Peter wrote:

Dillen wrote:

Check the ceramic resistors at the regulator, and check that the regulator is running on both 2N3055.

BTW nice backdrop on that last photo. 🤒

Martin

I have a suspicion that the back drop is become some thoughtless so and so hasn't been to pick things up! Sorry Craig - on holiday next week and will try to come down! I have something you might like mending and then keep for amusement - I don't think the other collectors will have one!

Peter

I will be around all next week, but working through the day...finish 13:00 Friday, tou have piqued my curiosity ;¬)

Craig

got finished on the drivers this morning...…..all caps and trimmers replaced. I don't know if I should replace the transistors as Charles suggests, I haven't done this in the past unless one is faulty....I know Frede recommends replacing the black cap units as they are prone to failure.

Had this delivered yesterday, after struggling with the idea of spending £15 on a screwdriver I finally decided if I can spend it on a bottle of wine I can spend it on one of these...….don't know why I didn't do it years ago, so usefull

Craig,

I'm not recommending you replace the drivers; I recommend you check the HFE and match them as close as possible to each other, per channel.

So how does one do that?......isn't the hfe a fixed value that varies quite a bit from one device to another? so if if two had different hfe how would one match them without replacing one or the other.....you can see I haven't done this before 😮

You have the right tool for that purpose Craig, your DCA75!  This tool will measure and show the HFE if you scroll down the display.

The HFE is the amplification gain of a transistor and when used as complementary pairs, ideally they should have similar characteristics.

So first, for a given transistor, make sure it meets the minimum recommend manufacturer HFE.  When used in pairs, ideally, the HFE difference between the two should be around 10-20%.  Above 50% , the weaker will work much harder and may create distortion.

Before removing the transistors, mark their position and part number.  Since a pair is a combination of an NPN and a PNP transistor, you don't want to end up with the same kind as a pair...

Let me know if you need further clarifications.

Charles

As this is new to me im happy for any advice, is there a thread detailing this process? pics etc......which transistors should be checked as a matter of course, if they do turn out to be above 50% apart would one need to replace a pair?

Craig

Craig,

I don't recall seeing a thread discussing the subject.  As a standard procedure, i'm checking all transistors and checking mostly the HFE and leakage but also, the transistor has to be properly identified the DCA75.  I've seen transistors identified as diodes so this is clearly not good.

In a complementary configuration (Pairs), we are looking at a 10-20% HFE difference... if greater than 50%, look if one is below the minimum or close.  This is the one to replace, you don't have to replace both.  If you decide to buy transistors, go and buy the two kinds (PNP and NPN) in quantities of ten (approx), this will give you flexibility in your matching.   

Ok....I have done some searching through the threads and Sorens BL5000 saga provided a lot of information, I have since contacted Frede for some additional advice regarding replacements, he advises as follows:

BC 550B for npn

BC560B for pnp

When the Beolab was designed (before 1967) only a few transistors were available is really low noise versions – therefore B&O have chosen a number of different types of coarse also being able to be used and meeting the actual voltage level. But I have for many years used the above type in the pre-amplifiers.

And I am using the transistors type B because higher gain in classic electronics design is not a good idea (this is my opinion 😊)

B U T --- for the power amplifier and the power supply – I only use the original RCA types !!!

For the output power transistors and psu transistors --- I am only using the original Motorola or RCA (there are to many “fake” 2N3055 out there).

When I am writing the original it is actually the original because I re-use these transistors from Beolab 5000 which is scrapped.

However when I look around I'm having trouble finding them, can anyone recommend a suitable equivelent for them?

Hold that last request.....found some here 

reichelt elektronik GmbH & Co. KG
Elektronikring 1
26452 Sande