Author Topic: 5-pin preamp 015245 015244 Vibrato issues  (Read 348 times)

Offline Krischu

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5-pin preamp 015245 015244 Vibrato issues
« on: February 14, 2020, 12:07:23 PM »
I found that the amp (coming from an 88 suitcase) I have here for inspection shows some unwanted effects with vibrato.

I found that the output signal is still changing volume also when the vibrato switch is in the off-position.
When vibrato is "on", the amplitude change isn't really smooth, it's kind of jumping.

Is this normal? Could the LED-Photoresistor units be aged?

Offline sean

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Re: 5-pin preamp 015245 015244 Vibrato issues
« Reply #1 on: February 14, 2020, 11:01:37 PM »

Christoph,

The stereo tremolo is not a gradual swell from side to side, it is a surge in volume that switches back and forth (with some subtlety provided by the intensity knob).

I don't know if I can add much except common sense help with fixing the vibrato issues, but if I had this beast on my bench, the first thing I would do is to verify that the power supply portion of the circuit was healthy (by replacing C12, C13, C14, and C15 with fresh new electrolytics). 

I suspect that since you are indeed getting vibrato that the LDRs are fine.  They are expensive to replace, so they should be your last resort.

I feel that if you are still getting tremolo when the vibrato switch is turned off, the circuit must be passing the oscillator output or current load through the power supply.  New C12, C13, C14, and C15 might resolve that. 

Then I would check to see if the oscillator output at pin 7 of the 1458 was a symmetrical and stable squarewave.  Then I would turn on the vibrato switch, set the speed slow and hope to find bad behavior at the LDRs - but I doubt that I could measure the LDR resistance while they are still in circuit.  Maybe it would be easier if we completely stop the oscillator (I would try this by grounding pin 6 of the 1458, or maybe simply switching the vibrato off, and tying the junction of the switch and R26 to the positive or negative supply).  You should be able to tell if each side is working by ear, maybe.  I guess I would feed the preamp with a test tone, and watch the left and right output with a two channel scope (I love my PicoScope 2204A). 

Anyone that has actually had a Janus preamp with sliders on their bench care to speak up?

Sean
« Last Edit: February 14, 2020, 11:04:47 PM by sean »

Offline Krischu

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Re: 5-pin preamp 015245 015244 Vibrato issues
« Reply #2 on: February 15, 2020, 02:05:25 AM »

Christoph,

The stereo tremolo is not a gradual swell from side to side, it is a surge in volume that switches back and forth (with some subtlety provided by the intensity knob).

...

Anyone that has actually had a Janus preamp with sliders on their bench care to speak up?

Sean


Just want to mention: the Vibrato *is* anti-phase, thus providing a panning effect (you may  call it stereo)

In the video I've uploaded one can see the antiphase vibrato as well as the problem. It starts with vibrato off, inmidst I switch to vibrato and after that I switch it off again.

« Last Edit: February 15, 2020, 03:13:41 AM by Krischu »

Offline Krischu

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Re: 5-pin preamp 015245 015244 Vibrato issues
« Reply #3 on: February 15, 2020, 03:59:24 AM »
Ah, I understand. Your stress laid on "not smooth". Yes, that's due to the rectangular character of the vibrato voltage, a bit smoothed at the edges.

I noticed that my amplifier is REV A and the circuit diagram in the Fender Rhodes manual doesn't match the PCB layout there. R23 is 1K, R39 not present in the schematics.

--
Christoph

Offline Krischu

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Re: 5-pin preamp 015245 015244 Vibrato issues
« Reply #4 on: February 15, 2020, 04:00:33 AM »
Foto posted.

Sean, I now saw that you had mentioned the newer (reviewed) schematics in your other post .

That one fits my PCB layout (REV A).
« Last Edit: February 15, 2020, 04:21:46 AM by Krischu »

Offline sean

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Re: 5-pin preamp 015245 015244 Vibrato issues
« Reply #5 on: February 15, 2020, 07:15:54 PM »



AAACK!  I got confused and I was focused on the other schematic.  Let me scratch my head and re-think my hunches.  Crud.  I think I have to edit my previous post about how to stop the oscillator.  (Because the switch is on the other side of R26.)

Give me a bit...

Sean

Offline Krischu

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Re: 5-pin preamp 015245 015244 Vibrato issues
« Reply #6 on: February 16, 2020, 05:42:57 AM »
My power supply is a lab supply at the moment so this is definitely not the cause. With the 100 Ohm resistors is it quite normal that there is a drop in PS voltage but this drop is being suppressed by the opamps anyway due to their PSRR.

One can easily see that with S1 to GND, die LDRs are still steered thus leading to the still noticeable volume change.

It is probably possible to shut down vibrato totally by turning speed to 0Hz.

Another circuit change would be to use a dual switch and cut the wire after R39. This would mute the LDRs totally.

But where does one find such a nice Knitter switch with that flat shaped lever?


« Last Edit: February 16, 2020, 10:39:46 AM by Krischu »

Offline sean

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Re: 5-pin preamp 015245 015244 Vibrato issues
« Reply #7 on: February 17, 2020, 11:15:41 PM »


Pick your favorite electronics supplier, Mouser, Digikey, Arrow, Allied, Avnet, Farnell, Newark/Element14, whoever...

Take a look at anything like:
NKK   M2022ES1W01    https://www.digikey.com/product-detail/en/nkk-switches/M2022ES1W01/360-1832-ND/1006911

C&K  7201P3HZQE    https://www.digikey.com/product-detail/en/c-k/7201P3HZQE/CKN9732-ND/2055010

NCE  8C2012-Z    https://www.digikey.com/product-detail/en/nidec-copal-electronics/8C2012-Z/563-1665-ND/5086630

E-Switch  100DP1T6B11M1QEH    https://www.digikey.com/product-detail/en/e-switch/100DP1T6B11M1QEH/EG2408-ND/378877


Digikey's website does a good job on the photos of the switches, so it isn't too have to scan through them to find one with a paddle instead of the round baseball-bat.


Sean

Offline Krischu

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Re: 5-pin preamp 015245 015244 Vibrato issues
« Reply #8 on: February 18, 2020, 01:33:16 AM »
Thanks, Sean, for the support in finding the device. Digi-Key do a good job in supplying comprehensive information, datasheets etc., albeit awkward and expensive when it comes to buying single quantities.

Besides that, do others observe this subtle influence on volume with vibrato switched off? And what do you think of replacing the vibrato shut off circuit by a double switch, one switch for the indicator LED, the other for cutting off the steering voltage to the Vactrol cell.

« Last Edit: February 19, 2020, 02:36:15 AM by Krischu »