How do i fix vintage germanium amplifier from a record player ?

[OldDIY]

BC255🙂

 

[gabripeic]

IMG-20200830-212442-2 — ImgBB
IMG-20200830-212436-2 — ImgBB
IMG-20200830-212427-2 — ImgBB

Here is what i came up with...
I am not that good at revers engineering... But i hope this helps. What do you all suspect is most likely dead. Also I don't know what that component mounted to one of the transistors is i think it is a diode?

 

[U230421]

Reverse engineering is a fruitfull steep way mastering electronics, and germanium antiquity stirs a lot of brilliant dinosaurs here to help you out. Who's going to post the proper circuit drawing first?
And I've checked my local supplier: AC180 and AC181k or GE replacements are still available.

 

[JMFahey]

But I came across a transistor that i can't find any information about it is a iskra 924 bc255p

transistor BC255 datasheet - Texas Instr. - ISKRA

 

[OldDIY]

Almost correct🙂
Is the "diode" connected in series with 27 ohms or in parallel?
Most likely Ac127 / 128 is closed.
It would be nice if you measured the voltages across the bce (relative to the total) across all transistors (working and faulty) for comparison.

 

[trobbins]

Have a look at the schematic link - your circuit is not exactly the same, but it indicates how to better draw the output stage transistors. Your circuit doesn't have AC180/181 emitter resistors, and the transistor mounted on to one of them is in the base bias circuitry to normalise output stage current as the output transistors heat up.
Registratore a cassette G19/151 Ampl/Mixer Geloso SA; Milano

 

[poynton]

It's almost the same as this.

The object attached to the output transistor is a thermister - negative temperature coefficient device - to keep the bias in check when the amp warms up.


.

 

[gabripeic]

Oh i got a lot of feedback. I will look at all the details tomorrow. It's 2:46 am in Croatia lol

 

[trobbins]

It won't be a thermistor - it will be a diode (or a transistor with one leg cut off, as the photos don't show that).

 

[gabripeic]

Almost correct🙂
Is the "diode" connected in series with 27 ohms or in parallel?
Most likely Ac127 / 128 is closed.
It would be nice if you measured the voltages across the bce (relative to the total) across all transistors (working and faulty) for comparison.

Basically "diode" or what ever it is is connected between bases of both output transistors so yeah it is in parallel with 27 ohms resistor, As far as measuring voltages goes do you mean from ground to each leg or across the legs of the transistors? I will do the measurements later and update you all.

 

[gabripeic]

OK here is what i found out. Basically fault seems to sometimes get worse and sometimes better. I was testing the amp and it worked decently. Faulty channel was still quite and distorted but it worked and after powering it off for a few minutes oscillating is back at only half a voltage. I recorded a video of the fault.
Sorry for my bad English i get really confused when i speak English in front of the camera lol

Broken amplifier - YouTube

 

[gabripeic]

One more find !
When i turn it on i have to wait for it to stop oscillating and then rise the voltage and then wait again until i reach full voltage and if I disconnect the power for more than a few seconds it starts oscillating again for a short period of time once I re connect it. The longer it's unplugged the longer it is oscillating when power is applied. And when i get it to full voltage working channel is nice sounding and strong but failed channel is weak and "dirty " sounding

 

[poynton]

Have you changed any capacitors on the affected board ??

It sounds to me like a cap is charging up slowly.


Andy

 

[gabripeic]

I replaced all the capacitors except these plastic tube with foil type looks caps. I don't know what type they are.

 

[poynton]

What is the big white capacitor?

Why is it not the same as the other board?

 

[gabripeic]

That big white capacitor is replaced it is a weird electrolytic of a same capacity as the other. I don't know why some components are mismatched but that's how it was. I will upload another video in 10 minutes explaining everything.

 

[poynton]

Can you get a better photo of the markings on the white capacitor?

 

[gabripeic]

Big white capacitor only says 500-15v and a part number? 235-6944kb
Take a look at this video

Broken amplifier 2 - YouTube

 

[gabripeic]

Here is a closeup of that weird capacitor. It looks like polarized electrolyte but in plastic??? Not important anymore. It has been replaced as you can see in the video
IMG-20200831-171752 — ImgBB

 

[U230421]

gabripeic, can you verify / redraw the components around T4? I've the impression something is missing (from the base of T4 to the power rail). Also the 330, 27 ohm resistors and the ?-component (whatever it is).