Hitachi HMA-7500 - anyone repaired one?

mossmanr

Member
Many years ago I stumbled across an old HMA-7500 that was gathering dust in the back of a 'retro' shop but didn't give it much thought. Eventually the shop moved to another part of the city and the owner has given it to me. I tested it out and I got absolutely nothing out of it. Power comes on, lights work but no relay click at all. Since then it's been sitting in the shed for the last 2 years and last night I've decided that I'm going to breathe some life back into it.

DBT tells me there are no shorts, however the protection relay hasn't clicked so there must be an issue in there further on.

So, while I'm removing and testing out the components and taking stock of what I have to order, I got to wondering if anyone has repaired one of these tanks before? I have repaired a few amps now, but haven't touched anything with FET's in it (there's an old Sony sitting here as well) so is there anything I should be wary of? Most posts here deal with the MkII and not the MK1

Rob
 
anyone has repaired one of these tanks before?

I'm currently using a MK1 that I repaired as a regular runner. There may have been more wrong with it than I remember, but for sure one thing was aged-out fusible resistors. I'm guessing you may have seen those parts mentioned in other threads. I think it is warranted to at least check, and perhaps preemptively replace, all of 'em.

The MOSFETs were OK in my example. If you are removing them for testing, or doing any disassembly that will result in any of the output devices being disconnected, you'll want to take precautions against static buildup.

Other than the output stage itself, I'd say there isn't anything about this amp that's way different from many non-FET amps. I'm not at all sure I understand some of the fault-protection circuitry, but I think it's unlikely that any of that is troublesome.

HTH,

chazix
 
Greetings;
What were your preliminary diagnosis? Test with speakers and input fm? etc.

Could very well be the fusistors.. gone open. They may test okay? but under power they may go open. Sony as well.

Relay click or protect circuit: Then something in either pre or to driver is sending "no" signal to protect so relay isn't coming on. Cap, transistor and diode is pretty standard circuit for power on for a relay. Spagetti circuitry so a lot a tracing involved.
bink
 
Greetings;
What were your preliminary diagnosis? Test with speakers and input fm? etc.

Could very well be the fusistors.. gone open. They may test okay? but under power they may go open. Sony as well.

Relay click or protect circuit: Then something in either pre or to driver is sending "no" signal to protect so relay isn't coming on. Cap, transistor and diode is pretty standard circuit for power on for a relay. Spagetti circuitry so a lot a tracing involved.
bink

Thanks Bink
Initially tested with DVM (zero output reading) and then speakers. Both a CD and an Iphone input.
Haven't done a signal trace.

I've replaced the fusable resistors on the audio board and am now in the process of testing transistors & resistors. Ive replaced the aging lytic caps on the audio & power boards, left the filter caps as they are.
Also cleaned the relay contacts as well.

I've removed the outputs so when I start it on a DBT next I can trace voltages.

Rob
 
I've removed the outputs so when I start it on a DBT next I can trace voltages.

I'm not at all sure that this amp will be able to control the output point voltage with the outputs removed, so voltages may look out of whack.

(If it was me, I'd tack resistors from the gate connections of each output MOSFET to the output point of the amp. Say, 100 ohms or so. That would close the overall feedback loop and should enable the amp, if it's healthy, to control the DC voltage at the output to close to zero volts.)

Sounds like you're well on your way to having a nice amp back in action!

Cheers,

chazix
 
I'm not at all sure that this amp will be able to control the output point voltage with the outputs removed, so voltages may look out of whack.

(If it was me, I'd tack resistors from the gate connections of each output MOSFET to the output point of the amp. Say, 100 ohms or so. That would close the overall feedback loop and should enable the amp, if it's healthy, to control the DC voltage at the output to close to zero volts.)

Sounds like you're well on your way to having a nice amp back in action!

Cheers,

chazix

alright - good to know. Thanks Chazix
 
I'm not at all sure that this amp will be able to control the output point voltage with the outputs removed, so voltages may look out of whack.

(If it was me, I'd tack resistors from the gate connections of each output MOSFET to the output point of the amp. Say, 100 ohms or so. That would close the overall feedback loop and should enable the amp, if it's healthy, to control the DC voltage at the output to close to zero volts.)

Sounds like you're well on your way to having a nice amp back in action!

