Marantz 1090 DC

Depaj

Member
2015-08-05 5:31 pm
lyon
Hello,

A few weeks a go I got my hands on a non working marantz 1090.
It powered up ok and the preamp stage worked fine connected to an other power amp.

The Power amp stage had one bad output and, bad drivers on one channel some leaky capacitors, which were actually badly installed on a previous repair attempt, and two shorted diodes. Once that was all replaced and the Marantz was able to sing again on its own I went for a full recap, set the bias and checked for dc. This is the part I could use some help with.
With the main amp and preamp disconnected I had 11mV on one channel and around 43mV on one channel which isn't great but still reasonable. When I connect the jumpers I have a reading between -20mV and +120mV on one channel and between -4mV and +100mV on the other channel. It doesn't stop changing, sometimes from one extreme to the other.
I listened with some suicide speaker and the amp is very quiet even with volume almost to max and it sounds really good so this DC doesn't seem to affect the sound but I would still like to get it sorted.

As there already is a little to much offset on the main amp alone I guess there might be a problem in both pre and main amp, what should I be looking for ?
 

Depaj

Member
2015-08-05 5:31 pm
lyon
Little update, without changing anything I now read a constant dc offset of 11mV on the left channel and 43mV on the right channel, with either main and pre connected or disconnected. Still those 43mV, even if not harmful, seem a little high.

Also on the right I have quite a lot of music coming through the speaker even with volume at 0.
 

Depaj

Member
2015-08-05 5:31 pm
lyon
There aren't any trimpots for the offset, only bias can be adjusted.
It's not really unbalanced, the channel that still gives when volume is at 0 stays at exactly the same level until the other channel reaches this same level, then they are properly balanced and grow louder together.
 

Depaj

Member
2015-08-05 5:31 pm
lyon
Yes those have been replaced by good quality multi-turn trimpots and the quiescent current is very stable. After those first measurements the offset did stabilise and is now at 11mV for the left and 43mV for the right channel. I removed Q702 and 704 from the right channel and the gain is 362 for one and 369 for the other, close enough no ?
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Depaj,
Your input pair are not balanced well enough. It is extremely difficult to match input pairs with a transistor tester. You must keep all the case temperatures exactly equal. Therefore you need to build a transistor balancer tool.

Replace the original bias controls with similar good quality single turn controls. They should be more reliable in this application. Never mind the fact that bias current is strongly a function of temperature.

-Chris
 

Depaj

Member
2015-08-05 5:31 pm
lyon
Hello Chris,

The input pair is the original one, I haven't replaced anything there, is the "factory match" good enough or should I consider trying to get closer to a perfect match ( honestly I don't really have the equipment and finances to go buy a whole bunch of transistors to find a perfect pair).
However q714 (2sc945) appears to have been replaced in a previous repair.

Jonsnell, c733/4 are both new.
 

Welcome

Member
2013-03-11 10:52 am
I think that's excessive. Matching input pairs is easy. Today most transistors have almost exactly the same hFE anyway*. I just pick a transistor and put it in the tester, with the same speed every time. That way, they all get equally warmed up by my fingers. If I test it again (thus warming it up more), it gets a higher hFE.

If I match transistors this way, and use those with a hFE rating within -/+ 5, I get excellent offset levels, every time.

* Of all my genuine Fairchild KSA992/KSC1845 transistors (hundreds), they all have a hFE value of about 450. This pretty much renders matching a practice in futility - just grab some out of the bag and you're good to go. Also note that they no longer sort transistors by hFE rank - you get this, and this only.

When restoring vintage amplifiers - always replace the input pair! This is a very common source of problems, and on paper, it's a definite upgrade to get that old noisy junk out of there.
 
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Depaj

Member
2015-08-05 5:31 pm
lyon
okay so I retested the input pair, as you said, with the same amount of time between my fingers ans I got 362 and 365 (I guess the first time I struggled to get that second one in the tester) so the input pair is definitely close enough and not the cause of this 43mV dc.
 

Depaj

Member
2015-08-05 5:31 pm
lyon
No They are not coupled, but I guess that can be easily done as they are back to back on the board.

Now then the offset aside, I still have that volume problem, I measured the resistance between ground and the different pins of the volume pot and got a difference between the first and the second potentiometer, is it possible that the pot is at fault here ?
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Welcome,
Over 35 years of experience differs from what you have found. Careful matching reduces distortion as well as making the DC offset closer to what you would calculate from the schematic. The diff pair(s) is (are) the most critical place for matching and also affects distortion. If you're going to do a really good job, using the hFE function of a meter is only useful for pre-sorting the parts. The fine match is done by creating a diff pair, tying the two prospective transistors together and looking for equal collector voltages. It matters, but if you haven't done this, you can't tell whether it is a benefit or not. It makes such a worthwhile difference that I built a jig for this work. Besides, it is the only way to match darlington transistors.

Hi Depaj,
Your yields are much better with larger populations of parts to match. The lower number you start with, the less likely you will get a close match. Often, the quantity breaks occur at 10 or 25 pieces. Just look at different suppliers to see what quantity pricing is like. Of course, it also depends on your expectations and how much you expect to do.

Most "tweaks" have marginal performance increases. This is one that may not measure (THD) greatly, but it sounds a lot better, and the harmonics on a spectrum analyzer are reduced.

As for 43 mV, not something to worry about. Its the other stuff that is worthwhile.

-Chris
 

Depaj

Member
2015-08-05 5:31 pm
lyon
The amp is from a friend and he at first just wanted it repaired, I suggested a recap which he finally accepted but I don't think he's wallet is ready for much more.
I will definitely keep this in mind for my own projects.

What exactly do you mean by "tying the two prospective transistors together and looking for equal collector voltages" ?

Any ideas on that unbalanced volume knob ?
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Depaj,
Volume controls often do not track well near the bottom of their adjustment range. This is quite normal and the main reason why there is a -20 dB mute button on many sets.

What exactly do you mean by "tying the two prospective transistors together and looking for equal collector voltages" ?
Make a differential pair circuit using 100R resistors as the collector loads. I use 1K0 resistors from each base to common. Stick the two transistors in contact and isolate them with foam (make a hole in a small piece). This will keep them in their own little environment. Let them settle in (about 5 minutes each pair) and measure the voltage from collector to collector. A good match will have little voltage between those points. Transistors are extremely sensitive to temperature, and by making them the same temperature, temperature drops out of the mix when they are matched.

-Chris