Re Cap on my Hitachi HA-1100 today

RuffzGuts

Super Member
What a beautiful Amplifier...Who ever Engineered this one wanted it to last for ever..!

This one was well loved or used and abused, ...Ive had it on the shelf for years as it had high DC offset, around 120Mv each channel...
after full recap including main filters which were way under par, 18000uf...@ around 12000uf....replaced with 22000uf..

What is special about this amp is the engineering, its a treat to work on, the pcbs are high quality, you have component numbers both sides of boards..high quality all the way...I have been fighting with this Marantz receiver and have formed a low opinion regarding design , and in comparison...actually, I think Hitachi is bench mark in this price range..hats off to who ever designed this amp...its a piece of art...

So anyway, after recap, I had hoped to see that DC offset cured...not the case....pretty even both channels at around 115mV...

So impatient i plugged into my dummy loads...and damn it sounds good...seriously good...couldn't help my self and plugged into my yammy NS-10s...too be honest i find it hard to believe this could sound any better...completely blown away....

I want to chase this offset down...I can replace the input pairs, but i gotta wonder, how good can it get? i had a hearing test the other day and scored surprisingly perfect...

i will definitely chase this down regardless...its just interesting i think, respect to echowars on his valued thread regarding dc offset...

all i am saying is this amp sounds frickin amazing...and i have 120mv both channels...
 
Im not familiar with this amp - Is the DC offset adjustable via small pots on a PCB? Can you find and download a service manual to give you the DC offset adjustment procedure? If the DC offset is adjustable, then the issue isnt that serious as you can adjust it.
 
Im not familiar with this amp - Is the DC offset adjustable via small pots on a PCB? Can you find and download a service manual to give you the DC offset adjustment procedure? If the DC offset is adjustable, then the issue isnt that serious as you can adjust it.

there are distortion trimmers, they don't alter the offset at all though...i will change the pots before i do anything
 
2SD478 and 2SB568 are drivers.

I had a quick look at the manual, and there are some things you should read.
Page 8 point 4 -DC voltage
Page 10 point 4 -speaker protection say +- 150mV.

As you know there is no trimmer for DC offset. (VR702 Distortion Adj needs a scope, Don't even look at touching it.)
If you want to try and reduce DC you can replace Q701,Q702 left channel and right channel)
the manual says they are 2SA1345 -F
http://www.datasheetarchive.com/dl/Datasheets-IS14/DSA00275483.pdf
You had better check with your unit to see if they are the ones.
Thats high gain 600-1200hFE
You could try BC546C
http://au.rs-online.com/web/p/bipolar-transistors/8031071/
These transistors NEED TO BE MATCHED!
 
2SD478 and 2SB568 are drivers.

I had a quick look at the manual, and there are some things you should read.
Page 8 point 4 -DC voltage
Page 10 point 4 -speaker protection say +- 150mV.

As you know there is no trimmer for DC offset. (VR702 Distortion Adj needs a scope, Don't even look at touching it.)
If you want to try and reduce DC you can replace Q701,Q702 left channel and right channel)
the manual says they are 2SA1345 -F
http://www.datasheetarchive.com/dl/Datasheets-IS14/DSA00275483.pdf
You had better check with your unit to see if they are the ones.
Thats high gain 600-1200hFE
You could try BC546C
http://au.rs-online.com/web/p/bipolar-transistors/8031071/
These transistors NEED TO BE MATCHED!

Hmm, Thanks for pointing me in the right direction, I took another look at the schematic this morning and noted two .47 caps on the input of the differential pairs...I failed to mention that they were the only two capacitors in the amp that i hadn't replaced..didn't think i had the value at the time...

I had a scratch around in my box and found a couple of elcheapo sultans (jarcar)...I removed old caps, sanyo's, both had huge esr readings, also miles apart....

So after installing new caps, it has brought the offset down on one channel... to around 60mV

other channel, still the same....I will order some good quality caps to replace these but I am wondering if the value is important here...would 1uF suffice ?

I am also going to spend some time checking over all resistors related to that part of the circuit and soldering...

Might be just a good idea just to replace these transistors though i think
 
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60mV perfect. if thats what it is i wouldn't mess with the inputs trans just yet. You could pull the originals and see how closely matched they are for your reference thou.
Note: those .47uF caps are aluminium solid. I'd stay with the same value but get the tollerance tight.
Also try replacing R701, R706 , and R713 with matched pairs (1% 0.5watt).
 
