Elkatone Amp with problems

hi guys, i'm new here.

I have received an old Elkatone leslie-style amp, it has a big solid state amplifier (100 Watts RMS). Unfortunately, it's not working.
When I tried it, there was some hum coming from the speakers. It did output some music trough the tweeter, but not nearly as loud as it should be and with a lot of distortion.
i got some help from a guy on another website. I changed some bad resistors and transistors, but now were stuck.
Here are some pics:
An externally hosted image should be here but it was not working when we last tested it.

P1050494.jpg

P1050497.jpg


you can see it has 3 IDENTICAL power amplifiers, one for the tweeter and one for each woofer. I made a schematic:
An externally hosted image should be here but it was not working when we last tested it.


The "jack switch" closes when no speaker is connected. I'm now testing without the speakers connected so the big 10 Ohm resistor acts as load.

As you can see this amp is auto-biasing using 2 diodes (1N3754) placed on the heat sink.

the problem we face now is that the quiescent current through transistor X (about 75-80mA) is not the same as the current through Y (+- 40mA). Also, there is a center voltage of 0,4V-0,5V.

Maybe you guys have a clue on this?

If you need some more information, just ask it i can measure it right here.

I don't know very much about the operation of audio amplifiers, so excuse me if i don't understand everything ;D

PS: sorry for my English, i'm belgian. the translation of some technical words is not easy.

Thanks!
 
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Welcome to DIYAudio

and thank you for your interesting post. You certainly have tackled a very difficult project, knowing how many parts will be long past their useful life.
You don't mention any electrolytic capacitor replacements, but they appear to be old types. These deteriorate and should be routinely replaced in domestic applications after 20 years, sooner if in commercial or hot running gear.

In some cases, failures of caps take other parts with them but these are just symptoms of a the worse problem with old caps. Replace them first, considering how expensive this will be and then we can look at the DC conditions that may need adjustment. The bundles of caps fitted already may not be necessary with today's more compact parts and building up a board to mount them vertically as in a power supply may be convenient and neat.

Consider replacing the amp. modules with just 3 small and bulletproof chip amp pairs. These are a good, modern solution to small musical instrument amplifier ruggedness needs, as here. Looking at the heatsinks fitted, 70W might be about the limit for continuous output, there.
 
Hi, thanks for your reply!

I was thinking about replacing the amps, but then again... this is a vintage piece of equipment and I would like to keep it original. You can call me an idiot :D
Also, the amps are now working. The amps are amplifying what's going in, so it would be quite a waste to get rid of them. It's just that quiescent current that's not ok.

btw, the preamp isn't working properly either. It's generating 10 VDC on the output, so just buying 3 new amps would not solve the entire problem.

I know about the capacitors. I will certainly have to replace some caps, as a friend of me told me that one of the caps might be the cause of my quiescent current issue. I'll have to replace the large power supply caps too.
Fortunately, most of the elco's are placed in an unimportant DC circuit (like the motor power supply for the leslie motor), so they don't affect the sound.
There's still one question i have: do i need to replace the other caps too? Like the tantalic caps and the ceramic caps?

PS: this amplifier was built around 1970.
 
Yep, every single electrolytic capacitor, especially the power supply capacitors, are going to be well past their useful life. If you're serious about repairing it, this is where you should start.
I know those caps are important, but i measured every single one of them and not one has shorted. Their values have probably changed a bit and maybe there's some leaking current. But for example some leaking current in the power supply caps is not going to cause a quiescent current problem in the power amps.
I'm planning to replace all important caps when my preamp is working.

The offset you report would skew the x and y currents...the bigger question is...is the output still distorted?

Can't tell, there's a problem with the preamp and i don't dare to connect it again to the power amp. However, without the preamp connect, the amp is running without hum and if i put my finger on the input, there's and very, very loud hum, so the amplifier's volume seems ok.
 
I wouldn't worry about the power amp for now...it seems to be working well enough...It's probably time to look at the preamp issue...

some clarification would be good of the preamp offset problem, that could be owing to a shorted coupling capacitor...

The business of replacing all the caps...I'm probably going against the grain, but if it were for my own use, I wouldn't bother...if they're ok...they're ok. If I were repairing it for someone else, I might do it, as it might make the reliability for them better, but until you have the whole thing kind of working, replacing all the caps might cause more harm (from unintended accidents during the replacement) than good.
 
some clarification would be good of the preamp offset problem, that could be owing to a shorted coupling capacitor...

The preamp is based on a TBA231 opamp. TBA231 datasheet and application note, data sheet, circuit, pdf, cross reference, pinout | Datasheet Archive
There's +-10Volts DC on some of the pins. The difference between the inv. and non-inv. input is 10 Volts, too much.
This indeed could be a shorted cap. I should draw a schematic soon, but the print is hard to reach.
Maybe someone has a schematic of another preamp based on this TBA231? That would be a good starting point.

btw, there's also some kind of leslie-simulating-effect for the woofers (there's no spinning drum, like in the real deal. Instead there's this simulator). I made a schematic of that print. I'll post it here, maybe it will be usefull to someone in the future.
i added it as an attachment.
 

