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Old 18th June 2011, 06:50 PM   #501
Mooly is offline Mooly  United Kingdom
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Quick update... thinking some more on this.

The -8.8 volts on the IC being low will probably be compromising its performance so we do need to do something. It's running below its internal regulator drop out voltage.

What do the MJ outputs say exactly. Are they MJ21193G and 21194G. I am wondering if they are not the best examples... I'm not saying they are faulty or fake or anything... just the absolute minimum spec. CPC list MJ21193/4G's with a quoted gain of 75. I'm wondering if these are better specced than the ones fitted. You could fit them and find they were no different however so no absolute guarantee. I am surprised it seems so critical on this but it does...
It's a problem coming up with alternatives.

You could try 3.9k resistors in place of the 4.7k's which would give a couple of milliamps more drive... fractionally more than with the 27k's bridging them. 1 watt components would be advised although if you had 0.6 watt ones they would be OK medium term to see what it was like.

Or to give the same result having tried the 27k's now go a bit lower to 22k (which makes around 3.87k) and see if the bias adjusts smoothly to its correct value.

Reason for going a bit lower to 3.9k is to see if it brings the adjustment within the more linear part of the presets travel.

Remember to start with the bias on minimum whenever you change anything.
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Old 18th June 2011, 07:04 PM   #502
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Hi Mooly - they're just standard MJ21193/4s I think, as there's no letter on the end. They're definitely 'the real mccoy' too (been reading up quite a bit on fakes), but from what you say might not be ideal. Sound level does seem to get loud with very little volume turned, but perhaps that's because it was designed before digital sources with their 2v output were the norm.

Checking them from pics I took today, underneath the MJ21193/4 model numbers, they do say BM0233 and BM0231 (or could be 8M0233/1 - pics not too clear). I think that's just the date/manufacture code though?

Just before you posted I actually just finished setting the mV (21mV per channel after warm up) and nulling the DC offset.

Working fine at present, though running is at a gentle volume level for a few hours incase any nasties show up.

I'll definitely get some 22k resistors as you suggest, and will make sure they're 1 watters.
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Old 18th June 2011, 11:06 PM   #503
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I'm terribly late to the party--apologies!

I got around to displacing the tube amp that lives on my Sony and measured getting 22mV on each channel. Need me to check anything else while I've got the top off?
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Old 18th June 2011, 11:53 PM   #504
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No not late at all - and MANY thanks for checking out the mV reading. I take it yours has the original Hitachi output transistors? (Mine doesn't hence a few measurement anomalies).

If it's not too much trouble, you could take a voltage reading from the side of R340 and R390 (the sides connecting to the 100K adjustment pots). I imagine if you're running original output transistors you'll get a -8.8V reading...

Mucho thanks
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Old 18th June 2011, 11:56 PM   #505
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No not late at all - and MANY thanks for checking out the mV reading. I take it yours has the original Hitachi output transistors? (Mine doesn't hence a few measurement anomalies).

If it's not too much trouble, you could take a voltage reading from the side of R340 and R390 (the sides connecting to the 100K adjustment pots). I imagine if you're running original output transistors you'll get a -8.8V reading...

Mucho thanks
Yup, all four devices are Hitachi branded! You want those readings relative to chassis ground?

*edit* am I reading this right? Sony used the ICs heatsink as a jumper?! Hilarious!

Last edited by thefragger; 18th June 2011 at 11:59 PM.
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Old 19th June 2011, 12:07 AM   #506
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8.75V off of pin 14 on each channel (should be 8.8V)
-8.58V off of pin 4 on the right channel (should be -8.8V)
-8.62V off of pin 4 on the left channel (should be -8.8V)

Should I care that they're just a smidgen off?


*edit* one thing to note: the voltage hit 8.8V at the initial application of power, however once the voltage settled down to 8.75V, then the protection relay clicks-in.

Last edited by thefragger; 19th June 2011 at 12:12 AM.
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Old 19th June 2011, 12:07 AM   #507
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Yup, all four devices are Hitachi branded! You want those readings relative to chassis ground?

*edit* am I reading this right? Sony used the ICs heatsink as a jumper?! Hilarious!
That'd be great - mucho thanks

Looks like! Haha - good old fashioned form follows function engineering!

It's 1am in the morning over here so going to hit the sack - will check in again tomorrow (I mean today!). Cheers for your help!
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Old 19th June 2011, 12:11 AM   #508
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Quote:
Originally Posted by thefragger View Post
8.75V off of pin 14 on each channel (should be 8.8V)
-8.58V off of pin 4 on the right channel (should be -8.8V)
-8.62V off of pin 4 on the left channel (should be -8.8V)

Should I care that they're just a smidgen off?
Ah you've done them already haha!

They look correct to me - some of those carbon film resistors set the voltages too so they may well be a bit of variation (I intend to replace R306/307/356/357 with more accurate/modern resistors next week).

Anyway many thanks for those - confirmed that the original transistors are very much tied in with the voltage readings one gets (as Mooly stated a few posts back). It's an intriguing design, this amp!

Thanks a mil for your help, Sir!
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Old 19th June 2011, 12:14 AM   #509
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Oh please, I'm just a kid with an' old amp and a set of Fluke multimeters

Mooly's the chap whom deserves all the praise!
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Old 19th June 2011, 06:54 AM   #510
Mooly is offline Mooly  United Kingdom
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Morning folks... some more thoughts on this.

The more I look at this design the stranger it becomes... everything is in the detail.

If you want to try the 22k's then any small wattage will be fine. It was only if you fitted 3.9k's that 1 watt were advisable. Its easy to work out... assume worst case and say there is 50 volts dropped it. Watts = V squared/R so thats 50*50 / 3900 which is 0.64 watt. For a 22k in parallel its 50*50 / 22000 which is 0.11 watt.

I think there could be another better way though.

If you look at the circuit around the output transistors there are small differences between the resistor values for the NPN and PNP devices. R326/327 and R317/318. That's very unusual to see that and the only reason would be to try and compensate for the difference in characteristics between the NPN and PNP output devices. We always call them complementary but they aren't really.

So this could be another possible route to improving this. I wondering whether increasing slightly R317 and 318 might have the desired effect. And that means leaving the original 4.7k's in place feeding the IC. Perhaps initially leave R318 at 22ohms and increase R317 to 10 or 12 ohm and again see what the effect on the current setting is. Always start with the pot on minimum current though. The MJ devices are more complementary than most... this was a big feature of them. R317/318 are effectively across the B-E of the driver devices. If we increase the value of these resistors then the drivers will be able to conduct slightly more for the same level of drive from the IC.
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