Author Topic: RCA Victor SHF-8 Amp Motorboating ? Need some Help.  (Read 5469 times)

HiFiFun

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RCA Victor SHF-8 Amp Motorboating ? Need some Help.
« on: August 18, 2019, 12:33:18 PM »
Hello everyone,
1. I have finished rebuilding the record changer,
and done recapping of the amp.
Replaced selenium rectifier, and replaced tubes with NOS.
Resistors all check within 10 % tolerances.
Voltages all close to schematic specs, except 12AX7.

12AX7 V1B side :
Pin 7 Grid    neg -0.2 V  ( 0 V )
Pin 8 Cathode   0.7 V  (1 V)

2. There is a very low-volume pulsing, oscillating sound
from the speakers. Amplifier sound, though, is clear
and nice when records played.

3. I need help on where the "motorboating" may be originating.
Tried different searches on the net, but I'm a newb to this
kind of problem.
I don't have a signal generator or scope to help diagnose.

Thanks,
HFF


ed from Baltimore

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Re: RCA Victor SHF-8 Amp Motorboating ? Need some Help.
« Reply #1 on: August 18, 2019, 04:38:35 PM »
           I think that Sams Photofacts voltage readings are a little deceptive. The larger voltages were rounded to the nearest multiple of 5. 58 would become 60, 103 would become 105, etc. The lower voltages would be rounded to the nearest 1/2 volt or so. 0.1 would become 0. .6 would become .5. The 50C5 tube grid would have either a slightly positive or slightly negative DC voltage on it, depending on if the tube was slightly gassy or the .056 coupling capacitor had slight leakage. That would reflect to the 12AX7 grid (pin 7). The 12AX7 itself might be slightly gassy too. But unless the DC voltages measured are jumping around, indicating they are part of the oscillations, it should be OK. I guess Sams didn't want to confuse troubleshooters by putting .87 on the schematic when anything between .5 and 1 was OK.
         Is that capacitor definitely a 0.1 and not a 1.0 uF at the junction of the 2 plate load resistors and the 100K resistor.??  0.1 would be 105 and 1.0 would be 106 on a modern capacitor.  Also is the 5 uF capacitor on the 12AX7 cathodes definitely that value ?  If it was way too small, the 12AX7 might turn into a cathode-coupled multi-vibrator circuit.
           Also it's strange that an oscillating circuit would be generating such a low level of oscillation the it is barely audible in the speaker. Is anything strange connected to the same power outlet like a large older TV set. The vertical oscillator in an old TV with a CRT (not a flat-screen) might leak into the selenium rectifier power supply.
     Look at the tubes with the room completely dark and see if you see pulsations in the glowing cathode area of the tube. Kind of a purplish colored haze that pulsates with the puttputt sound.

HiFiFun

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Re: RCA Victor SHF-8 Amp Motorboating ? Need some Help.
« Reply #2 on: August 18, 2019, 05:23:56 PM »
           Is that capacitor definitely a 0.1 and not a 1.0 uF at the junction of the 2 plate load resistors and the 100K resistor.??  0.1 would be 105 and 1.0 would be 106 on a modern capacitor.  Also is the 5 uF capacitor on the 12AX7 cathodes definitely that value ?  If it was way too small, the 12AX7 might turn into a cathode-coupled multi-vibrator circuit.
     Look at the tubes with the room completely dark and see if you see pulsations in the glowing cathode area of the tube. Kind of a purplish colored haze that pulsates with the puttputt sound.
1. Yes, C12 is a 0.1 mF cap installed.
2. 5 mF (4.7mF) on the 12AX7 also confirmed .
3. There is nothing on that house circuit, but this amplifier.
4. I forgot to mention that I replaced the selenium rectifier with a 1N4007 diode.
The voltage following that rectifier is 123 V.
I was advised the voltage was acceptable; if not, I'm ready to add
a dropping resistor.
5. Will observe tubes when it gets dark enough.
Thanks, Ed !

electra225

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Re: RCA Victor SHF-8 Amp Motorboating ? Need some Help.
« Reply #3 on: August 18, 2019, 10:50:03 PM »
I don't have a copy of the schematic for this amplifier.  So I will bat in the dark here.

