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Hitachi HA-4700 Repair and Restoration
Hitachi HA-4700 Repair and Restoration
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Old 3rd June 2019, 01:32 PM   #11
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Good job, guy. I suggest as an idea, to place a short piece of aluminum paper or sheet, between the leds and the darkened wall behind the leds, in one of the last pictures.
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Old 10th June 2019, 11:36 AM   #12
ubergeeknz is offline ubergeeknz  New Zealand
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Bit more work tonight. Test fit a mini switch in place of the one I broke. I'll need to 3d print an adapter but it should work here.


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Originally Posted by Osvaldo de Banfield View Post
Good job, guy. I suggest as an idea, to place a short piece of aluminum paper or sheet, between the leds and the darkened wall behind the leds, in one of the last pictures.
I pulled the burnt plastic piece out, gently warmed and straightened it out and gave it a good clean, a light sand, and a shot of gray primer. We'll see how it turns out, if not good, I'll fashion a replacement piece of aluminium or something. Maybe another job for 3d printing?

Still haven't figured out a practical solution for the rust hole in the top cover. I think brazing a small piece in may work but it's going to be rather fiddly.IMG_20190610_214051_674.jpgIMG_20190610_214701.jpg
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Old 31st August 2019, 07:12 AM   #13
ubergeeknz is offline ubergeeknz  New Zealand
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Buttoned up and calling it a day until I find another cover or the weather fines up so I can strip this one bare and tidy it up properly. I haven't updated for a while so here goes.

Gave it a power test into 8 ohm load. 27v RMS @ 1khz (one channel driven) before clipping - which is over 90 watts! Not bad for a 50W rated amplifier. Both channels achieved this, albeit seperately.

Had to retouch a LOT of solder in this thing. As I went, joints kept failing. All okay now though.

I also ended up increasing Miller capacitance to 220pF after it started oscillating again when playing into a load... That seemed to stabilize it. I haven't been able to test yet if it's hurt the performance, but it sounds absolutely fine.

1. Pushbutton replacement - after a few failed attempts at 3d printing an adapter, ended up using hotglue... Actually it works just fine.

2. Lid on and operating

3. Internals

4. New CDE filter caps

5. LED indicators

6. Knobs came up okay, not great thoIMG_20190831_145702.jpgIMG_20190831_175420_237.jpgIMG_20190831_180121.jpgIMG_20190831_180157.jpgIMG_20190831_180227.jpgIMG_20190831_180130.jpg
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Old 31st August 2019, 10:19 AM   #14
Ian Finch is offline Ian Finch  Australia
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Hitachi HA-4700 Repair and Restoration
I've only listened to much older Hitachi amplifiers than this one. Those were CFP output stage designs and sounded quite good compared to other typical Japanese consumer audio designs of the 1970s-80s period.

What are your impressions of the sound quality of this one (looks like NEC's version of RET power transistors in the pic) and was it worth the many hours of hard work in restoring a budget model like this one?
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Old 31st August 2019, 11:38 AM   #15
ubergeeknz is offline ubergeeknz  New Zealand
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Originally Posted by Ian Finch View Post
What are your impressions of the sound quality of this one (looks like NEC's version of RET power transistors in the pic) and was it worth the many hours of hard work in restoring a budget model like this one?
So far I've only listened in the garage. No obvious problems but I'll need to move it indoors to get a better read on the "sound" as neither the speakers or acoustics are up to the job.

It has a lot of power (more than rated), the output noise is very low indeed, and If the specs are to be believed it offers low (0.02%) distortion and reasonably flat response across the audible band. So it "should" sound quite good. I intend to perform some more thorough testing once I get my rig together, and I will post it for interests sake.

The phono stage offers MM or MC input and seems to have been carefully designed to minimize noise.

This was the 2nd tier Hitachi integrated in that year's lineup, the top model was rated 80wpc and used a mosfet output stage. This model and it's smaller brother the HA-3700 used a "superlinear" topology with BJT outputs.

The following year most of their integrateds were using IC output stages, and offered worse specifications. I guess this was the tail end of the "golden era" for Japanese hi-fi.

This amp had plenty wrong with it. I had to replace *every* fusible resistor, the solder was bad and I probably ended up touching up 90% of the joints due to cracking. I had to bypass some of the internal connectors which just weren't reliable even after cleaning. It loves to oscillate at several MHz and I had to increase the miller capacitance to make it stop. And this specific amplifier had suffered some pretty serious corrosion to the chassis and cover.

The potentiometers are nice and smooth and after a quick spray of electrolube have no crackles (though the volume control is neither "heavy" nor stepped like the higher end recievers), the push switches are sturdy and have a nice feel to them, the low profile is impressive for something with this much power.

Worth noting, this unit was assembled in New Zealand, so I don't know to what degree, or if that had any impact on the build quality. This was common due to punitive import levies at the time.

