate any form of advice. TIA

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Old 27th July 2012, 09:54 PM   #2
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Why not just disconnect all of the secondaries from the board and carefully check the voltage of each secondary with the transformer powered up?

The voltages should be a bit high since the transformer is completely unloaded, but that should answer your question.

The 5V rectifier and 6.3V heater windings can be difficult to measure since they are usually less than an ohm.

For reference, I just measured a 273BX (350-0-350 @ 175ma) and the B+ secondaries are 77R across the ends, and about 39R from either end to the CT. The 5V and 6.3V measure about .4R end to end.

Last edited by boywonder; 27th July 2012 at 10:01 PM.
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Old 27th July 2012, 11:57 PM   #3
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Quote:
Why not just disconnect all of the secondaries from the board and carefully check the voltage of each secondary with the transformer powered up?
Good idea, and leave it powered up for an hour or so. It should not get hot or buzz. Mildly warm is OK.


It is extremely rare for a transformer to die from sitting on the shelf. Electrolytic caps and rectifier tubes are far more likely to die from non use.
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Old 30th July 2012, 01:13 AM   #4
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Hi Guys,
Really appreciate all the inputs.
Will measure the secondaries without load tonite.
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Old 4th September 2012, 01:26 AM   #5
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Had been housing the current wooden case SSE to Hammond Alu case all this while.

Done with the secondary measurement (WITHOUT load) and reading as follows:
Red-Red: 800V
Red-Red/Yel: 400V
Green-Green: 6.25V
Yel-Yel: 5.24V
Click the image to open in full size.

However, upon connecting to the SSE's PCB (tried all combination of setting be it 4/8ohm spk out, Triode/UL or with or without CFB), the 374BX hum (vibrate) and can be heard and feel. By just removing the Red-Red & Red-Yel wires (#2 from pic attached - No Load on High Tension) from the SSE's PCB no hum (vibrate) of the 374BX can be heard (feel).

Question:
Without load no hum (vibrate) with the above measure on the secondaries. But with load 374BX hum (vibrate) upon power ON. Is the 374BX defective and what else can I do?

Appreciate any kind of inputs and advices/suggestions.

Thanks
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Old 4th September 2012, 02:37 AM   #6
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To add on, the power ON was done without the tubes.
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Old 4th September 2012, 07:50 AM   #7
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Is it possible that I had fried the FRED Diode (D1 & D2) that resulted in the shorted Red-Red wire of the 374BX resulting in Hum (vibrate).

I measure the continuity of the diode and was found that the diode body (k-cathode) was shorted to the anode as beep beep sound was heard. (no tubes inserted into any of the tube sockets).

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Old 4th September 2012, 02:23 PM   #8
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Junior: What's your B+ voltage and heater voltages when all of the tubes are installed and a proper load is connected to the speaker terminals?

If any of your voltages are out of spec (low), that would indicate a short condition and would make the transformer hum. Are the screws clamping the end bells on the transformer tight?
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Old 4th September 2012, 05:24 PM   #9
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Quote:
Is it possible that I had fried the FRED Diode (D1 & D2) that resulted in the shorted Red-Red wire of the 374BX resulting in Hum (vibrate).
Yes this has happened in the past. Remove the diodes, the amp will work fine without them. They are only needed when using the solid state rectifier option.
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Old 5th September 2012, 12:38 AM   #10
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hi boywonder,
checked all the screw on the power transformer is all secured tightly.
i mounted it on top of an aluminium Hammond chasis.

Hi Geroge,
Desolder both D1 and D2 (both are ISL9R8120P2) and check via DMM.
D2 is "dead". replace both D1 & D2 with DSEI12-12A.
Hope I had nailed down to the culprit.

Will try it out tonite and report back.
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