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Replace any tantalum capacitors you find. These have a limited life span and when they fail, they go short circuit and typically explode! Though i dont actually see any on the photos. Metallised polyester and ceramics are fine.

Electrolytic capacitors in the signal paths going bad could cause all sorts of weirdness. You really can't diagnose anything else until you rule out failed caps by replacement.

The leslie simulator looks like some sort of EQ circuit
 
Ok guys, thanks for the help!
I'm going to mesure the TBA231 pins later this evening.

There are some tantalum caps (16 of them) but they are hard to reach. They are placed on the vertical prints that are soldered to that big effect print (bottom of photo 1).
You can see them here: http://i51.tinypic.com/21kgtiw.jpg
They all seem to be working, but i'm going to replace them as soon as i'm finished with the preamp.

So this is the schedule so far:
1) find preamp problem
1) put new elco's in the power amps (signal path)
2) repair preamp
3) replace power supply elcos
4) replace tantalum caps

PS: here's a direct link to the leslie sim schematic, it's easier to read ;)
http://i56.tinypic.com/fk9ogo.png
 
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okay, did the tests:
INV. INPUT B: not stable. when i put my test leads to this, it reads a few volts but then it drops down to almost nothing. Like some capacitor is decharging over my voltmeter.
NON INV. INPUT B: -0.136V
INPUT LAG B: 6,00V
INPUT LAG B: 8,34V
OUTPUT LAG B: 10,93V
OUTPUT B: not stable. See INV. INPUT B.
V+: 11,08V
V-: -10,82V
INV. INPUT A: -0,08V
NON INV. INPUT A: -0,07V
INPUT LAG A: 7,29V
INPUT LAG A: 7,27V
OUTPUT LAG A: 10,07V
OUTPUT A: 0,0148V
 
Opamp A seems ok.

Opamp B is either defective, or the feedback from the output to the input has opened up. There should be a resistor from the output to the inverting input, typically 100K or less....with power off, can you measure that resistance?

I made a schematic of the opamp. The tweeter preamp is A, woofer preamp is B.
There's a large capacitor in the tweeter amp between the inverse input and the output. It's a tantalum cap. I followed the color markings, it says 180µF, but is seems much too much in comparison with the 3600 pF cap in the woofer preamp. That's why I put question marks next to it.
You can see i made an interupted connection between output a and the output of the preamp: that's a connection on the circuit board that has been cut through with a knife!

In the woofer preamp, there's a coil near output B. I don't know if that is a coil, it looks like one (see attachement 2). There's no number or code on the outside of this element.


I dont have a lot of time, can't measure the resistors today. will do that tomorrow.
 

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Okay guys, i'm going to post an update here:
All poweramps are OK (though they have a VERY high quiescent curren, about 100 mA). I replaced the important electrolytic caps.
The tweeter channel is working nicely now. Only problem is that the input sensitivity is not correct. Maybe someone knows how to adjust the input sensitivity to the right level?
The cutoff frequency is too high, but i'm going to replace the filter caps so that'll be OK.

The woofers aren't working and that's probably because the TBA231 isn't working correctly. I'll try to replace it with a uA739 but they have become quite rare.
I want to replace all the caps around the opamp to be sure, but i don't know the types.
An externally hosted image should be here but it was not working when we last tested it.

The yellow ones are polyester caps, the blue ones are electrolytic caps of course and there's one tantalum cap.
But I don't know what kind of cap those big white ones are. Same problem for the 'Arco' cap and the '3400 J' cap.
 
Hi guys!

I have a little problem with my Elkatone 305 and i'm not gonna open a new thread for it. This seemed like a good thread for my question, so here we go.

The pedal for leslie control on my amp is completely broken (because of that i recieved the amp for free :D) My circuit reading skills are rubbish, so what I want from you is to post a picture of the pedal guts, so I can rewire it. I don't know if it helps, it says elka amp 5 on the circuit in pedal.

Thanks a lot!

Sorry if my english is a bit rusty.
 
Hi guys!

I have a little problem with my Elkatone 305 and i'm not gonna open a new thread for it. This seemed like a good thread for my question, so here we go.

The pedal for leslie control on my amp is completely broken (because of that i recieved the amp for free :D) My circuit reading skills are rubbish, so what I want from you is to post a picture of the pedal guts, so I can rewire it. I don't know if it helps, it says elka amp 5 on the circuit in pedal.

Thanks a lot!

Sorry if my english is a bit rusty.

Hi! First an update on my project:
My Elka is working okay now, the preamp IC was indeed faulty. Unfortunately, the amp does not sound very good. It is also very noisy. The speakers are good, the leslie spin effect is good, but the amplifier itself isn't very linear. This probably has something to do with old caps and components (FETs) in the first stages, but the amp is also very old and an early solid state amp. This one came from an era where proper PNP output transistors didn't exist or were very expensive?
I don't think i will get it to sound as clear as a new Hifi amp. So i'm thinking to build a passive crossover and connect a new Yamaha amp to it.

