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Beolab Penta 6603 MK1 restoration project

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StuckE30
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StuckE30 replied on Wed, Sep 10 2014 2:47 AM

While I'm certainly not going to argue with any of the points you've made the fact remains that the components are bought, paid for, and installed. No sense doing anything else at this point.

I was aware of that fact that only one of the 68uF caps needed to be changed but I'm a little OCD and can't have things I'll likely never see again mismatched. I doubt such a personal problem is uncommon around here.

I haven't had a chance to power up and set the trimmer pots yet as more important things have come up. I shall update though when I have something to do so with.

Beobuddy
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Beobuddy replied on Wed, Sep 10 2014 8:56 AM

Die_Bogener:

There is no need to use 2 of the 68uf caps on the one board. Only 1 of the two caps is responsible for the sound of the woofers. The other one is used for linearisation of the load for the amplifier.

Sorry to disagree.

But this is in my opnion not true. If you look more carefull at the filter, then you will see that the first filtering (2nd order) is done by L1 and C1.

Then the second stage L2 and C3 (combined with R1) is the second 2nd order filter.

In essence 2 2nd order filters combined makes it a 4th order. Only B&O has chosen to choose 2 different cut-off frequencies. And that's done to linearise the frequencies around the 700Hz.

Every filter has it's overshoot around the cut-off frequency. So, by choosing to different cut-off frequencies you can linearise that overshoot.

It has nothing to do with the load for the amplifier.

This kind of filtering does B&O also in it's active filtering with active speakers like the bL6000 and BL8000. Instead of using standard calculated capacators and resistors in these 4th order active filters, B&O uses modified values to achieve the same linear behaviour around the cut-off frequency.

Leslie
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Leslie replied on Wed, Sep 10 2014 11:57 AM

Wow, following this discussion with much interest guys!

Brengen & Ophalen

Dillen
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Dillen replied on Wed, Sep 10 2014 1:18 PM

I agree that all electrolytic caps on the crossover boards form part of the filtering.

Martin

StuckE30
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StuckE30 replied on Fri, Oct 10 2014 4:48 AM

Well sorry for the lack of updates, not that I think anyone is on the edge of thier chair to hear about whats come of this project... but some things came up in life and I had to box the parts up for a few weeks.

 

This evening I soldering in the last capacitor I had been waiting on, I referenced my photos of the untouched board and checked it against my work to make sure each capacitor was correct and not reversed. All was well, so in my case anyway the PCB was labeled correctly.

I attached test leads across the emitters of TR27 and TR32.

I plugged the amplifier in and kinda stood back. Nothing blew up, but the power LED was red.

I figured it needed a source to turn on, so I attached an ipod as an input.

I plugged the amplifier back in.

The light turned green for about one half second, followed by smoke, ozone smell, and... flames being shot off R99 and R100. The same resistors that were burnt up last time.

 

I'm so sad. Does anyone have any advice as to where to look for the problem?

 

Dillen
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Dillen replied on Fri, Oct 10 2014 7:02 AM

Sorry for saying this, but I wonder why you are surprised, - you haven't repaired anything yet!

Please let me copy my post of aug 22nd:

"Anyway, I think, I would have diagnosed and repaired the bad amplifier module before upgrading and
replacing a lot of components.
... And in case the amplifier module turns out to be a complete write-off, you've lost money and time too."

Søren Mexico posted something to that effect as well.

New capacitors will not fix a blown output stage. - Even shiny ones.
Repairing a DC-coupled complementary power amplifier can at times give even trained tech guys a good run for their money.
In short; Replace the output stage transistors and their emitter resistors, the driver transistors and all other bad components.
The latter being the problem part.
The constant current generator will often have suffered and the idle current trimmer series resistors etc. could also have had it.
When you think, you are done replacing burned components, power the amplifier up using a dual DC-voltage bench powersupply with
current limiting (set to almost nothing initially) and monitor the idle current and the center (output) rail DC-voltage to catch eventual
remaining faults without blowing everything up again.

Martin

StuckE30
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StuckE30 replied on Fri, Oct 10 2014 6:31 PM

I'm not too worried about the small amount of money put into the amplifier components. 90% of the monetary investment was in the crossovers, and ultimately if I can't get the amplifier working I will run the towers bypassing the amplifier.

