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Old 10th June 2014, 05:41 AM   #1
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Default Hitachi HA-250 Amp no output

Hi there,
Anyone had experience with this amp.
A love job for a friend it appears the speaker protection relay is not operating at turn on. The mains led lights up but no relay click. Friend said the same symptoms happened previously with amp & was repaired. So can someone point me at what components could be causing this, cannot find a schematic.

Cheers Johno
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Old 10th June 2014, 07:29 AM   #2
sesebe is offline sesebe  Romania
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Usualy this hapends when are DC present to output.
It is not a simple repair like change this and this component.

First is need to check if DC it is present on OUTPUT (before relay). If yes, check the final transistors and prefinal transitors. If the final transistors are broken, check the emitors resistors too.

BUT, if you do not have experience in repairs, better ask for a experienced person.
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Old 10th June 2014, 07:30 AM   #3
Mooly is online now Mooly  United Kingdom
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I haven't a circuit but would venture to say its an easy fix.

Check for DC offset at the amplifier outputs. If present check for failed output devices. If these are short then most likely reason is intermittent failure of driver and VAS devices. So replace all if needed. Low value emitter resistors could have suffered so check these to. Identify the type of output stage, EF or CFP and set the bias in the right ballpark for such a stage. And use a bulb tester while diagnosing and working on the amp.
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Old 10th June 2014, 09:33 AM   #4
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Default Final stage

According to user guide is a emitter grounded inverted darlington pair from 1970's. The finals are a pair per channel of 2SD180/2SA626.

Can confirm the relay coil for the o/p protection has no supply across it.
Without a cct I think I'm stumped is it worth taking some voltage measurements across the output devices & posting them ?

Cheers Johno.
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Old 10th June 2014, 11:22 AM   #5
Mooly is online now Mooly  United Kingdom
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The two things to initially check are the DC offset of each power amp. That's the DC voltage at the amplifier output before the relay. And with the amp off you could check to see if any of the outputs are short circuit collector to emitter.
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Old 11th June 2014, 06:51 AM   #6
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There's usually plenty of old threads here with references to any available info.
Hitachi stereo amplifier HA-250
Have a look at post #20 suggestion and others.
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Old 11th June 2014, 07:51 AM   #7
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Default Thanks

I found that attached thread very helpful cheers.

Another post I found helpful too. Specifically comments at the bottom of thread. Hitachi HA-250 | whathifi.com

Found that SCR which controls the relay coil for the speaker protection & removed it. The SCR is located left of the power transistors - TO-220 packages (2 off) now the relay kicks in (very good, but why ?)

I then measured the DC offset on spkr outputs & there was 90mV on 1 channel & 45mV on the other. I removed the 4 output devices & they measured OK dc wise no collector / emitter shorts. Found the idle current as you suggested across the output emitter resistors was high approx 50mV (50mA) on both ch's, have moved this back to 22mV (22mA) as recommended. The amp seems stable enough over a 4 hour period of running I wonder if you think that 90mV DC offset is too high I guess this would amount to 1/8W of spkr coil power across 8 ohm spkr. Is it likely to be a problem ? Other than that it appears to sound OK through speakers

Cheers Johno.
PS Nice part of the world Coffs Harbour.
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Old 11th June 2014, 08:01 AM   #8
Mooly is online now Mooly  United Kingdom
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An offset of 90 mv isn't excessive for a product design of that era. Power in an 8 ohm load is (0.09*0.09)/8 so 0.0010125 watts.

Would have to see a circuit to get a clue as to why the part you removed is inhibiting the relay drive.
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Old 11th June 2014, 10:16 AM   #9
sesebe is offline sesebe  Romania
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Quote:
Originally Posted by johno210 View Post
Found the idle current as you suggested across the output emitter resistors was high approx 50mV (50mA) on both ch's, have moved this back to 22mV (22mA) as recommended.
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. 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.

Last edited by sesebe; 11th June 2014 at 10:19 AM.
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Old 11th June 2014, 03:19 PM   #10
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Default My typo on previous post

I set bias current to 22mV across the 0.22ohm emitter resistor which should set the idle current to 100mA per channel, you agree ?

Cheers Johno
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