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 Post subject: Rebuilding RCA Orthophonic SHF-5. Hum after recap.
PostPosted: Oct Mon 21, 2019 3:26 am 
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Joined: Oct Mon 21, 2019 2:54 am
Posts: 4
So I picked up an SHF-5 to rebuild. This is my first ever tube amp restoration, so I basically know nothing. I'm even having a hard time reading the schematics. Anyhow, when I bought the unit, the guy plugged it in and turned it on. (which I didn't want him to do) anyhow, I looked to see the tubes start to glow, rubbed my finger on the needle enough to get some scratches and quickly powered it off.

I took the amp out and did a full recap using polyester film caps for the old wax caps, and cut the old filter cans with a dremel and soldered in new electrolytics, cleaned out the cans and placed them over the new caps and used some aluminum tape around them so it looks more original. (basically I restuffed the old cans)

Anyhow, after the recap I rebuilt the turntable. Today was the day it all went back together. Turned the unit on, and after about 10 seconds I notice a low pitched hum I am assuming is 60hz. There is no hum with volume at 0 and gets louder as the volume control increases.
When playing a record, the hum disappears until the needle lifts off the record, then the hum returns after 2 seconds. The unit has a couple other inputs. Tape, Tuner (which I don't have yet) and Stereo. I found that the stereo input (which is actually mono) doesn't pass through the amp at all, an input there doesn't increase volume with volume control.
I switched the input to Tape and get the same hum. I connected my laptop to this input and as soon as I do, the hum goes away. I can play music through the laptop and through the amplifier with no hum, but immediately if I disconnect the input the hum comes back. Same thing with the tuner input.

Anyone have any ideas as to what this could be or how to troubleshoot it?

Thanks!
-J


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 Post subject: Re: Rebuilding RCA Orthophonic SHF-5. Hum after recap.
PostPosted: Oct Tue 22, 2019 3:53 am 
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Joined: Jan Wed 16, 2013 12:04 am
Posts: 1616
Location: 77001
Welcome to the Phono Forum.

1. I am attaching a schematic jpg. It may not have the best
resolution due to limitations of file size here.
SHF-5 found in Sams Photofact 387-13.

2. As the hum is volume dependent, source is indicated in
that part of the circuit prior to the volume pot.

3. The reason that the hum goes away for a short time after
the tonearm lifts up, is that the record changer has a mute
switch. When the tonearm lifts at the end of the record,
the mute is engaged for tonearm travel back to tonearm rest.

4. I'll await some other members' thoughts on the troubleshooting
for you.
Hotwax


Attachments:
RCA SHF-5.jpg
RCA SHF-5.jpg [ 230.07 KiB | Viewed 678 times ]
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 Post subject: Re: Rebuilding RCA Orthophonic SHF-5. Hum after recap.
PostPosted: Oct Tue 22, 2019 5:03 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2667
Location: Haledon NJ USA
Is there any change if you reverse the AC plug in the wall?


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 Post subject: Re: Rebuilding RCA Orthophonic SHF-5. Hum after recap.
PostPosted: Oct Tue 22, 2019 7:39 am 
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Joined: Oct Mon 21, 2019 2:54 am
Posts: 4
No change if the plug is reversed. I tried grounding the chassis to outlet ground, no change there either.

So I put a 10Kohm resistor between the input's and ground and the hum goes away. (this explains why when my laptop is plugged in the hum isn't there) Can't do that permanently obviously.

I also pulled V1, the 6CG7 tube which is the preamp tube and the hum goes away. so I assume the hum is from the preamp side or the 215V or 235V power supply.
Then I swapped the preamp and phase inv tubes (both 6CG7's) and the hum is the same. So I think that probably means the tubes are not the problem.


Last edited by miatawnt2b on Oct Mon 28, 2019 3:15 pm, edited 1 time in total.

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 Post subject: Re: Rebuilding RCA Orthophonic SHF-5. Hum after recap.
PostPosted: Oct Fri 25, 2019 12:35 am 
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Joined: Aug Sat 27, 2011 1:59 am
Posts: 169
Location: Kansas City area
Does the hum go away when you unplug the changer from the amp? (The cartridge signal line. Not necessarily the AC.)


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 Post subject: Re: Rebuilding RCA Orthophonic SHF-5. Hum after recap.
PostPosted: Oct Mon 28, 2019 3:17 pm 
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Joined: Oct Mon 21, 2019 2:54 am
Posts: 4
@Blast
No, the hum is still there with the changer completely unplugged (both signal as well as it's A/C)


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 Post subject: Re: Rebuilding RCA Orthophonic SHF-5. Hum after recap.
PostPosted: Oct Tue 29, 2019 1:02 am 
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Joined: Aug Sat 27, 2011 1:59 am
Posts: 169
Location: Kansas City area
Let's have some fun with meter's and data. Fun, because scientific it WON'T be.

I've got a RCA SHF-4. So, same console, mine has a tuner, yours does not. Probably the same amp (RS-151B), same changer. Mine totally recapped in 2011. Cartridge is a replacement stereo Pfanstiehl wired for mono but I've having trouble remembering which model I put in it. Existing turntable wiring (A/C and cartridge).

The only scientific way I can think of to compare hum is to use a sound level meter. I have a Realistic brand sound level meter from the 80's.

At ambient room noise, console turned off, I read no higher than 58dB with the sound level meter placed directly at the front of one of the 12" woofers of the SHF-4.

Under the following conditions:
    SHF-4 on
    Input set to PH (phono)
    RCA plug from the changer unplugged (cartridge disconnected, no music playing)
    BASS control to full
    TREBLE control to full
    VOLUME control to full

I hear both a 60Hz hum and a 120Hz hum/buzz. As I probe/move the microphone around for the loudest position in front of one of the woofers I can obtain 74dB on the meter. Difference: 16dB louder.

With 74dB showing on the meter I plug the changer into the SHF-4 amplifier the sound level meter drops to 64dB (down 10dB).

Now I turn on the changer motor. Hum increases. We're back up to about 72dB.

I cycle the changer. The mute switch kicks in. The volume goes up 2dB due to some extra 60Hz hum 74dB. That needs to be addressed.

So, not really much difference between changer muted or unmuted.

To continue... without touching any controls and leaving the mic in exactly the same position I play a record on the changer and see 114dB on the meter!


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 Post subject: Re: Rebuilding RCA Orthophonic SHF-5. Hum after recap.
PostPosted: Oct Tue 29, 2019 5:52 pm 
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Joined: Oct Mon 21, 2019 2:54 am
Posts: 4
Yea, I can tell you for certain that my hum is significantly louder. When the mute switch activates the hum completely disappears. If I were to turn volume bass and treble all the way up the hum would be extremely loud. So loud you'd see the woofers moving.

I'm certain I have a ground loop issue, I just don't know where to start looking, nor how to troubleshoot to figure out where it's coming from.


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 Post subject: Re: Rebuilding RCA Orthophonic SHF-5. Hum after recap.
PostPosted: Oct Tue 29, 2019 6:51 pm 
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Joined: Jan Wed 16, 2013 12:04 am
Posts: 1616
Location: 77001
1. Have you checked resistors' for shorts/out of spec ?
Do the checks with amp powered off.
2. Check for bad solder joints with a wooden chopstick
3. Try gently twisting the audio preamp tubes in socket
to see if any change or noise, indicating oxidized socket
or bad solder points.
4. Have you checked tube voltages ?

Use electrical high voltage personal safety rules for any checks on chassis when powered up.


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