Vintage Graphic EQ with static rumble sound

[Studiogearlover]

Hey guys.. a quick advise needed please. I have a vintage graphic EQ which I have recapped. A very rare one so no schematics sadly (Lindsay 7706). Recapped it due to old leaky caps and produced some static rumbling/windy noise, with drop outs at lower volume. It has a 50k pot on it too to increase and decrease the signal. Cut it short, it still has this weird rumble and static noise even when no signal is passing through it, and this static rumble responding to the pot, especially when I turn the pot down to zero db.

I did clean the pot with a contact spray many times, and checked for cold solders and old loose wires...nothing. All looks good. Poked every component with a chop stick, no reaction...

the EQ is full of opamps... LF351Ns and NE5534N..could an op amp do such weird noises and drop outs?

Any thoughts on this would be greatly appreciated!

 

[Khron]

Poking around the opamp outputs with an oscilloscope could at least point out where the noise starts to show up, in the signal path.

 

[JohnRoberts]

Look for a solder bridge... that symptom noise could be from an op amp running at high noise gain.

JR

 

[Studiogearlover]

Thanks guys...JR solder bridge you mean that accidently shorting something after soldering in a component..or an op amp short /burnt out? I did check all the soldering and the op amp sockets, seems to be OK. Switched it on again, to listen to the symptoms again and the same whoosh/ static noise but now I also hear some weird "galloping" pattern....could this be some oscillation? this pattern is repetitive and does not increase or decrease with the volume pot...but the static noise does... could be the pot? Sadly I don't have a scope, i know without it I might be stuck with this...

 

[NOON]

Doesn't sound like a pot, sounds more like an active component misbehaving. If turning the pot down reduces the noise it means the source is before the pot, the other noise sounds like it's after the pot. There's lots of good free audio analysis software available that can be used like an oscilloscope. You'll just need to make a probe that plugs into your audio interface and be careful of higher DC voltages.

 

[Studiogearlover]

Doesn't sound like a pot, sounds more like an active component misbehaving. If turning the pot down reduces the noise it means the source is before the pot, the other noise sounds like it's after the pot. There's lots of good free audio analysis software available that can be used like an oscilloscope. You'll just need to make a probe that plugs into your audio interface and be careful of higher DC voltages.

thanks NOON..will investigate this further... attached a pic of a resistor...the resistor itself is not carbonated however the pcb around it looks like had some heat....is this normal? it did not happened just now, it was always like this since I have this unit (never fully worked since I had it and I dont know the history of this)...

 

[Lampie519]

You could try to heat up (or freeze) some components you suspect and "listen" (best is to use an oscilloscope) to see if it then changes behavior. During this "shock treatment" you will see DC values change like crazy as well in case you hit the defective component(s).

 

[JohnRoberts]

Thanks guys...JR solder bridge you mean that accidently shorting something after soldering in a component..or an op amp short /burnt out? I did check all the soldering and the op amp sockets, seems to be OK. Switched it on again, to listen to the symptoms again and the same whoosh/ static noise but now I also hear some weird "galloping" pattern....could this be some oscillation? this pattern is repetitive and does not increase or decrease with the volume pot...but the static noise does... could be the pot? Sadly I don't have a scope, i know without it I might be stuck with this...

an accidental short to ground at the - input of an op amp can make it operate at full open loop gain. The input noise of an op amp when amplified enough to be audible can sound a little like you describe..... or not.

JR

 

[Ike Zimbel]

That resistor could easily be the series resistor for the pilot lamp. I've seen many older devices with signs of long-term heating on the board that were just that.

 

[Studiogearlover]

an accidental short to ground at the - input of an op amp can make it operate at full open loop gain. The input noise of an op amp when amplified enough to be audible can sound a little like you describe..... or not.

JR

thanks JR...I did some very close checks on one of the 5334... attached...i think that is toasted or at least cracked broken up. I need time to get some as there are 4 of these and 3 of LF351Ns next to them ....maybe all of these are fired. The other problem is I don't know who worked on this unit before and with no schematics, there is no guarantee that these the right op amps and J FETS are in place... need a few days to get new ones tho and report back if any improvement... have a nice weekend to you and to ALL of you!

 

[AnalogPackrat]

thanks NOON..will investigate this further... attached a pic of a resistor...the resistor itself is not carbonated however the pcb around it looks like had some heat....is this normal? it did not happened just now, it was always like this since I have this unit (never fully worked since I had it and I dont know the history of this)...

The green wire directly soldered to the resistor lead on the component side looks like a user mod of some kind. The board either got overheated by poor soldering technique or as a result of high temps of those components during operation. The associated light and dark brown wires may also be involved. Where do these three wires attach at the other end?

Does the 1456 get hot when the power is on? What about the nearby resistors?

Just observing the layout it seems like the row of DIP-8 devices along the board edge (op-amps) are likely input and output related. In particular, the one nearest the burned area has two TO-220 parts adjacent to it. Can you see the markings on them?

It might be worthwhile to try to get a picture of the bottom of the board around the suspect area and then trace out the circuit.

 

[Studiogearlover]

The green wire directly soldered to the resistor lead on the component side looks like a user mod of some kind. The board either got overheated by poor soldering technique or as a result of high temps of those components during operation. The associated light and dark brown wires may also be involved. Where do these three wires attach at the other end?

Does the 1456 get hot when the power is on? What about the nearby resistors?

Just observing the layout it seems like the row of DIP-8 devices along the board edge (op-amps) are likely input and output related. In particular, the one nearest the burned area has two TO-220 parts adjacent to it. Can you see the markings on them?

