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.
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.
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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.
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.
Attachments
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?
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.
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.
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.
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.
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Here are some photos of the schematics:
And PCB(it is a bit too large to upload it here but can be viewed by clicking on the link):
ELFA DISTRELEC tehnikafoorumid
An externally hosted image should be here but it was not working when we last tested it.
And PCB(it is a bit too large to upload it here but can be viewed by clicking on the link):
ELFA DISTRELEC tehnikafoorumid
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.
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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This is not the first device I repair(it is the hobby of a sort) so Current Limiting Device is in my toolbox for long time already. I am in the stage with this receiver where all test-point voltages are acceptable - everything runs a bit(10%) hot due to our mains is 10% higher(240V).
I have some crappy car speakers I buy for 10cents a pair that I use for testing. Have not lost more than one so far. So when I hook these up the I get normal (radio)sound through the speaker terminals.
I have 5R and 10R 20W resistors I can use for load or said 4R 100W speakers. However I do not understand this entirely. Can you explain step by step?
I have some crappy car speakers I buy for 10cents a pair that I use for testing. Have not lost more than one so far. So when I hook these up the I get normal (radio)sound through the speaker terminals.
I have 5R and 10R 20W resistors I can use for load or said 4R 100W speakers. However I do not understand this entirely. Can you explain step by step?
The resistance of a lamp is not constant, it depends on the current flowing triough it. So if you put any of them as an amplifier load, if the amp is OK, the lamp will not bright until signal of sufficient amplitude is "outputting" from the amp. If lamp brights with no signal, the amp is defective. In any case it is cheaper than a good speaker, and more secure for the amp.
I made a serial of three 10W 12V (halogen)lightbulbs. When connected to speaker terminal there is nothing - no light what so ever.
I am a bit worried about the 2W resistors R805/6/7 getting hot(60*C) or maybe it is normal. And I did not replace the blown resistor R709 with fusible. Or should I?
Otherwise it seems to be working normally.
I am a bit worried about the 2W resistors R805/6/7 getting hot(60*C) or maybe it is normal. And I did not replace the blown resistor R709 with fusible. Or should I?
Otherwise it seems to be working normally.
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