Elkatone Amp with problems

[PsychoM]

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:




you can see it has 3 IDENTICAL power amplifiers, one for the tweeter and one for each woofer. I made a schematic:


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!

[Ian Finch]

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.

[djoffe]

The offset voltage you report isn't so bad...a quick back of the envelope calculation says maybe at its best it might be around 100 mV...

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

[PsychoM]

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
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.

[jaycee]

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.

[PsychoM]

Originally Posted by jaycee

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.

Originally Posted by djoffe

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.

[djoffe]

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.

[PsychoM]

Originally Posted by djoffe

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.

[djoffe]

ok...pretty straightforward opamps...if you just measure and report the dc voltages on all the pins, that would be a great hint!

[jaycee]

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