Could be a capacitor problem, that can almost never be ruled out but
it could also be a blown output stage or even the transformer itself.

You can isolate the transformer by lifting off its secondary leads.
You can also put an ohmmeter across the output stage transistors (unpowered)
and compare one channel to the other.

Martin

Thanks Martin,

turns out to be blown output transistors. Any recommendations what to use instead of the original BD697/BD698 Darlingtons, which are no longer available (not easily, anyway) ? As a matter of fact, in my kit these transistors are not originals - they have been replaced with BD899/BD910 and TIP125/TIP120 pairs.

-jari

First see if you can find out why it happened. Check the DIN sockets, the solder tags are often very close
and a short here will kill the output stage.
Next, check if other components are damaged too; Resistors, driver transistors, diodes, trimmer etc.

Two different ways of mounting was used on Beocenter 1400 cooling fins and the transistors
housings varies accordingly. Some have a fairly large countersunk area around the screw hole.
Can you provide a photo or closer description of the transistors ?

Martin

DIN socket and solder tags look OK to me.

Checking around it seems that at least the driver transistor and trimmer of left channel need replacing... actually looks like a lot of the components have been changed/tampered with!

Here is a photo of the transistors, BD899/BD910 pair on the right (left channel, the bad one) and TIP125/TIP120 pair on left.

-jari

Oh, looks like someone with very little soldering skill did something here.
Can you provide a photo or two of the component side too ?

Martin

Here is a close-up of the left channel components. A number of caps and a trimmer has been changed sometimes, too.

Going to change all the caps anyway, and comparing with the other channel (assumed to be OK) all suspect components around the output stage.

On the power darlingtons: from some charts on the internet I found recommendations to use BD652/BD651 or BDX53B/BDX54B pair (instead of BD697/BD698). Any thoughts on that ? (I am not very familiar with this audio stuff, more at ease with digital/computer electronics.)

One peculiarity of this design are the output stage driver darlingtons (2TR21 and 29, MPSA13), which are bend down thru a hole in the PCB to a lump of cooling paste. looks like an original feature, but never seen anything like it before!

-jari

jrantala wrote:

One peculiarity of this design are the output stage driver darlingtons (2TR21 and 29, MPSA13), which are bend down thru a hole in the PCB to a lump of cooling paste. looks like an original feature, but never seen anything like it before!

I'm wondering this too.
I'm familiar with potting two transistors together for thermal tracking. But this is just one transistor potted to the PCB.

Best to change all the transistors.The MPSA13 is quite crucial to the stable operation of the output stage.It's mounted through the board,and encased in thermal conductive paste.This means the temperature of the output stage is monitored,and the dc conditions stabilised as necessary.

As Martin say's,this output stage is "touchy"and easily blown.I alway's used to rebuild them by replacing all the devices and the two emitter resistors (the big 2w jobbies)

To be extra safe I'd use a variac to slowly wind up the supply voltage while monitoring.Not something generally available I realise.I still managed to get through several devices in some cases,before sucessfully sorting it!

Take care with it.Nice sounding amp,worth the effort

Partial success!?

Changed all 5 transistors, 4 diodes, the 2 big resistors and 4 capacitors of left channel. At least 2 diodes were blown (as well as the output darlingtons), cannot say about the driver darlington as couldn't get it out in one piece.

Now the fuses stay OK when I power up, the 32V is OK but I cannot adjust the idle current on that channel (right channel idle current is OK). I get a reading around 750mV which is way too much, and the trimmer doesn't seem to affect that at all. What could be wrong there ? Could it be the big 3000uF output capacitor ? Or do I just change all the remaining parts (resistors mostly) ?

I also measured some voltages to compare with info from the service guide: TR22 base 20V (25V in service guide), TR23 base 17V (22,5V), 18V (23V) between the emitter resistors and 14V (19V) on TR19 base. I used BD651/BD652 pair as power darlingtons, do they have any effect on these reference voltages ?

-jari