Is there a cathode bypass cap on the output tube?  If yes, you replace it?  Ditto the cathode bias resistor if any.  You might also confirm the value and condition of the output filter cap while you are at it.  I got kicked by that horse one time.  If there is no cathode bypass capacitor, the output filter cap serves as cathode bypass.  If its value is low or if it is leaky, it could cause motorboating.  Make sure you have the filter caps wired correctly.  I would sub the output tube against the chance that it is gassy.  Then sub that 12AX7.  They are not as problematic as a 6EU7, but it won't hurt since you are chasing a problem.  Good luck.
I don't need Google.  My wife says she knows everything.

ed from Baltimore

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Re: RCA Victor SHF-8 Amp Motorboating ? Need some Help.
« Reply #4 on: August 18, 2019, 10:54:48 PM »
There is a sot of weird effect that occurs in vacuum tubes at about 105 VDC which is where your 50C5s are right at. There is a sort of internal ionizing discharge, or fluorescing or something, that starts up at that voltage, and is either there or absent. They say that preamp tubes with plate voltages slightly above 105 VDC or so will have it start up. causing a  increase in internal tube noise. If your voltage is right at the threshold to start  up this extra noise,  it might change the current draw from the power supply and oscillate back and forth from noise to no noise. It has been years, and I don't remember if I heard it word of mouth or read it somewhere. That's another reason to look at the tubes I n absolute darkness, looking for purple discharge around the glass itself, not near the grid or cathode. If it has the oscillation, lowering or raising the AC line voltage slightly may get it out of the zone of oscillation back and forth.
      Also, swap the two 50C5 tubes in their sockets. If one is weird in some way, and it's grid goes to the 12AX7 grid pin 7, moving it to the other socket may get rid of the -.2 volts, if that helps.
Definitely a weird problem, especially with new tubes and capacitors, and such a standardized circuit.
          Could the spring mounted record changer be bouncing up and down, when a record isn't being played ?

ed from Baltimore

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Re: RCA Victor SHF-8 Amp Motorboating ? Need some Help.
« Reply #5 on: August 18, 2019, 11:06:13 PM »
Hi, Greg. Looks like our posts crisscrossed. There is a link to the schematic in HFFs first post, or at least there is on my computer screen, though it's not in the summary of posts you get after hitting reply.
  The oscillation being of a low audio level is what puzzles me, it should be pretty loud since oscillations like that usually swing from grid saturation to plate cutoff, in other words, the full peak to peak swing possible with the given power supply voltages. Maybe the push-pull nature of the voltage driver and output circuits is cancelling a lot of the voltage swing and only the differences are getting through. You would still think a record wouldn't sound when played, though.

electra225

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Re: RCA Victor SHF-8 Amp Motorboating ? Need some Help.
« Reply #6 on: August 18, 2019, 11:42:11 PM »
Thanks Ed.  My eyes will be the SECOND thing to go in my old age!   ;) :)

Just for grins, disconnect the cartridge from this amp and try it.  See if it motorboats with no input connected at all.  I'm still suspicious of that 20uf "blue" section in the filter cap.  I believe I'd connect a 20uf electrolytic (temporarily) from the cathode of the 50C5's to wherever the filter caps negative is connected then see what that does.
I don't need Google.  My wife says she knows everything.

HiFiFun

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Re: RCA Victor SHF-8 Amp Motorboating SOLVED !
« Reply #7 on: August 19, 2019, 03:31:16 PM »
Hello everyone,

The problem is identified !

The serendipity discovery related below:

I was starting some more troubleshooting as you all
and others recommended.

I made a minor tweak on a capacitor, and then
turned on the record changer with motor in neutral position.

There was no regular pulsing/oscillation, all quiet
through the speakers.
Then, I engaged the 33.3 rpm speed selector, and BAM !
the pulse/oscillation came back.

I said, what the hell ?! So, I moved the speed to 45 rpm,
and the pulse/oscillation went up in frequency.

I thought, wait a minute, there's one possibility here.
That platter is made of steel, and the paint has worn off
all around the inside circumference, exposing the steel.

To confirm, I found another RCA RP-205 platter that still
has the paint on it. Tried it out, and no pulse/oscillation.

I surmise that the metal going through the motor's electrical
field must be generating enough current/voltage to be picked
up in the audio circuit. I even took out the cartridge, and the
sound was still there with the exposed steel platter.

I have saved all the suggestions for future reference for this
type of troubleshooting. Learned a lot from you guys, and
glad I still have some observational powers left !
Thanks to all,
HFF