Beolab 5000 (60:th), 2N3055 in the power supply breaks when speaker is connected

Reply

Gunnar Faith-Ell Posted: Wed, Aug 11 2021 9:39 AM

rated by 0 users

I am repairing a Belab 5000. All capacitors have been changed. The Power supply is repaired with new transistors, new 18V zenner and show correct values (60V, over the 33V Zenner diode about 34V).

The problem is when I connect a load, a pair of speakers (RL150 or others) then at power on it's a "pop" sound in the speakers and the two 2N3055 transistors in the power supply breaks emediatly. The voltage then raise to about 76V (short circuit in the 2N3055). The fault occure indipendent of volume.

The amplifier works, I have checked it whith tone generator and osciliscope. It also works when the power supply gives 76V with the speakers. I checked the 2N3055 in the amplifier with a Huntron Tracker (desoldered). 

Some earlier repair has been done on this unit, one of the 38647 in the amplifier has been replaced with another transistor type.

Any good ideas on where to start?

My thoughts are:

1) Problem with the batch of 2N3055? I have checked the 0.22ohm in serie with the transistors.

2) Problem with the current reduction through the 0.22ohm in the power supply, maybe change the resistor to a higher value?

3) Problem with the 3000uF caps to the speakers? They have been changed by someone earlier to correct value but much smaller type. (They measure ok).

Reply

Dillen replied on Wed, Aug 11 2021 12:07 PM

rated by 0 users

1. Most likely
2. The emitter resistors are not for current reduction.
3. Not likely if they measure fine.

Where did you buy the 2N3055?
And can we see how they look?

Martin

Reply

Gunnar Faith-Ell replied on Thu, Aug 12 2021 7:57 PM

rated by 0 users

Thank you Martin for a fast reply.

1) The 2N3055 come from a workshop that I bought so its not possible to trace the actual source. What do you recommend for make of them to be sure its original? Here is a picture of them:

2) No it's not the emitter resistors, there is one more in the power supply with the same value as the emitter resistors. According to the service manual it should reduce the current at 7A. My idea was to change it temporary for fault finding. But first option is to check the 2N3055 and maybe order new ones.

Gunnar

Reply

Gunnar Faith-Ell replied on Thu, Aug 12 2021 7:57 PM

rated by 0 users

Thank you Martin for a fast reply.

1) The 2N3055 come from a workshop that I bought so its not possible to trace the actual source. What do you recommend for make of them to be sure its original? Here is a picture of them:

2) No it's not the emitter resistors, there is one more in the power supply with the same value as the emitter resistors. According to the service manual it should reduce the current at 7A. My idea was to change it temporary for fault finding. But first option is to check the 2N3055 and maybe order new ones.

Gunnar

Reply

Dillen replied on Thu, Aug 12 2021 9:30 PM

rated by 0 users

That transistor is a counterfeit/fake.

See this:

playing with bits: Counterfeit 2N3055 Transistors from eBay (bradthx.blogspot.com)

Can you wipe off the lettering with IPA and a cotton bud?

Martin

Reply

Gunnar Faith-Ell replied on Fri, Aug 13 2021 4:53 PM

rated by 0 users

ok, I will order new transistors.

Thank you 

Gunnar

Reply

Dillen replied on Fri, Aug 13 2021 5:55 PM

rated by 0 users

Perhaps it would be a good idea to show them to us before ordering.
Or find NOS or even good used ones.

Martin

Reply

Gunnar Faith-Ell replied on Sun, Aug 15 2021 5:10 PM

rated by 0 users

Ok, I will see what I can find.

Gunnar

Reply

hemenex replied on Mon, Aug 16 2021 10:33 AM

rated by 0 users

I just wanted to add that not all of  ST's 2N3055's are counterfeits.

I did order a bunch of them from a distributor back in 2013 when I heard about ST givin' up the production.

After reading here I tried to rub-off the printing with alcohol; it faded somewhat but was not erasable. See lthe left front transistor

And I notice a bit more "digital" print font on the transistor compared to the other picture in this thread.

Does anyone have a description of the printing on it? I searched the net and my datasheet collection but couldn'd find a valid one for the 3055 (Martin?). I think these are 2013 Week 24 and from Malaysia.

