PSU and amp module update

Started by Will, January 14, 2022, 03:11:48 AM

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A few months ago my piano amp was not working at all. I assumed the power supply was faulty. I ordered parts to restore the power supply and amp modules and made the change. It is working fine now. Here is a report of what I did, for the record.

My piano is a 73 suitcase Rhodes Mark I piano ref. 3077. No upgrades have been done on this power supply or on the amplifier modules so far. It was time to change all the electrolytic capacitors. I also changed the twisted wires to suitable 0.5ohm resistors. I did not change the transistors.

You will find a picture of the finished product here. Zoom in for details :

The part list. At the end of each line, what I ended up choosing.

Amp   capacitor   Electrolytic   C1   4,7uF / 25V NP       AXIAL    >>> Mundorf BG50 4.7uF 75V      
Amp   capacitor   Electrolytic   C2   1uF / 16V NP          AXIAL   >>> Mundorf BG50 1uF 75V   
Amp   resistor   Wirewound   R18-R19 R20-R21   0,47 ohm  -  5W      >>> Royal 0,47 ohm 5W
PSU   capacitor   Electrolytic   C1 C2   1000uF – 35V - 60 mm AXIAL   >>> Jamicon 2200uF 35V
PSU   capacitor   Electrolytic   C3 C4   500uF  -  15V - 40 mm AXIAL   >>> Vishay BC-021 470uF 25V
PSU   capacitor   Electrolytic   C1 C2   11000uF – 40V – diam 50 mm    >>> F+T GMB 33000uF 40V

It costed me 52 euros inc. VAT + 8 euros delivery = 60 euros in total ie. US$ 69 in jan 2022.

The Vishay 1000 uF were not available at the time of purchase so I ended up putting Jamicon spare cap I had from a PSU kit. Not the best choice. I will change them next time.
I left a gap between the resistors and the PCB when I soldered them. Those components are likely the get hot so they need space to dissipate heat.
I soldered an insulated terminal to the wires connected to the 33 000 uF capacitors. They have bolts.
I cleaned the PCB with 95% alcohol, a toothbrush and a tissue to remove flux.
The amp +/- rail shows 28.6V and the preamp rail shows +/- 14.6 V. Good !

Feel free to ask if you need more details.


Commander Fluffypants



Some comments on the choice of components:

I looked for the same value in capacitance (F) and operating voltage (V) on my supplier's website, often in vain. Then I chose the next higher value. The same components with the same values and sizes may be difficult to find due to a change in manufacturing standards. My choice was also oriented towards manufacturers renowned for their reliability, restricting the choice.

About capacity:
These electrolytic capacitors have the function of smoothing the rail voltage (+ ground -). Replacing these capacitors with others of higher capacity is not a problem. In fact, the higher the better (in the Hi-Fi field you can find crazy capacitance values). In product design, the value is chosen taking into account a reasonable capacity for the needs of the device (i.e. providing current to the speakers when hiting a key at full volume) as well as other considerations including headroom and cost. Here, a capacity 2 or 3 times greater is not that much greater.

About operating voltage:
An operating voltage 2 or 3 times higher than required does not do justice to the device. The capacitor will not perform at its best and would be susceptible to wear out (not to mention wasting headroom and money). However, never replace a capacitor with one that has a lower operating voltage value. There is a risk of failure here.

About size :
As you can see in the photos, some replacement capacitors are smaller than their original counterpart.
Some are bigger. I made sure they would fit on the PCB before purchase.
Some had to be the same diameter: C1 and C2 because they are mounted on a clamping ring.
The PSU + AMP unit is not in an enclosure so the height of the components is not an issue.


Interesting observation regarding higher operating voltage. A lot of techs will (for example) just keep 50 or even 100 volt caps around in a particular value and use those in everything at or below that rating, rather than having to stock 6.3v, 10v, 25v, 35v, 40v etc. It's something I've observed on YouTube, with the reasoning that it's nothing more than a higher max capacity, plus you can now buy 100v with the same small footprint as an old low voltage cap. Always wondered if this one size fits all convenience approach overlooked any performance characteristics.


Good point.
I was tought electronics in the early 90s. I learned that some types of electrolytic capacitors were designed with a self-healing feature. This required voltage to be applied to them regularly, at a reasonably high level, for the self-healing process to occur, thus preventing early wear.
On the Electrolytic Capacitors Wiki page I see that most modern electrolytic capacitors "can be stored for up to 10 years without problems" (i.e. without any voltage applied). My point about the voltage rating is therefore debatable/obsolete. Indeed, it seems that having a stock of 100V capacitors to meet all needs is a reasonable choice today.


That's good news because I am absolutely guilty of doing just that  ;D

Tried reforming the original 11000uf caps with an old Eico 950b leakage tester that can run full test voltage up to 500v. Leakage meter goes down the longer you apply full working voltage, which is 50v in this case. More of an experiment than a permanent fix. Anyhow, tried that but the power down problem still persists. New 33000uf caps are on order. Will also up the 1kuf cap to 2200 as indicated. Hope it does the trick!


Since this old thread comes up when googling this issue, I'll post my results here as reference for future techs.

Definitely glad I replaced the big leaky caps, but my issue ended up simply being a cold solder joint on the pin header where the main voltage regulator board puts out plus and minus 15 volts. Looked fine to the naked eye at a glance but up close it had a faint version of the circular crack you commonly see on jack and header pins. Bad connection had me at +20/-10 which somehow caused the oscillator to affect the power at slow speeds even with vibrato switched off - just as it did on yours, Will.

Always suspect the power supply first!  ;D