Eros voltage reading problem

[Paul Joppa]

OK, looks like the 1085 is dead all right. Contact Eileen on Monday and she'll send a new one; there's at least a good chance that will fix whatever was wrong.

Meanwhile you can try to remove the old one. Once it's out, read the resistance to ground from pins 1 and 2; they should be 499 and 623 ohms respectively - you'll have to pull the tubes in order to make these measurements (the heaters will short these resistors, which are the voltage divider that sets the 6.3 volts). This check will help assure that this portion of the PC board is working correctly, giving I hope increased confidence that the new 1085 chip will work when installed.

I still have no good theory what happened to the chip, but if a new one works then you don't have to care!

[Steve Reese]

Paul, let me see if I understand this correctly... Once I take out the 1085, you want me to do an ohm test on it, or once I have the new one in place, that's when I do an ohms test. Cause you're talking about making sure the tube is out cause the heater will short the resistors. Or am I to take the readings from the power supply Pins 1 and 2...?

[Paul Birkeland]

You want to take readings on the PC board with the tube pulled out.

-PB (the other Paul)

[Steve Reese]

oh, where Pins 1 and 2 are respectively... and powered off correct?

[Paul Birkeland]

You can find the pin assignment on page 2 for the TO-220 package:

http://cds.linear.com/docs/Datasheet/108345fg.pdf

The drawing assumes that the writing on the chip is facing up, so if you have the writing facing up and the hole in the metal tab facing left, the pins go 3, 2, 1, down the right edge of the chip.

-PB

[Steve Reese]

Okay.... I have tried like crazy to get that huge heat sink out, but no luck. I've tried to remove the two small resistors near the edge of the board, but can't get them out cause for me it's hard try to heat two leads in order to remove them because my thought was to get those out then clip out the 3 pronged 1085 resistor. If anyone would have any experience in removing these parts, please share. I've been out of town for while but plan tackle this again when I get back. Thanks.

[Paul Birkeland]

I'd leave the heatsink there, it doesn't need to come out.

Go buy a spring-loaded solder sucker and suck all the solder out around the pins on the 1085, then remove it (removing the hardware that holds it to the heatsink as well).

Same for those two resistors, you can suck the solder out around them, then pull them out.

-PB

[Steve Reese]

Okay... I've gotten the 1085 resistor in place, along with the two 10uF capacitors which I had to have Eileen send me some after basically destroying them trying to get them out. Tried to get a reading, and once again I get the same high reading on D1. It hovers around 307 for a good 5 seconds, then stops between 325 and 332. It gets the same when I remove the tube. I'm at a loss.

[Steve Reese]

Okay, I'm begging now... if anyone has an idea what I should check next, please tell me. I keep looking at the big can capacitors wondering if something may be compromised with them. I didn't do anymore  checks other than D1 and D6 which were same, so I thought that any other readings I'd be asked previously would also be the same. Any thoughts are welcomed. Thanks.

[Atom Shop]

Hi Eros fans --

I've built from scratch and modded more gear than I can remember. I'm now in the process of smoke-testing an Eros kit I built for a friend. I would think that following Bottlehead's extremely well-documented manual would result in a first-trial success story. Uh, not quite.    I'm at a point where I can't seem to see the forest for the trees. 

The left channel uses the led-biased input stage (to experiment) while the right channel follows the original Eros servo design. I have 1.8v at the L cathode; the R sits at 1.5. The plate voltage on the left is 97, the right is 94. A bit low in each case. BUT, the plate voltages on the EC88 are both 107! That's way off!!! B+ from the power supply card is 221 and filaments are at 6.2. Close enough.

My wiring appears to be spot on. So, what is my problem?  (Besides the likelihood of overlooking something all too obvious)

Atom

[Paul Joppa]

So, what are the grid and cathode voltages on the ECC88? And the screen voltage on the EF86's? This is a direct coupled two-stage amplifier, and all the voltages interact.

One possibility, for example, is the bypass capacitor on the triode cathode, which if reversed (it happens!) will drive the tube into saturation. The voltages will give a good hint.

Incidentally, 1.8 volts is high for an HLMP-6000 LED, which is normally around 1.55 volts. 1.55v is a good bias for an EF86 that meets the original specs, but many modern tubes sold as EF86s need an actual bias that is lower; I believe I've seen as low as 0.8 volts. That's why there is a servo in the stock design. Direct coupling is a pain!

