Redplating jcm900

[madmartigan]

This is the correct schematic, but its hardly legible. I noticed the pcb # didnt match up on the other.

[dynaman]

That's the same as what I posted earlier. Ya gotta scroll down in the file I posted.

[madmartigan]

yeah sorry i tried to delete that post after i realized but was unsuccessful

[Structo]

Not sure if this one is any better but it has the input on page 3 that is clear.
http://www.schematicheaven.com/marshall ... xx_50w.pdf

[madmartigan]

Well... Got a new set of tubes and more of the same. Red plating like crazy.
Anyone care to take a whack at which cap is the bias cap?

Thanks for everyones input so far.

[Cliff Schecht]

C13 and C14?

[ampgeek]

Yep. The 10 uf/100V characters with one end going to the ground rail.

Do note the polarity if you intend on replacing them. D1 creates a negative (with respect to ground) voltage. So...the two bias caps have their positive (+) side connected to ground.

In fact, you might want to check the downsteam side of D1 to demonstrate that. DVM set to ~200 VDC, positive DVM lead at the intersection of D1 and C14 and negative DVM lead to ground. Your meter should read a negative value as es335 predicted earlier (-90VDC IIRC).

Good luck,
Dave O.

[Cliff Schecht]

The standard bias is at -90V or is that maxed out? -90V is practically turning the tubes off. What is the typical bias voltage/current in a JCM900?

[ampgeek]

This is the raw bias voltage directly after the diode so it will be significantly greater than what gets to the control grid of the tube. Note the 15k dropping resistor and bias control pot before heading off to the grid in the schematic.

2X the final bias voltage here is not unusual and my best guess was ~-45 VDC final which is probably about what you were guessing.

Dave O.

[es345]

Well... Got a new set of tubes and more of the same. Red plating like crazy.
Anyone care to take a whack at which cap is the bias cap?

Thanks for everyones input so far.

Hey,
please don't kill your tubes. Do the measurements in the way I proposed - again: without the power tubes and both switches on - and tell us the results. This step by step procedure will help us to find the root cause.

[madmartigan]

The only way I get a reading on pin 5 with everything turned on is if I set the mm to 2000m vdc. It starts at about 50 and then keeps climbing past 300 but by then the tubes are red hot and smoking so I shut it down.
This is with the tubes in.
Without tubes I get about -8 millivolts dc.

On Pin 5 (grid 1) you should have something between -40V to -50V. If you have only -8 mV then definitely your bias circuit has a defect.
Pull out the power tube, switch on (both switches) and start measuring.
Reference is the JCM 900 50 W power amp diagram.

1. AC voltage at the connection C15,R7: should be something around 100V AC
2. Connection D1,C15,R29: should have around -90V
3. Connection R27, R26: around -42 to -48

Well... maybe I need to go back to elementary school to learn how to use my meter , but these are the readings I got..

1. 170.1 (meter set to 200mVAC)
2. -11.5 ( set to 200mVDC)
3.-10.4 (200mVDC)

[ampgeek]

How about an ac reading upstream of C15 which is also the input side of the standby switch. That ought to be in the 100'ish VAC as well.

You might also want to stick you meter into a wall socket just to make sure that it is reading correctly (e.g., ~115 VAC to 120 VAC).

I am getting the feeling that something is funky with C15!

Dave O.

[madmartigan]

c15 is 170vac. Same as the standby switch like you said.

oh, and wall socket is 116vac

[ampgeek]

Cool! At least you know that the meter is good in that AC voltage range.

Soooo...170 volts AC upstream of C15 and 170 millivolts AC downstream of C15. Yes?

If that really is true than it seems like adequate current isn't passing through C15 or R7 is leaking too much to ground.

Could the "170" in both of those readings simply be happenstance?

I would next measure the voltage difference across C15 and across R7 to get a different "perspective".

Truth be told, I don't really know the purpose of C15. "Cleaning" up the AC before the diode? Blocking renegade DC? Hopefully someone will chime in on that one.

Not sure how much you have invested (monetarily or emotionally) in this amp but I don't think that jumpering C15 would be a high risk venture for a quick test. Followed by lifting R7 to assure that adequate voltage is getting to the diode.

Good luck!
Dave O.

[Jana]

My money is on C15 being toast. Seen it happen before.

It is there because there isn't a separate bias winding and the B+ for the mains is derived from a full wave bridge. Hence, there is no "ground" reference directly on that winding of the PT secondary. It's a cheap way of creating a faux tap for the bias. It works, mostly, for a low current application like for the bias.