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Old 11th June 2014, 03:54 PM   #11
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If you even found 50 mV (> 200 mA when measured over one 0.22R resistor -- I = V/R) originally, chances are that the bias pot has contact issues and should be cleaned / replaced (in which case I'd contemplate going multiturn). Not uncommon in Hitachi amps. Excessive quiescent current makes the output transistors much more susceptible to secondary breakdown due to heating.

Most Japanese amps run 30-50 mA of idle current, mainly for thermal considerations.
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Old 11th June 2014, 06:00 PM   #12
sesebe is offline sesebe  Romania
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Please check what I wrote here:
Quote:
Originally Posted by sesebe View Post
I do not have/know exatly the schematic for HA-250 but from photo looks exatly like HA-330 which I have opened on my working table for revival.

For HA-330 the idle current need to be adjusted to have 8mV±4mV on emitor resistance which it is by the way 0.22Ω.(according to service manual)
This means that you set the current to 100mA and not to 22mA for 22mV.

On my amplifier I put supplementary in parallel with the original circuit, a multiplied diode thermaly connected with the heatsink of the power transistors for a better idle current stabilization (original circuit it is thermaly connected to predriver and not with power transistors).

At HA-330 the SCR it is for output short circuit and SOA protection and not for DC out protection. Check the values of resistors around this SCR.
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Old 11th June 2014, 06:59 PM   #13
sesebe is offline sesebe  Romania
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In my case the red circle resistors are devalued and need to be changed.

For your case with high DC offset, change the blue marked capacitor.

I already change most of small filtering capacitors and I must change the yellow marked capacitors.
The yellow marked I want to change because now it is sound very nice but can not be compared with my tuned Yamaha A-6A. The sound it is somehow noised even if I do not have any noise without signal.

You can see my modification too.
Attached Images
File Type: jpg final amplifier.jpg (330.0 KB, 38 views)
File Type: png idle current.png (28.6 KB, 36 views)
File Type: jpg P1180370__.jpg (585.5 KB, 32 views)
File Type: jpg P1180371__.jpg (497.7 KB, 32 views)
File Type: jpg P1180372__.jpg (834.5 KB, 31 views)
File Type: jpg P1180374__.jpg (106.8 KB, 19 views)
File Type: png Changed schematic.png (171.7 KB, 27 views)
File Type: jpg P1180369__.jpg (543.5 KB, 27 views)
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Old 12th June 2014, 02:29 AM   #14
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Default Thanks for the feedback

Pictures explains a thousand words.
Am I correct in thinking the physical removal of SCR901 allows the base of Q901 to go low which allows the coil of RL1 to conduct thus operating the relay ? Yes will investigate the replacement of VR701 with your workaround.
Cheers Johno
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Old 12th June 2014, 07:24 AM   #15
Mooly is offline Mooly  United Kingdom
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When SCR901 is triggered it removes the +14 volt rail by placing what is effectively a "short" across zener diode CR802. That 14 volt rail is what turns on relay driver Q904 via base current supplied by R903. Q901, Q902 and Q903 provide DC offset protection and can disable the relay by pulling the base of Q904 low.

So there are two mechanisms. Over current detection via the SCR, which once triggered removes the 14v rail until the amp is powered off, and DC offset protection as described above.
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Old 24th July 2014, 01:58 PM   #16
sesebe is offline sesebe  Romania
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johno210 how is your amp?

I manage to finish to change all the marked components on my photos.

Quote:
The sound it is somehow noised even if I do not have any noise without signal.
All "noise" and distorsion disappeared after I replaced the marked electrolitic capacitors with film capacitors 1u/63V WIMA (even in place of 3.3u and 0.47u).
Only C705 was replaced with an electrolitic.

After I replaced all devalued resistors was needed to correct resistor VR701 value from 68 to 120ohm in my workaround and I set the voltage drop across R720 to 4-5mV.

Now the sound of the amplifier it is simply awesome.
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