I don't see why 1uf as a replacement wouldn't work. .47 though is one of those cap numbers that keep coming up in schematics and parts stores are selling them. might replace as orig design maybe beef up the voltage rating if service caps. or filtering. 1uf can filter a bit much.
 
IMO 1uF replacing .47uF for the amp section input caps would be fine and probably better than the original. This position is frequently increased a little. You should get either Panasonic ECQ-V or Wima MKS2 film caps, either a significant upgrade over electrolytic.


RE something mentioned earlier: those trimmer pots shouldn't be turned unless you know exactly what the effect is. If the bias trimmers are turned it might not take much to cause the amp to overheat, or, if turned the other way, could negatively impact sound.

I have an HA-7700 which may (or may not) be similar to your amp. They were only a couple years apart. HA-7700 has MOSFET outputs, do you know if the HA-1100 does?

My HA-7700 is a beautiful sounding amp. It is "only" rated 65 watts but it is a beast of a 65w amp at around 35 lbs. The DC offset and bias setting procedures, per the service manual, are complex compared to most amps. The DC has to be set in two different places. There are other complexities I do not presently recall, disconnecting wires and inserting an ammeter? The point is, the adjustment procedures for your amp may (or may not) be similarly complex.

One other FYI is that Hitachi used fusible resistors in some of their amps from the era (sample1 sample2 sample3). Fusible resistors have often been known to increase significantly in resistance (or go open) and can cause issues. I don't know whether your amp, or mine, has any of them. < my amp is not in rotation and I have been working on other projects>
 
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60mV perfect. if thats what it is i wouldn't mess with the inputs trans just yet. You could pull the originals and see how closely matched they are for your reference thou.
Note: those .47uF caps are aluminium solid. I'd stay with the same value but get the tollerance tight.
Also try replacing R701, R706 , and R713 with matched pairs (1% 0.5watt).

Yes i would like to get transistors out to see how balanced they are, I should also have mentioned both channel off sets were in the negative range...that could mean a few different things i guess..I don't have a hfe tester on hand, but ive just purchased one of these Chinese component testers online...maybe ill sneak into jaycar with them in my pocket in the meantime...:scratch2:, resistors have all measured very well...

I don't see why 1uf as a replacement wouldn't work. .47 though is one of those cap numbers that keep coming up in schematics and parts stores are selling them. might replace as orig design maybe beef up the voltage rating if service caps. or filtering. 1uf can filter a bit much.

Yes well i was of the understanding that if the caps just filtering DC in the signal path, a slight increase doesn't hurt...

IMO 1uF replacing .47uF for the amp section input caps would be fine and probably better than the original. This position is frequently increased a little. You should get either Panasonic ECQ-V or Wima MKS2 film caps, either a significant upgrade over electrolytic.


RE something mentioned earlier: those trimmer pots shouldn't be turned unless you know exactly what the effect is. If the bias trimmers are turned it might not take much to cause the amp to overheat, or, if turned the other way, could negatively impact sound.

I have an HA-7700 which may (or may not) be similar to your amp. They were only a couple years apart. HA-7700 has MOSFET outputs, do you know if the HA-1100 does?

My HA-7700 is a beautiful sounding amp. It is "only" rated 60 watts but it is a beast of a 60w amp at around 35 lbs. The DC offset and bias setting procedures, per the service manual, are complex compared to most amps. The DC has to be set in two different places. There are other complexities I do not presently recall, disconnecting wires and inserting an ammeter? The point is, the adjustment procedures for your amp may (or may not) be similarly complex.

One other FYI is that Hitachi used fusible resistors in some of their amps from the era (sample1 sample2 sample3). Fusible resistors have often been known to increase significantly in resistance (or go open) and can cause issues. I don't know whether your amp, or mine, has any of them. < my amp is not in rotation and I have been working on other projects>

I actually have some Panasonic films in that value...they are 630v though, so a little big...I thought about throwing in a couple of 1uf in series...then i thought, I'm over thinking this, 1uf should be ok....This one doesn't run fets no, and youre's look like a beautiful amp by the way...but only 60w per channel...? it looks more like 200w... looks like it has adjustment for dc input and output...which might be why its a little more complicated....

I am going to have to replace the idle current trimmers on mine as can not get it to sit on 14mV...wants to jump a mile either side when touched, so a ide say the other trimmers need changed also..will need to figure out how to make that adjustment...
 
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@RuffzGuts

I got to thinking about the idea of upping that input cap to 1uF. So I grabbed a copy of your SM.