Now for your problem:
The pedal on a 305 isn't exactly the same as the one on my amp, but it probably works in a similar way. I don't have pictures of the pedal (i might take some soon). Even if I had pictures, you'd probably wouldn't be able to rewire it because there are a lot of wires in the pedal (about 14) and the colour code is probably not the same. But I can tell you how it works:

Inside my pedal, there are not a lot of different components. You've got 2 switches for the belt motor (Motor on/off, Fast/Slow) and one switch for the reverb, and some (variable) resistors. I think your Elka doesn't have a reverb, so you can ignore that one.
The motor on/off switch, is a simple DPDT (double pole) switch. One half of it acts as a switch that cuts the power to the belt motor.
HOWEVER, the other half of the switch activates a special electronic circuit that 'emulates' the rotating drum for the woofer (there is no mechanically rotating drum for the woofer). In my amp, this circuit is quite complicated. Please note, the major part of this circuit is located inside the amplifier cabinet. Something wrong in the pedal, might damage something inside the amplifier too. The circuit consists of 2 analog oscillating circuits, they control the bias voltage of an array of FETs (transistors) that are preamplifing the audio signal for the woofers.
One oscillating circuit runs at slow speed, the other one has a higher frequency. This is where the 'Fast/Slow' DPDT switch comes in: one half of this switch is able to switch between both oscillating circuits, thus switching between the emulation of the drum spinning at high or low speed.

This is the electronic 'emulating' speed circuit. You can see it in this picture, in the bottom center. It is the largest print on the picture. The small wires on it come from the pedal. http://i892.photobucket.com/albums/ac125/OlivierS/P1050494.jpg
This effect is only audible through the low frequencies (woofer), it sounds like some kind of vibrato. If something is wrong in this circuit, it only affects the bass frequencies (unless you've got a short circuit - then, the fuse will blow).
I have posted the schematic of this circuit, but it is complicated (look on the other page of this topic).

The other half of the Fast/Slow and Motor on/off switches, are used for adjusting power supplied to the belt motor. This one is also quite complicated... The motor current is controlled by two large 2N3035 transistors (one for each speed, they are also located inside the amplifier cabinet). The transistors are connected to the motor. The fast/slow speed is achieved by controlling the bias voltage on these two transistors. The variable resistors in the pedal can adjust these preset fast/slow voltages.
This is a picture of the circuit in the amp cabinet. The two large transistors are mounted on the outside of the chassis, you can see the pins coming through. Please note the black cable with all the small brightly coloured wires. These wires come from the pedal.
http://i892.photobucket.com/albums/ac125/OlivierS/P1050495.jpg
If something is wrong with this circuit, it CANNOT affect the amplifier (both tweeter and woofer). Even if there is a short circuit, because this circuit has its own fuses.

This circuit wasn't broken in my amp, so i didn't need to draw a schematic for it and (i'm sorry for you), i'm not gonna do it, because i need to unsolder a big bunch of wires to be able to reach it.

Now, what might be important to you:
- if you have sound from both the treble driver and woofer, and you can adjust some kind of 'vibrato' coming from the woofer by pressing the motor on/off and fast/slow switches, the 'emulating' circuit is okay. You should not mess with that part. Is this true for you?
- If your horn is not spinning or doesn't change speeds, you've got a problem in the motor power supply circuit. Is this true for you?

Please answer those 2 questions. I might have an idea how to fix your pedal. If you have pictures of the insides of your pedal and/or amplifier, please post them.
 
You don't mention any electrolytic capacitor replacements, but they appear to be old types. These deteriorate and should be routinely replaced in domestic applications after 20 years, sooner if in commercial or hot running gear.

Hi , Ian

Looking at the pictures it seems that they are typical 70 s Phillips
and these are quite reliable caps.
I replaced a DC connector on an old Grundig Music Boy portable
radio , (FM/MW/LW/SW , 2W rms , 28mA Iq , all this with 7 bjts !!)
and it has those capacitors , all are fully working , since 1968.....
 
I'll be honest, the power amp circuit is nothing special, and is indeed from the bad old days of solid state. If this was me, I'd probably be inclined to forget repairing it, but gut it and build my own in there.

The important parts for the "sound" are the preamp, Leslie simulator, and the speakers themselves. You could build these with modern parts without too much hassle at all. The power transformer and the heatsinks could be reused, along with the casing. There's three power amps in there - two woofers and the tweeter ? Probably not neccesary nowadays.
 
The important parts for the "sound" are the preamp, Leslie simulator, and the speakers themselves. You could build these with modern parts without too much hassle at all. The power transformer and the heatsinks could be reused, along with the casing. There's three power amps in there - two woofers and the tweeter ? Probably not neccesary nowadays.

You are completely correct. However, I'm gonna try it with my external hifi-amp first. I just need to build a crossover and that takes less time.

@wahab: 80% of the electrolytic caps have been replaced by now (not shown on the picture). These were old Procond caps. None of them were faulty, and replacing them didn't help much.

I will try to solve Lin689's problem first. The Elkas have become rare, there's not a lot of information available (i had to draw the schematics), so let's see if he's able to get his amp up and running with the help of this forum, that would be great.