Hitachi HMA-7500 MkII project

Bert 1100

Super Member
I bought this power amp about a month ago, from a local hoarder. It wasn't cheap (300$) but the fact that I could see it in person and test it beforehand ....and it has great big meters and MosFET, and twin power supply....OK, OK, I couldn't help myself :biggrin::rolleyes:.

Anyway, It's all working reasonably well. The only few things are burnt-out bulbs, intermittent leds (4 Ohm - 8 Ohm - BTL) and a non-adjusting idle current for the left channel (oops, that had me worried !)

That idle not adjusting turned out to be just a very dirty trimpot, the wiper just made zero contact onto the track...after desoldering to test and a thourough clean with deoxit it's adjusting just fine.

This model is older than I first thought, from 1980, soon 40 years old.....

The goal is to restore it fully : replace all electrolytic caps, all fusible resistors, most ceramic caps (more likely not needed but as long as I'm in there, and it's cheap enough...).
I'll also redo the lighting to leds.
And the main and secondary rectifiers will be changed to soft recovery ones.

Right now it's pretty much all disassembled (a bit frightening :) ), and I have all of the parts. For the main caps (8 of them...) I chose Nichicon LS series, upping the capacitance from 5600uF to 6800uf.

Next is a good wash for everything apart from the transformers, relays and meters.

IMG_4491.JPG

IMG_4492.JPG

IMG_4495.JPG

IMG_4493.JPG

IMG_4494.JPG

IMG_4643.JPG

IMG_4647.JPG

IMG_4648.JPG
 
Now a quick question : does anybody know of drop-in replacement relays ?
I couldn't see any with the proper pin placement, the contact legs are offset.
the originals are DEC type AP2U 24VDC...
See the photo below, the relay was unsoldered (photo from my Marantz PM-750 DC, same relay)

IMG_4144.JPG
 
Throw your old bias trimmers in the garbage and buy new ones.
That pic isn't particularly helpful.
 
I bought this power amp about a month ago, from a local hoarder. It wasn't cheap (300$) but the fact that I could see it in person and test it beforehand ....and it has great big meters and MosFET, and twin power supply....OK, OK, I couldn't help myself :biggrin::rolleyes:.

Anyway, It's all working reasonably well. The only few things are burnt-out bulbs, intermittent leds (4 Ohm - 8 Ohm - BTL) and a non-adjusting idle current for the left channel (oops, that had me worried !)

That idle not adjusting turned out to be just a very dirty trimpot, the wiper just made zero contact onto the track...after desoldering to test and a thourough clean with deoxit it's adjusting just fine.

This model is older than I first thought, from 1980, soon 40 years old.....

The goal is to restore it fully : replace all electrolytic caps, all fusible resistors, most ceramic caps (more likely not needed but as long as I'm in there, and it's cheap enough...).
I'll also redo the lighting to leds.
And the main and secondary rectifiers will be changed to soft recovery ones.

Right now it's pretty much all disassembled (a bit frightening :) ), and I have all of the parts. For the main caps (8 of them...) I chose Nichicon LS series, upping the capacitance from 5600uF to 6800uf.

Next is a good wash for everything apart from the transformers, relays and meters.

View attachment 1112732

View attachment 1112733

View attachment 1112734

View attachment 1112735

View attachment 1112736

View attachment 1112737

View attachment 1112738

View attachment 1112739


Really nice amp.

Yes get rid of those trimpots.

:lurk:
 
Throw your old bias trimmers in the garbage and buy new ones.
That pic isn't particularly helpful.


I got new Bourns trimmers, 6 identical ones in there, 500 ohm. They are very easy to adjust, so I went with single turn ones.
About the relay photo, I'll unsolder one and give actual dimensions.
 
Ok, these are the relays I'd like to find a drop-in replacement for...anybody ?
They are working well, but in a rebuild like this having new ones would be good. If not I have some Schrack relays that I can glue upside down on the board next to the original placement and run wires to them....but if there are drop-in replacements, that would be much nicer :)

dimensions are in millimeters.
IMG_4650.jpg IMG_4649.jpg
 
The 2 boards and the heatsinks are now cleaned up ( dishwater soap, hot water, then alcohol to remove the old flux, then soap and water again, then compressed air)

IMG_4654.jpg

I've now removed the glue, was quite a pain, it took a good soaking with acetone.

