National Panasonics SU-50A - first rare transistor amp 1969-Adjust

[pakko]

Hey everybody!

I just finished reworking a rare Technics/N.P. amp from 1969. A National Panasonic SU-50A which should be the first transistor amp from Technics at that time.
The amp is running very good and the sound is surprisingly nice and smooth.
I recapped the whole unit with new caps incl. the main caps.
Last step now is to adjust the old fellow.
The Service Manual gives me only informations for the Bias Adjust - but there are two more trimmer I still did not touch. Maybe someone here give me a hint how this works!
We have three trimmer on the power amp board.
I think RV 19, 20 are for the Bias Adjust to 10mV over the Emitter Resistor.
RV 15, 16 are at the input of the Power amp and
RV17, 18 are after the two input transistors.
So - what and how can I adjust RV 15 - 18?

Thank you....
Here are the files from the SM:

 

[dB happy]

Cool looking old amp!

If you look at the schematic, there are voltages in little boxes by those pots. Pretty sure you adjust those pots for the proper voltage. What exactly dc unbalance and collector dc balance means I don't know. Don't know for sure, maybe somebody more knowledgeable will chime in. Don't do any adjusting until you get confirmation.

 

[pakko]

Thanx.
Here are some more information for everybody who's working on this one.

 

[pakko]

Maybe some can translate ...

 

[Powertech]

Looking at the circuit diagrams (they are not precisely the same) it's quite an odd design by modern standards and the drawing layout is unusual. It appears to have a long tailed pair at the input with a current mirror of sorts and symmetrical followers. Presumably TR25,39 and 41 are all on the same heatsink for thermal stability. The adjustments looks like the following:-
The 1st adjustment (#1 in red) (just below TR17 on the drawing) is for DC offset. This should get the voltage across the speaker terminals close to zero.
The 2nd adjustment (#2 in red) is the quiescent current adjustment for the output driver pair. This is measured as volt drop across the emitter resistors (measured between points 80/81 and 81/82). The voltage stated should be the minimum you set it to.
Carryout both adjustments with the amplifier switched on and warmed up, but with no inputs or outputs connected. Make adjustments very slowly and leave it to settle and then measure again.

 

[pakko]

Looking at the circuit diagrams (they are not precisely the same) it's quite an odd design by modern standards and the drawing layout is unusual. It appears to have a long tailed pair at the input with a current mirror of sorts and symmetrical followers. Presumably TR25,39 and 41 are all on the same heatsink for thermal stability. The adjustments looks like the following:-
The 1st adjustment (#1 in red) (just below TR17 on the drawing) is for DC offset. This should get the voltage across the speaker terminals close to zero.
The 2nd adjustment (#2 in red) is the quiescent current adjustment for the output driver pair. This is measured as volt drop across the emitter resistors (measured between points 80/81 and 81/82). The voltage stated should be the minimum you set it to.
Carryout both adjustments with the amplifier switched on and warmed up, but with no inputs or outputs connected. Make adjustments very slowly and leave it to settle and then measure again.

Thanx a lot!
I was working on this amp today.
Yes, you are right TR 25,39,41 are on the heatsink.
And yes, the adjustment for the Bias is (#2 red) or trimmer pot RV 19/20 - it should be 10mV.
I could adjust this today perfectly.

My DC offset right now is: - 50mV on both channels the same. I can change the offset with trimmer 17/18 (#1 red), but not all the way to zero (minimum - 33 mV).
But with trimmer pot you can although adjust the 34V at R106. So, what I did today is to adjust this on both channels the same (in my case to 32.5V)
If you are right with the DC offset at this point I need to change the Diff. Input pair TR 17/18 to a matched pair transistors like BC 557 to get the offset all the way to zero.?!

And I still have no idea what trimmer RV 15/16 is doing...!

 

[stoN_Cold]

 

[Powertech]

It adjusts the base voltage to TR25. Thin in turn increases or decreases the standing bias current to the 2 output stage driver transistors TR35 & TR37. These in turn maintain the bias current in the 2 output transistors such that they never quite turn off. This is what makes this a class AB amplifier.

 

[pakko]

Sorry we had this already.
Pot RV 19 is for the Bias Adjust. That's clear.

Open are those two other trimmer pots:

 

[stoN_Cold]

RV 17 DC offset, VR 15 look likes some sort of NFB adj.

 

[Moving Ahead]

A unique look. How does the back lit face work in real life? I suppose those bulbs are a candidate for LED. Maybe a color changing rainbow affect might get your girlfriend jealous.
I like the matsushita badged trannies. A good time to be working at National. Are the outputs labeled with the triad's mark?

 

[maxhifi]

It looks to me like RV15 is going to affect the gain of the amplifier, however why it is set up like that, and not like a normal voltage divider, I have no idea, because it also controls the DC current though the input stage, aside from varying gain. I don't really get it, maybe it was to compensate for variations in transistors?

I would do the following:

1. Set bias current using RV19

2. Set DC offset using RV17

3. Measure gain of one amp, and then tweak RV15 slightly to make sure left and right gain match exactly.

4. Do steps 1 and 2 again, since tweaking RV15 may affect those settings.

 

[sregor]

RV15 has only slight affect on gain. My guess it would be to help balance the input pair. And it will change the offset too. I would start by adjusting vr15 so that both bases of the input pair are same voltage, then rv17 to adjust offset. However, it looks like both controls will have some interaction. I would go for lowest offset - a matched pair of input transistors will help.

As an aside - the schematic is different than what I am used to. Negative voltage is at top (like some old british schematics) and the input pair and second pair of transistors are (to me, anyway) drawn sideways. My 2 cents.

 

[RobinTW]

Can someone send me a copy of the service manual? Or tell me where to download it. I have been searching online with no luck. Thanks a lot.

 

[NAD80]

RobinTW ,I would try to get ahold of member Pakko. He does have a SM. Direct message him maybe. He is still a member. Last seen at 11:32 here.

 

[RobinTW]

@NAD80 , I have got the SM from @pakko. Thanks for the info.

 

[restorer-john]

Lovely and quite rare amplifier.

 

[merlynski]

This is my thought: RV17 adjusts the bias of the long tailed pair, to put them in their best linear region and reduce distortion. It would not be negative feedback because it effects both sides of the LTP equally. RV15 affects dc balance and RV19 adjusts bias as others have noted. Maybe @rcs16 will give us his opinion on what RV17 is for.

 

[RobinTW]

I finally scored an SU-50A. It's broken and not cheap, but I really like the style. Anyway, I'll fix it when I have time.

 

[RobinTW]

I finally had time to open it up today. First thing first, I hooked it up with my DBT and it passed. Nice! It turned out to be not broken, but missing the pre-out/main-in cable. A simple fix with a new cable and it sings again.

From what I can tell, it's all original without any sign of work done on it. However, the output is no name 2SD218 (supposedly NEC's) whereas SM shows MATSUSHITA 2SD189 in stead. I guess this is a later undocumented factory change. Anyway, I am happy to have a working SU-50A and now I need to find time to get it fully recapped.