Worth it? I will put it down to a good learning experience, the amp was cheap but I have spent more on parts and materials than I would like to admit. Trouble is, the more recognised japanese amplifiers/recievers tend to go for stupid money - even if they are not working.

A problem with the small form factor and lightweight heatsinks, the heatsinks get real hot if it's pushed hard, and in the absence of good ventilation, the amp section becomes an oven. I am sure this contributed to its former death, that or the oscillations (or both in tandem).

If I find a cheap ha3700/ha4700 with good front controls and case, I may grab it just to finish the job properly. I wouldn't bother repairing a 2nd one, although I might raid it for parts.
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Old 8th September 2019, 06:48 AM   #16
ubergeeknz is offline ubergeeknz  New Zealand
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Managed to wire up a test rig using REW and an external sound card. THD was basically to spec, which was a nice surprise. At 1W and at 50W was around 0.02%, both channels driven with 8 ohm loads. 1khz and 10khz. I'd call that a win!

There was noise but it seemed to be mostly from the test rig, rather than the amp. I have plans to improve the rig which will allow me to perform more conclusive tests.

Now in the lounge with it hooked up on my Mission 700LE speakers and the Chromecast audio. Having the power meter has prompted an interesting observation, the first LED (which says 0.04W) barely lights up at the volume I dare listen when the family is at home Hitachi HA-4700 Repair and Restoration So far I haven't had it even up to a Watt. What Nelson Pass says about the first Watt certainly rings true here! To be fair it's a very quiet environment and a pretty open plan house.

I did notice this before, but the switch I replaced makes a nasty thump sound through the speakers when engaged/disengaged. It's the tone defeat. I'm thinking maybe it's not make-before-break and the old one was, or there's some other issue in that section, but the tone controls work perfectly, it's both channels, and I went over all the solder joints etc, so I do suspect the former. I wonder if there is a good way to fix this aside from finding the right switch which so far has proven difficult Hitachi HA-4700 Repair and Restoration

L-R balance was out of whack at low volumes, with left channel lagging by well over 3dB in parts. Found the volume pot was way off below halfway, as measured. It's a quasi log pot with a large "deadspot" around halfway, which I guess is to provide fair warning you're entering 1W+ territory. For now, I bodged 2x680k resistors from the hot end to the wiper on the left channel, they track much closer now.

The sound?

Initial impressions, clean, punchy, detailed. If any criticism, perhaps slightly brittle/bright in the top end? But I need more time with it to really assess, and to listen louder. So far, it seems to have more clarity and better imaging than my other setup.

It's also introduced a bit of a hum issue with my phono preamp, maybe the input is more sensitive, or maybe there's a ground loop. No hum or noticeable hiss with other sources.

For now I think I'll stick with it and pull my other pre and power amp for analysis and servicing. And fit RCA plugs to the turntable so I can use the built in phono stage.
IMG_20190908_142034_487.jpg1567921049123.jpg
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Old 9th September 2019, 12:19 AM   #17
mjona is offline mjona  New Zealand
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There is too much solder in those joints in the first image. A correctly made join will have a shiny appearance like silver not dull like aluminium.

There is too much solder to reheat the joints so it will adhere properly inside those blobs. The best thing is to remove all of it and start again heating the lead to be soldered first and then bringing the solder into contact.

If you don't do this the component will transfer the heat away and the solder won't flow properly and you will get another blob.
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Old 9th September 2019, 01:05 AM   #18
ubergeeknz is offline ubergeeknz  New Zealand
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Quote:
Originally Posted by mjona View Post
There is too much solder in those joints in the first image. A correctly made join will have a shiny ......
Should have known better than to post less-than-perfect looking joints here Hitachi HA-4700 Repair and Restoration

Thank you for the pointer, but in my defense, your honour, I was over it after bodging 4 different values in to get it right, and decided to "just send it anyway".
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Old 9th September 2019, 07:35 AM   #19
Ian Finch is offline Ian Finch  Australia
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$50 fine and 3 months probation!
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Old 9th September 2019, 10:38 AM   #20
ubergeeknz is offline ubergeeknz  New Zealand
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Found the source of the hum, not my preamp, but the proximity of the tonearm / phono cartridge to the power transformers in the Hitachi. Anyway the preamp has a (very small, but still concerning) wub-wub oscillation every so often, so I need to take it to the workshop anyway.

Got more time to listen today. I'm still finding it bright, but I must say I'm impressed with the dynamics and accuracy of the soundstage. I'm wondering if the channels in my other setup are well matched, phase wise. Now I've got them in the workshop I can check.

In my other preamp, I had added a bass boost which boosted the response from about 100Hz down, to extend the effective bottom end of the speakers. The bass dial does not seem to be doing the same. I have a JBL 12" sub waiting in the wings for it's amp plate, which should help, and/or I will build a bass boost module and stick it on one of the tape loops.

I realised I forgot to test the frequency and phase response before I brought it inside! I will do so this weekend and post, for posterity I guess (and completeness)
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