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This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hi everyone!
One of my Pentas Mk I just said poof. The led is still green but the speaker is silent. I was feeding it line-level through the RCA socket.
I took the Beolab 150 apart to have a look. There is no particular smell, and nothing obvious is burnt. I found the 2A fuse is toast, but the 40mA fuse is fine. I suppose the fuse could have just given up, but it seems more likely that it went for a reason.
I've been meaning to recap and check the levels on these, but I haven't gotten around to do it. I guess this is the time to do that. :-)
Anyway, does anyone have any ideas on what might be wrong, and where to start looking?
Thanks!
/ Johan
Johan, I can give you a link how it looks like when the Penta 1 has been taken apart, if you already didn't.
I had some fried components on the main board and crossover that has been replaced, replaced the big black caps as well and then my friend Ipaul took care of the rest and voila, it's alive!
Brengen & Ophalen
It's always good to have a reference.
Though, I've already taken it apart, mostly. :-)
No one? Or is the answer just to recap the BL 150 and hope for the best?
I would check the output transistors, and driver stage. More than anything, most penta amps will be due a new coat of thermal paste on the transistors as this tends to dry up, and I consider this a necessary service requirement now!
Olly
Step1: I would check the output transistors, and driver stage. More than anything, most penta amps will be due a new coat of thermal paste on the transistors as this tends to dry up, and I consider this a necessary service requirement now!
Right! New thermal paste sounds like a good idea. But would that cause the fuse to blow? Well, I guess a shorted transistor could, but still..? There is no obvious damage.
There will be likely no visible damage, and yes there will be s/c transistors. While you are in there you best off replacing the two trimmers.
Step1: There will be likely no visible damage, and yes there will be s/c transistors. While you are in there you best off replacing the two trimmers.
Cool. I guess it's time to get my hands dirty.
Would be "cool" if you could show us pictures of the repair!
Leslie: Would be "cool" if you could show us pictures of the repair!
I'll take some pictures as I progress. But today I'm going to a wedding, so there won't be much Beo-ing happening. Perhaps tomorrow.
Indeed Olly, most of the output transistors are screwed. I've done some googling but I can't find a source for those. Now, I found a forum post somewhere that says that Toshiba stopped making them. The post also mentions 2SA1943 and 2SC5200 as replacements. Is this correct? Which have you used?
Those complimentary transistors will be fine, available from all good electronics retailers ;-)
Also I recommend at the very least when powering up for the first time, instead of replacing the fuse in the carrier, connect a 60W light bulb. This will indicate any further problems (although the amp should also go into standby if there is still a fault condition, You can bypass protection but not advised to do so if you are not 100% comfortable!) and prevent you blowing fuses, but will allow the amp to work fine at low power whilst connected to a load :-)
Step1: Those complimentary transistors will be fine, available from all good electronics retailers ;-)
Great! Yes, Mouser has them.
Step1: Also I recommend at the very least when powering up for the first time, instead of replacing the fuse in the carrier, connect a 60W light bulb. This will indicate any further problems (although the amp should also go into standby if there is still a fault condition, You can bypass protection but not advised to do so if you are not 100% comfortable!) and prevent you blowing fuses, but will allow the amp to work fine at low power whilst connected to a load :-)
The only problem is that 60W bulbs are no longer legal so sell in the EU. :-)
Johan: Step1: I would check the output transistors, and driver stage. More than anything, most penta amps will be due a new coat of thermal paste on the transistors as this tends to dry up, and I consider this a necessary service requirement now! Right! New thermal paste sounds like a good idea. But would that cause the fuse to blow? Well, I guess a shorted transistor could, but still..? There is no obvious damage.
Amps aren't blown by dryed paste.
Mostly the amps are faulty because of the trimmers on the mainboard. As they become dirty/open, they lose their function and the ouput stages are getting hot caused by the incorrect idle current, which are way to high. Supplying signal to these stages and the transistors are clipping which will make them even hotter and will be fried.
Thermal paste doesn't have to be replaced.
Only if you seperate the transistor from the heatsink, then you have the re-apply new paste.
And even then you'll have to apply a small amount of paste. The function of thermal paste is only to fill the "gaps" between both metal surfaces.
Thermal paste itself doesn't have a cooling capacity. So, the bigger the layer of paste between the transistor and the heatsink, the bigger the change of a faillure again.
Thanks Beobuddy!
Being on this forums is educational. Obviously I will replace the trimmers and make the adjustments.
Now, speaking of heat. There are these thin pieces of transparent plastic film between the heat-generating components and the heat sink. There is thermal paste both on the "transistor side" and the heat sink side of these plastic pieces. That, to me, seems weird if you want to get rid of heat. I would imagine you'd want direct contact. What's the story here?
The back of the transistor, the metal part, is connected with one of legs of it. Mostly, if not always, connected to the "collector" of the transistor.
With power-regulators, they are connected with ground.
