Great River MP-2 , info, schematics & discussions

[okgb]

the part that shows the testing circuit also has the wire colors

Brn /5 primary
red /2 primary

Org/8 secondary
Ylw / 11 secondary

brn / Org for positive polarity to keep " in phase "

white is case , blk is inner shield , hope that helps

 

[okgb]

And for the output Xfmr Dans sez

wire as per test circuit 4 on the cinemag info

http://cinemag.biz/output/CMOQ-1.pdf

basically windings in series , 1:1

 

[Sredna]

The small transistors are getting very hot, is this normal?

Same thing on my two boards. A ruff check on the voltages shows ok.
A check with tonegenerator and scope shows no oscillation.
Haven't listened to the signal yet though...

Thanks to Dan for sharing!

Anders

 

[okgb]

they do throw some heat as a class A circuit , too hot to touch ?
See if Dan can get in on this one

 

[Sredna]

Yeah, they're almost to hot to touch.

Signal looks ok though.

Signalgenerator goes to line-input, scope from unterminated output.

Offset voltage is <0,5mV but changes when you fan the small transistors...

The preamps was connected for hours and no smoke yet so I presume it's
normal.

Thanks,

Anders

 

[Dan Kennedy]

Hi all,

Yeah a couple of the small signal transistors run pretty warm.

Worst one is Q7 which dissipates about 360mw, which is within ratings.

If I can leave a finger on a part, I figure it's OK.

The next worst run about 225mw, so everything's should be fine.

No failures in 16 years.

 

[mata_haze]

Hi everybody.
I have just fired my 2 GR preamps and this is what I get at the output of both.
above sine is INPUT the bottom is the output.



I did a measurement of the static voltages and this is what I am getting:

R10: 1.156 V
R11: 1.171 V

R19: 0.217 V
R20: 0.212 V

R29: 0.9 V

R30-R33: 0.13 V

I have followed the signal and it actually gets bad after C21,while it's perfect before Q1.
does this mean that there is something wrong with the discrete input stage?

any tip would be much appreciated,
Best,
Mattia.

 

[Dan Kennedy]

Those voltages all sound about right, my guess is there has to be a wrong part somewhere, done twice. Maybe the compensation cap is off by an order of magnitude? 680pf instead of 68?

 

[mata_haze]

[quote author="Dan Kennedy"]Those voltages all sound about right, my guess is there has to be a wrong part somewhere, done twice. Maybe the compensation cap is off by an order of magnitude? 680pf instead of 68?
[/quote]
I have used Ptownkid KITS...not sure if there was a wrong part ...but I'll double check..I have the feeling it's something silly...
:?

 

[Ptownkid]

[quote author="Dan Kennedy"]Those voltages all sound about right, my guess is there has to be a wrong part somewhere, done twice. Maybe the compensation cap is off by an order of magnitude? 680pf instead of 68?[/quote]

Nope, couldn't be a mistake on that one, there's only one and it's definitely 68pF

 

[madriaanse]

Oh yeah, speaking of Ptown parts, I need to order a bunch more. I have a few extra 2SK389 and inductors and would like to build some more channels. Ptown, Pls tell me you still sell those handy kits?

M.

 

[Ptownkid]

I will be once Dan has edited the pcb file to use the replacement part for the 2sk389

 

[mata_haze]

[quote author="Ptownkid"]Dan Kennedy wrote:
Those voltages all sound about right, my guess is there has to be a wrong part somewhere, done twice. Maybe the compensation cap is off by an order of magnitude? 680pf instead of 68?


Nope, couldn't be a mistake on that one, there's only one and it's definitely 68pF[/quote]


well then....I am lost again...

 

[Ptownkid]

well, by all means check the cap...but the single mica cap is a 68pf.

 

[mata_haze]

[quote author="Ptownkid"]well, by all means check the cap...but the single mica cap is a 68pf.[/quote]

yes,that's right. all the caps (mica one as well are correct)

I have noticed that pin 2 (hot) on J3 is connected to ground ..is that correct??

that means that my "hot" pole on the secondary transformer is always connected to ground unless I swap the phase..is it just something silly I am saying?

Best
Mattia.

 

[okgb]

At the same time it grounds one side it also selects the other as input ,
simply reversing the polarity , that seems fine .

Start either at the input or back from the output , stage by stage
and see if you can find out where the signal goes wrong , stops
amplifying ?

good luck & skill

 

[mata_haze]

[quote author="okgb"] Start either at the input or back from the output , stage by stage
and see if you can find out where the signal goes wrong , stops
amplifying ? [/quote]

here are some datas:
Vin= 2 Vpp
Vsec=6 Vpp (secondary windings of trafo going to PCB)
Vout=0,5 Vpp (output of GR gain set to minimum.

I am using a OEP A26A3E trafo and I am testing the boards without side and top lid of the case.
wired as follows (NOTE the pictures shows a OEP A26A2E instead of a OEP A26A3E):


The distorted signal appears at PIN3 of the Hi-z input (before R9) and without the jack connected it has an amplitude of 10mVpp, while with the jack connected it's 75mVpp.
PIN 2 it's just fine and it has an amplitude of 6 Vpp.

Does it looks like I am doing something wrong with the connections??
could anyone please post how they connected the components off board?
IE: trafos DI input etc.. I think that will clarify many things.

many thanks.

 

[Dan Kennedy]

By your description it does seem to be a connection issue.

The Hi-Z jack wiring can be bypassed by simply putting a "shorting jumper" from pin 2 to pin 3 of the jack.

The fact that you are getting 6 volts out of the transformer but nothing at the other pin of the Hi-Z jack indicates a wiring problem.

It could be that what your scope showed in the picture was just air-borne crap. Not too noisy though...hmmm

The wiring is supposed to be pin 1 ground, pin 3 tip, pin 2 to the switching contact. This way the signal comes out of the transformer, passes thru the switching contact to the tip when no plug is inserted, then feeds the opamp. When a plug is in, it feeds the opamp directly.

 

[mata_haze]

thank you very much Dan,
I'll double check that!

 

[mata_haze]

GOT IT!
thank you Dan it was exactly that! I have actually shorted pin 2 and 3 unless the jack is inserted and it works!
brilliant!

all the Best,
Mattia