Pioneer m90 repair, jfet stage confused

[Jakobsson]

I replaced alot of damaged resistors and amplifier is powering on but i am a bit confused about why i am not getting proper current through the 2sk129a pair.
i have about 1.3ma on each side which gives a total of 2.6 and to my understanding from schematics it should be 2*2ma=4ma.
Q103,Q105 and Q107 replaced.
I was thinking of adjusting the value of R105 to get more current but i have not tried yet.
32.8v on schematic is about 38v.
I am mainly concerned because vgs is about -0.45v on the jfets and i think it should be closer to -0.35v to be on pair with operating range.

 

[sgrossklass]

Verify that R105 has not gone high or suffers from bad solder joints. If the reference current is not correct, nothing else will be right. If in doubt, vary R105 as needed.

Apparently 2.0 mA + 2.0 mA + 0.32 mA = 4 mA. Shall we call that Pioneer logic?

I wouldn't get too invested into Vgs, as FET parameters (like Vp and Idss) tend to vary quite a bit.

 

[Mark Tillotson]

What's the voltage across R117? That sets the standing current in the pair.

 

[Jakobsson]

What's the voltage across R117? That sets the standing current in the pair.

0.55v

 

[rayma]

0.55v

That's 0.55V/150R = 3.6mA total, seems ok.

 

[Mark Tillotson]

Unless R117 has changed in value, or something is leaking somewhere.

 

[U230421]

Did you replace the 2SK129a pair too?
If so, consider the endnote of #2.
Even more, dc at R109 and R113 are (hopefully) equal but to high.
Then the replaced Jfetpair has a to low Idss; adjust R117 accordingly
Try 120Ω or solder a 680-620-560Ω resistor upon R117.
Go step by step to a the proper needed value.
The aim is to come as close as possible to the desired 32.8V on R109/R113.

 

[Jakobsson]

Amplifier drifts for the first 20minutes then voltage is
Over R117=0,5V
R109/R113=35.1V
R105=150.4V
R259=50V
R111=68.4V

The 32.8V is about 40.5V after 20min.

Both channels measure very similar, but left channel is abit lower in volume then the right channel especially in the low freq area.
My idea was to swap the C105 but i have not tried yet.
2SK129A is original, but when googling it looks like many repair jobs include swapping them.
Looks like very difficult to find.
I have not changed any of the 1/2w resistors, could that be the reason for the 20min drifting?

 

[U230421]

I guess your fellow diy's on this topic assumed something was wrong with the amp, but if it works according to specs there's no real issue. You don't have to bother about the difference in the values given in the service doc and the real thing. Again according to #2, jfets have a substantial spread, up to 50% in its own range (2sk...A). So better not change anything.
If you don't like certain aspects of the audiable experience, I'm pretty sure it is not because of a malfunction of the amp.
R119-C105 have a cutoff frequency of << 1Hz. Try recap C015 (and the other channel).

 

[Jakobsson]

C105 fixed the channel inbalance.

 

[guswin95]

Hello,

I am working slowly on a complete rebuild of a couple M-90 from eBay. I removed both PCB and am doing a full recap and partial replacement of resistors diodes and so.

Regarding C105, I noticed its significantly bigger than a normal 470uF/50V electrolytic capacitor and suspect is very special kind of capacitor.
Looking at the specs it's an ACH-455 type, which in the Pioneer lingo (Pioneer Tunning Fork) means a metalized paper capacitor. It's unclear what type of modern capacitor would be recommended there. And unclear to me why is it that crucial for this capacitor to be there. Obviously Pioneer thought it important enough to specify a very different kind of part and gave it a lot of space in the PCB to accommodate it.

Any comments are welcomed.

 

[rsavas]

I’d have to look at the schematic to see what it does but if it is not electrolytic then it probably does not need to be changed
Out of interest look up the price of a film cap of that size, not cheap

 

[rsavas]

Could recommend a nichicon UES type for that place, could put a 1uF stacked film in parallel too

 

[huggygood]

 

[analog_sa]

Looking at the specs it's an ACH-455 type, which in the Pioneer lingo (Pioneer Tunning Fork) means a metalized paper capacitor.

While this is easily the most important cap with regards to sound, it is certainly not a paper capacitor. Use the best sounding for your taste electrolytic cap and all will be well.

 

[InspectorGadget]

Yes, without even looking at the amp, I can tell you it isnt a 470µF/50V film cap. Any company bothering to manufacture a 470uf film cap, isnt going to only rate it for 50V. It wouldnt just be 'significantly bigger' either. It would have to be enormous! where did you get this dubious information?

 

[rsavas]

Sure they available in that size and V, not for this app, wima makes some but interesting, where would they be required is more the ? esp at that price
Op got the info from the pioneer tech manuals but some definitions are a bit misleading esp for specialty components

 

[InspectorGadget]

Yeah, perhaps some metalized plastic film type, but voltage that low is pretty rare in film caps. a search at mouser in that range results in only the wima metalized plastic films, which themselves are huge, but is dominated by even more enormous 1000V and higher rated caps. Nothing in metalized paper is that low voltage, certainly nothing ive ever seen. Sure, perhaps they exist for some rare niche.

Agreed, not sure why they would be required, especially as they wont be film and people seem happy tuning them with electrolytics.

 

[guswin95]

Before I answer some questions, I wanted to post a couple pictures.

 

[guswin95]

So, now to some comments. The description of the part with the ACH description in the Service Manual, as per Pioneer would be a Radial, letter A, and CH being metalized paper (from Pioneer Tunning Forks Capacitors). They use CE for electrolytic. I have never heard of such device, therefore the question. Film capacitors are also not polarized, so obviously the one installed originally in the M-90 and M-91 are polarized electrolytic by appearance. High-End film capacitors are very large physically and they don't make them in values in the 100's of uF. Another clue. In addition, most are Axial and not radial. Metallized Polypropylene is mostly incased in plastic forms, with Wima one of the best.

The strange part it's that is bigger than normal. (I posted in the picture a Muse 470uF/100V next to it and a cheaper 470uF/35V from Yaggeo as I don't have any 470uF/50 at hand). And again, by description it's the only ACH capacitor in the M-90. I was hoping for some insight on the choice of such parts originally.

In the circuit, it would appear its used as a filter, and then again why would Pioneer use a one of kind cap in that position. Fortunately, in this case it appears to be within tolerance, so and I am not planning on replacing it, but it peaked my interest. I am lined up to do probably 2 more of these, so preparing myself for the next one in case they are bad.

The M-90/91 is a difficult beast to open-up and replace parts and test if I can hear a difference. I am mostly hoping to get it right from the beginning by doing lots of research. Although I may venture later on trying out parts on one of the future recaps.

Thank for all the comments and suggestions.