Haven't used one of them yet but this one being in a lot of electronics from times gone (power supplies, amplifiers...) I decided to get enough for the rest of my electronic life :-)

I'm hoping that they weren't counterfeits already in 2013 ;-)

  hx

Reply

chartz replied on Mon, Aug 16 2021 3:32 PM

rated by 0 users

I’m certain those are genuine, unlike the joke above ☺️

Reply

Dillen replied on Mon, Aug 16 2021 3:41 PM

rated by 0 users

I agree, but polystyrene is not exactly ESD-safe, and it even caries a warning label ...

Martin

Reply

hemenex replied on Tue, Aug 17 2021 8:52 AM

rated by 0 users

It's the original packaging box from ST

And it's really non-conductive - I measured

strange...

Reply

chartz replied on Tue, Aug 17 2021 9:16 AM

rated by 0 users

Dillen:

I agree, but polystyrene is not exactly ESD-safe, and it even caries a warning label ...

Martin

Reply

Gunnar Faith-Ell replied on Tue, Aug 17 2021 1:31 PM

rated by 0 users

When trying to find a larger lot than 1-2 of them most looks like the fake one.

Perhaps it's easier to find a good replacement for 2n3055 which they dont copy?

Reply

chartz replied on Tue, Aug 17 2021 8:50 PM

rated by 0 users

Just stick to 3055’s. The whole amplifier is built around them, like the Quad 303.

Reply

Gunnar Faith-Ell replied on Wed, Aug 18 2021 9:05 PM

rated by 0 users

How about this from Farnell:

2N3055 - Multicomp Pro - Bipolär (BJT) singeltransistor, Allmänt ändamål, NPN (farnell.com)

Reply

Gunnar Faith-Ell replied on Wed, Aug 18 2021 9:08 PM

rated by 0 users

How about this one from Farnell?

2N3055 - Multicomp Pro - Bipolär (BJT) singeltransistor, Allmänt ändamål, NPN (farnell.com)

Reply

Gunnar Faith-Ell replied on Thu, Aug 19 2021 11:39 AM

rated by 0 users

I need more like 50 pcs...

Reply

chartz replied on Thu, Aug 19 2021 11:54 AM

rated by 0 users

For the Beolab?

Reply

Dillen replied on Thu, Aug 19 2021 5:01 PM

rated by 0 users

I agree, the ones Jacques links to look fine.
If you need 50 transistors you could probably buy 25 pairs, - 
but it's in the UK so customs and import duties/VAT will have you for dinner.

My advice would be to keep an eye on Ebay etc. and grab good lots of original RCA or Motorola.

Martin

Reply

Gunnar Faith-Ell replied on Thu, Aug 19 2021 6:30 PM

rated by 0 users

I ordered the 2N3055 from CPC in UK and I will open one when they arrive and let you know if they can be trusted as a replacement. Otherwise I will send them back

Reply

chartz replied on Fri, Aug 20 2021 3:17 PM

rated by 0 users

I bet they won't do in the Beolab 5000. I hope to be wrong.

Reply

ablaumeise replied on Wed, Sep 1 2021 11:09 AM

rated by 0 users

When I come across a blown 2N3055, I usually replace it with a KD503 (essentially the eastern bloc variant of the 2N3055). Haven't had any issues so far.

You can easily find surplus from the 80s in big quantities, and they aren't expensive. And, the most important thing: nobody has started counterfeiting these yet ;)

Reply

Gunnar Faith-Ell replied on Mon, Sep 6 2021 10:31 AM

rated by 0 users

So, I got the new 2N3055.. The one to the right.

Reply

Gunnar Faith-Ell replied on Mon, Sep 6 2021 10:33 AM

rated by 0 users

The new one is 11gr and the fake 9 gr.

Reply

Gunnar Faith-Ell replied on Tue, Sep 7 2021 6:47 PM

rated by 0 users

Now an inside picture from the new and from the fake 2N3055

Reply

Gunnar Faith-Ell replied on Tue, Sep 7 2021 6:47 PM

rated by 0 users

Now an inside picture from the new and from the fake 2N3055

Reply

Gunnar Faith-Ell replied on Tue, Sep 7 2021 7:55 PM

rated by 0 users

And finally the Beolab 5000 is producing 60V and fully working. So the problem was the fake transistors.

Reply

chartz replied on Thu, Sep 9 2021 7:53 PM

rated by 0 users

Thanks for the update!