ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hi All,
finally managed to find a pair of Beolab Penta MK1 locally - as hardly anyone is prepared to ship
these very big/heavy speakers (sensible !!).
while I am waiting for Martin's service kit plus the "traditional" set of new foam surrounds for the midrange
bank I did a pre-check on the pair: one is working, one isn't producing any sound ☹️
a first check on the Beolab150 amp of the non-working speaker showed the following:
T1 was still wired for 220 VAC mains voltage, so I changed that to the 240 VAC setting as per SM.
no- load current adjustment did not work as no +/-68 VDC present, however 2 x 50 VAC from T1 present.
so, that looks like a dead KBU 6D in-line rectifier ?!! That one is now on order.....waiting again ?!
Before the +/-68 voltage rails are restored there is not much else I can do at the moment.
any comments and suggestions are most welcome ...so please....
ALF
Quick update:
I checked the suspect bridge rectifier off-board with my DMM and it actually tested fine, but is it possible it breaks down under load ??
shall wait until the replacement comes in.
also did a check on the two big reservoir caps (10000uf/i80V) - both were within spec at around 9740 uF but did not register a ESR reading
other than 0.00 with the Peak tester ??
can someone shed some light on this please ?
more to come shortly.......
Fuse OK?Relay activated?
Martin
Hi Martin,
Both fuses seem ok, however the strange thing happens is:
mains plugged in, the red light is on, turning on the amp nothing happens - indicator stays red, no relay click until I move the sensitivity switch,
then indicator turns green and I can hear the relay click.
This stays on (green) for a little while until a brief change to orange and ending in red - relay click is audible.
measuring again the AC points of the rectifier board I am only getting about 32VAC on both points with the rectifier still off board !!
what however should be there is 2 x 50VAC 😥
not sure what not to trust, the rectifier (probably good?), the T1 or something else I have not yet duscovered.
back on the Beolab 150 amp:
in the meantime I replaced the D2 bridge rectifier and no-load current trimmer pot R95 ... just to be safe.
I also checked (off-board) on
PCB 2 TR8 / 9 / 10...... all fine !
PCB 1 R35 (NTC), TR8 / 15 / 16 / 23 / 25 / 24 / 29 ......all fine !
from reading previous posts I feel uneasy about 1IC1 4066 as well as 1IC 2 .
of course and can get all parts off the board and check but there has to be an easier way of finding the culprit.
why am I not seing the +/-68 VDC ???
lots of pointers and support needed here..... just keep it coming....pleease
Hi everyone,
while I am waiting for parts I decided to replace the output power transistors and was inspecting the crossover boards 14 & 15.
mine are consistant with the ones shown in the MK2 service manual.
the questions I have are:
1. Do I have to replace those film caps at all ? and if YES,
2. could they also be replaced with bipolar electrolytic caps ?
most of the higher grade caps seem a total overkill, showing 250V up to 400V as there is no cap on those boards being specified higher than 100V ?! !
some advice would be very much appreciated - thank you
All has been answered while contacting Martin about possible service kits for the x-overs.
As suspected I will leave the MKT caps and focus on the electrolyte caps.
What happened in the meantime ?
well, all SA1302 original Toshiba transistors measured fine but one of the SC3281 transistors failed.
I was tossing it up whether to re-install the 1302 but decided to replace the complete set and checked they all connected to their destinations ok.
however, hope took a nosedive as the overload indicator still comes on 🤬
what am I not seing here ?? ??
I mentioned earlier that the green light comes on very briefly when the sensetivity switch is operated
while the power switch is set to ON, but the indicator turns quickly to orange.
checked basically all transistors along the bolded signal path.....does it mean the only way of triggering the overload indicator is a short somewhere !
as Martin mentioned in one of his replies to someone: : "......that typ of amp can give even a trained tech a good run for his money....."
but surely, it can be repaired...or what ??
c'mon guys, any ideas or pointers for me ? Could really do with some assistance here.
thank you....
ALf
Ok, back at the bench.
tested the transformer, and yes I am getting about -/+65 VDC....so the big new rectifier does its job.
so far I tested almost every transistor from board #1 & 2 - off-board of course ....no negative findings except the one previously mentioned
output power transistors - new set has been fitted !
one dead capacitor was replaced - 1C22
Another strange thing:
Mk1 circuit diagram has no mention of 2TR15 nor has the parts list, yet it is fitted on board #2 ?? The space for 2TR16 is vacant ?!
the MK2 circuit digram shows both of these transistors ???
