Author Topic: Zenith power transformer failure  (Read 506 times)

Motorola Minion

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Zenith power transformer failure
« on: October 11, 2016, 10:28:01 AM »
I was contacted to fix this unusual model MP500 by someone who saw some of my consoles for sale when I had a stand at an antique mall. The Chassis 4K22 stereo amplifier is single-ended 6BQ5. The tuner-preamp is stereo also. The speakers swing out and the hinges make the connections. There are RCA jacks to remove speakers but cords need to be made.

When I picked it up, I noticed the power cord was cut off and soot on the chassis near the trans. I suspected an issue other Zenith owners have had but said nothing initially. A quick check revealed tubes were strong and record changer looked like new. That's the good news.

I found a 6-amp fuse in the line fuseholder. :(  The primary should measure 3 ohms DC per the Zenith schematic, it measured 9 ohms on my Fluke DMM??? So, I powered up to be sure. With the 5Y3 out, it pulled about 3 amps with 65 AC volts going in from the variac. It got warm and that was it. Diagnosis complete! I wonder why the DC resistance went UP when it failed. I expected shorted windings to have lower DC resistance.

Luckily, I picked up a stripped 1959 amp chassis with a good transformer :D, so we can proceed to restore this HiFi. The high voltage is 30 volts lower as it was a p-p 6BQ5 mono amp. The 5Y3 will not be used saving 10 watts on the transformer primary. A 2 amp fuse, pair of silicon rectifiers and surge-limiting resistor will accompany the capacitor replacements. 
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Dave

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Re: Zenith power transformer failure
« Reply #1 on: December 05, 2016, 02:39:32 PM »
The transformer was installed and with new power supply capacitors and SS rectifiers used instead of a 5Y3 tube. A few mylar film caps also.

In order to drop the voltage down to 368 vdc, a 300 ohm 5 watt resistor was added in series. The amp plays good.

The 2g changer needed a new idler wheel. In place of the little o-ring that holds the platter on, there was a C-clip  so I got a few O-rings from V-M.
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Dave

19and41

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Re: Zenith power transformer failure
« Reply #2 on: December 05, 2016, 05:55:56 PM »
I haven't seen a unit in that layout before.  It looks like they took a cue from European equipment.  Nice.
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TC Chris

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Re: Zenith power transformer failure
« Reply #3 on: December 05, 2016, 06:34:14 PM »
Did the transformer die from a filter cap failure, or from being overworked and under-spec'd?

Chris Campbell

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Re: Zenith power transformer failure
« Reply #4 on: December 06, 2016, 11:56:14 AM »
I did not even voltage-check the multi-cap before replacement but I will since its still on the chassis, just for my customer's peace of mind, who bought it as not working. I hope removing the heater leads to the 5Y3 reduces inrush current and run load.

I hope it did not croak due to being undersized but I do have two other Zeniths with a similar amp chassis power trans (a 4L21 and 5B29) and both seem to run about the same temp as the Mag's much larger AMP175 does, using its 5U4. What I will do is take temps on all these with a probe. Hopefully this helps you all to make a decision on your own units.

Based on evidence found so far, a baked-to-hell trans and crispy idler wheel, the phono stalled on the change cycle after playing a record and power was left on indefinitely. If the lid was closed, the dial lights were not seen. If a unit does not have a cabinet pilot light, install one. 

I worked on the tuner this morning. Its tight in there and has its own transformer. Though I did not test it beforehand, the selenium rectifier is the first to get replaced and then a cap cluster on a terminal strip. It has a 3-section with a 60uf and two 150 uf, all 150 WVDC. The 22 ohm resistor in series with the selenium will need to be changed once I measure how much higher the B+ is with the 1N4005... 

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