Redplating jcm900

[ampgeek]

Thanks Jana! Nice little "learning" thread here.

So...would jumpering around C15 for a quick test cause any catostrophic failures and/or lead to misleading results? It seems that one would still retain the that winding ground tap path as long as R17 was reasonably healthy.

Cheers,
Dave O.

[Structo]

I wouldn't do that.

If you have another cap that is close to the value and voltage, just sub it in and see if that fixes it.
Of course, ideally you would want the same value and voltage part.

You can usually go a bit higher in value without harm, make sure the voltage rating is at least as high as the existing or higher.

[Jana]

Do Not jumper C15. It has to be there.

I forgot what a class X cap is but if memory serves me right (rarely does) it is designed to fail in an open condition and not a short circuit.

[dynaman]

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)

I don't understand why you have your meter set to measure millivolts when your trying to measure stuff much higher. Have you replaced the battery in it lately?

[Structo]

Man, I would buy a newer meter.
They aren't that much money.

You don't have to buy a Fluke although they are nice.
Modern DVM's auto range the setting so you don't have to worry about overloading a lower setting.

[madmartigan]

When I have it set to volts I dont get a reading. Its less than a volt. But when I stuck it into the wall socket it read fine. The only way there is any reading but zero is in millivolts. I would suspect a faulty meter as well but doesnt good reading at the wall socket confirm that it is functioning correctly?

[Jana]

Maybe the meter is telling you the truth--there aren't any volts there?

The readings you are getting would certainly explain the redplating.

[madmartigan]

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

Actually its 170V on one side and 0V on the other. Reads about 174mV at the junction of c15, r7, and d1. Would this indicate that c15 is bad?

[es345]

Ok, I propose the following sequence (power tubes out, disconnected from mains)

1. disassemble D1, measure it. if D1 is dead(low resistance in both directions), you have found one root cause, D1 needs to be replaced
2. measure R7 (56K): if not ok, replace it.
3. power on (both switches) , measure again at the connection D1,R7,C15. Remember: D1 is still not in the circuit. If there is still nearly 0V AC then C15 needs to be replaced. Do it.

Put in D1, measure again. Tell us the results.

[madmartigan]

Ok I will do as you suggest. Thanks for the advise. Anyone know where I can get one of these c15's if I need to? It looks like a white rectangular box. Or will any cap with the same values work?

[es345]

you must put in a class X capacitor as written in the schematic which is from my understanding a self healing type.
Take the voltage class which is written at the capacitor. If you get one which is able to handle more voltage, no problem. but NEVER a lower voltage class

[es345]

Hi,

interesting to know for myself. Was the fix successful?