Realistic 31-2020A 10 band Equalizer HUM problem

[flinx]

I have had this for about 23 years, a few years ago it developed a hum. At the time I was not interested in or had any idea how to fix it. Now I would like to get it working right. I googled for information and found a thread on another forum of someone with the same problem. The last entry from 2010 was that the OP was going to recap the power supply. There are no further entries as to whether this worked or if the person ever got theirs working right.

I recapped the power supply in this unit and there was no improvement in the hum. The hum gets worse if you push in the EQ record, IMX expander, Tape monitor 1 or 2, or tape dubbing 1 or 2, the hum is reduce a small amount by pressing the EQ bypass button. The hum (according to the spectrum display) is centered around 120hz. Moving the sliders for 125hz or around it does not have any effect on the hum. The hum is present whether anything is connected to the input or not.

I hooked up my oscilloscope and did some random measurements. I found that there is either a sign wave present almost everywhere I tested, or a sawtooth wave, with the sawtooth wave being more prevalent.

Zoomable schematic:
31-2020A

Attached is a PDF file of the Schematic.

The scan is the best scan I can get as the one in the back of the book is rather small and detailed.

[lexwalker]

Re: Realistic 31-2020A 10 band Equalizer HUM problem

I have had this for about 23 years, a few years ago it developed a hum. At the time I was not interested in or had any idea how to fix it. Now I would like to get it working right. I googled for information and found a thread on another forum of someone with the same problem. The last entry from 2010 was that the OP was going to recap the power supply. There are no further entries as to whether this worked or if the person ever got theirs working right.

I recapped the power supply in this unit and there was no improvement in the hum. The hum gets worse if you push in the EQ record, IMX expander, Tape monitor 1 or 2, or tape dubbing 1 or 2, the hum is reduce a small amount by pressing the EQ bypass button. The hum (according to the spectrum display) is centered around 120hz. Moving the sliders for 125hz or around it does not have any effect on the hum. The hum is present whether anything is connected to the input or not.

I hooked up my oscilloscope and did some random measurements. I found that there is either a sign wave present almost everywhere I tested, or a sawtooth wave, with the sawtooth wave being more prevalent.

Zoomable schematic:
31-2020A

Attached is a PDF file of the Schematic.

The scan is the best scan I can get as the one in the back of the book is rather small and detailed.

Well, the hum could have originated from a broken common GND/Earth line as well (since the hum is around the multiple of the mains frequency). You may want to look for signs of solder cracks (cold solder joints) and internal broken wires. Also what were the brand/model/series capacitors used for replacing the original ones?

[Longbow]

Re: Realistic 31-2020A 10 band Equalizer HUM problem

Where exactly is the hum? At the input to the regulators, at the output of the regulators? Not on the power rails at all??
.....

[Longbow]

Re: Realistic 31-2020A 10 band Equalizer HUM problem

what were the brand/model/series capacitors used for replacing the original ones?

I think he's safe to assume that power supply capacitors are not causing the hum, especially if the symptom did not change. And more especially since this unit uses a non-switching power supply which does not make extreme demands upon the quality of the filter capacitors.

[budm]

Re: Realistic 31-2020A 10 band Equalizer HUM problem

120 Hz (twice the line frequency) is what you get as the AC ripple on the DC out put after the full wave rectifier circuit.
So look at the AC ripple at the filter caps. You may also have ground loop, bad grounding of the circuit, etc. Make sure to tighten up all the board mounting screws since it may use as circuit grounding.

[flinx]

Re: Realistic 31-2020A 10 band Equalizer HUM problem

here are the measurements of the transistors in the power supply and diode information:

zener d504 gza15(z) rated min 14.85vdc max 15.60vdc
q505 d288 y-923
e 14.95vdc
b 15.52vdc
c 21.74vdc

zener d503 qza22(x) rated min 20.8vdc max 21.8vdc
q504 a614 y-922
e 21.56vdc
c -24.54vdc
b -21.27vdc

zener d502 qza22(x) rated min 20.8vdc max 21.8vdc
q503 d288 y-923
e 20.68vdc
c 24.43vdc
b 21.27vdc

I checked both resistors r511 and r512 at the request of someone (unsoldered one leg) and they tested about 4.4k. I resoldered them and then checked all the power supply soldering. I powered it up and noted that the display did not show a hum, but the power led was dim. I was about to wonder what was going on when I noticed the tiny wisp of smoke coming off the power supply. I shut it off and found that the resistors were hot. I double checked the soldering again and verified all capacitors are installed correctly. I powered it up again and got the same results. something is shorted now. I am going to go over it again here in a few minutes.

[flinx]

Re: Realistic 31-2020A 10 band Equalizer HUM problem

I found that when I had resoldered one point, from the angle I was sitting I did not see that 2 pads that are not joined got joined.

I fixed that and powered it up with no more smoke, but still a hum. I checked the 2 resistors again and they were now measuring higher, so I replaced them. powered it upain, and... The display maxed out and there was a clicking sound. NOW WHAT.

Powered it off and on again and no hum, but no display... The unit passes music, but IMX expander sounds distorted when enabled.

Now the voltage coming off of the transformer at C516 is only about 1.3vac

While pondering this I noted that I could smell transformer, and went to touch it. It was really hot 140°f.

sigh™ I do not know why I even bother working on transistorized stuff.

Just going to leave it alone for today.

[ben7]

Re: Realistic 31-2020A 10 band Equalizer HUM problem

I found that when I had resoldered one point, from the angle I was sitting I did not see that 2 pads that are not joined got joined.

I fixed that and powered it up with no more smoke, but still a hum. I checked the 2 resistors again and they were now measuring higher, so I replaced them. powered it upain, and... The display maxed out and there was a clicking sound. NOW WHAT.

Powered it off and on again and no hum, but no display... The unit passes music, but IMX expander sounds distorted when enabled.

Now the voltage coming off of the transformer at C516 is only about 1.3vac

While pondering this I noted that I could smell transformer, and went to touch it. It was really hot 140°f.

sigh™ I do not know why I even bother working on transistorized stuff.

Just going to leave it alone for today.

Could be that there is a short, overloading the transformer.
Perhaps you have a bad rectifier diode, or a leaky diode. (leaky electrically, not physically!)

[Longbow]

Re: Realistic 31-2020A 10 band Equalizer HUM problem

Now the voltage coming off of the transformer at C516 is only about 1.3vac

That would be a problem. Desolder either pin 4 or 5 of the transformer and check the secondary. If OK, find the short in the diode bridge or in the regulator. Q505 is responsible for the operation of the display among other things.

I do not know why I even bother working on transistorized stuff.

You mean you prefer to work on tube gear?

[flinx]

Re: Realistic 31-2020A 10 band Equalizer HUM problem

That would be a problem. Desolder either pin 4 or 5 of the transformer and check the secondary. If OK, find the short in the diode bridge or in the regulator. Q505 is responsible for the operation of the display among other things.You mean you prefer to work on tube gear?

Yes, so far tube gear is more forgiving.

Went to a hamfest today and what did I find? The Optimus version of this same EQ. It has one issue that I will not mess with, the 16khz led display does not work, but the 16khz channel does. So in to the parts bin goes my old one. I may try to work on it later.

[Longbow]

Re: Realistic 31-2020A 10 band Equalizer HUM problem

So in to the parts bin goes my old one. I may try to work on it later.

Now that's the spirit!

[flinx]

Re: Realistic 31-2020A 10 band Equalizer HUM problem

Now that's the spirit!

I suck... I admit it.

I will most likely work on it again, but not this week.