1060 - Can't get half voltage at output caps

[david winter]

Hi,

I have done this model more than once and don't understand the problem.
It's fully recaped as usual with Nichicon / Elna.
I replaced all 2SC458/1000 on the phono stage, as well as both 2SD438 on the output board (the two new ones I have in stock have a gain of 152-155 so no idea if this is too high).
Trimmers wer replaced by Bourns multiturn.
So far the 1KHz sine wave used for testing gives a nice output. However, main DC is 67V and I only have 26V-27V at output capacitors even though bias can be adjusted to 7.5mV across one 0.24R resistor (say 15mV across both). If I drive the amp a bit high, bias will drop down to 7.5mV at +/- 1mV.
Adjusting the symetrical trimmer won't allow more than a 2V change at the outpu caps so I'm wonder if there's any typical known trouble.
I only get a very small clip at the bottom 1KHz sine wave signal when volume is near the maximum. This can't be really perfected but at the same time I made sure to not drive the preamp too high.

I tested all four Marantz 2SC897 output transistors using my Peak DCA75:
Hfe @ 5mA is 36 - 57 - 54 - 58 (first appears a bit low)
Vbe @ 5mA is 0.564 - 0.587 - 0.578 - 0.590
I get 0mA leak on all four

Are the above outputs responsible for the trouble ? I find their Vbe a bit low.

Drivers test (will eventually replace with cross refs):
H709 2SC959: Hfe @ 5mA = 65, Vbe @ 5mA = 0.68
H711 2SA606: Hfe @ 5mA = 124, Vbe @ 5mA = 0.7
H712 2SA606: Hfe @ 5mA = 99, Vbe @ 5mA = 0.7
H710 2SC959: Hfe @ 5mA = 63, Vbe @ 5mA = 0.69

Pre-drivers test (ok IMO):
H706 2SC371 Hfe @ 5mA = 121, Vbe @ 5mA = 0.76
H707 2SA562 Hfe @ 5mA = 144, Vbe @ 5mA = 0.74
H706 2SA562 Hfe @ 5mA = 122, Vbe @ 5mA = 0.74
H706 2SC371 Hfe @ 5mA = 112, Vbe @ 5mA = 0.76


Thanks !

 

[catrafter]

I think your amp is working fine. it is very difficult to set the clipping symmetry perfectly. If you don't drive the amp to clipping it is kind of a moot point.
I wouldn't replace the semiconductors in an attempt to achieve the 1/2 power supply reading.
The half voltage recommendation was just a cheat for those without the means to do the adjustments properly (an o'scope'). I should have probably made it clear that 'somewhere around' 1/2 the power supply voltage such as your 26 to 27V is fine..
Keep up the good work.

Tom

 

[david winter]

Thanks for your hints. I already ordered the parts (MJ21194G for the outputs and the 2Nxxxx for the drivers).
To be honest I don't like the idea of having one channel with such a difference between both outputs (gains 36 vs 57). Also the drivers don't show the same gains between both channels.
In other words, I prefer having the same thing on both sides.
It's amazing to see that if you set the symetrical trimmer to a specific position, the bias will have to be set again and the amp will work fine unless you raise the volume to the clipping level.
I got the lowest clipping by adjusting the trimmer close to one end whereas the originals were set at about 33%.
The real question is: how were they factory adjusted considering the time it can take ?

David

 

[catrafter]

The real question is: how were they factory adjusted considering the time it can take ?

David

I have wondered about this myself. It would be interesting to find out how things were done 'at the factory', they must have had some shortcuts.

Tom

 

[david winter]

I have replaced the drivers by 2N equivalents testing close to each other (like 98 / 101) and also put matched outputs. Same thing so I curious to know why the output DC stays slightly out of the middle.
I left all others in place.

 

[sregor]

djusting the symetrical trimmer won't allow more than a 2V change at the outpu caps so I'm wonder if there's any typical known trouble.

Based on the schematic I looked at, the trimmer for the DC is 100K ohms and I would think it should give you more range. The Vbe and Hfe of H703 and H704 are also important. (H705-708 are just current limiters, and shouldn't make any difference unless they are bad). Caveat - I don't have one in front of me, so I could be off base. YMMV

 

[catrafter]

I think your amp is working fine. it is very difficult to set the clipping symmetry perfectly. If you don't drive the amp to clipping it is kind of a moot point.
I wouldn't replace the semiconductors in an attempt to achieve the 1/2 power supply reading.
The half voltage recommendation was just a cheat for those without the means to do the adjustments properly (an o'scope'). I should have probably made it clear that 'somewhere around' 1/2 the power supply voltage such as your 26 to 27V is fine..
Keep up the good work.

Tom

Quoting myself, wait, I hear everything twice!

Tom