As for finding the actual problem I fear quickly this will fall outside my scope. My diagnostic equipment is limited to a digital multimeter.

 

I'm not sure which transistors are output vs. driver.

I'm also not sure how to go about testing any of the transistors to evaluate which ones are or could be bad.

 

StuckE30
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StuckE30 replied on Fri, Oct 10 2014 6:51 PM

OK so please correct me if I'm wrong but TR25-27 (SC3281) and TR30-32 (SA1302) are the output transistors? Initial investigation looks like both of those transistors are NLA from Toshiba and difficult to source unless you want the cheap Chinese knockoffs. (I don't)

Which are the drivers?

What is the constant current generator?

I have spare emitter resistors to replace R99/R100, everything was ordered in double quantity to give the other amplifier the same upgrades.

 

 

 

 

chartz
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chartz replied on Fri, Oct 10 2014 7:11 PM

Sorry to intrude, but do you have any solid notions in electronics? You seem at a loss here Sad

Jacques

StuckE30
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StuckE30 replied on Fri, Oct 10 2014 7:26 PM

No, I'm sure I've said as much earlier in this thread.

I have an interest, learn quickly and have steady/capable hands. I knew I could handle the recapping of the crossovers and that is what I originally set out to do. That part is done and sounds great. I figured I would give repairing the amplifier a shot too, and that didn't go so well as to say I'm right back where I started.

 

I assume the transistors need to be removed from the PCB to test across the leads. I don't know what to test for, and / or if the test can be done with a DMM or if I need equipment I don't have.

 

tamtapir
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tamtapir replied on Fri, Oct 10 2014 8:06 PM

Leslie:

Wow, following this discussion with much interest guys!

X 2

/***

 

Søren Mexico
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StuckE30:
I assume the transistors need to be removed from the PCB to test across the leads. I don't know what to test for, and / or if the test can be done with a DMM or if I need equipment I don't have.

Here

Collecting Vintage B&O is not a hobby, its a lifestyle.

chartz
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chartz replied on Fri, Oct 10 2014 9:26 PM

Good link Søren. Our comrade here will have to learn the hard way by himself! But then motivation is everything!

I prefer to use the diode position, however.

I suggest our friend here should nonetheless get a nice book about the basics. However, nothing will replace a good teacher... like you Søren!

I have no merit, I learned at school. Stick out tongue

Jacques

Johan
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Johan replied on Fri, Oct 10 2014 9:44 PM

StuckE30:

OK so please correct me if I'm wrong but TR25-27 (SC3281) and TR30-32 (SA1302) are the output transistors? Initial investigation looks like both of those transistors are NLA from Toshiba and difficult to source unless you want the cheap Chinese knockoffs. (I don't)

They are discontinued but have been replaced by new versions (and new numbers). I can dig out what they are if necessary, but you should be able to google it. The new ones are readily available.

/  Johan

Søren Mexico
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chartz:
I have no merit, I learned at school

I learned at school too, but mechanics Big Smile, the electronics I had to learn the hard way, that why I only passed the link, just as a reminder, I found all the theory and basics, I have needed until now, on the internet. And I hope StuckE30 will follow the hint.

I started out with a BM 901, and after reading about it on this forum, I got the impression, that it was easy, just change a couple of caps, clean a little, and done. Well not so, the next one was a BM 2400, before I even started on that one, I decided that every time I changed or run into something I didnt know, I would look it up, like a diode, what is it, why is it there, and for what. And as a result the BM 2400 took 3 weeks of and on, before it was working. (Martin would have done it in a couple of hours). And even so I had to get help from the forum. 90 % of the time for the BM 2400 was spend on the internet. I still need a lot of help, but now at least I understand what the experts, like you Jacques, are talking about.

So StunckE30, dont give up, but start reading, there are no fast way to do it, only the right one.

Collecting Vintage B&O is not a hobby, its a lifestyle.

StuckE30
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StuckE30 replied on Fri, Oct 10 2014 11:28 PM

I haven't had any luck finding a superseded part of the transistors via google. Just a pile of links to Chinese junk knockoffs.

