Hitachi HMA-7500 MK11 idle adjustment clarification needed ! (MOSFET amp)

[Bert 1100]

Hi all,
I bought this nice looking power amp yesterday. (No pics yet, sorry )
It's all working OK, but before trying it out in my main system I would like to make sure the idle currents are right. No DC offset adjustment here, a servo is at work

First question : is it OK to use a DBT on MOSFET amps ?

This is from the service manual :





So we have 2 trimpots per channel. One channel shown on the schematic.
The pots are not different values in series so this is not to make a finer adjustment easier.
My guess is the procedure is to balance the idle the proper way within the circuit of each channel ?

The manual procedure calls for : (green are my comments)

1) all pots to the left (I assume counter clockwise ??)
2) power on for min. 8 minutes.
3) shift all 4 pots to the center position (OK, clear, but that's already a bit frightening, I'll monitor the mV)
4) rotate pot 1 (NOWHERE does it say which pot is 1 or 2, I assume it does not actually matter ??) in the direction where the mV go UP.
5) next point, now, mentions pot 2 at the leftmost position, when did we move pot 2 back to the left ???? or was 4) just to find out the direction of rotation needed, after which pot 2 is turned back left...
6) Now adjust pot 1 to read 30mV OK, that's clear...
7) Then adjust pot 2 to read 32mV
8) Then adjust pot 1 again to finally read 33mV.


So, are any of you familiar with this adjustment in this amp ?
Am I understanding the procedure right ?

To make things more confusing, the german text says in point 4 to adjust pot 1 as to get the needle of the (analog...) VM to move LEFT, meaning down.....
(in english and french : "where the voltmeter pointer deflects in a clockwise direction", meaning UP / MORE )

I haven't yet measured the idle current as it is now, of course I will before touching anything. I'll also mark the pot's positions before touching them.

Thank you !

 

[Bert 1100]

I measured the idle "voltages", I have 30mV on the left channel and 39 mV on the right. And the right is noticeably warmer.

It's now playing in my main system (Rega Apollo CD, Kenwood Basic C-2 pre (rebuilt) and Monitor Audio RX6) and it sounds very good.
It has a very dynamic, present sound and yet is not aggressive, nice from very bottom to very top !

 

[Bert 1100]

A couple of pictures...





Thinking about that idle current procedure, I believe the manual explains what should be simple in a complicated way :

I think all there is to it is this (for one channel) :

- pots all the way counter-clockwise
- power on, wait > 8 minutes
- turn one pot till you read 30mV
- turn second pot till you read 32mV
- turn first pot again till you read 33mV

This procedure of turning one pot then the other is I guess due to this :
The idle needs to be 32-33mV in both parts of the circuit, but there is only one reading point, and one pot's adjustment "hides" the second pot's adjustment.
With the procedure, you bring one pot up, then the other one a bit higher so you "see" that second pot's adjustment, then again a bit higher with the first pot to finalise...

When I get to do it I will report back.
The pots as it is now (untouched) sit at about 3/4 of the way to full clockwise.

The amp in time will get restored properly, that will be a new thread.

 

[ekuna]

I have the same issue. Still don't know which pots are #1 and #2. Did you ever get this worked out?

 

[Bert 1100]

@ekuna I did work it out, it is as described in post #3. it does not matter which pot you turn first, just decide which is first and which is second.
Did you have a look at the thread on the full restore ? here it is :
https://audiokarma.org/forums/index.php?threads/hitachi-hma-7500-mkii-project.809803/

 

[ekuna]

Bert, Thank you. Ed