Volumne Not Balenced

[VicN]

Hi
Just finished assembling the Eros kit. Measured the resistances and they looked OK and the voltages were close. So I plugged in the CD through a reverse RIAA circuit/attenuator into the Eros and then into a small amp in workshop and out came music! Terrific, or so I thought. I left it playing on repeat for the best part of a day (with the amp off) to start the burning in process. Came back later that day to have a listen, and the channel balance seemed off, with most of the sound seeming to come from the black channel. Did a quick and dirty by measuring the output with the multimeter set to AC while it was playing. The Black channel was jumping between 0.4 to 0.6 V while the red was jumping between 0.2 and 0.3 V. Hmmm! I swapped the input leads, but the black channel was still noticeably louder. I swapped the EF86 tubes around, replaced the 6922 with another one I had, but still no joy. Decided to see just how far off it was, so I pumped in a 1 kHz signal via the reverse RIAA circuit and hooked the outputs to a scope. The black channel was about 0.5V peak to peak. Plugged the 1 KHz signal into the red channel and got about 0.25V, which was pretty much the same as the quick and dirty I did with the multimeter. I rechecked all the components on the 2 servo boards, wetted all joints on the boards with a touch of extra solder, checked and  resoldered the tube plugs and each signal path from start to finish and measured the resistances to make sure they were actually what they were suppose to be. Alas, the problem persists. I

 

[Paul Joppa]

...When you tun off the amp, the Kreg voltage on the black channel decreases a lot faster than the red channel

 

[VicN]

Hi Paul

Thanks for the quick response, but no joy. I had some smaller 1000uF caps, so I used 2 in parallel to replace the 2200uF cap, but the same result - lower volume and very slow discharge on the red channel. I even jumperred the negative leg of the capacitors directly to earth in case their was an issue with the earth connection.

So if it's not the capacitor and it's not the earth connection, could it be the 2N222A transistor on the other side of the  2200uF capacitor that's acting up? Can that cause a lower than normal volume? Mind you, not knowing how long it normally takes the transistor leakage current to drain the capacitor, I'm not sure if it's the black channel that's draining too fast, or the red channel that's draining too slow! Is it possible for a faulty transistor to cause too much gain???

Also, just to clarify the transistor pin nomenclature, K is the collector, A is the emitter and r is the base? Is that right?

Thanks again

Vic

 

[Paul Joppa]

The fact that the 100v points (breg and OB) are very close indicates that the servo bias is working; that's the 2N2222 transistor. If the 2200uF caps discharge differently, but they are good and correctly connected, then perhaps the 100uF/160v cap is not connected on the channel that discharges slowly. But check both, I may need another cup of coffee this morning!

 

[VicN]

Hi Paul
Nope. Checked the 100uF cap on both servo boards. Both negative legs were 0 ohms to ground, both charged to about 96.5 V and both discharged in less than a minute when the power was turned off, so they look OK.

I tried another tact and removed both servo boards and swapped them over to make sure that the issue was with the board and not something else. Took the opportunity to again wet all the solder joints on the underside of the boards and all the tube pin connectors.

When I turned the amp on, I had the multimeter (fortunately) connected to kreg on the former red channel servo board (ie. the one that was giving me the low volume), just to see what the voltage would do. The 4 leds on each servo board lit up as they should, but then several things happened simultaneously about 10 seconds after switching on. The 4 leds on the power board lit up (normal), the 2 leds on the A side of the former red channel board went out and the kreg voltage on the former red channel board suddenly started going up quickly. I managed to turn the power off at about 2.6V.

Assuming I

 

[Paul Joppa]

Not enough information for me to guess anything. All the voltages will do odd things during the turn-on transient, and will vary depending on how fast each tube heats up. There are three tube sections each channel so it gets complicated. Are you using the stock tubes?

Let it run until the voltages are stable, and let's see if things are working then. Watch the LEDs carefully; there is a difference between getting dim (indicates too much current going out) vs. going out completely (indicates the R2 has failed).

 

[VicN]

Hi Paul
Yes, using the tubes the Eros came with.

What worries me is that the leds on the servo have never gone out during warm-up before and they stay lit on the other board. Also, the kreg voltage slowly climbs to about 0.2V, but when the power board leds light up it rockets (and I mean "rockets") up. I'm afraid it'll rise to more than the 4V rating of the 2200uF cap. Again, the voltage on kreg has never done that before.

The servo leds light up immediately when the power is switched on, but then the A side ones go out as the power board leds kick in.  They also looked like they were out to me rather than just dim.

Cheers
Vic

 

[Paul Joppa]

OK, my new theory is that the 6922 is not performing at full emission - either it's a bad tube, or it is not yet fully broken in (the cathode is not fully "formed"). Sometimes it takes several tens of hours to get the cathode working properly - it does seem to be a problem with modern-production tubes. I came to this conclusion after I reviewed my detailed analysis. But having spent some time working it out, I've left the analysis below. You can let it run and check voltages as suggested below, or try another 6922 if you have one. Or ask Eileen for another tube.

Notice below that I originally concluded the tubes are OK, forgetting that the servo bias will force an under-performing tube to get the right voltages, but it may only happen when the bias is too small - which leads to the 6922 grid loading down the RIAA network, giving loss of gain.

======== here's the original analysis ==========

Yes, you may lose the 2200uF cap. But the alternative at this point is to build a new board.

Since you have swapped the boards, we know that all the tubes are OK, hence the problem must be on the board. (I am assuming all the connections are done correctly, because you have been careful about checking and re-checking them. There is always a chance that something has been consistently missed, but I can't do much with that possibility so I won't try!)

