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

31 replies
GF
Offline
Gunnar Faith-Ell
Bronze
Posted August 11, 2021 at 09:39 GMT
#2213410

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).

DI
Offline
Dillen
BronzeModeratorFounder
Posted August 11, 2021 at 12:07 GMT
#2213411

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

GF
Offline
Gunnar Faith-Ell
Bronze
Posted August 12, 2021 at 19:57 GMT
#2213412

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

GF
Offline
Gunnar Faith-Ell
Bronze
Posted August 12, 2021 at 19:57 GMT
#2213413

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

DI
Offline
Dillen
BronzeModeratorFounder
Posted August 12, 2021 at 21:30 GMT
#2213414

That transistor is a counterfeit/fake.

See this:

GF
Offline
Gunnar Faith-Ell
Bronze
Posted August 13, 2021 at 16:53 GMT
#2213415

ok, I will order new transistors.

Thank you 

Gunnar

 

 

DI
Offline
Dillen
BronzeModeratorFounder
Posted August 13, 2021 at 17:55 GMT
#2213416

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

Martin

C(
Offline
chartz (archived2)
Bronze
Posted August 14, 2021 at 09:27 GMT
#2213417
Hi,

I hadn’t seen this thread.

Yes, as Martin says, definitely counterfeit or fake 3055’s. You would be surprised if you opened one 😎

Even real noughties ones tend to pose problems on devices that used the original RCA ones.

The best bet is to buy untouched old 60’s and 70’s amps that used RCA or Motorola ones. They are aplenty and usually very, very cheap.
GF
Offline
Gunnar Faith-Ell
Bronze
Posted August 15, 2021 at 17:10 GMT
#2213418

Ok, I will see what I can find.

 

Gunnar

hemenex
Offline
hemenex
BronzeFounder
Posted August 16, 2021 at 10:33 GMT
#2213419

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

or
BeoWorld Gold Membership card
Join BeoWorld

Register to access the forums, reference material, and exclusive member benefits — including our monthly prize draw.

Become a Member

December 2026 Prize Draw

Gold Members Only

As a Gold member you're automatically entered into our exclusive prize draw — your chance to win iconic Bang & Olufsen products. A unique benefit of BeoWorld Gold membership, every draw brings a new opportunity to win something special.

View Prize Go For Gold