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Chip Amps Amplifiers based on integrated circuits

Hitachi SR-4010 repair
Hitachi SR-4010 repair
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Old 10th December 2019, 12:34 PM   #1
vanakaru is offline vanakaru  Estonia
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Default Hitachi SR-4010 repair

I got this receiver as not powering up. However it does power up, but I get 25V DC on both channel output(speaker) terminals. With headphones I can hear faint radio. And I did not attach speakers to blow them up!
Is it correct to assume that the Darlington STK00300's are dead-shorted? Or something else is possibly wrong?
Transformer puts out 2x25V AC and gets rectified to +/-34V. Output circuit breakers are not blown.

Last edited by vanakaru; 10th December 2019 at 04:51 PM.
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Old 10th December 2019, 09:52 PM   #2
Douglas Blake is offline Douglas Blake  Canada
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Do you have the schematic? Please post it here.

Yes, first blush, it sounds like you have blown output transistors.
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Old 11th December 2019, 02:27 PM   #3
vanakaru is offline vanakaru  Estonia
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I can not find the schematic. There is a basic 30W amp schematic using STK0030 and it looks similar
STK0030 power audio amplifier schematic
Also the 400R 1W resistors heat up in 5min but not to scorch. On the picture marked with red stripes.
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Old 11th December 2019, 05:34 PM   #4
Douglas Blake is offline Douglas Blake  Canada
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Quote:
Originally Posted by vanakaru View Post
I can not find the schematic. There is a basic 30W amp schematic using STK0030 and it looks similar
STK0030 power audio amplifier schematic
Also the 400R 1W resistors heat up in 5min but not to scorch. On the picture marked with red stripes.
Without a schematic and not being there to probe around, I can't really be certain, but I'm thinking it most likely is the outputs.

What test equipment do you have?
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Old 15th December 2019, 12:15 AM   #5
sgrossklass is offline sgrossklass  Germany
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Having the same voltage on both outputs may also indicate a broken ground connection or a power supply issue (like a bad rectifier diode). Usually if the outputs are dead short you are seeing close to a supply voltage. It seems unlikely (though not impossible) that both STKs would have failed in the exact same manner.

So I would check supplies and try to find the feedback network and where its ground connection is, or alternatively find the input pair and determine the differential voltage between both inputs.

At least one of the marked resistors appears to be feeding a zener diode (ZD802), some dissipation is entirely expected there. You're good as long as the zeners aren't shorted.
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Old 19th December 2019, 07:17 AM   #6
vanakaru is offline vanakaru  Estonia
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Thanks! I will check further.
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Old 30th December 2019, 07:56 PM   #7
vanakaru is offline vanakaru  Estonia
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Well, I got the service manual today and the main problem is fixed. There is a R709 100R fuse resistor that was blown(showed 24K instead). I replaced it with temporary resistor and all the V's are close to normal. Output speaker terminal show 0.26V and 0.4V. I hooked up my test (junk)speakers and I got sound.
I traced the test voltages to this little thingy. Without manual it would have been too hard.
Thanks for all the support. When I get to the scanner I upload the manual and put the schematics here as well.
Some of the V's are a bit too high. Maybe it is due to 220V transformer getting 240V. There's supposed to be about 30.6V powering most of the transistors, but I get 34V. Also left and right channel is a bit different voltage wise. Should I worry or leave it?
Also the 2W resistors are getting warm still. I did not run it for long so I don't know how hot these will get but usually I have seen these being about 50-60*C.
BTW I ordered replacements for transistors C548 that supposed to be flaky. These were very cheap(substitute BC549 1EUR for pack pf 10) so I may just replace these. Moreover someone has done some repair on one of these and damaged PCB rails.

Last edited by vanakaru; 30th December 2019 at 08:05 PM.
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Old 31st December 2019, 12:10 PM   #8
vanakaru is offline vanakaru  Estonia
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Here are some photos of the schematics:

Click the image to open in full size.

And PCB(it is a bit too large to upload it here but can be viewed by clicking on the link):

ELFA DISTRELEC tehnikafoorumid
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Old 6th January 2020, 04:38 PM   #9
vanakaru is offline vanakaru  Estonia
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Before I shall put it all together: do you have any clue what possibly went wrong that blew the fuse-resistor?

Last edited by vanakaru; 6th January 2020 at 04:47 PM.
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Old 6th January 2020, 05:46 PM   #10
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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It would be important that when testing any kind of such devices, use a current limited power supply. What is this? Well, OK, simply attach a filament lamp in series with the power cord, for example getting a lamp holder with two short pieces of wire and a couple of crocodile clips in their end. Remove the fuse and connect the lamp in the fuse holder. Much like in the Mission Impossible series, when the black man make illegal connection to phone lines.

Ok, when such a device is ready, place a lamp of once to twice the power expected to be draw by the amp. When powering up the DUT, the lamp will show a short flash while charging the supply capacitors, and then it will go almost dark. This is the normal condition of the limiting action. As the filament increases quickly its resistance with temperature, further dependent on current trough the lamp, it will protect the DUT if something tends to go wrong. So you protect amp (or DUT) and yourself if an error occurs.

Once it is OK, then, attach a resistive load to the amp. of the same magnitude of the amp needs. Say, let the amp be loaded with 4Ω @ 10W, then use a 4.7Ω @ 15 or 20W, you can parallel or series resistance to get the power and resistance needed. In case of lamp, a combination of car lamps (12V) will suffice to match the buses voltages (Say, bus = 30V, then 3 lamps will do the job.).

If with no signal, the load lamps bright or the resistances becomes hot, then the series lamp will increase the bright, telling you that anything is bad.

Do this and tell us what happens. Keep us posted.
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Last edited by Osvaldo de Banfield; 6th January 2020 at 05:48 PM.
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