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 Post subject: measuring sensitivity on an RCA 816K unrestored
PostPosted: Oct Tue 10, 2017 3:00 pm 
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Location: Monroe,NY
I'm in process of restoring an 816k and wanted to have a starting point because the alignment instructions look so daunting( look at the link, starting on page 165). First I replaced the electrolytics to make it safe then brought it up slowly. At first, I had to fix the motorized tuning and a clean some pots, but it seems to work now, so I wanted to measure what I have now before replacing all the paper caps. I have an HP 8662a generator so I hooked up my homemade selectible impedance matching trans to the antenna and started measuring the sensitivity of each band ( 7 of them ) by injecting a modulated signal into the set . Of course, alignment is off so leaving the dial at 1000 kc position on BC, I found each band at around mid-dial by using the frequency scrolling function of the generator and not moving the dial on the set, just using the band switch to select different bands. This way I could measure the sensitivity for each band ( based on a 10db S/N ratio measured at the speaker) and save the settings for that band in memory of the generator . To my surprise, BC measured about 1 uv sensitivity! A few measured around 10uv and some around 20-25 uv . Not bad for 80 years old!. My hope is to replace one cap at a time and turn the set back on after each, to see if I threw something out of whack or did it improve. Although tedious, this way I won't just start replacing parts, then find that I screwed something up and spend weeks trying to find it.
My thought is if this was an AA5, I wouldn't bother, but with this complex a set, I thought a slow approach was better.






http://www.americanradiohistory.com/Arc ... s-1937.pdf

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 Post subject: Re: measuring sensitivity on an RCA 816K unrestored
PostPosted: Oct Wed 11, 2017 1:04 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
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Yikes! what a project. I wholeheartedly agree, although I am certain there will be opposing viewpoints, on changing one part at a time since your slow powerup didn't smoke anything. Might want to do a quick resistance test on the remaining caps tho ... don't want to smoke any tubes or transformers needlessly. Although you may be safely beyond that point already.

Good luck!


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 Post subject: Re: measuring sensitivity on an RCA 816K unrestored
PostPosted: Oct Thu 12, 2017 3:35 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Philadelphia, PA
maynardb wrote:
My hope is to replace one cap at a time and turn the set back on after each, to see if I threw something out of whack or did it improve.



Don't take offense to this, but why in the world would you do that?

You are going to replace one part then turn the set on run it until you can make a S/N measurement then turn it off. You will be stressing all those 80yr old parts you are going to replace anyway. When one of those screen bypass caps shorts out because you have cycled the set on and off a few dozen times, now you will have one or more screen resistors to replace too. Or maybe something even worse.

Yes, it is fun to see the improvements you are making in the restoration. So take the first measurement as you have done, then replace all the failure prone components that will cause major headaches when they do fail. Make another set of measurements afterwards to see if there was any improvment, then do a complete alignment of the set, if it still isn't where you think it should be you can spend the time troubleshooting each stage without worrying about something failing on you.

One thing that is great working on RCA sets is the documentation available in the Red Books, if you remove a component and got side tracked and don't remember where to connect the new part, just look at the chassis pictorial.

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 Post subject: Re: measuring sensitivity on an RCA 816K unrestored
PostPosted: Oct Thu 12, 2017 8:18 pm 
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Location: Monroe,NY
I should have explained better. I don't intend to measure sensitivity each time, just listen for the modulation tone. This way I can understand how each cap effects tuning. Also if I make a mistake, I'll know it before going on to the next cap. I also noticed that the spread bands were more off from center dial than the other bands. I suspect that as I replace caps in the front end , tuning will move around a bit.

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 Post subject: Re: measuring sensitivity on an RCA 816K unrestored
PostPosted: Oct Thu 12, 2017 8:29 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 359
I am still on the side of one cap at a time. Even at the risk of smoking a resistor or tube, which seems unlikely. Always a crap shoot .... do it this way and risk something frying, or shotgun the caps and risk days of troubleshooting if you put one in wrong, bump a part so it shorts and you can't see where, or any number of other small errors that lead to hours and hours of later "fixing".

I do suppose it all depends on whether you believe in the stock market, or Vegas ... which has better odds :-D

b


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 Post subject: Re: measuring sensitivity on an RCA 816K unrestored
PostPosted: Oct Fri 13, 2017 12:05 am 
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Location: Maryland 20709, USA
I never turn a set on until ALL old caps and out-of-tolerance resistors have been replaced. It's quick and easy to check the resistors while you're replacing the caps.

Every time you turn that set on you risk a catastrophic failure that could damage it severely.

Replace one part at a time, and one lead of it at a time.

Cut one lead loose from the body. Point the severed lead and the body both straight up.
Remove the severed wire and connect one lead from the replacement to that terminal.
Remove the other lead and body, and connect the replacement.

NEVER disconnect more than one part from a solder terminal at a time.
Always solder a terminal when all new wires are attached. If that's not going to happen immediately, wrap the wires around the terminal so they cannot come loose.

There's no excuse for making a wiring error. Follow a standard practice and pay attention.

I've repaired and restored countless radios since 1950, and never once had a wiring error.

Plus, your basic premise is faulty. You're not dealing with a modern(ish) radio that has a single component failed. You're dealing with a set that has multiple degraded components.

- Leigh

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