[Atom Shop]

So, what are the grid and cathode voltages on the ECC88? And the screen voltage on the EF86's? This is a direct coupled two-stage amplifier, and all the voltages interact.

One possibility, for example, is the bypass capacitor on the triode cathode, which if reversed (it happens!) will drive the tube into saturation. The voltages will give a good hint.

Incidentally, 1.8 volts is high for an HLMP-6000 LED, which is normally around 1.55 volts. 1.55v is a good bias for an EF86 that meets the original specs, but many modern tubes sold as EF86s need an actual bias that is lower; I believe I've seen as low as 0.8 volts. That's why there is a servo in the stock design. Direct coupling is a pain!

I made a voltage chart, Paul.

[Paul Joppa]

Thanks, Atom. You are right, this is strange and something is not working right. It's still hard to tell what, because the voltages and currents don't add up.

So ... some possibilities:

* I assume since you haven't said anything to the contrary that the LEDs on the C4S boards are all lit with similar brightness - a little brighter on the A side than the B side.

* If the R1 resistor on the C4S boards is off, that would change the current. Check the resistances, R1 (A side) and R1 (B side) which should be 113 and 301 ohms respectively.

* If the resistors are off, that would cause the currents to be off. Check for 27K from C3 to ground, and from C8 to ground - these are paralleled with the 100uF / 160v cap, so it will take time for the resistance reading to settle. Make sure that cap is correctly oriented (yeah I know - you've probably already done that several times by now. But it's still a significant suspect.) Then check the 47K resistors, C1 to ground and C6 to ground.

* Long shot - you posted several times "EC88" which is a single triode - not useable in Eros. I have assumed you meant ECC88 which is also called 6DJ8. The circuit is actually designed around the 6922 or 7308, though the 6DJ8/ECC88 is similar enough that it ought to work.

OK, it's nearly midnight and that's all I have right now.

[Atom Shop]

Thanks, Atom. You are right, this is strange and something is not working right. It's still hard to tell what, because the voltages and currents don't add up.

So ... some possibilities:

* I assume since you haven't said anything to the contrary that the LEDs on the C4S boards are all lit with similar brightness - a little brighter on the A side than the B side. [ ------------------ By my subjective observation, Paul, they all look very much alike.]

* If the R1 resistor on the C4S boards is off, that would change the current. Check the resistances, R1 (A side) and R1 (B side) which should be 113 and 301 ohms respectively. [ ------------------------ I measured every resistor on the C4S boards and all were within fractions of their indicated value.]

* If the resistors are off, that would cause the currents to be off. Check for 27K from C3 to ground, and from C8 to ground - these are paralleled with the 100uF / 160v cap, so it will take time for the resistance reading to settle. Make sure that cap is correctly oriented (yeah I know - you've probably already done that several times by now. But it's still a significant suspect.) Then check the 47K resistors, C1 to ground and C6 to ground. [ --------------------------  All OK there. ]

* Long shot - you posted several times "EC88" which is a single triode - not useable in Eros. I have assumed you meant ECC88 [ ---------------- Yup! Definitely an ECC88. ---------------------- ] which is also called 6DJ8. The circuit is actually designed around the 6922 or 7308, though the 6DJ8/ECC88 is similar enough that it ought to work.

OK, it's nearly midnight and that's all I have right now.

Well, Paul, thank you very much, that was lot of good late night advice. I ultimately found my problem! The resistor check had me stumble over it.

I'd said previously that following the manual's instructions (kudos to its authors) should result in a first-attempt success at a functioning Eros. I still strongly abide by that.

In MY case, however, I didn't quite follow some important details.    The MPS4250's are to be installed so that "the flats of the bodies [are] facing in toward each other." Those words and the associated picture make it quite obvious how these li'l guys should sit on the card. But, n-n-o-o-o-o-o.... I had to do it differently. I mounted both A-Q1's so that they're marching off the card in the same direction as the B-Q1's. Wrong! What a bonehead!!! 

Correctly reinstalling the two transistors brought everything up to spec. Almost. (Both channels sound very good!) The cathodes' led bias and servo bias both sit at 1.56. But my B+ only came up to 158, still short of the designated 170. I'm wondering if the transistors took a beating by being installed backward.

I'm so close! 

Atom 

[Grainger49]

Here is one Dan didn't include.  I need often myself:

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