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Old 2nd April 2012, 07:18 AM   #1
ccheung is offline ccheung  United States
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Default HELP, repaire Cyrus LFAU similar to Cyrus II

I just acquired a pair of Mission 767 speakers with LFAU, an bass amp modified from Cyrus II according to Mission. I accidently shorted the speaker cable while seting them up. Now, the amp only has "hum" sound output. All fuses are fine. Does I fried the output transistors? What do I need to check. Other then then two ST PT77 transistors per channel, there is a small transistor in the middle of the ST PT77 attached to the heatsink, is it driver transistor?

1. Does my problem only related to these three transistors attached to heatsink?
2. Can I just replace them to solve the problem?
3. If yes, what replacement part no. for them?
4. Do I need to tune anything after replaced those transistors?

If anyone has service manual for LFAU or Cyrus II, please let me know.

Thank in advance for any advice.
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Old 6th April 2012, 01:00 PM   #2
lohk is offline lohk  Europe
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The PT77 is a customized transistor made for these amps - you wont get that in any store. Besides - don't get me wrong - it seems that you do not have much experience with repairing amps. I suggest to go to a qualified expert or dealer.
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Old 13th April 2012, 08:26 PM   #3
richto is offline richto  United Kingdom
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Hi,

The equivalent transistor is the BUV48A

If you change the output transistors, you must also re-adjust the bias current to 8mv at the left and right channel bias test points.

Also likely you should replace the large electrolytic capacitors.
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Old 14th April 2012, 08:12 AM   #4
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They were not custom transistors as stated above, simply relabelled , usual hifi manufacturers trick .
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Old 14th April 2012, 11:45 AM   #5
richto is offline richto  United Kingdom
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nb - I have the service manual and schematics for it. Give me an email and I will send you them if upload them somewhere / link to here.

Out of interest, how much did you pay for the 767s? After a pair myself.
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Old 14th April 2012, 04:05 PM   #6
ccheung is offline ccheung  United States
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Quote:
Originally Posted by richto View Post
nb - I have the service manual and schematics for it. Give me an email and I will send you them if upload them somewhere / link to here.

Out of interest, how much did you pay for the 767s? After a pair myself.
My email is cktchifi@charter.net, please send me the service manual. It only has "de de de" sound, even I replaced the output transistors and drivers mje243 and 253. Do you have any idea?

I got the pair for $1200 in locally sale.
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Old 24th April 2012, 05:30 AM   #7
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I own a pristine pair of Mission 767 Speakers and love the way they sound . I would greatly appreciate a copy of the service manual and schematics for the LFAU.
Understanding it's primary purpose is as a sub woofer amp and controller, I have always wondered if the LFAU had crossovers directing signals to the High and Mid range drivers or does it merely to facilitate bi-wiring. Another questions I have always had is do the speakers have their own crossovers? Another questions can a Cyrus Two Amp be converted to a LFAU?
Kindly advise - charlesmcdermott@hotmail.com
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Old 1st May 2012, 06:35 AM   #8
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Default Seeking info on Mission 767's LFAU

Quote:
Originally Posted by richto View Post
nb - I have the service manual and schematics for it. Give me an email and I will send you them if upload them somewhere / link to here.

Out of interest, how much did you pay for the 767s? After a pair myself.
I ran across your recent post and was filled with hope. As owner of a pair of Mission 767, I would greatly appreciate a copy of the service manuals and schematics for the CYRUS LFAU.

My speakers are pristine but I need a LFAU. I've purchased two Cyrus II amps (different versions), with the hopes one could be converted to a LFAU.

At present I am bi-amping using one amp to power the high and mid-range drivers. THE 767's subs are driven by the earlier Cyrus II via a NHT "Sub-One" controller. Bass is not my problem, not knowing what actually goes on inside the LFAU is.

Understanding the LFAU supports the 767's tri-wire posts leads me to the assumption that: 1.) There are no crossovers in the speakers. 2.) I should expect there should be crossovers in the LFAU for the high and mid-range drivers in addition to those that filter bass signals for the LFAU to amplify.

Currently I have the high and mid drivers bridged despite knowing the LFAU provided separate connections for the hi, mid and low frequency drivers. Plenty of experimenting has gone into tuning this configuration and expect it could sound even better. If only I knew how the designers intended the LFAU to work, I'm sure they probably would sound even better.

Look forward to your response,
Cheers
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Old 3rd June 2012, 03:40 PM   #9
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Hi,

Hope this helps. I have owned a 767/LFAU setup since early nineties and have done several tweaks to significantly improve sound quality and timing.

The LFAU's three outputs only add to contact resistance in the hi-mid signal paths, inside the LFAU its a straight through bar for hi-mid outs. The only signal of interest to LFAU internals is bass. Bear in mind the 767 has no crossover for bass input, its straight to the four speaker drivers so its the LFAU that profiles, boosts and shape the bass signal . If you are driving your bass drivers direct from a "normal" amp such as Cyrus 2 then you are putting full spectrum into the bass drivers, you may want to consider adding an active or passive bass filter before the bass input, the bass will be much tighter and cleaner. For a quick test most car audio stores have relatively low cost bass/sub crossover filters to hand.

767 hi and mid inputs go via a two way crossover inside the 767, I bi-wire wire direct from power amp to 767 hi and mid inputs respectively and a third wire via LFAU for bass. The hi-mid sound is a lot cleaner and better time aligned without all the contacts in the way.

Being a bass amp my LFAU always suffered from mains hum from the internal transformer. I understand later LFAU's had an input option for the Cyrus external PSX supply to deal with this. I had an electronics expert build a custom outboard power supply and disabled the internal one. Its clean of mains hum now and bass timing, positioning and separation is much better.

My 767's need light maintenance once a year, first tighten all the driver screws a quarter turn, not too much though, but they do loosen over time due to vibrations. Secondly I un-clamp all input cables , give the wires a quick once over with fine sandpaper and re clamp. The mids and highs benefit audibly. Finally I give the mahogany a once over with decent wood polish.

I usually listen with the 767 fronts off, it does make a difference, however remember to put them back on if any toddlers are about, they cannot resist poking in the mids. !!

The 767's are about timing, separation and complexity. With the right amp driving them they can reproduce very complex sounds cleanly, my amp is 90W class A so the power is always on tap to drive the demanding speakers effectively. Try rhythm of the saints by Paul Simon, you can almost "see" each individual drum in the sound stage, its the first track on the album that turns some other speakers to mush.

Been lurking for a year or so and this is my first post, do hope i don't offend anyone.
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Old 3rd June 2012, 04:49 PM   #10
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Quote:
Originally Posted by lohk View Post
The PT77 is a customized transistor made for these amps - you wont get that in any store. Besides - don't get me wrong - it seems that you do not have much experience with repairing amps. I suggest to go to a qualified expert or dealer.
That is not true. Only the labeling type number is a customized. The associated industry type number is the BUV28 (TO 220). There is also a TOP3 version with the same internal die. I don't know this type number.
http://www.datasheetcatalog.org/data...cs/mXqzrqx.pdf
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