Hitachi HMA-8500 II

[von.ah]

Hitachi HMA-8500 II help requested

Hello, honored AK.
I have a Hitachi HMA-8500 II that has been giving my tech trouble, so I'd like to ask for more AK advice. I know few specifics about the problem(s), but I know that he was having difficulty getting the amp to respond to bias adjustments. Some components have been replaced already.
So I ask: have any of the AK techs had much experience working on these amps? Or, has someone owned one of these and had work done on it?
I was only able to have a few minutes of listening pleasure with this amp before it went schizoid a year and a half ago. I've been itching to soak up the mosfet goodness of this big-metered beast ever since.
Here's a few pics from that awesome Japanese restoration site (amp8) of an 8500 for the visually inclined.

Happy Friday!

 

[von.ah]

Clarification:
Is there a Dr. MOSFET in AK, ala Merrylander is Dr. Yamaha? Would someone like to offer service for my ailing amp?
Of course, closer to my area (Baltimore) would be preferable, but if need be, I can ship far and wide.
Thanks! :thmbsp:

 

[tarior]

God, that is one sexy amplifier!!
I've never worked on one of those, so I'm no help.

 

[HMA8500]

Hma8500

I realize that this is an old post but I was just wondering if you ever had any luck getting your Hitachi fixed? I have one of these bad boys. It gets used daily. I would love to get another one.

Darrell from Moose Jaw.

 

[HMA8500]

Hma

Oops. I guess it wasn't an old post. I was looking at your join date. duh
Darrell

 

[von.ah]

Haven't had it worked on yet. I'll be taking it to a local shop soon. I just realized that they're an AK advertiser, so I should be in good hands!
If mine is fixable, I'm pretty sure it will be my main amp for a long time...

 

[von.ah]

resurrection

I'm reviving an old thread, cause I recently got this amp back from the pro shop.
They gave up on it, but I may not.

The tech said they went "above and beyond" trying to fix the problem, but in the end could not get it to behave. They traced the problem to the input section, replaced some parts, and got it to perform beautifully under a dummy load. Hooked up to some speakers and a music source, though, they said it would play fine at first, then produce a loud shrieking at the speakers at which they'd shut it down.

I was lucky enough to not have to pay for any of this extensive diagnosing and troubleshooting, so I still want to see if I can get it fixed.

Any ideas? Would freaking out under a reactive load indicate a bad transformer? Something else?

Thanks, AK! :thmbsp:

 

[Stanga]

That sound like oscillation to me, and tracking that down ain't going to be sunshine. Can you get hold of a dummy load? I used some finned resistors that I mounted on heatsink. The trick is to put the scope probe on the output and keep your eyes on the trace in order to catch the start of any build up of oscillation tendency. A tin or two of freezer spray are required so that you can freeze say the output MOSFET transistors one by one to see if they are the cause. Also check the zobel network to see if the coil is soldered in or has a dry joint.

 

[von.ah]

Can you get hold of a dummy load?

The dummy load is definitely me in this case...:stupid:

Thanks for your suggestions, but most of that is way over my head. I do know that the tech said all the outputs checked out. I imagine that I can examine all the boards and connections for bad solder joints and fix any that I find, but I don't have a scope or even reliable schematic-reading skills...
Which reminds me, the shop forgot to give me back my original service manual and extra Hitachi mosfets... Gotta get those back.

 

[Stanga]

but I don't have a scope

:scratch2:. I can see two scopes in the 2nd picture you posted....

OK, step back: does the sound breaks up on both channels, or just on one channel?

By the way, handle those 2SK and 2SJ MOSFET transistors with anti static protective measures. So DON'T TOUCH THE PINS.

 

[von.ah]

Ah, those pics are from a japanese site. not the actual amp in question.

 

[Stanga]

Ah, those pics are from a japanese site. not the actual amp in question.

Don't forget my question: does the sound breaks up on both channels, or just on one channel?

 

[von.ah]

When it happened to me, I recall it was both channels. I cannot remember whether the tech said it was both or one channel that would go bad.

 

[Stanga]

When it happened to me, I recall it was both channels. I cannot remember whether the tech said it was both or one channel that would go bad.

Right. What that tells us is that the fault is unlikely to be in the amplifier output stage or even in the pre-amp stage.
What it does tell us is that it is a common fault to both channels, which means that it is likely a ground issue or a power supply issue.
Put a nice clean soft cloth or an old blanket on a table. Make sure the amp is not connected to the mains (you'll be amazed how many engineers make the mistake of leaving the amp plugged in ). Leave amp unplugged from the mains by at least a few hours! This will allow for the power reservoir caps to discharge completely or at least to a safe level. A belt from a charged one of those can throw you across the room, burn part of the flesh off where it came in contact with you, and even knock you out. So treat them with the utmost respect.
Put the amp upside down on the cloth with the legs facing upwards. Part of the bottom of the case should be removable. It is screwed down by a couple of screws. Undo those screws and remove the unscrewed plate. You are now looking at the solder side of the power supply and driver stage PCB. Use a clean/unused paint brush to bush away the dust on the PCB. Check for dry joints, and for soldered components legs where the solder is not shiny. i.e. it might be dull and the solder is kind of wrinkled. I normally remove that solder with a solder sucker, and then I apply new solder. A common place to find those types of dry joints is on the legs of the two large bridge rectifiers ( in the 1st picture they are the big rectangular things with a hole in the middle).

With that behind you, clean the PCB again with the brush to make sure no tiny bits of solder was left on the PCB. Now screw the bottom plate back on, and turn amp over. Without putting any signal through it, switch it on and make sure you can hear the speaker relays click on after a few seconds. If that works OK, then switch amp off and connect it back up and check to see if fault is cured or not. If it isn't, you better get that schematic scanned and emailed to me.

 

[wualta]

Do what the man says, von. After you get this thing up and running you'll be very glad you took the trouble. It's a MOSgravy classic, as you well know.

 

[von.ah]

Such amazing advice! Thank you, stanga. I will try to do this soon and report back.

Wouldnt it be great if it was only a bad solder joint...

 

[SteveM]

Amp repair

Any luck on getting that puppy working, or have you
temporarily given up, and stored it away until later?

If you did manage to get that service manual back,
get in touch with me and I may be able to help.

I have found a pdf of the schematic page,
as well as the "Adjustments" page, if that's
any help.

 

[Deane1]

Just picked up the Hitachi HMA-8500 MKii and matching Pre amp HCA-8500 at Goodwill for $25. They had no idea how it worked. My gain. Beautiful condition. Lamps out on one meter, but just ordered replacements. Can't wait to get this hooked up to my Ohm Walsh-II's.