Eros Tape Resistance Check Discrepancies

[kevemaher]

I've just completed the wiring for an Eros Tape. I've found a few differences from the manual during the resistance check. Here they are:

1. Tabs 4 and 29 read 200K not 130K both switch positions.
2. Tabs 19 and 27 read 100K not 47K both switch positions.
3. Tabs 16 and 30 read 200K not 130K with the switch toward the output jacks.
4. Both input jacks read 100K not 47K.

Other readings were within 5% of the manual values.

I performed a complete check of the parts list prior to assembly, including reading values of all resistors and capacitors. All were correct.

This appears to be a conscious design change and not my error. Am I correct?

What effects do these changes have? It looks like an eq time constant was changed.

Please help!

Thanks,
Kevin

 

[Doc B]

I'll take a look when I have a minute, but I suspect it's just differences due to the different EQ values that didn't get changed when the phono version of the manual was edited for the tape version.

 

[kevemaher]

Thanks Dan.

Further tests:
- Socket wiring checked out.
- Voltages at test points are perfect.

I will assume that the build is done correctly. I will now begin testing with sine waves. Gain, EQ, distortion, etc...

Cheers,
Kevin

 

[Paul Birkeland]

If your voltages are perfect, than these are likely artifacts from the EQ differences not quite making it into the manual.  The important thing about resistance checks is to not get a really low resistance where you aren't supposed to, and your discrepancies are all high resistances.

-PB

 

[kevemaher]

Thanks Paul. I'm not concerned about the high resistances, just making sure I haven't made mistakes.
I have just put some very clean sine waves in. Good looking sine waves coming out, although they do ride on a bit of noise, which looks like a mix of a square wave and higher freqs. I will put an FFT on this shortly.

Very high gain!

Looks like I have an operating system. I'm now in the process of characterizing it.

Any idea about how to suppress the noise? I'm using single ended RCA throughout.

Kevin

 

[Paul Birkeland]

You'll want to get the Eros and tape machine installed in your system.  Zero feedack tube circuits do have some self noise, but we have found with the Eros that the playback noise of both tape and vinyl is quite a bit higher than the noise floor of the tube circuit.

-PB

 

[kevemaher]

You're probably right.

Moving the power cord to the same strip as the preamp helped quite a bit. Noise dropped a good 20dB and all the very high freq went away. Ground loop. Probably can't hope for much better.

 

[Doc B]

I would caution you to be careful about assumptions made about the performance of a piece of gear based solely upon a test lashup. I think one should use their ears to analyze what the circuit is doing as well as measuring. Drawing conclusions strictly from scope traces can be misleading because of the different power and signal connections than might be used in the actual listening setup.

 

[kevemaher]

Understood.

Test gear does help in trobleshooting  major problems. Helped me with the ground problem, for example.
After the tape deck is hooked up to the Eros and to the rest of the system, I plan to record some sine waves and pink noise.
Playing these back should be a realistic test of the performance of the entire system.
Obviously I can paste my ears to the speakers to get an idea about the residual background noise. Ears are the final judge.

I've designed quite a number of optical and electronic instruments. I've found in development that it pays to characterize hardware as it is built rather than rush to complete the entire system. Just my style.

I hope that everyone who reads this correspondence understands this. I'm not putting the Eros down. Not by any means. It is a very good design and kit (and assembly manual) if I can build the very complicated system and have it work properly with only a minimal amount of troubleshooting.

Cheers,
Kevin

 

[jrsrover]

During the resistance check process on the Eros Tape, I have a discrepancy re: the nine pin sockets. I realize from the advisement at the top of page 79 of the manual I should expect a fluctuating reading at Pins A4, B4 and C4. However, after a brief fluctuation, the meter settles to precisely .624K (624 ohms)at these three pins. This is also true at D4 and D5 and at the +6.3 VDC joint on the power supply board. As this is relatively close to 0 ohms it is concerning. Wiring is correct and joints are solid. Also, The black wires running in the same STPs and connected at  -6.3 VDC joint, D9, C5, A5 and B5 are all at 0 ohms as one would expect based on the manual.

I'm at a loss to determine why this condition exists and would welcome any suggestions for resolving. Or is this condition acceptable?

Thanks and best regards,

Jim

 

[Doc B]

This is not really a discrepancy. If the meter reading briefly fluctuates when you test it you are experiencing the fluctuation described. As to the reason for the meter eventually settling at 624 ohms - if you look at the schematic for the heater voltage supply regulator you will see a 124 ohm resistor and a 499 ohm resistor in series from the +6.3V rail to ground. That is what is creating the settling to a 624 ohm reading as the 10uF capacitor in parallel charges. So it's behaving correctly. I always encourage builders to consult the schematics to help determine what the resistance reading might be at a certain terminal.

 

[jrsrover]

Many thanks, Doc, and much appreciated!!