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 Post subject: ZTO H500 required DC voltage Range from Rectifier
PostPosted: Jun Tue 11, 2019 10:26 pm 
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Joined: Dec Sat 22, 2007 7:12 am
Posts: 2641
Location: Austin, Texas
Dear Fellow Members-Work has really been a bear and led me astray from this fascinating hobby. Before I recapped another ZTO, I had paid another technician to recap one of my H500's and it worked great for several years (light use). Last time I turned the radio on using 120 VAC, there was lot's of snap , crackle and pop but no stations. On battery mode she works great on AM! Wondering what range of DC voltage I SHOULD be getting off of the rectifier, as I replaced it awhile back with a dropping resistor after replacing the several series rectifiers that the original restorer did after removing the Selenium Rectifier. Thanx for any and all feedback, as I am going to repair her on my next vacation:)-Gearhead222


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 Post subject: Re: ZTO H500 required DC voltage Range from Rectifier
PostPosted: Jun Wed 12, 2019 5:42 am 
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Joined: Sep Sat 27, 2014 4:24 am
Posts: 1833
Location: Oxford, MI
The power supply input voltage is best measured between the first filter capacitor (C25 below) and B- where it should be 105V DC ideally. It is probably not best to measure directly after the rectifier as the waveform at that point is not clean and will give different readings on different meters.

Really, these sets can tolerate wide variances in the high voltage power supply section as long as the filament string voltage remained between 8.4-9.0V. The military H-500 manual specified that batteries providing voltages from 90-120V could be utilized, so you have some wiggle room on the upper end.

My usual procedure is to adjust the value of the resistor after the rectifier until the filament string voltage is within the above range, and the B+ is usually close enough.

Note: the schematic below shows the Rev. C chassis with low line voltage compensation switch, your radio may or may not have this depending on which version it is.

Attachment:
Power Supply.jpg
Power Supply.jpg [ 195.32 KiB | Viewed 285 times ]

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Caretaker of Portable Tube Radios


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 Post subject: Re: ZTO H500 required DC voltage Range from Rectifier
PostPosted: Jun Wed 12, 2019 4:28 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3201
Location: Lexington, KY USA
Your symptoms seem to point to some problem beyond the B+ voltage being a little off.

Do check the B+ and filament voltages first, but a bad contact at the AC / Battery switch, or a resistor going south might be the cause of your trouble.

Note that an open switch contact, or resistor, feeding the filaments would result in higher B+ nearer the rectifier, because most of the B+ load would be missing.

Ted


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 Post subject: Re: ZTO H500 required DC voltage Range from Rectifier
PostPosted: Jun Mon 17, 2019 2:43 pm 
Member

Joined: Dec Sat 22, 2007 7:12 am
Posts: 2641
Location: Austin, Texas
Dear Fellow Members-Appreciate the detailed feedback. I have rebuilt the power supply and recapped 2 other tube ZTO's, so this is not my first walk in the park:) I did replace the Selenium Rectifier with a 100 ohm 5 W resistor and then a 1 amp rated silicon rectifier, besides replacing all of the filter and axial capacitors. Will go in and see if there's anything obvious, because this set worked fine on 120 VAC for over 8 years. Will keep you all updated-Gearhead222

Update-On 120 VAC she receives around 6 AM stations well, although the sound is "clipped", like she doesn't have quite enough voltage to create a complete waveform. Considering that I have three tube ZTO's and the other ones work fine and this one works great on battery, I am leaving well enough alone-not enough free time to work on it-got other stuff, but thank you people!:)-Gearhead222


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 Post subject: Re: ZTO H500 required DC voltage Range from Rectifier
PostPosted: Jun Mon 17, 2019 3:19 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5817
Location: Near Brandon, Iowa
gearhead222 wrote:
...as I replaced it awhile back with a dropping resistor after replacing the several series rectifiers that the original restorer did after removing the Selenium Rectifier...


There are a couple of reasons why the "Original Restorer" may have done this: first, having several silicon diodes in series may have been an attempt to mimic the forward voltage drop of the original (selenium) rectifier. This voltage drop would have been around 7 to 11 volts for a functional rectifier (failing ones tended to have high forward drops). With each silicon rectifier having a fixed forward voltage drop of 0.7V, the net voltage drop for the series string would have been 0.7V times the total device count.

Second, selenium rectifiers can burn out and "go open" but they are not known to short out. This is not the
case with silicon rectifiers: they not only can open up, they can sometimes short out, anode to cathode. This is a bad situation for the electrolytic caps downstream as it can subject them to reverse biasing, which will quickly destroy them. Perhaps the Original Restorer was attempting to avoid this scenario.


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