Neptune mixer model 611 - Works for a few minutes then fades out and distorts

[Zonzi]

Hi everyone, sorry about the delay, I was busy these last few days.

So, I got under the board and tested the pots in circuit. They read approx. their value, which is indicated to be 125k. They seem to work as intended, no broken track, otherwise they'd read open circuit after a certain point, right ?

Also I don't seem to be able to find any info on these... They're 125k pots, and that appears to be a non-standard value, given that I can only find some on ebay for like 15€ each ?

The writings on them are similar for all level pots : LM4245A 125k 1377849 (EDIT : I've found that they're CTS pots made in 78 after their code, but still no idea if they're linear or log, and 125k still seems impossible to find nowadays)

Now google and bing give me NO results at all for these numbers ! Not even a single page. So I don't know what's up, but I guess they don't make these anymore (whoever did at the time) ?


Then what should I do to test them ? Take one out of circuit ? I don't have any means of replacing them with same value pots, and if I have to replace them all I have to make sure the covers fit...

I'd be very grateful if anyone who knows what's going on could shed some light for me !

[Zonzi]

Sorry for posting several times in a row, but I've found some disturbing stuff on the board, that makes me question if the job was done properly by the previous owner.

Around the power supply and the first IC (1458) there are resistors that don't match at all the ones marked on the board.


The power supply is supposed to have two 100Ω resistors, one for each polarity, but the ones in place are 47Ω.

Around the 1458 chip, ALL resistors have wrong values. I'll first mark the value the board says, then the actual value in place right now.


100Ω -> 15Ω
56kΩ -> 47kΩ
10kΩ -> 1kΩ
39kΩ -> 3.9kΩ
1.5kΩ -> 15kΩ

There's almost a pattern with a factor of 10 but it's not consistent on all resistors. This confuses me. Could it be the reason one IC would malfunction ?

Or, second idea, could it be that the transformer was never switched for a proper 240VAC to 15VAC, and therefore the previous owner switched the resistors to accomodate for the overvoltage ?


Should I test the transformer with the leads disconnected from the board ? I'm not sure if I feel confident enough to work with mains with my current ability. Wouldn't want to take unnecessary risks.


Anyways let me know what you think

[Zonzi]

Hi everyone, it's been a while but I've finally taken the time to measure the transformer output.

So I'm happy to tell you it HAD BEEN switched-out to adapt to european power : when plugged on 240V it reads 20V to ground, 40V positive to negative. It's not ideal since it should have been 15V to ground, or 30V positive to negative, as specified on the opamps, but at least I haven't been running it at twice the recommended voltage. That's good to know.


The last question for me before putting everything back together, is regarding the resistors I mentioned in the previous post.

It appears the two resistors on the Power Supply side of the board only drop a voltage when the unit is plugged in, but switched off. Switching it on makes the voltage drop disappear. Also they drop 7.5V which I'm having trouble to wrap my head around.

They're 47Ω resistors but the board expected 100Ω resistors. Does this halving help in any way in converting the unit from 120V to 240V ? If it's only a load resistor to bleed the capacitors, what's the point of changing their value ?


I'm quite lost, for sure I'm lacking experience... If someone was kind enough to chime in I'd be very grateful.


Have a nice weekend everyone, cheers

[Zonzi]

Welp, as a conclusion for now, I've switched all the resistors going to and coming from the 1458 opamp and it now works. Only the first two channels crap out when going past noon, sound disappears, but that'll be a subject for another post. All the others work well, reverb is nice and dark and I like it a lot overall.

Thank you to those who shared a bit of their knowledge with me ! I'll mark this solved as the primary issue is fixed.

EDIT :I can't find how to mark as solved lol

[eccerr0r]

20V dual rail (40V total) is a bit too high for the LM1458 and probably others on the board, and likely why it's causing problems. Usually these op amps are specced to no more than 16v dual rail. going past this will cause damage or at least unexpected behavior due to heating.

[Zonzi]

Hi eccerr0r, sorry for the late answer, I moved to another place in the meantime. Would a good solution be finding a more adapted transformer ? Or maybe trying to add a homemade voltage divider to the power rails ? I'm alright with these, it's just that finding just the type of transformer I'm looking for, has proved rather difficult in past times ! If you know foolproof ways to find some, I'm willing to give it another try

[eccerr0r]

what power supply does it have, is it just transformer bridgerectifier filtercap, and no regulator, and it fully depends on PSRR of op amps?

Photo of the power supply section?