 

While Google and the internet in general is a great resource sometimes seeking sage advice from an individual familiar with the exact product you're working on, more so familiar with the common failure modes of its components is infinitely more valuable.

Where I'm sitting it seems prudent to replace all 8 transistors that are attached to the main heatsink body... but would that even fix it? I don't know.

What about all the smaller free standing transistors on the PCB? I wouldn't *think* them the problem based solely on my understanding that they aren't handling the kind of current that can ignite a resistor like TR25-32. Again perhaps my understanding is wrong. Just thinking outloud.

From my rudimentary understanding of the diagrams in the service manual I have it it seems reasonable (to me) the problem would be in TR24/TR29 or TR25/TR30 as the emitter resistors on TR25/30 are the ones going up in smoke. That, or somewhere up stream from there.

I may be able to find the time this weekend to remove the transistors and check them out with a DMM, it looks exceedingly simple, its just the time/labor involved.

 

 

DMacri
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DMacri replied on Sun, Oct 12 2014 1:48 PM
I have some experience with electronics and I can tell you the small signal transistors can fail and cause the output transistors to fail as well because they are not providing the right operating conditions. So yes, even the small signal transistors can have a dramatic effect. You would be wise to take Soren's advice on continued research. Your patience will likely be rewarded.

Dom

BeoSystem 5000, BeoSystem 6500, BeoLab Penta 2, AV 7000, BeoSound 4000, Playmaker, BeoLab 2500, S-45, S-45.2, RL-140, CX-50, C-75, CX-100, 3 MCL82 link rooms, A8 earphones, A3, 4001 relay, H3, H3 ANC, H6, and ambio 

pikacz5
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Hi , im would like to restore my crossovers , but i don't know what part to order as i don't have any schematics for them.  I have uploaded picture of crossover boards on my profile. Thanks a lot for you help 


Leslie
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Leslie replied on Tue, Dec 16 2014 7:28 PM

Try to contact Beobuddy, member and Penta expert...

Brengen & Ophalen

pikacz5
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pikacz5 replied on Tue, Dec 16 2014 7:31 PM

Ok thanks:)

Beobuddy
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Beobuddy replied on Tue, Dec 16 2014 8:34 PM

Sent some info. But wondering if your tweeter still functions ;-)

StuckE30
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Sorry for several months passing before an update, but I was waiting on someone else.

I ended up dropping the amplifier off at a well regarded shop locally to have it looked at. The company I work for sends him assorted items to repair so I figured it would be worth a shot.

It took him about 3 months to finally look at it and fix it. It looks like he replaced TR25 and TR30. Both replacements are Toshiba branded of the same part numbers.

He said it was fixed so I picked it up. Upon getting it home and connecting it I've found it is not fixed.... sigh.

 

The amplifier doesn't roast the emitter resistors anymore, and the green light stays illuminated when it has an input. There is no output though. The only speaker than makes any audible noise is the tweeter, using the high level inputs, with my receiver driving a +15dB (max) volume into the Penta amplifier. Even then its barely discernible as the music selection I had playing when I hold my ear directly to the tweeter.

I tried the RCA input with the same results.

Oddly though if I connect or disconnect the RCA input while the amplifier is powered on I get a VERY strong HUM through all drivers as is to be expected when only the center prong of the RCA is connected to any other amplifier. It seems the amplifier is capable of amplifying the hell out of that signal....

StuckE30
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StuckE30 replied on Sun, Jul 10 2016 4:05 PM

Well after a year and a half I finally was able to recover my amplifier from the shop I'd left it with. What an absolute nightmare.

After paying them for the botched repair and bringing it back they said they would get it working. One excuse after another it just never happened.

Looking at the amp as its been returned to me I can honestly say I don't see any evidence they did any further work on it.

Now I'm in the same situation as so long ago, with a non functioning amplifier. The wife has pretty much had it with the non functional Penta's in the bedroom so I either need to get this unit repaired and into service... or get rid of the Pentas as it.

Anyone have a suggestion as to who I could mail this off to for proper repairs?

Beobuddy
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Beobuddy replied on Mon, Jul 11 2016 10:03 PM

Send it over.

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