Hitachi HMA-7500 MKII distortion

[forgonati]

Wow. To say the least, that is not the sort of distortion I had imagined. I had assumed that there was just a non-linearity in the amp's response, but this seems more like the amp is breaking into oscillation at points that are at least loosely correlated with the phase of the input signal.

Truthfully, I have no solid ideas. You should double-check that all connections, especially ground connections, are good - but I gather that you've already checked that. You might also try tapping the relays at the onset of symptoms, to see if the symptoms change.

I think one thing I would try is to disconnect the collectors of Q759 and 762, which are meant to detect excessive current in the output stage and, if detected, disable the signal drive to the output stages. It would make sense to first check the output MOSFETs' source resistors, the 0.22 ohm parts. (You won't be able to accurately measure such low resistance values, but they should measure as less than 1 ohm.) The big flaw with this line of thinking is that it seems unlikely for an over-current detection problem to affect both channels.

Maybe my description "distortion" wasn't accurate for this issue, sorry i didn't know how to describe it better. I will check every connection one more time, maybe i will find something. The amp definitely had an accident before, probably it was dropped because the big board was broken around the relays, and someone has repaired the broken traces. But that section was the first what i checked with multimeter and i couldn't find any broken path, everything seems all right there...
The amp is acting really wild, i haven't seen such thing before, probably that's why i could not describe it better than "distortion"...

Yes the driver transistors were replaced by me. On right channel it had the original ones, Q757 and Q759, 2SD669 and 2SB649. But on the left side were different transistors (i can't remember their type but i know they weren't proper complementary pair. So i decided to replace all four, i used 2SC2690 and 2SA1220. I thought they will be good replacements. Your thoughts on that? By the way the amp had this issue before i did replace those transistors.

 

[chazix]

I think the driver replacements are OK, and presumably not related to the distortion since that was present before the replacement.

I don't have any other bright ideas. Or even dim ideas .

 

[forgonati]

I think the driver replacements are OK, and presumably not related to the distortion since that was present before the replacement.

I don't have any other bright ideas. Or even dim ideas .

Thanks so much for all the effort. I will check it very thoroughly one more time, maybe i will find something. If not, then i will start to pull every component and check them out of circuit, i have really no other idea... Very strange that it happens on both channels since this amp is really 2 separate mono amps and it has only very few parts in common... A mystery really.
I wonder why the previous owner gave it to me for such low price after he invested money into it to replace all the caps... I have a strong suspicion that he couldn't figure it out either and eventually he gave up on it. I had no chance to try it out and the price was just too tempting.. At least it's preamp does work perfectly. I will try to fix this amp even if it takes long time, i think it's worth saving. I just noticed that the 2 preamp IC-s (IC711) are in sockets so probably he tried to replace them (the IC-s themselves don't look new so maybe he just dropped new ones there and when that didn't help he just dropped the originals back but this is just speculation)...

I will report back if i find something.

 

[forgonati]

Greetings guys, i today i had time to mess around with this amp. I took out all the small transistors, checked them, i found no problems with them. Except Q759, their gain was pretty low on both channels, around 80-90, so i replaced them with the same transistor, brand new ones. Of course this did not solve my problem. In fact the situation has gotten worse, when i fired it up to make some voltage checks, it didn't come out of protection. After some poking around i found the problem, the cable RN was broken, probably from the lot of movement, turning the panel way too many times... Now the amp comes out of protect, BUT now the right channel has no bias current, and turning the pots doesn't change it. I can measure around 2mV on the testpoints, while on the left i do have 33mV. I checked the power FETs, every resistor, diode on right channel, i found 1 resistor R764 10 Ohm, that had around 3 Mega, so i replaced it with new one, to no avail. I swear i checked this resistor a few days ago and it was OK. I have no idea what might have happened. I did some voltage measurements, you can check it here on the picture. Some of the voltages are way out of spec, for example on Q753 collector i suppose to have over 10V, and i have 0.45 (have checked on the left channel, it's 0.84V there). Is that a typo or i really suppose to have over 10V there?

Could you check out these voltages? Maybe you will have some ideas what might be wrong here? That would be great.

Oh and the voltage selector is set to 220V here in EU we have 230V so the rail voltages are a bith higher, i don't think that would be a problem.

 

[KeithTr]

Greetings guys, i today i had time to mess around with this amp. I took out all the small transistors, checked them, i found no problems with them. Except Q759, their gain was pretty low on both channels, around 80-90, so i replaced them with the same transistor, brand new ones. Of course this did not solve my problem. In fact the situation has gotten worse, when i fired it up to make some voltage checks, it didn't come out of protection. After some poking around i found the problem, the cable RN was broken, probably from the lot of movement, turning the panel way too many times... Now the amp comes out of protect, BUT now the right channel has no bias current, and turning the pots doesn't change it. I can measure around 2mV on the testpoints, while on the left i do have 33mV. I checked the power FETs, every resistor, diode on right channel, i found 1 resistor R764 10 Ohm, that had around 3 Mega, so i replaced it with new one, to no avail. I swear i checked this resistor a few days ago and it was OK. I have no idea what might have happened. I did some voltage measurements, you can check it here on the picture. Some of the voltages are way out of spec, for example on Q753 collector i suppose to have over 10V, and i have 0.45 (have checked on the left channel, it's 0.84V there). Is that a typo or i really suppose to have over 10V there?