Cheers,

chazix
Hello. Could you help me. I have same issue here. Thanks.
 
Many years ago I stumbled across an old HMA-7500 that was gathering dust in the back of a 'retro' shop but didn't give it much thought. Eventually the shop moved to another part of the city and the owner has given it to me. I tested it out and I got absolutely nothing out of it. Power comes on, lights work but no relay click at all. Since then it's been sitting in the shed for the last 2 years and last night I've decided that I'm going to breathe some life back into it.

DBT tells me there are no shorts, however the protection relay hasn't clicked so there must be an issue in there further on.

So, while I'm removing and testing out the components and taking stock of what I have to order, I got to wondering if anyone has repaired one of these tanks before? I have repaired a few amps now, but haven't touched anything with FET's in it (there's an old Sony sitting here as well) so is there anything I should be wary of? Most posts here deal with the MkII and not the MK1

Rob
Sorry I know this is old. But I need help here with the same issue Thanks
 
Check voltages around the protection chip IC 901 HA12002.
 
Hello. Could you help me. I have same issue here. Thanks.

Welcome to AK.

There are many different issues that could cause the amp to not engage the protection relay. Avionic's suggestion would be a great place to start. Checking the fusible resistors makes sense, too.

If the suggestions that are already in this thread don't make sense to you, or just aren't enough to point you to a solution, you might consider taking the amp to an experienced repair person.

Cheers,

chazix
 
Check voltages around the protection chip IC 901 HA12002.
Hello. And thanks for the reply. I have a replacement chip in stock. So I will check the voltages later today like you suggested. I have another issue here. There are two diodes underneath the power amp board one for each channel. One is fried stuck to the insulation strip. And ther one is missing. Looks like someone has been here before. Also. I have checked and replaced bad fusible resistors My question here is. How do I know what to use to replace those diodes when they are beyond recignition We’ll i cant see this on the schematic so I’m taking a shot at you guys. I have uploaded a picture so you’ll know what I’m talking about Thanks
 

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Hello Fenix479. Welcome to AK. The Service Manual for the amp is here on HiFiEngine.com. https://www.hifiengine.com/manual_library/hitachi/hma-7500.shtml Also is it a MK1 or MK 2 amp? Think you have a MK2 after looking at the schematic. More pics of the area might be needed to see where they were located.
Yea. Sorry about that. It is a mkII. I CANT see it on the schematic and the board is not marked. Like I said. The left channel diode is fried stuck on the insulator tube and the right channel diode is missing. They didn’t even took the time to disolder it. The just cut the leads out. One more thing. I need to replace a few bad transistors. As follow
2sd756a
2sb716a
2sa1015
2sc1740
2sc2389
2sa1038
Can any one point me on the suitable subtitules for them. Thanks in advise.
 
https://www.el-component.com/bipolar-transistors/2sd756a-d https://www.el-component.com/bipolar-transistors/2sb716a https://www.el-component.com/bipolar-transistors/2sa1015 https://alltransistors.com/transistor.php?transistor=12311 https://alltransistors.com/transistor.php?transistor=13154 https://alltransistors.com/transistor.php?transistor=6833 Found these to check out to compare specs to find the best replacement I hope. a quick look I did. Can you tell me which parts the diode were across? Bee careful with the mosfets as they can be sensitive to ESD shocks (static charge) and blow the device.
 
For the power mosfets there aren’t many options I’ve read that buz900 and buz905 could work

Profusion sells a drop-in replacement mosfet that's supposedly better than the original, it has higher wattage and amperage ratings anyway. The one linked below is the higher voltage rated replacement for the SK135, which will also work for your SK134. The complementary part is listed at the link below.

https://www.profusionplc.com/parts/ecf10n20
 
Looking at the board layout in the MkII manual suggests the (Zener) diodes are possibly ZD751 (L & R) - both listed as "HZ-15-2" in the parts list.
 
Profusion sells a drop-in replacement mosfet that's supposedly better than the original, it has higher wattage and amperage ratings anyway. The one linked below is the higher voltage rated replacement for the SK135, which will also work for your SK134. The complementary part is listed at the link below.

https://www.profusionplc.com/parts/ecf10n20
Thanks for the support brother. About Newark’s BUZ900. And BUZ905. Would those work out for this application as a drop in installation or do they have to be modified somehow.
 
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