So, the logic I have is the following:

* Inititially the shunt reg board LEDs are out because all the transistor bias current is going through the base-emitter junctions, which have a lower voltage than the LEDs. Once the shunt reg tube cathode starts emitting (around that 10 second point), then the shunt reg tube draws enough current to allow the LEDs to light. This drops the output voltage to the regulated 225 volts.

* Initially the applied voltage to the audio circuit is higher, say 350v. That's enough to allow both C4Ss on the servo boards to operate, since they have resistors from plate to ground.

* When the shunt regulator kicks in, it drops the voltage to 225v, which is no longer enough to drive the full 7.4mA until the 6922 cathode starts emitting. This could cause the LEDs on that side (the A side) to drop out until the 6922 finishes heating up.

* If the 6922 cathode (C3 or C8) is not drawing current, then the servo transistor 2N2222 does not see any base voltage, and is an open circuit. When the EF86 cathode starts emitting, its current charges the 2200uF capacitor. This creates a bias voltage that shuts off the EF86, allowing its plate voltage to be high. Once the 6922 conducts, that high voltage at the grid of the 6922 will cause it to conduct current through the 27K and 174 ohm resistors, providing bias to the base of the 2N2222 which will pull down the voltage on the 2200uF capacitor.

So, some speculations - perhaps the 6922 heats up slowly on this side, and if you let it run, it will stabilize at more normal voltages. Or, perhaps the servo feedback resistors (27K and 174 ohms) are not providing the necessary continuous path - but since the steady state voltages were correct before the board swap, we know it was working correctly before. That leaves a possible error in connecting to the tube at C3 or C8 (not sure which one goes with which channel, but C1-C2-C3 goes with one board, C6-C7-C8 with the other board.) You should be able to measure 27K from that pin to ground.

That's why I suggested letting it go - if the voltages stabilize correctly and the LEDs come back on, then the slow-heating theory is correct. If not, then something that was working is now broken, and the voltages will give us a clue. I have given a detailed reasoning in hopes that it might stimulate you or another Forum denizen some idea which has not yet occurred to either of us - this is a tough one.

For what it's worth, on my own Eros the servo board LEDs come on immediately and never waver in intensity; the regulator board LEDs come on at about 10 seconds and remain on thereafter. I have an EH 6922, which I think is still the stock tube. So there is something different with your Eros.

I have said slow-heating, but it may be the cathode is not well formed yet - sometimes it needs several tens of hours before a new tube is emitting at full performance.

 

[VicN]

Hi

Latest update. Replaced the 6922 with another one and it made no difference. Also pulled out all the tubes and disconnected one leg of the heater wire from the 6.3V supply on the power board to check that nothing got broken or shorted out.

So I decided to collect a little more data. Hooked one digital meter to Kreg so I wouldn

 

[Grainger49]

Vic,

Booger!  You are in a fix since the cap is going to rated voltage before any of the other points get to settle out. 

Replacing everything on the board is a huge PITA.  If you are not already skilled with PCB soldering you can lift a trace and then you are really in trouble. 

Hopefully Paul can check his Eros and see what he sees as maximum voltage on that 2200uF cap.  It would be safe for a quick overload, surge voltage, but not for sustained voltage over its rated 4V.  BTW, I went looking for a similar low voltage capacitor and I couldn't find any upgrade.  Matter of fact, I couldn't even find the 2200uF value at the usual suppliers.

Good luck with this.

 

[Paul Joppa]

There is a (largish) 27K resistor in the R4 outline connected to breg. This resistor should go to the 6922 cathode B3 or B8, which should settle at about 100 volts. breg is also connected to the positive lead of the 100uF capacitor, with a short 1" wire (manual p. 29). It also connects to pin 1 of the EF86 - black wires, B3 to A1 and B8 to C1. If breg is staying at zero volts, then either it is not connected to the 6922, or the tube is not conducting. The servo reduces the voltage on the 2200uF capacitor when breg gets to 100 volts, so the failure of breg to rise explains the voltage at kreg.

I'm sorry this has turned into such a long saga! Take each step carefully and patiently (as you have been); we almost always resolve these things eventually.

You should be able to measure 27K from B3 to ground, and from B8 to ground. This is paralleled with the 100uF capacitor, so it may take some time for your meter to settle at 27K. And you should measure zero ohms B3 to breg and B8 to breg, on the respective adjacent boards.

 

[VicN]

Success!!!
Ok, a slight oversight. When I originally made up the first servo board, I accidentally soldered the breg connecting wire to the +reg pad next to it. I picked it up when I checked the wiring and moved it where it should have been. Unfortunately, the blob of solder was still on the +reg pad. So when I swapped the boards and soldered back the connecting wires, I spotted the solder on the empty +reg pad and well

 

[Paul Joppa]

Woo-hoo! Cause for celebration!

I know I often give more information than seems needed; this is the reason: pretty often it's something neither of us thought about, and it's the guy with the hardware in front of him and the most information in his head who will (eventually) find it. Always an extra challenge when there are TWO problems...

I have some 4x flip-down magnifiers, which go in front of my 2x bifocal reading section - focal length 6.5 inches. Any closer and I'll burn my nose with the soldering iron, but it sure helps sometimes!

 

[Doc B]

Congrats on the repair! Once again I will implore builders to inspect every single solder joint with a magnifier before assuming a part is dead. I do the inspection myself on every piece of gear that leaves this place.