Alpage AL-300

[Quadmatrix82]

There's a design flaw on the AL-300 that affects playback sound.
Found it when working on one with a very noisy playback.
So I've printed the schematic in high resolution and traced the signal path and power supply lines.
View attachment 80944
Replaced several capacitors and muting transistors (there's tons of them).
But still got noise and with a 10kHz bump that looks suspicious.
View attachment 80945
Finally found out the test tone oscillators are constantly turned on !
View attachment 80946
When removing the power of them, no more noise it's even the best AL-300 I've ever heard !
View attachment 80947
So I've found a way to modify the circuitry to have the test tone oscillators working only in calibration mode.
View attachment 80948
After some adjustments of the bias and rec eq that deck is capable of a 20kHz frequency response with metal tape at -20dB.
Can even goes to 30kHz at -25dB with a 1982 TDK MA !

Is this mod as simple as just removing the diode d148?

 

[20tajk7]

No need to remove D148, just pull R289 leg and connect it to D103 anode.
I might have some photos of how to do it, there's wrap pins so it's even easier to do the mod.

 

[Quadmatrix82]

No need to remove D148, just pull R289 leg and connect it to D103 anode.
I might have some photos of how to do it, there's wrap pins so it's even easier to do the mod.

Awesome if you have a photo that would be great otherwise those instructions are pretty clear

 

[20tajk7]

Here ya go.

R289, the leg with the black mark should be pulled out.


Pin #18 with purple and grey/white wires.


R289 and pin #18.


Solder a wire to R289.


Connect to other side of the wire to pin #18.


On more wire to the rat nest, now the oscillators will only be on during calibration, less noise on playback and no more 10kHz bump.

 

[Quadmatrix82]

Thankyou! Seems straight forward and a well worth upgrade

 

[vince666]

There's a design flaw on the AL-300 that affects playback sound.
Found it when working on one with a very noisy playback.
So I've printed the schematic in high resolution and traced the signal path and power supply lines.
View attachment 80944
Replaced several capacitors and muting transistors (there's tons of them).
But still got noise and with a 10kHz bump that looks suspicious.
View attachment 80945
Finally found out the test tone oscillators are constantly turned on !
View attachment 80946
When removing the power of them, no more noise it's even the best AL-300 I've ever heard !
View attachment 80947
So I've found a way to modify the circuitry to have the test tone oscillators working only in calibration mode.
View attachment 80948
After some adjustments of the bias and rec eq that deck is capable of a 20kHz frequency response with metal tape at -20dB.
Can even goes to 30kHz at -25dB with a 1982 TDK MA !

Great stuff here!

anyways, I see I am not the only one who prints schematics at big size onto more A4 sheets attached together and traces the schematic with a few coloured pencils!

 

[J!m]

I use crayons. The end I haven’t eaten is usually sharp.

 

[Quadmatrix82]

Does anyone al-300 have a safety capacitor across the power switch? Mine makes a snap through the speakers when switched on and off I wonder if a x2 cap would solve this and if so what would be the best value?

 

[20tajk7]

On single voltage model (American market) there's a 0.01µF/125V capacitor.
No capacitor on multi voltage model (other countries), but the switch has two poles.

 

[Quadmatrix82]

On single voltage model (American market) there's a 0.01µF/125V capacitor.
No capacitor on multi voltage model (other countries), but the switch has two poles.

Mine is multovoltage I wonder what the harm is to have one added to save the switch and stop the noise on start up and power down?