So, these metal parts carries currents and/or voltages (sorry, didn't know how to put it the exact way), and therefore they have to be isolated from the heatsink. The heatsink itself is grounded. So without the thin plastic part between the transistor and the heatsink you would have a massive short-circuit.
But the same story here. Apply as little as possible thermal paste.
Ah, of course. Makes sense. Thanks a lot!
Exactly which type is it I should get? There seem to be several versions.
Is this what I should order?
2SC5200-O(Q): http://se.mouser.com/ProductDetail/Toshiba/2SC5200-OQ/?qs=sGAEpiMZZMtQ1ytNl72Bk61bpVKF%252bR%252bC
2SA1943-O(Q): http://se.mouser.com/ProductDetail/Toshiba/2SA1943-OQ/?qs=sGAEpiMZZMtQ1ytNl72Bk5WDs8KvOf9C
I didn't mean to suggest paste aloneg would be responsible, but judging by the state and amount of goo B&O originally used in some examples it certainly isn't going to help, especially in something as tightly packed as a penta amp!I have some pictures with plenty of examples where there is dried up paste in patches over the transistors on pentas and 8000's, so some of the metal would have had some contact (all be it with a thick layer!) and other bits very little or non at all! The transistors also sometimes simply fall off when the clips released.Also the thick compound that b&O often let squelch out of the transistor side has a habit of attracting dust and other contaminants which could effect the properties of the grease....
I just don't think it should be overlooked but I guess that is my opinion. I have also seen single transistors fail and have been unable to find any other reason other than poor contact with the heatsink, and that includes thoroughly driving the amp (at high levels) for a good few hours into a dummy load afterwards...
Next time I wade through the 10,000 pictures (I am starting to label them :-/ ) I have on my PC and come across some of the heatsinks I have cleaned up I will post them here :-)
I should have emphasized above how necessary it is to replace the trimmers though, so I apologise to the OP..
Beobuddy: Johan: Step1: I would check the output transistors, and driver stage. More than anything, most penta amps will be due a new coat of thermal paste on the transistors as this tends to dry up, and I consider this a necessary service requirement now! Right! New thermal paste sounds like a good idea. But would that cause the fuse to blow? Well, I guess a shorted transistor could, but still..? There is no obvious damage. Amps aren't blown by dryed paste. Mostly the amps are faulty because of the trimmers on the mainboard. As they become dirty/open, they lose their function and the ouput stages are getting hot caused by the incorrect idle current, which are way to high. Supplying signal to these stages and the transistors are clipping which will make them even hotter and will be fried. Thermal paste doesn't have to be replaced. Only if you seperate the transistor from the heatsink, then you have the re-apply new paste. And even then you'll have to apply a small amount of paste. The function of thermal paste is only to fill the "gaps" between both metal surfaces. Thermal paste itself doesn't have a cooling capacity. So, the bigger the layer of paste between the transistor and the heatsink, the bigger the change of a faillure again.
Olly, that's fine. I didn't think that the thermal paste alone would be the cause of the failure. And I can confirm that there is more than enough thermal paste and a more than healthy amount of dust stuck on the transistors and heat sink.
I would really like to know if the transistors in my previous post are the ones I should get? I'd like to finish my order today, if possible.
I figure these speakers are really old now and deserve a good service, so I'm recapping, replacing the trim pots, rectifiers and output transistors.
Yes they are fine :-)
Step1: Yes they are fine :-)
Great! I was pretty confident, but it's always nice to know for sure.
Thanks a lot!
I'll be damned! The darn thing works again!
I replaced all electrolytic caps, trimmers, rectifiers and power transistors, and I made the adjustments in the manual.
Unfortunately, Leslie, I was too eager to see if I could get it to work that I forgot to take action pictures. I'll do that when I give the same treatment to the other speaker.
Thanks guys! Your help was most valuable.
Beobuddy: Mostly the amps are faulty because of the trimmers on the mainboard. As they become dirty/open, they lose their function and the ouput stages are getting hot caused by the incorrect idle current, which are way to high. Supplying signal to these stages and the transistors are clipping which will make them even hotter and will be fried.
I can confirm that this indeed was the case. The idle current-trimmer was way off. I took it out and measured it. Then I adjusted the new trimmer some 40 ohms or so higher. Even with that higher value the idle current was around 160 mV instead of the 30 mV it's supposed to be. Poor transistors.
Johan: Unfortunately, Leslie, I was too eager to see if I could get it to work that I forgot to take action pictures. I'll do that when I give the same treatment to the other speaker.
Have the same experience Johan! Before you know the job is done without taking any pictures!
Hi Johan congratulations :-) just one thing though, if you increased the value of the new trimmer, this would have had the opposite effect and increased the Idle curret!
Step1: Hi Johan congratulations :-) just one thing though, if you increased the value of the new trimmer, this would have had the opposite effect and increased the Idle curret!
Yes, I realized that after I wrote the post. I got suspicious, and went back to look at the schematics.. oh well, it works and I'm happy. :-)