Hello, I'm trying to fix my friend's amplifier. It's an old Hitachi hta-4000 .
He said it won't play anymore. I have turned it on, no output, Vu meter is not working either but on this amp it shows the output power in watts, so no output , it does nto show anything. I measured DC on output it is 0.4v on both channels, that's kind of high.
I tear it apart, output transistors are alive. I have the service manual. it's a weird amp at least for my eyes.
It uses some " metal fuse resistors " I never heard about until now. I bet some of them are burnt, Can I replace them with regular resistors , same value? but then it won't have " protection" anymore. Please take a look at the manual / schematic. Maybe you can guide me a bit on what could be wrong.
I suspect these fuse resistors, since the output transistors are alive.
He used it with a fan on the heatsink, so no over heating issue, but I bet he listened very loud for a long time and something went wrong .
Thank you, Bruno.
He said it won't play anymore. I have turned it on, no output, Vu meter is not working either but on this amp it shows the output power in watts, so no output , it does nto show anything. I measured DC on output it is 0.4v on both channels, that's kind of high.
I tear it apart, output transistors are alive. I have the service manual. it's a weird amp at least for my eyes.
It uses some " metal fuse resistors " I never heard about until now. I bet some of them are burnt, Can I replace them with regular resistors , same value? but then it won't have " protection" anymore. Please take a look at the manual / schematic. Maybe you can guide me a bit on what could be wrong.
I suspect these fuse resistors, since the output transistors are alive.
He used it with a fan on the heatsink, so no over heating issue, but I bet he listened very loud for a long time and something went wrong .
Thank you, Bruno.
I checked most of the transistors, resistors, " fuse resistors" they seem fine. Something wrong with the protection ic I think because I can not hear the relay work , when I press A or B speakers. If I measure DC at speaker output it shows 0, when I press speakers A for example and measure at speaker A i had 0.5v but as soon as I connect a speaker it shows 0. Tweaked the pots very slowly now I have 150mV at the output, same behaviour when speaker connected it goes to 0. but you can not hear any relay when pressing speakers A or B . So i still think it is in protect mode. and not letting it turn on.
It uses the HA12002 .
It uses the HA12002 .
Of course on the output from the output transistors BEFORE the relay, to be sure there´s no offset larger than +-50mV.
But I guess you knew that already 😉
The minus rail capacitor stays charged, the plus rail doesnt. After turning the amp off . 1v on the positive rail and -44 on the negative rail.
Can this tell me something?
Can this tell me something?
I have desoldered the jumper of the defect output channel , the one with 38v dc offset. let only the other channel. Relay turns on now, I have sound on that channel !. So the Left channel is bad, I have to check why.
Two resistors were burned open circuit. And one diode .
I replaced the diode 1s2473 with 1n4148 hope it's fine.
And the fuse resistors 100 ohm with ordinary 1/4w 120 ohm as that was all I had. Hope is fine .
I will get 100 ohm tomorrow but Im not sure I can find fuse type.
Anyway, amp works great.
Just need to consult the manual again to set proper quiss current.
Thank you @Boydk for the suggestions.
I replaced the diode 1s2473 with 1n4148 hope it's fine.
And the fuse resistors 100 ohm with ordinary 1/4w 120 ohm as that was all I had. Hope is fine .
I will get 100 ohm tomorrow but Im not sure I can find fuse type.
Anyway, amp works great.
Just need to consult the manual again to set proper quiss current.
Thank you @Boydk for the suggestions.
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Hi xXBrunoXx,
Those are fuse resistors. It isn't uncommon to see them used neart the top of their ratings. Running the amp hard would increase inside temperatures even with a fan. The others may also fail. I would replace them with one wattage size up, metal oxide would be fine.
Now about running the amp hard with a fan. Continue doing this and the next thermal fuse that will fail is the one in the power transformer (not replaceable). I've seen this so often as a technician. There really isn't a good solution except to use a more robustly built amplifier.
-Chris
Those are fuse resistors. It isn't uncommon to see them used neart the top of their ratings. Running the amp hard would increase inside temperatures even with a fan. The others may also fail. I would replace them with one wattage size up, metal oxide would be fine.
Now about running the amp hard with a fan. Continue doing this and the next thermal fuse that will fail is the one in the power transformer (not replaceable). I've seen this so often as a technician. There really isn't a good solution except to use a more robustly built amplifier.
-Chris
No real problems. You are just testing. These may run warm, I haven't calculated the actual power but know the general use. I would suggest metal oxide types, but you could use metal film as well. Carbon film - naw.
Just let your friend know that running it hard like this will eventually open the power transformer. That would be a shame.
Just let your friend know that running it hard like this will eventually open the power transformer. That would be a shame.
@anatech Thank you !. I also from time to time " abuse" my amplifier, about that , I was a bit worried when I touched the heatsink because it was hot to the touch but after testing with several thermometers, highest temperature was around 55 degrees celsius. By my hand I would've said something like over 70 , wow. Used it at a party for several hours, the transformer was not even hot, slightly warm like touching your forehead. This thing is build great ! I love it, I'm also thinking it might need re-cap soon as it is from 1993, Pioneer A-702r.
I didn't knew that you can't change the " internal fuse " , " Continue doing this and the next thermal fuse that will fail is the one in the power transformer (not replaceable) " I saw a few transformers with a fuse u described but the leads were outside so you could " bypass it " and use the transformer just fine, with the risk of burning it for sure. But I think these are under sized chinese transformers, that run hot even at idle . I don't need to worry if it's damn cold even after long periods of use should I?.
I didn't knew that you can't change the " internal fuse " , " Continue doing this and the next thermal fuse that will fail is the one in the power transformer (not replaceable) " I saw a few transformers with a fuse u described but the leads were outside so you could " bypass it " and use the transformer just fine, with the risk of burning it for sure. But I think these are under sized chinese transformers, that run hot even at idle . I don't need to worry if it's damn cold even after long periods of use should I?.
Well, transformers have high thermal mass. They take a while to heat up, but the internal heat will be higher. Then they stay hot for a long time.
Yes, they started to undersize transformers in the 1980's. That bit them! Some transformers you can get to the fuse, but here it is illegal to short them, you must replace the fuse with the same rating and reposition them properly.
Basically, if you are really heating up the heat sinks, you should be concerned about the power transformer. At least check it. Avoid opening the thermal fuse, they got smart and began making it extremely difficult to get to them without destroying the transformer.
Yes, they started to undersize transformers in the 1980's. That bit them! Some transformers you can get to the fuse, but here it is illegal to short them, you must replace the fuse with the same rating and reposition them properly.
Basically, if you are really heating up the heat sinks, you should be concerned about the power transformer. At least check it. Avoid opening the thermal fuse, they got smart and began making it extremely difficult to get to them without destroying the transformer.
Nice find...... and congrats with fixing it.
Isn´t it nice to have 2 cheap fusible resistors blow instead of the output transistors?? 😉
Anyway... @anatech has said it all. When replacing the 100 ohm with 1 watt fusible or metal oxide, you´re done. 1N4148 as replacement
is fine. Read the manual for how/where to measure and adjust idle current and output offset, and warn your friend to use the volume control
reasonably.
Nice job 👍👍
