SX-880 basket case, with lots of issues. Wiil need some advice

What is the Idss grouping? Not even the 'GR' is supposed to have a current above 6.5mA. I thought you bought 'Y's, which should be between 1.2 and 3mA?

So many bogus parts out there...

You can change the current by putting a resistor on the source, like the attached pic. You'll have to experiment with values, but probably somewhere between 47 and 100 ohms. Your goal is between 2 and 3mA.
 

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What is the Idss grouping? Not even the 'GR' is supposed to have a current above 6.5mA. I thought you bought 'Y's, which should be between 1.2 and 3mA?

So many bogus parts out there...

You can change the current by putting a resistor on the source, like the attached pic. You'll have to experiment with values, but probably somewhere between 47 and 100 ohms. Your goal is between 2 and 3mA.

They are all stamped "K30A Y" on the case.

OK - I'm scratching my head here.....

I had the DVM set to 20mA, DC... but the red probe was plugged into the red V/Ohms jack..... so it looks like I was reading voltage drop.

But..... I plug the red probe into the red mA jack, and I get no reading at all.....

I'm not doing something right here.
 
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You may have a blown fuse in your meter.

Most DMM's have two separate jacks for the red lead...one for voltage, diode, and resistance measurements, and another one for reading current. The current function is fused, and it doesn't take much of a false move to blow it.
 
You may have a blown fuse in your meter.

Most DMM's have two separate jacks for the red lead...one for voltage, diode, and resistance measurements, and another one for reading current. The current function is fused, and it doesn't take much of a false move to blow it.

I didn't know about the fuse - thanks for that. I checked it just now, and it ohms good. The 9v battery was bulging a bit, and was only reading at 3.94v. Put a fresh battery in, but still get 0mA on the meter with the test. :scratch2:

I am plugged into the red current jack with the read lead, and the common one with the black lead.

The diagram you supplied is pretty simple - could I be misinterpreting it somehow? Following the N channel diagram - I have the black lead on the drain, the red lead on the "+" battery terminal, and the S and G on the "-" battery terminal.
 
What type of ammeter?

You can check a DMM on the 2A scale by connecting it in series between the 9V battery and a dial illumination lamp or similar load. A 194 auto lamp should indicate about 250 mA on a good 9V battery. If the meter checks out, verify the lead arrangement on the FET.
 
What type of ammeter?

You can check a DMM on the 2A scale by connecting it in series between the 9V battery and a dial illumination lamp or similar load. A 194 auto lamp should indicate about 250 mA on a good 9V battery. If the meter checks out, verify the lead arrangement on the FET.

I have a Fluke 8021 B multimeter. Let me go find a bulb, and conduct your test.
 
It's YOUR test, and good choice on the meter. I have a few of the 80xx series myself and they have lasted about as long as the gear we are working in.

Fluke8020Series.jpg
 
It's YOUR test, and good choice on the meter. I have a few of the 80xx series myself and they have lasted about as long as the gear we are working in.

Thanks. I've had this meter so long, how I acquired it is a lost memory. Sometime in the early 80's, when I was a Field Engineer for the late great DEC.

Anyway - problem solved. The meter is not passing current. With a bulb wired in series through the meter and a 9v battery, there is no reading and the bulb is dark. With the bulb directly on the battery, it glows fine. :(

The GOOD news is I have a monster backup - something that came into my possession in the early 90's but I never used - A big ol' bench top Keithley 179A TRMS Multimeter. I just dusted it off, plugged it in, and tested the three SK30's per EW's diagram/instructions:

1.93mA
1.98mA
1.94mA

So.... the big question: Is that close enough to the stated goal of 2-3mA? Do I have a green light here?
 
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Good enough.

DEC huh? Must have spent as much time in MA as I did.

Good enough - aye. Thanks for your patience and help.

I worked for DEC in Northern CA (mostly at Lawrence Livermore Labs), and went to the Mother Ship (Bedford, MA) about 3-4 times a year for training. I was there for 12 years, right up until the lights went out.
 
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Practical trueism? (See above) I'd say that 1.98 mA is for all intents "2 mA" in this case.

Once the 880 is up and running, take a close look at the solder joint at the base of the probe connection barrel(s) in your 8021. They are a common point of failure, and can easily be cleaned, fluxed, and re-soldered to solve any problems. The other possibilities are the fuseholder connections, and the shunt (and its connections). It's definitely worth repairing.
 
Practical trueism? (See above) I'd say that 1.98 mA is for all intents "2 mA" in this case.

Once the 880 is up and running, take a close look at the solder joint at the base of the probe connection barrel(s) in your 8021. They are a common point of failure, and can easily be cleaned, fluxed, and re-soldered to solve any problems. The other possibilities are the fuseholder connections, and the shunt (and its connections). It's definitely worth repairing.

Thanks for the tip, I'll look at it post-haste.

D-Day on the power-up is scheduled for tomorrow - hope I don't burn the garage down.

Stay tuned,and wish me luck!!
 
Good enough - aye. Thanks for your patience and help.

WAY OT

I worked for DEC in Northern CA (mostly at Lawrence Livermore Labs), and went to the Mother Ship (Bedford, MA) about 3-4 times a year for training. I was there for 12 years, right up until the lights went out.

Wow DEC I have not heard that in a while I worked for Intergraph we OEMed from DEC PDP 11XX thru VAX 85xx series.

Remember when the CDC 670MB drive was king....

How about repairing a CDC 300MB after a head crash......
 
We had two DEC PDP 11/34a's in different buildings, supposed to be identical.

One was buggier than a nuthouse.... (mine of course, which needed the uptime to be unbroken, also of course)

The only difference we could figure was one was on a north south axis, and the other on an east west axis.

We used the (80 Mb?) CDC 9762....
 
Well...... the good news is that I didn't smoke anything.

The bad news is.... some strange goings-on. I fired it up on the DBT, and everything seemed good and the relay clicked on. Connected a CD player to aux, and it played nice. Hooked up an FM antenna and pulled some stations in nice and strong. Checked some voltages, and they looked ok.

Listened to it about 3-4 minutes, and got a burst of noise and into protection. Shut it off, turned the volume down, fired it back up. With the volume down all the way, it plays at listening volume. If I turn the volume knob up, the DBT glows bright and I get the burst of noise and into protection. Also - no front panel lights. All the LED's work - AM, FM, PHONO, AUX, and FM

Since this was a replacement volume pot, I gave it a good De-Oxit and tried again with the same results.

So......I'm stumped. What do you wizards need to know to help me out?
 
With the volume all the way down to minimum and selector to AUX, is the DC offset stable at the speaker out? If not, try turning on Tape Mon and measure again.
 
With the volume all the way down to minimum and selector to AUX, is the DC offset stable at the speaker out? If not, try turning on Tape Mon and measure again.

With volume all the way down, set to aux, measuring on the + and - of Speaker A terminals with an 8 ohm resistor for load, I get 19 to 20mV. Same for Tape Monitor. When it comes out of protection on power-up, it is at around 35mV, and slowly ranges down to settle at around 20mV. If I turn up the volume it quickly shoots through the roof and the DBT lights up REAL bright.
 
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