I can't see the purpose of 2TR15 on board #2 when omitted in the circuit diagram and parts list ?! Explaination ?
sadly the amp still shows the orange indicator when the amp is on and 1P5 is plugged in.
I can set the amp from OFF to Auto, red indicator is on ......only goes very briefly to green when either Sensetivity or Bass level switches are shifted ?
the relay seems fine, with a coil resistance of about 820Ohm. Diodes were tested...without any negative findings.
one of the few remaining things that still bug me are the ICs 1, 2 and 3 on board #1 & 2.
i did check the new set of output transistors with my Peak tester and all looked ok.........
frankly I don't know hwere else to look - runn8ng out of components for sensible testing ?
can't be all of your experts have died from the virus - please don't be shy 🙄🤗
Ok, beginning to enjoy replying to myself, but.....none of the Beolab Penta experts seem currently available ?
anyway, kept on searching why there's permanent overload indicated ?
gone back to the big PS caps and checked their capacitance again...quite a difference betweem about 8300uF and 9800uF !!
But this should not trigger an 'overload'.
something springs to mind whether the replaced output transistors were fakes, although all showed trustworthy hfe readings ???
somewhere is a short I can't find.......so far all transistors I tested were ok, as well as all diodes.
so, just in case one of the followers has a brain-wave.....let me know.
the search continues
Next step ??
you guessed right: waiting for parts 🤗🤗🙏
alf
hi there,
very quiet reply, don’t want to wake up any experts here - a pair of new big filter cap has finally arrived from the UK - installed but sadly no luck !
not that they would do any damage but still, hope were there to finally stagger over the finish line.
in the meantime a suspicious fuse has been discovered and a new waiting game has started: coming from ...where ? you know the answer !
i could have ordered one domestically for $7.12 !!! speechless ....!! a bloody cartridge fuse 40mA🙄 used the waiting game of replacing most of the
ageing capacitors.....not that much left to replace 😁
as always, open to suggestions (but not my own, please!).....
any news, ...you will hear it
ALF: gone back to the big PS caps and checked their capacitance again...quite a difference betweem about 8300uF and 9800uF !! But this should not trigger an 'overload'.
It can.
During shutting down, both powerrails are decreasing to zero. When these rails are not equal decreasing, it might cause some voltage on the output, which causes at that moment a orange led. But most orange leds are caused by a almost ceased Tr3 on the front pcb.
Lot of output transistors on ebay are fake. When you measure the hfe of these, they are somewhere between 200-300. A genuine transistor lays far beneath the 100. Somewhere around 40-50 is more usual.
Thank you kindly for coming on board:
Understood, but in the meantime the two 10000uF caps have been replaced and should not create issues.
I also revisited 2TR3 and my peak tester said BC337 ok with about hfe = 340.
as far as the large output transistors concerns I am aware of that rather sad fact of fake components 🤬 it happened to me before.
my new sets were not purchased on ebay but from one of our large electronic suppliere (rs-components) for that very reason......?!
out of interest I tested both of the original Toshiba sets (2SA1302 and 2SA3281) with my Peak tester again:
2SA1302 set showed hfe = 100-103.
the two remaining 2SC3281 showed hfe = 55 - 72
one of the new MJL3281A showed hfe = 79 which could indicate they are indeed genuine transistors ?!?
I am very tempted to re-install the original Toshiba set of 2SA1302 and see if any change occurs followed up with the 2SC3281 originals but would have you supplement one of the new MJL3281 types.
lets find out if there is any change - really, I do not have anything else to work with.....all old caps, trimmers, big rectifier and ICs have been replaced including the relay.
Hey,
I hope you don't mind me posting this here for you and others to find, I've posted a repair video of my amps to YouTube
https://youtu.be/psY_RV1NjHM
It might have some helpful information for you and others?
Bristol based B&O fan with a collection including the following: BC9500, Beogram 9500, Beosystem 3, BeoVision Avant DVD 32, AV9000, Beolab 3500, Beolab 3000s, Beolab 4s, Beolab 6000s, Beovox Pentas, Beovox 3000s.