The inputs caps are part of a high-pass filter, along with a resistor. Changing capacitance value there does not make as much difference as one might think because the resistor is part of the equation. I put down what I know in this diyaudio thread along with a couple questions.

Bottom line, unless my calculations are incorrect, your high-pass filter is already pretty low at around 1.5Hz. Increasing the uF value would likely not offer any benefit.

So if you keep the value of .47uF you may be able to fit a polypropylene cap in there. That would be even better than the two polyester types suggested earlier. In that position, I bet you could hear a nice improvement over the original electrolytics with film of any type, but especially polypropylene if they would fit.
 
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@RuffzGuts

I got to thinking about the idea of upping that input cap to 1uF. So I grabbed a copy of your SM.

The inputs caps are part of a high-pass filter, along with a resistor. Changing capacitance value there does not make as much difference as one might think because the resistor is part of the equation. I put down what I know in this diyaudio thread along with a couple questions.

Bottom line, unless my calculations are incorrect, your high-pass filter is already pretty low at around 1.5Hz. Increasing the uF value would likely not offer any benefit.

So if you keep the value of .47uF (or .56 or .68) you may be able to fit a polypropylene cap in there. That would be even better than the two polyester types suggested earlier. In that position, I bet you could hear a nice improvement over the original electrolytics with film of any type, but especially polypropylene if they would fit.

I appreciate your input Roger, thank you, to be honest increasing the capacitance wasn't so much about improvement, it was more about what i have here on hand, I have a box full of Nichicon FG 1uf here..

But its interesting to hear what you are saying, and i will keep an eye on that other thread...as far as board placement, its kind of tight in that area due to the orientation of the legs...though i could probably stand an axial polypropylene on its end..I will see what I can find to put on my next shopping list:scratch2:
 
Panasonic ECQ-V are pretty small in .47uF/50V, only slightly larger than an equivalent electrolytic, and have long legs. Should be able fit those.

Good luck finishing up your project. I bet that HA-1100 is a great amp :thmbsp:
 
If the amp has a differential pair at the input then adding a trimmer to control offset should not be a problem. Of course, the better differential transistors are matched the better. Anyway, without seeing the circuit diagram it's difficult to seriously comment on potentially desirable mods.

cheers,
 
Panasonic ECQ-V are pretty small in .47uF/50V, only slightly larger than an equivalent electrolytic, and have long legs. Should be able fit those.

Good luck finishing up your project. I bet that HA-1100 is a great amp :thmbsp:

Thanks Roger, i will come back to this when hfe tester comes in...

If the amp has a differential pair at the input then adding a trimmer to control offset should not be a problem. Of course, the better differential transistors are matched the better. Anyway, without seeing the circuit diagram it's difficult to seriously comment on potentially desirable mods.

cheers,

the schematic is easily available via hifi engine if you wish to contribute..adding a trimmer pot too input pairs is an interesting subject
 
Attached is circuit diagram of one of my amps. Differential pair is formed by Q1 and Q2, R15 and R16 are emitter resistors and R17 is a trimpot controlling the offset. It's value should be 10-20 times the value of one of the emitter degeneration resistors. I use 2k with R15=R16=121 ohm. I hope it will help.

cheers,
 

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Attached is circuit diagram of one of my amps. Differential pair is formed by Q1 and Q2, R15 and R16 are emitter resistors and R17 is a trimpot controlling the offset. It's value should be 10-20 times the value of one of the emitter degeneration resistors. I use 2k with R15=R16=121 ohm. I hope it will help.

cheers,

interesting, though is that not more of a restrictive way of doing it? I've been reading through Echowars thread on offset, Page five he's supplied a circuit... trying to get my head around it...

At the end of the day, i would think the right way is getting components as close to spec and zero tolerance as possible...though if circuits are being added to cure offset..i guess there must be a reason..
 
No, it's not more restrictive at all. There is a number of ways one could controll offset depending on amp's circuit. You can choose whichever one you want.

In the circuit I have presented trimpot is acting as two resistors in parallel to emitter resistors of LTP. By varying emiter resistor values one balances currents through them. This is the simplest way to control offset with one differential pair input and requires adding one part only. You could replace one emitter resistor with a trimpot and then vary its value until offset is zero, measure that value and then replace trimpot with exactly the same velue resistor.

cheers,
 
Sorry, my thoughts were it would be better to adjust the bias of the transistors via the base...and adding a resistance at the emitter would see one transistor working harder than the other...

I need too put some serious time into revision on my electronics theory...
 
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