Barn find ST70

Im working on a barn find - video coming shortly. Im trying to keep the unit stock with the exception of some 7199 converter sockets. I have a nice modded ST70 for regular use, this one landed in my lap and will be used for testing and understanding to compliment my new Hantek 5072P


A few questions however.......

The two 50MFD caps on the power supply. Rated at 75V. These need to be replaced. One has an ESR off the charts and measures in at 11MFD. What do you feel a good replacement should be?
Should I stay at 50MFD and 75VDC rating or could I benefit from a higher capacitance?

The output transformer inductance readings show the following. The right jack seems to have higher inductance. Does this reading look ok?

LEFT
16 OHM - 70mh
8 OHM - 28.8mh
4 OHM - 14mh

RIGHT
16 OHM - 85mh
8 OHM - 33mh
4 OHM - 17mh


The RF choke measured 65 OHMS at the multi capacitor can and .87H.. I cant find the specification for this

Whats a good way to measure the selenium rectifier. I generally replace them, but want to keep it in this unit.


If you are still reading this... does anyone have the NO-LOAD voltage readings for the power transformer... That is to say. what it reads with all tubes removed.


Thanks much, in advance


Jordan

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Heres a video of some of the work ive done thus far.......

with any restoration, I feel, always start at the power supply, especially the caps and older resistors, then work your way to and through the rest of the circuit, again giving good attention to caps, solder joints, wires etc..
Once up and running properly, then maybe fine tune it with better coupling caps, maybe some good quality film resistors and tube rolling as well.

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Unfortunately the 7199's (at least) one of them cannot be tested in curcuit as it has a short.


Im going to load in the EL34's and turn up the voltage to see what I get for heater and grid bias

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All the points about power fluctuations should be moot if you're using a circuit interrupter with a manual reset? Any brown out strong enough to cause problems should trip that.

Theoretically, of course.

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I ve reached an issue today. while setting up the grid bias I have no voltage on the 1.56 terminals nor pins 1 or 6 on the EL34's



Changes to the trim pots cause a change in voltage to el34 pins 5 and 6 which show about 33 volts.

Also I was able to test the selenium rectifier good which matched the manual at top 65vDC and bottom 50VAC

Wait a minute.....

The manual says to set up the bias before putting in the rectifier tube.  Without the rectifier tube there is no high tension DC.  Therefore no current flow... so there would be no way to set up the bias.



Just to be sure I pulled the rectifier from my other ST70 and sure enough, no readings on the 1.56 terminals without the rectifier.   Why would they write that in the manual?



EDIT:   Ahhh ok , it only says to center the pots without the rectifier..... not that you would get a reading... only after installation would the reading be present when current flows


I find it interesting that I cant find a 50uf 75v or 100v capacitor on any of the dyna parts websites

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Hello Bob,



So what you are saying is to use 100uf vice the original 50uf ones in the amp?

Is this an electrolytic cap... i see that its polarized....

Part II posted..... I should have realized that without the rectifier there would be no current flow.....

Very nice video! If I read the date codes on the bias pots correctly, this amp was kitted in 1963. Both pots have similar date codes, which supports 1963. I like the patina on the amp. Any idea how long it sat idol?

Mike

I am told 40 years.... Mostly as a display piece and possibly in storage too

So Ive purchased some go-no-further parts for the unit.

2x 100uf 450v caps

20/20/20/30 cap -   I figured, if im gonna stay original.....


Since Im going no further till the parts arrived I though I might ask a few questions.  In looking at the schematic...and wondering

1.  The input terminals TIP/RING are tied together with a 470KOHM resistor.   What is its purpose, and why 470KOHM




2.  With the discussion about the 50uf caps:

    The AC connection RD-BL supplies 50vAC to the rectifier with 65VDC out.  The output after both Caps goes directly into
the 10K POTS    to connections 6-21   onward to connections    1-2   and 22-23    to the EL34's pins 5-6.  So any voltage
change with the larger caps can be offset by the bias pots.    Correct? Also I imagine the caps are flipped around because the negative voltage is lower than the chassis....




3. What biases the 7199's?  There is no negative value for any test points on the tube. Pin 2 plate of the 7199 connects to (A) 305VDC which is connection 19
after a further drop in voltage by a 270KOHM resistor but its also connects to the grid pin 9 on the triode side of the 7199. does that 82mmf cap stop the DC flow into pin 9?




4.   Since there is no coupling cap between pins 9 and 2 of the 7199, am i to assume the DC Plate voltage from pin 2 is injected into the grid at pin 9
or is there some internal capacitor to separate the DC from the audio signal within the tube.  It seems to me that the plate voltage of the pentode side[A] is
different from the plate voltage on the triode side [B].

5. Is a reliable way to test the choke C354 by measuring  pin 8 on the GZ34 [435VDC] and expecting a 20VDC from on the output of the RF choke where it connects to the 20uf cap?

6.  What is the purpose of the two 270K OHM resistors in series that connect the output signals to the EL34 grid after the .1mf coupling capacitors? Does this match the peak to peak size of the wave forms going to the EL34 after they are phase inverted?



7. what is the purpose of the 1K resistor before the input signal into the EL34. Is it to match the expected output signal to an appropriate input level for the EL34?




8.  The pin 4 and pin 3 get a positive dc voltage from C power which is 415v  i see that pin 3 is lower probably due to the resistance of the audio transformer coil
and should read 5 volts less as a result.  Where is the return path for the primary side of the audio transformer, is it fed back into the EL34




sorry for all the questions, just curious

Jordan