Fender piano student kb1273 amplifier

[mikeydee77]

Hi, wonder if anyone has any alternative schematic to the common schematic that comes up for the student, or fishtail, model?

Need I say that my experience with Fender Rhodes is virtually nil.

The schematic that is available in the Fender Rhodes service manual and widely on the Web has a transformer coupled output stage however the one I am looking at is a simpler design. With a pair of chassis mounted t03 transistors and a few drivers. Most of the parts are difficult to identify the markings.

Think the keyboard dates from the late 60s but not sure.

[J M Fahey]

Are you sure there is no driver transformer somewhere nearby?
I see a single large heatsinked TO66 driver , probably a relabelled 2N3053, which was a popular interstage transformer driver.
Itīs also a single supply, capacitor coupled output.
What is the actual problem or symptom?
Please post the "commonly available" schematic, at least as reference.

[mikeydee77]

Hi Juan,

Thanks for replying...

The commonly available schematic comes from a Fender Rhodes service manual that covers many models.



There is a separate preamplifier control box and power amplifier unit coupled together with multiway connectors.

The fault with the unit is distortion. On the scope there is noticeable crossover distortion although the input signal post-preamp looks good.

The majority of CC resistors had drifted northwards in the range of 20-50 percent and I have replaced these and a couple of ruptured electrolytic caps.

[mikeydee77]

Also...
Dating info from the owner:
If it helps it's a 1967 Fender Rhodes Student Model (Jetson's Avocado Green) it's earlier than most of the green student models (they don't have the white vertical pin stripe on the tail)

I've attached a picture just in case.

[J M Fahey]

Ah, then itīs working as intended.

No kidding , crossover distortion is unavoidable and "a feature" from day 1 , it left he factory that way.

Itīs certainly transformer driven and biasing is horrible but worst is that it has no thermal tracking so it either suicides in thermal runaway or distorts because it has to be way underbiased to stay alive.

Best option is to pull that very dated power amp and mount a chipamp , an LM3886 will perform very well there.

Do not worry about originality or mojo because there is none there, itīs a cheesy poor transistor amp, the kind that gave SS its (today undeserved) bad name.

What +V rail voltage do you have?

EDIT: this schematic shows split rails, +/-30V , but your picture shows a single supply output capacitor one so it must be even older.
Check for some Rhodes power amp schematic showing such architecture.

[mikeydee77]

Hi Juan,

Unless the transformer is something different different to the typical EI transformer I just don't see it (hope I don't get corrected!). I guess that it would be quite a small item but I just see the usual array of passive components. Also the other clue is that this PCB features a zener diode CR1 which is not mentioned on that schematic not to mention also another clue is that none of the resistor values tie up. The component location references are non-existant on the diagram but still the obvious 2W watt, 820R seem to be 470R on this design and many others too.

The amp has a a 30v supply for the output stage and a 20v rail for the pre amp from my observations. There are many other schematics out there so I will review again in case I missed something.

Yes I agree on the idea to go for a replacement amp as there are a couple of other issues around PCB burn and transformer wiring damage that give me some safety concern. I am exploring the options. Velleman are a module/kit company in Euroland that I was looking at. Given the possible transformer issue as well this might be a cost effective way to go.

Cheers, Mike

[J M Fahey]

Oh, you have the actual ampliier before you , so observation beats deduction
But Iīm curious about it.

Transistor codes do not mean much for us because they are house numbers, but at least and to start with something, measure voltage at power transistors, maybe take a picture or two showing them and the wires coneccting them to the PCB and a couple PCB closeups.

Measure E/B/C DC voltage relative to ground for each of them , also label them in the picture so we can say "Q5" or whatever instead of "the one above and to the left of the red thingie" , and Vbe , so we know polarity and whether they are Ge or Si.

Follow transistor wires backwards to see where do they lead.

Either to some driver transistors (no Darlingtons way back then) or a driver transformer ...... or ......

Also measure E/B/C and Vbe for that TO66 transistor on the board.

As of Velleman, I love their kits and very clear explanations.

But if you have a single supply, not sure many modern Companies offer kits for that (except 12V car amps) so you might have to kludge something on Veroboard or perfboard, or mod an existing PCB.

A TDA2050/LM1875/LM3886 can fit the bill.

[TigerAmps]

Very cool! I never seen one before...
Where does the vacuum come in - I don't see it referenced in the schematic... (kidding)

[mikeydee77]

Hi Juan, I took your advice and have roughly sketched out the output stage. It is accurate on the output devices but less so on the driver stage which was a bit too confusing.



The Vbe voltages suggest Ge devices to me - what do you think?

However one of the output transistors looks original with an RCA and house code on it whereas the other has a Motorola logo and it is marked SJ6357 (180v T03 NPN Si device). That's odd, coud it be an earlier repair with one Ge and one Si transistor?



There have clearly been multiple repairs on this board so the history is very much in doubt. It does not sound too good but it is working and puts out about 30-40watts. However there is particularly noticeable crossover distortion on sine waves (organ sounds too ) but on music or speech it just sounds a bit crunchy and at low levels there is severe distortion that seem to break through into clear at times.

The client is very keen on keeping it original so my suggestion of putting in a chip amp or is not going to happen.

[J M Fahey]

The client is very keen on keeping it original

Ok, the let him enjoy original crossover distortion.
Or bias it out of it, but then advice him it will self when playing loud for any length of time.

Because the circuit does not have thermally compensated bias.
IF you mod it, it wonīt be original any more ....

Thanks for drawing the schematic, itīs always very useful.
With what you found it's enough to understand the architecture: itīs definitely an improvement over the transformer driven one, but still crude.
It does have very poor biasing ... just like the transformer one, it does crossover distort ... same thing, probably designers found it slightly better than the old one and left it there.

Owner "wants it original" just out of superstition, because itīs a *poor* design and performance amp.

He probably believes you are replacing his high quality US made old parts, "they donīt make them that good any more", with cheesy cheap chinese junk.

Well, the modern Asian made one is 1000 times better than the original, and if USA manufacturing base moved there ... itīs not your fault (or his).
And being cheaper is just the icing on the cake.

EDIT: if you want to improve the original, then we need the *full* accurate schematic ... will the owner pay for the extra time? At least equivalent to extra couple hours bench time.

[mikeydee77]

Yep, I know chasing the mojo!

Mind you this amp repair is part of complete keyboard restoration and the rest of the unit looks stunning. Perhaps when the client starts enjoying playing it he will consider the benefits of a new amp. Fortunately it's only for home use so unlikely to be pushing it too hard.

You have been a great support on this one Juan I really appreciate your advice and experience.

Mike

[Tom Phillips]

...Perhaps when the client starts enjoying playing it he will consider the benefits of a new amp. Fortunately it's only for home use so unlikely to be pushing it too hard...

Mike,
Does the piano you are working on have provisions to drive an external amp like the higher end Rhodes piano? Using an external amp would be a good way for your customer to compare the original sound with the possible better sound of a better amp (and speakers too). If the power amp drive signal is not presently available on a connector it may be easy to discreetly add one. Something to consider anyway.
Cheers,
Tom

[Enzo]

SJ6357 is a Peavey house number for an MJ15003, so that is not original transistor anyway. That is NPN by the way.

So we have seen the commonly available schematic from online, but did you try the commonly available phone number for Fender and ASK a customer service type there if they could find the old schematic file for that thing? Nothing to lose there.