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Old 4th Mar 2021, 10:42 am   #21
Max Ripple
Pentode
 
Join Date: Oct 2016
Location: Near Coventry, West Midlands, UK.
Posts: 183
Default Re: Beolab 150 overload fault

Back to basics time?
You mentioned earlier that you have replaced the main psu caps.Can you make sure that the supply is producing +68 and -68 voltages?
It s along shot too,but the supply relay(it only switches the +68/-68 on)may be faulty,so that in fact only + or -supply is provided?
The output stage has to "balance" in dc terms,and settle at zero dc voltage at the output.
This zero point is "fine tuned" by R59 to achieve a null of only a few millivolts.
You don't mention whether you checked IC1,and I'm still mystified about the output stage emitter resistors that you refer to as 470mOhm,instead of 0.47ohm?
The circuit should run without the new expensive output devices in place,to avoid damaging them,so that the voltage levels can be assessed around the circuit.
Other than that,it's down to further close inspection for cracked print,and of course,doubly checking every thing you have done.
As mentioned earlier we all get set's that defy logic,and then we are shame faced to discover that we have been "chasing our own tails" and "can't see the wood for the tree's"
Metaphor's are us?
Nick
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Old 5th Mar 2021, 2:34 am   #22
cloudrock
Triode
 
Join Date: Aug 2020
Location: Perth, Western Australia, Australia.
Posts: 19
Default Re: Beolab 150 overload fault

Yes, quite right !

I can measure the +/- 68VDC at P5 Pin1 & 3, which is present pretty much instantly

Also, again: all emitter resistors have been replaced with wire-wound
5% 0.470 Ohm resistors.

5IC1 - the MPSA13 transistor - measured at hFe > 32000 on my Peak tester.

From memory, I believe I already replaced the PSU relay, but having -/+68VDC present should eliminate the relay as a suspect, shouldn't it ?!

As for broken tracks on board#1 that has been checked over and over again with numerous continuity measurements.

So, pretty much back to basics ?!

And again: the moment I connect P5 to board #1 the protection kicks in !

A next step is to re-check all replaced capacitors for correct polarity - no one is totally imune from those silly mistakes.

I earlier indicated the slim possibility of a faulty protection circuit ??

More after the work......

Regards
Cloudrock
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Old 7th Mar 2021, 3:17 am   #23
cloudrock
Triode
 
Join Date: Aug 2020
Location: Perth, Western Australia, Australia.
Posts: 19
Default Re: Beolab 150 overload fault

Ok, some further work completed:

I did a polarity check on all! previously replaced electrolyte caps - all correct !

Next step was to re-test and reflow the following transistors: TR23/24 & TR28/29
All good.
The PSU relay - a potential canidate for delivered unbalanced power as Nick pointed out - had indeed been replaced and believe we can discard any contact corrosion as a cause ?!

I noticed a problem with the +/-15VDC supplies and replaced the old voltage regulators 1IC5 & 1IC6.

In one of my first suspicions I had replaced the big rectifier - a very fiddly exercise I may point out - but lost one contact during continued work and one soldering eye on that tiny flimsy pcb its mounted on.

As the rectifier and its connecing wires have no connection with the housed IC1, the MPSA13, the new rectifier will be wired separately to its connecting wires.

A reminder, all B&C from all output transistors are still not connected !

This is a far as I got so far - will report back about the next steps.

As always, thanks for following

Cloudrock
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Old 10th Mar 2021, 6:58 am   #24
cloudrock
Triode
 
Join Date: Aug 2020
Location: Perth, Western Australia, Australia.
Posts: 19
Default Re: Beolab 150 overload fault

Hi all,

Short delay in further progress due to a couple of minor accidents with the big reservoir caps, discharged the "wrong way" - now damaged.....
And a subsequent damage in the main rectifier !

Replacements are on the way - more attention & caution next time, silly me

I'll be back

Regards, cloudrock
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Old 10th Mar 2021, 10:49 am   #25
vinrads
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Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
Posts: 3,737
Default Re: Beolab 150 overload fault

Hi I have just read this from the start , can't wait for the next episode , and I hope a positive outcome ,good luck , Mick.
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Old 26th Jul 2021, 2:50 pm   #26
david656
Triode
 
Join Date: May 2008
Location: bristol
Posts: 19
Default Re: Beolab 150 overload fault

Hi, I'm really sorry to read the problems you're having with the Beolab.

I'm a long time forum lurker and haven't been on here for some time. when I logged in it said I last visited in 2009!

I've just uploaded a repair video of my Penta 2 amplifiers to YouTube, and whilst I think you're in deeper than what i've covered, I thought it might be of interest to you and others.

https://www.youtube.com/watch?v=psY_RV1NjHM

I had to replace the large transistors myself as I had one fail on a Penta 1. I cover and explain this in the video. I'd love (as i'm sure you would), to know what the issue is with your amplifier so that information could be added to the video to hopefully help others.

Good luck!
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