Aleph 5 DIY- with Dolphin sounds...Help please

[george a]

Hi All

My trusty DIY Aleph 5 has been running flawlessly for a few years but recently has started playing up..ONE channel only
First symptoms:
at turn on some dolphin sonar type chirping- then settled done after a few seconds.
the other day the suspect channel went dead- as in no sound - a ittle thump fixed this - (Dry joint?)

I opened it up to have a look to find that 2 of the output mosfets on the -ve rail have been running very hot - I have white stickers with writing on them from when they were matched.- the stickers having turned brown.
note each channel has 3 mosfets per rail - total 6 / channel.
voltage across each source resistor (1 ohm) by each mosfet on neg rail appr 1.4v , 0.6v 0v (yes 0v) (replaced this mosfet 3 times....)

pos rail all about 0.6v. across 1ohm resistors

Now i've checked for dry joints by reflowing each junction.

All transistors on board have been replaced
neg rail mosfets all replaced (matched).

result is 23v at speaker terminals with no load.
two hot mosfets (one very hot ) and one cold one..

Any ideas anybody?

Many thanks in advance

 

[Zen Mod]

schematic with exact measured values on it will help

use this one as guide what and where to expect

 

[AudioSan]

let me guess. the cold fet is on negative rail?
are you sure you dont have a short in the cold fet?

 

[george a]

ZEN MOD

thanks for the pdf, I've noted voltages on my uploaded copy, thanks in advance for any wisdom.

AudioSan
Yes cold FET on -ve rail.... - i've changed it 3 times and even left it out with no change..

Thanks

 

[AudioSan]

is the Source resistor for the Cold fet okei?

 

[george a]

Hi AudioSan
Source resistor is ok

.....
Thanks

 

[Zen Mod]

easiest way - desolder one end of all output source resistors , same as one end of all output gate resistors

power up - check voltages on input LTP ( voltage across R9 in Algar's schm will tell everything )

solder back resistors for one by one vertical pair of outputs ; power up and measure offset and voltage across source resistors

enclosed schmatic is'n exactly clear - did you measure some of voltages acroos resistors or you measure them ref. GND

 

[george a]

Hi ZenmOd

I will try what you suggest
voltages are measured across resitors as per orig pdf, others are voltages measured above grnd
Thanks

 

[Zen Mod]

sometimes is problem to find what's main reason for output offset , without isolating input LTP from outputs ; that's the reason why I wrote that

however , maybe for first step - just desolder one end of all output gate resistors ; that way , they will be unbiased , so you can check operation of LTP

if any of mosfets is damaged , it will draw enough current unbiased , so you will see excessive current through it's source resistor

sometime is best and fastest way - just unsolder all semis (one by one ) and check then separately

http://firstwatt.com/pdf/art_mos_test.pdf

 

[george a]

ZenMod
Time for me to look like an idiot...LTP - what does this mean?
- an abbreviation.?
I know enough to be dangerous...
Ta

 

[Mooly]

LTP +long tailed pair... the two input devices that form a differential amp.

 

[george a]

all 6 source resistors disconnrcted
voltage across R9 is 3.5v (should be 4-5v?)

Could it be a faulty zener?
ta

 

[Mooly]

I doubt it tbh

Is Q4 OK ?

 

[Mooly]

When you said zeners I thought you meant the protection ones.

You should have 9 volts across Z5 and zero (really 0.0000v across R107

 

[Zen Mod]

what's voltage across R106 ( same file ) ?

 

[Mooly]

I'll leave you to it 😉

 

[Zen Mod]

no need ....... more hands can do more

on the other hand - I always found Dolphin sounds as enjoying ;

maybe is best to leave it as is ?

 

[george a]

Hi Voltage across r106 = 5.75v...

I loved the show "Flipper " as a kid...

 

[Zen Mod]

that means you have 5V75/221R ~ 26mA through LTP , which is good

now - time for checking output mosfets

first with buzzer on DVM , to see is there any with substantial shortie between pins

then with diode test ( there must not be any conductance between pins )

then either per Papa's pdf , or connecting one by one vertical pair in amp itself

 

[george a]

Tried testing one vertical pair
Smoked 221R resistor on adjacent moset (disconnected)

bummer
will try the long way - mosfet testing individually.

Spare amp now in service - Digitech AA-0476 - bought cheap - see other thread.
Now i know what it sounds like....
Cheers