[Zonzi]

It's exactly that. No regulator that I can see. The pic is from before I switched the filtering caps that were corroded, but the new ones are basically the same.

[stj]

thats a bit shit,
do you know the current draw?
maybe a circuit with 7815 and 7915 regulators would be good.

[Zonzi]

Actually I don't ! I don't have a controllable psu but I can maybe do a current reading in series with one of the caps or something. Problem is I don't really know where would be a sensible place to check, and if I have to be playing music through the unit at the same time ?

If I were to add regulators, how would that go ? In series with the bridge rectifier ?

[stj]

regulators would go after the main smoothing caps

[redwire]

I would just add a DC 18-35V LM7815 + LM7915 + - 15V Dual Power Supply Voltage Regulator Module Rectification Bridge Power Supply Module $5.
Another SMT version https://www.aliexpress.com/item/1005001699436317.html
Worst case the regulators run too hot and need heatsinking to the enclosure.

The light blue electrolytic capacitors (Matsushita?) in 80's gear I'm finding are drying out, so I replace them.

[eccerr0r]

since it's using 1n4002's it's probably less than 1A and 7815/7915s will be fine, but likely you will likely be puking up to 2 watts so the heatsinks would be good, the onboard ones may be sufficient.
Do note that these boards suggested above have the rectifier on them so they technically replace the whole board you have minus keeping the transformer - but woefully underspecced in terms of filter capacitance. As they have only 100µF-220µF depending on which one you get, you'll need to increase to at least 470µF-1mF. Likely to get similar PSRR as before, match the 2.2mF that you have now.

[Zonzi]

Originally Posted by redwire

The light blue electrolytic capacitors (Matsushita?) in 80's gear I'm finding are drying out, so I replace them.

Oh God I sure hope they're not, there are like 120 of these exact ones on the board lol...


But anyway thanks everyone for your input, this is all really interesting. I'm leaning towards buying individual regulators with an added heatsink, so that I can just add it to my power board without switching it out. Plus I can get them at my local shop, which is the last one around here.

Just to be clear, that would go after the main filter caps, one to V+, one to V-, and they'll deliver clean, stable 15V DC to the rest of the board ?
That sounds very simple and I don't see why it wasn't built like that to begin with. Maybe regulators didn't exist in the 70s ?

[stj]

the regulators have 3 wires, input - ground and output
you connect ground to ground, the input to your existing power board,
and the output is obvious - BUT.
you need an extra capacitor or 2 between the ground and the output pin.
also because your going to use a pair, the datasheet recommends a 1A diode like a 1N4007 between the ground and output to protect it from reverse current.
check the examples in the datasheet.
specially page25 - 15v twin rail circuit!

[eccerr0r]

The regulators are not technically needed if the voltage is steady enough - and would be additional cost to add them and heatsinks.

And do mind insulating the heatsinks especially if you're going to attach them to the case. The 7915's heatsink tab is NOT ground and needs to be isolated, else you'll short the input to ground.

[stj]

i dont see any fuses?

[eccerr0r]

Looks like a chassis line fuse to the right of the picture?

[Zonzi]

You're right that's indeed a fuse on the right. Does than change anything ? I just realized... I can know the max. current draw just by looking at it. Sorry about this folks lol. It says T50mA 250V so guess it's a 50mA fuse.

Originally Posted by stj

you need an extra capacitor or 2 between the ground and the output pin.
also because your going to use a pair, the datasheet recommends a 1A diode like a 1N4007 between the ground and output to protect it from reverse current.
check the examples in the datasheet.
specially page25 - 15v twin rail circuit!

That's good to know. I also noted the fact that the tab is not gnd, thanks.

What are the caps for in this scenario ? To block DC from one leg to the other ? Or is there some other trick I don't know about ?


Thanks for all your continued help by the way, it is much appreciated.

EDIT :

Originally Posted by eccerr0r

The regulators are not technically needed if the voltage is steady enough - and would be additional cost to add them and heatsinks.

Why aren't they needed ? Wouldn't they be a good solution to my over voltage problem ? That is, without replacing the transformer ofc

[eccerr0r]

The premise is that 2x input voltage of a transformer = 2x output voltage.
At 2x output voltage you will dissipate significantly more power since power is related to the square of the voltage.

Most of the time when an older device is used for both 220 and 120VAC the transformer is modded to allow both, usually using a double winding primary. This way when switching between the two, you don't burn power. The caveat is that it isn't automatic and if you forget, you fry things or things won't work.

Today there are switching power supplies that can take either and automatically get the voltages you need without wasting the power.