Beolab 4000 Type 6638

[dahiyas]

Hi,

Need help with beolab 4000’s. Recently purchased a pair for 50$.

Symptom – When I play music from a source the status led turns from red to green but no sound. On checking the power supply, I found that fuse F3 and F4 are blown.

Disconnected the AMP PCB2 connections, replaced fuses and tried powering up again (in standby) and the fuses F3 and F4 blew again. The status led was red throughout. (even with the blown fuses). I am getting 12.36V on the secondary on the standby transformer.

I moved the PCB2’s to another BL4000 and the PCBs are good on both the speakers.

Is it a bad standby transformer ? It’s odd as the second speaker also has the same issue.

Any guidance will be appreciated.

Thanks

 

[Glitch]

dahiyas wrote:

Is it a bad standby transformer ?

Most likely. The transformer could be shorted internally. I believe the best way to check this is to pull the transformer off the circuit board and check to see if each of the primary coils is independent from the other.

Glitch

[dahiyas]

Thanks Glitch. I pulled out the standby transformer and yes looks like the primary coils are shorted :-(. I am getting continuity between the two primaries.

What could have caused both (speakers) of them to go bad at the same time ?

Any suggestions on replacement ?

 

[Glitch]

Burned-out standby transformers seem to be a pretty common problem for B&O speakers of that era. The transformers in my BL4000’s appear to be a bespoke part, only having B&O markings and no markings from the supplier that produced them. Based on the prevalence of identical failures, the part was poorly designed, but well manufactured (at least from a repeatability standpoint).

What is interesting about this particular failure is that a failed transformer still outputs a random voltage. This is why you still see the red light, which is pretty robust to voltage variations. The green light is more sensitive to the actual voltage value, so this may or may not work with a failed transformer.

I’m interested in hearing if anyone else has a already has a solution or knows of a replacement part. My research didn’t find much beyond people ordering a replacement part/board from B&O. I assume that that particular pile of parts is long gone.

I’m currently working on an alternative solution, but it is going slowly. Maybe one of the transformer candidates that come in with the next parts order will be a winner. Please let me know if you make any progress and I’ll do the same.

Glitch

 

[dahiyas]

Sure Glitch.

Yes, with the bad standby transformer , the status LED did turn green was signal was detected.

I did check with my local contact and he didn’t have the part. He does have a couple of 220v version (not sure why he has those in the US) but said that it would require the main power supply also to be converted to 220v and then use a step-down transformer….too much of a pain so I will wait for something to show up on ebay. I managed to reuse the amp pcb’s on two other BL4000’s that have developed foam rot related issues (static/hum).

Will surely update if I find an alternate solution. Thanks again !!

[Glitch]

I got my BL4000’s running. They had multiple issues including a bad standby transformer, fried relay, and foam rot corrosion. The corrosion not only dissolved some of the traces but also broke some of the connections between the SMD lead and the SMD component body. It was a challenging debug for all of the wrong reasons. 😉

I have a solution for the standby transformer that uses readily available parts. However, there are some (possibly safety related) caveats associated with it. Send me a PM if you are interested in the details.

Glitch

[dahiyas]

Thanks Glitch. PM sent.

[Author]

Posts