Could you check out these voltages? Maybe you will have some ideas what might be wrong here? That would be great.

Oh and the voltage selector is set to 220V here in EU we have 230V so the rail voltages are a bith higher, i don't think that would be a problem.

Did you check all your rail voltages? After I fried my 7500 Mark I, (not the outputs), when removing probes from setting the bias (these filter caps keep voltage for a long time), I discovered that I had fried one of the fuse resistors and the zener diode that controls the 13v (I think it's 12 or 13v) rail. Replacing the zener and the fuse resistor (and all fuse resistors should be replaced with 1/4 watt metal equivalents), mine worked like a champ. Start with power supply voltages and then move up from there.

 

[forgonati]

All right, the amp is back in the business again, both channels are working, idle current is OK now. I had a short between LN and LP, took me a few hours to find it... I was happy for a while than my probe just slipped, and bang, big spark, the amp stuck in protection again. Stupid me i forgot to discharge the main caps... 2 hours later i found 1 shorted diode, replaced it and now the right channel is up and running, bias is adjusted to 33 mV on both channels.
Now, back to the original problem that crazy distortion...

Damn, what an emotional rollercoaster this amp is... I like it but i hate it for so much headache... I like to learn but this is getting frustrating already.

 

[forgonati]

OK, some news, i took the multimeter and checked the voltages basically everywhere. I put here the photos showing the voltages in the preamp section and the left side power amp section.
In preamp section the voltages are more or less spot on, except at Q716 collector, where it's almost twice as it should be. Could be that the transistor itself (i took it out and checked, it's OK) or some other components? Resistors R716 and R724 below it are those carbon-composite resistors, i know they are notorious drifters, i checked them, they are a bit drifted, 35K and 28.5K, could that be a reason why that voltage is much higher??
At the bases of Q711 and Q712 i have 10.4 as per the manual, but as soon i touch that point with my probe the protection cicks in and disconnects the speakers. Is that normal or something is going on there?

On the power amp section some of the voltages are OK, some of them are out of spec. Especially at Q753 collector where i have 0.89 instead of 10V. Is that a typo or i really should have 10V there?
At the base of Q752 i have 0.20 instead of 0.44.
At the base of Q751 0.77 instead of 1.0.
Base of Q753 0.67 instead of 1.0.
Base of Q757 1.44 instead of 1.65.
LNG is very sensitive, as soon as i touch it with the probe it trips the protection.

Evrey transistor has been checked outside of circuit. Every diode has been checked. All electrolytic cap has been replaced. I have every connection and every solder joint checked twice, with magnifying glass. I really have no idea where to look.

Could you guys check these voltages, and share your thoughts with me? What should i check?

 

[forgonati]

Noone? Noone has any clues what's going on with those voltages?
The power FET's are disconnected, and with the headphone plugged in it sounds good, i cannot notice any distortion, and i can raise the volume to the level when it already hurts my ears. So it seems everything is working before the power FET's.
But i did some measurements with scope. 1st photo is the signal coming in, measured at leg 6 of FET711. 2nd is at N, 3rd is at P. 4th photo is at D and NF. Any opinions guys? I don't know why i'm seeing these strange signals at N and P.

Oh and this is happening on both channels.

 

[forgonati]

A little update on my long journey, the amp is working fine now. I managed to narrow the problem down to the very end of the amp, the power transistors. Because with headphones i could listen to it without problem, even at very high volume. As soon as the amp got under a bigger load with my Canton GLE-70s, or with my 8 Ohm dummy load, i got those weird distortions and eventually the amp went into protection. The root of the problems was in those damn grey connectors which Hitachi used to connect the power MOSFET's to the board. This amp has a socket where the power FET's are seated in, PLUS it has a connector to connect them to the board, so a couple of potential bad connections, screaming for troubles. Why they used these stupid sockets + connectors and haven't connected these FET's directly to the board is beyond me! I mean, how often should you disconnect these??? If the amp works as intended, you won't touch these connectors for decades...
I measured these connections a few times, and my multimeter showed stable connection. Probably the problem was, the multimeter didn't put enough load onto them to reveal the problem. As soon as the transistors got under heavier load, one (or more) of these connections became ustable. I did clean the connectors, that was one of my first steps weeks ago, but it seems like that wasn't enough. Yesterday, having no other ideas, i just grabbed a few wires and soldered the FET's sockets directly to the board, bypassing the grey connectors. Guess what... All my problems are gone now. Crazy. Lesson learned, don't trust your multimeter and in these cases make tests under load too.
I still have a few potential connection problems, because the transistors are seated into those black sockets, but i leave it as it is for now. It seems the problem was in those grey connectors. Maybe later i will find a way to bypass those black sockets because i don't like them at all.
The amp went through a long heavy testing with dummy load, and it's behaving very well, i have rock stable signal on the output.