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Messages - Will

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Hi 1way2life,
Quote from: 1way2life
"shunt the base of Q1 to ground when the vibrato is off"
I drew two schematics to think about it. Which one is suitable to you ?
The second one uses a DPDT switch.

Hi 1way2life,
Thanks for your reply.

Which wiring did you set up ? Out of phase (p.5)   or   in phase (p.3) as referred to in the vv document ?

I checked the rail without preamp (+14.58V  -14.58V) and with (+16V   -11V). Q1 tested (P>N 0.6V) and nothing wrong. A topic is opened at

"shunt the base of Q1 to ground when the vibrato is off" Which preamp version do you have, as referred to by sean ?

I have the same vibrato issue. I checked my speakers and they have been changed too to some 8 ohm speakers by previous owner, obviously. The original parallel wiring was kept too giving 3.5 ohm for the PA output. I changed to series config, to no avail to my issue. Still investigating.

By the way, I found this vv document which is getting me confused. In page 4, I don't understand why Janus 8 ohm speakers should be kept in parallel. The page 5 series and phase invert config seems best to me. Which I chose.

Preamps, Modifications & Upgrades / Rhodes MK I Janus preamp upgrade
« on: June 29, 2020, 01:45:29 PM »

I have the early version of Janus preamp ref. PART NO.013487 which I intend to upgrade. You can find the schematic diagram here.
These are my thoughts and questions about its upgrade which I submit to your comments.
You can find the part list here to follow the subject.

The LM1458 OpAmp on signal path can be upgraded. I have a pair of Burr Brown OPA2134PA which I intend to use. They have the same pinout config. And add their package.

The electrolytic capacitors need replacement after more than 40 years of services.  I will choose Panasonic (FC series), Nichicon (KA, FG, KW), ELNA (not Silmic II)... The relevant capacitors are C8 C20 C12 C13 C14 C15 C3  and non-polarised C4 C6 C16.

Black and round shaped capacitors seem to be polypropylic. If that is the case, they are good quality caps and age well so they must be kept in place, right ? Those are C11 C1 C9 C5 C7 C10.

Small and flat caps are ceramic. From what I know, they are not suitable on audio signal path. On the schematic, they seem to help stabilise the OpAmp (C2) and to suppress unwanted high frequencies (C17 C18 C19). Am I right ? Are they worth to be changed ? For what type ?

I don't know the quality of the present resistors and I am wondering if a change to modern low noise metal oxyde resistors would be beneficial. I have an assortment of them.

The transistor and diodes are not in the signal path and don't need to be changed, unless faulty. Same for the vibrato speed pot.

Linear bass and treble pots and vibrato intensity pot are on the signal path. I don't plan to change them as they make no crack.

Quote from: sean
With the preamp disconnected, what do the power feeds coming from the amp cab measure?
I get +14.58V and -14.58V. The drop in tension is coming from the preamp indeed. I also removed the A2 oscillator IC from its socket and I still have this -11V instead of -15V. So the OpAmp is not faulty.

Quote from: sean
check (...) continuity of the ground terminals
I checked all the bits with the multimeter and it is showing continuity.

Quote from: sean
Check that the 100%u03A9 current-limiting resistors are still in good shape (...) and close to 100%u03A9.
Yes to both, they are all fine.

Quote from: sean
take a good look at the PC board where the switch mounts.
I did and found nothing. However, I am suspicious about the switch, instinctively.

My to-do list :
- Draw up a components list to upgrade the preamp board (I will make a post very soon about this) and order the components
- Remove the components form the board. Remove the solder with a pump. Clean the board with care.
- Populate the board with new components and wires. Clean the flux from board. Look for any bridge between traces.
- Let you know...

Hi Sean,
I carefully read your post. I did few things this evening but I will do more this week-end. Some answers however :

A thick piece of plastic is already in place, stuck on the enclosure below the PCB. I don't know if it is from factory or if my friend put it in place but it is done very carefully.

The strips had some oxyde dust on them. They had some flux in some place too. So I cleaned them this evening. They need some more cleaning so I will start again this week-end. A leakage may occur here.

Switch case test still to be done.

The oscillator is actually running. I don't have a scope but at low frequency, my multimeter shows the voltage output swinging from + to - at OpAmp output pin 7. At top speed, the frequency-meter shows 15 Hz.

Removing the "vibrato indicator" LED causes the symptoms to occur, even if the switch is off.

I made further tests with interesting results :
When the vibrato is off :
...The positive power supply rail measurement is +16V (+15V expected from datasheet). The negative rail measurement is -11V (-15V expected from datasheet).

When the vibrato is on (at low frequency) :
...and when the oscillator output at OpAmp pin 7 is hight, the rails remain the same as above (+16V and -11V). The "vibrato indicator" led and the "pilote indicator" led are fully lit. The sound is normal.
...and when the oscillator output at OpAmp pin 7 is low, the power rails change to +26V and -0V. The sound becomes weak.  The "vibrato indicator" led is off which is normal. The "pilote indicator" led weakens too which is unusual. The reason is there is no more potential difference on its poles, between negative rail and ground.

There may be a ground reference issue because the + and - rail difference remain the same. Maybe an OpAmp failure...?

To be continued.

Hi Sean,
Thanks for your answer. The switch is connected to pin 7 of OpAmp IC A2 on one pole and to R26 resistor on the other pole so I deduce the Fig11-1 applies.
The board reference is
PART NO.013487
ASSY NO.013488
so yes, 013...

This is my first post.
I am the lucky owner of a 73 suitcase Rhodes Mark I piano ref. 3077 for two years now. I am a regular reader of this forum ever since and I would like to thank the members of this community for their contribution. I would like to pay tribute to my friend Pierre who offered me to be the second owner of his piano, and who passed away a few months ago. I have been confident enough to restore maintain and adjust my piano so far. But I now have an issue that I cannot fix by myself, which you may have gone through.
On the Janus preamp, I have a drop in power when I switch the vibrato on. The sound fades out and the LEDs light weakens. The switch seems to engage a short circuit. The power comes back to normal when the switch is turned back off. I don't make this test often as I would not like to cause a power supply failure.

I removed the preamp, opened the enclosure and I see nothing wrong. No smelly or burnt component. I tested the transistor and the diodes with a multimeter and they are fine. I sought a short circuit between +15V -15v GND and switch when the switch is on with no results.
One interesting fact is this phenomenon occurs also when I unplug the vibrato indicator led.

I could not find a topic about this. If you know of one please show me the link. Otherwise any advice or solution welcome.

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