I wonder why my leds were intermittent :rflmao::rolleyes:

IMG_4653.jpg

Q801 and Q802 were getting quite hot in their flameproof sleeves...

IMG_4656.jpg
 

Attachments

  • IMG_4654.jpg
    IMG_4654.jpg
    124.9 KB · Views: 29
I've about finished with the 2 boards, but as usual :no: :rant: I've forgotten a few parts in the order. This has been happening pretty much with all my latest projects :rolleyes:

I'm missing 2x 220uf 6.3V caps, I think I used them up to finish the Yam CR-620 I was doing last :whip::biggrin:
Also I failed to see 2 of the 10uf 16V were bipolar, so I don't have those either...

Anybody to help with the relays ? (see post N°6)
 
The 2 boards and the heatsinks are now cleaned up ( dishwater soap, hot water, then alcohol to remove the old flux, then soap and water again, then compressed air)

View attachment 1114261

I've now removed the glue, was quite a pain, it took a good soaking with acetone.

I wonder why my leds were intermittent :rflmao::rolleyes:

View attachment 1114262

Q801 and Q802 were getting quite hot in their flameproof sleeves...

View attachment 1114263
And I thought that only Yamaha is really good with provisioning these kinds of solder joints
 
Anybody to help with the relays ? (see post N°6)

All I can say is the relays are obsolete and finding a replacement is hard. My MCS 3285 uses 3 of these relays to run both sets of speakers and the headphone jack.

I ended up removing them and carefully cut the glued seem that hold the cover of the relay to the base. You have to be patient and take your time, but it will come off cleanly. I then used an old school Ignition point file to carefully clean the contacts. A ignition point file is very thin and produces a very fine cut. Then once they are clean of pits and shiny, I then used a dab of craft tacky glue on each end to put the cover on.
MCS 3285 relay cover removal.jpg
 
Thanks ! In my Marantz PM750DC with the same relay, I cut open some square holes in the top to access the contacts and polished them.

For the Hitachi, I ended up using some common relays and glued them upside down on a free part of the PCB.
I finished the amp quite some time ago and it is now in my system, together with my restored Kenwood basic C2 preamp.
I have to post some photos soon !
 
Agree with just wiring in a suitable relay "dead bug" style. You could even get crafty and use just the base of the old relay (keep the pins, scrap the rest) and fix it onto a suitable relay.
Great pics of bad solder BTW.
 
[QUOTE="Bert
Hello. I’m in the middle of restoring my mkII. I bought it non working and it’s a mess someone tried fixing it before. All the mosfets were gone and a lot of components were shot open. I tried using buz900 and buz905 to replace the mosfets. Anyway. Question here is can you tell me. What did you replace the fusible resistors with. Could you point me in the direction of where to get them thanks.
 
Now that I see my list, I'm not sure I used fusible resistors. please look at the parts in the list. good luck !
 
Now that I see my list, I'm not sure I used fusible resistors. please look at the parts in the list. good luck !
Hey thanks. For the info. I replaced all those fusible resistors with flame proof resistors. But it made no difference. I can get sound out of both channels one channel st a time. But as soon as I turn up the volume it cuts off and my dim bulb tester goes crazy. I tried hooking both channels up at the same time and relays goes crazy in and out and my dbt does too. I don’t know if those buz mosfets actually are a good replacement so I went and ordered The exicon mosfets. But I haven’t messed with it for about a week. I will try again this weekend. Hopefully those exicon mosfets will solve my problem.
 
The DBT is a pass / fail check for shorts - it limits the current to the amp to avoid further damage if a short is present.

If I understand your last post, you're trying to operate the amp with it still connected to the DBT - that won't work, you need full mains supply without the DBT to use the amp.
 
I seem to remember that a dim bulb tester is not a good idea with mosfet amps.
Not sure and I don't know why, maybe search a bit.
 
I seem to remember that a dim bulb tester is not a good idea with mosfet amps.
Not sure and I don't know why, maybe search a bit.
Well. Last time I plug it with out the dbt. It has the same issues. Relay just goes crazy. I have a variac also. Should I try that instead.
 
Back
Top Bottom