Nice job Morzh, I do believe this will be a challenge that hopefully will be worth it. The dial cord is intact and not alot of visible damage underneath. The power transformer has continuity so it should be ok, is there any ballpark values I need to look for? Havent tried any power on yet, still checking for obvious problems. Speaker coils have some resistance but havent pulled it out of cabinet yet.
Thanks for your inputs, help is always appreciated.
Geoge
Don't bother looking for values, just use a variac and see what the output does.
Remove all tubes, especially the rectifier, so there is no load, and simply see what the output high and filament voltages look like. The filament should come to nominal when the voltage is about 115VAC, the high voltage end-to-end (you could simply stick the leads into the two plate contacts in he rectifier tube's socket) should be around 4 or so times the input (500-600V).
As long as this is the case you are OK.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
On the spkr plug you can measure any 2 pins against each other and you should have less than 3.7K. The fc is abt 3.2k and it's in series with the center tap of the output transformer. So it's in some combinations you could measure the fc + 1/2 of the output transformer's primary. If you measure a resistance like 10 or 20k + there is a problem.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
(This post was last modified: 11-24-2021, 10:24 AM by Radioroslyn.)
I guess I jumped the gun by removing the cap can, now I have to wait for the new caps to test the PT. Wife wont let me use the oven (just got it fixed after 5 months of waiting for parts), I'll look for a used toaster oven heat gun not doing that good.
Speaker measurements are 282,540,3.2k,and 3.5k ohms. Think those make it ok
Thanks,
George
No you do not need the caps to test the PT. Without the rectifier tube it is not even connected to them. Simply do what I told you before with the probes in the rect tube plates.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
Happy Thanksgiving to all.
Morzh, the voltages were almost spot on (within 1 vac) and 568 vac on the rectifier. Thanks. Whoever made the notations on the schematic went to a lot of work and I for one really appreciate the effort.
Question for y'all, I neglected ordering the .015 cap that goes from 7 to 8 in the cap can when I ordered the solen caps. I would like to keep going with this since the caps came in today. Can I use a .015 Y2 box style safety cap for that? The only one I have on hand is a .01 orange drop.
Thanks,
George
I think it should be Y-rated cap anyway. Yes you are OK to use it.
In the link I gave you earlier you could see I used a yellow disk ceramic safety cap for it. it is also Y2 rated. I used 10nF value (popular for these); it is not really important.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
(This post was last modified: 11-29-2021, 06:45 PM by morzh.)
I agree. In the model 87 I recently restored, I used a .01 uF X1-Y2 safety capacitor to replace the .015 uF cap since one end of it connects to the AC line.
To Morzh and Ron,
Thanks for the reply I will keep going with this. Once again thanks for all that y'all do in providing help to those of us who need it.
George
Just a little progress, managed to open the small can and install two .47 caps inside. Now to mount the large on and hook it up and then do the push pull install, still worried it wont handle the load. I cant find a substitute for the 71A tubes. They are expensive and would hate to burn one up when I get to testing.
George
Yes, the 71A tubes are getting expensive. The cheapest I can find them on eBay is $18 plus shipping for good tested tubes. I advise you to check the filament voltage at the socket before you plug anything into it. The 71A has a 5 volt filament, so the voltage between the two fat pins should not be much above that. It can be AC or DC according to the spec sheet.
[Image: https://64.media.tumblr.com/aede1094fb9b...2e5b49.jpg]
You can also check the DC voltage from the plate to the filament, then check the grid bias voltage, and compare them to those shown on the spec sheet to see if they are in the ball park. Beyond that, about all you can do is hope for the best. Good luck.
Thanks for your reply. I plan on checking all voltages I can prior to installing tubes. I read on another forum you could sub the 71A with a 112 but cant find any more info on that. Later today I will hook up the cap can and install the transformer.
Thanks,
George
I would not sub a 112A for a 71A. The bias requirements are totally different. The 112A requires only -13.5 V bias while the 71A needs -40V. Subbing 112A's will throw off all the bias voltages since they are mainly determined by the current drain of the 71A output stage.
The 112A is actually a 201A battery set tube with an oxide coated filament. Its not really designed for use as an audio output tube.
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