It might be worthwhile to try to get a picture of the bottom of the board around the suspect area and then trace out the circuit.

Thanks for your help. I am away from the unit now but the nearest TO-220 to the burned area and the 1456 are one LF351N followed by a NE5534AN. Further on this row I have one more LF351n and another 3 of the 5534's and suspecting one or two are burned (above picture of one looks like it in my previous comment for example).

As soon as I am back next weekend, will test this further. Thanks again!

 

[AnalogPackrat]

Thanks for your help. I am away from the unit now but the nearest TO-220 to the burned area and the 1456 are one LF351N followed by a NE5534AN. Further on this row I have one more LF351n and another 3 of the 5534's and suspecting one or two are burned (above picture of one looks like it in my previous comment for example).

As soon as I am back next weekend, will test this further. Thanks again!

The TO-220 components are the black three legged ones with a metal plate on one side with a hole in it and standing up vertically on the board, not the DIP-8 devices in sockets.

Like this:

https://en.m.wikipedia.org/wiki/TO-220
Safe travels.

 

[Studiogearlover]

aaaaa those two next to the tropical fish cap... if I remember correctly to me those looked like just a copper pins in a socket..did not look like semiconductors/ICs but i can be very wrong here.... i did not understand the purpose of those. Much smaller than the TO220 and kind of an oval based shape...

 

[Studiogearlover]

Just observing the layout it seems like the row of DIP-8 devices along the board edge (op-amps) are likely input and output related. In particular, the one nearest the burned area has two TO-220 parts adjacent to it. Can you see the markings on them?

hi. Back to where the EQ is...checked those two TO-220 parts, very difficult to read, I only could make out one of them which is an MC MPS U55 or U95. These transistors tricked me as never seen any this size with golden pins and backplate. I thought these are completely other type components...

Still awaiting for new op amps ordered few of LF351N, MCP1458 and NE5534AN to see if any of these actually burned out (or not?)...

 

[AnalogPackrat]

Have you traced any of the circuit around the burned areas?

 

[Michael Tibes]

The basic circuit of such devices can be pretty simple. Maybe it is similar to this one: 10 Band Graphic Equalizer Circuit | Homemade Circuit Projects
Following that you could separate the individual bands to check whether the main amp is clean. Then reconnect the individual bands and check when the noise sets in. And always expect the unexpected - I've had cases where the silver foil caps caused such noise. Took me a while to figure that out ;-)

Michael

 

[Studiogearlover]

Have you traced any of the circuit around the burned areas?

It seems that the burned area and the resistors there are connected to the two led's or status light bulb and the two switches. One is the on/off and the other one at the bypass switch.

The basic circuit of such devices can be pretty simple.

Thank you... this circuit looks a bit more complicated- at least to me for sure - All i figured out so far is the above, and that the audio signal entering a resistor and the MC1458. That's where all the "fun" begins... I am also not really sure that this is PSU problem or audio signal problem. It does weird things.. sometimes pops with no signal going through it, then the whoosh windy sound, then a mix of very very high pitch of buzz and the mix of all of these.

If I switch the bypass on, all of these weird stuff is still there. When the bypass is off (engaging the EQ) and by turning the gain pot down, these noises are full on too, however, adjusting the gain up these noises are gone for a couple of seconds then fades in again...but if I turn the gain pot fully down to zero, these noises are constant.

Checked the soldering, for any loos connections, poked every component...no reaction. Only when I fiddle with the gain pot. Its frustrating... also knowing that I don't have enough knowledge of it...

 

[Michael Tibes]

Some more suggestions: I've just come across an old GDR equalizer: Vermona E2010. This is the simplest schematic of an EQ I've ever seen. Another classic is the DN27. This one has two gain stages, I assume in order to prevent interaction between the neighbored bands. These two EQs use L-C combinations (coil and capacitor) for the filter. They might help to understand the basic circuit and it's simplicity. Your EQ doesn't use coils but emulates them with a circuit needing an opamp - hence the huge amounts of opamps. I'm pretty sure, that the main signal path of your EQ (without the filters) will be similar to the DN27 schematic. I'd try to disconnect all the bands in order to verify, that the main signal path is clean. if the noise remains, I'd try a different power supply. You could also simply use two 9V batteries for a quick test. +/- 9V should be sufficient for the circuit to work. If the noise still remains, I'd swap out all parts of the remaining circuit of the main signal path. Or maybe do it the other way around, starting with the psu, it is easier to do...

Once the main amp is clean, reconnect the filters and check one by one if things stay clean.

Michael

 

[Studiogearlover]

Some more suggestions: I've just come across an old GDR equalizer: Vermona E2010. This is the simplest schematic of an EQ I've ever seen. Another classic is the DN27. This one has two gain stages, I assume in order to prevent interaction between the neighbored bands. These two EQs use L-C combinations (coil and capacitor) for the filter. They might help to understand the basic circuit and it's simplicity. Your EQ doesn't use coils but emulates them with a circuit needing an opamp - hence the huge amounts of opamps. I'm pretty sure, that the main signal path of your EQ (without the filters) will be similar to the DN27 schematic. I'd try to disconnect all the bands in order to verify, that the main signal path is clean. if the noise remains, I'd try a different power supply. You could also simply use two 9V batteries for a quick test. +/- 9V should be sufficient for the circuit to work. If the noise still remains, I'd swap out all parts of the remaining circuit of the main signal path. Or maybe do it the other way around, starting with the psu, it is easier to do...

Thanks Michael, I will play around with this and ordered an op amp tester just in case to see if any out of the 20+ op amps are faulty (or not...) I need to dig into this deeper. Will come back here later, once again, thanks to all of you for helping!