Test frequency?

[Bingtou]

If I want to quickly establish whether a connection problem between a tape deck and the Eros is more likely to be in the Eros or the tape deck, can I input a frequency from a signal generator to the Eros and read it from the output?

For my first connection attempt, all I was able to get is hum. So something is either wrong in the way I connected to the heads, or there's a problem in the Eros. As I did the voltage check of the Eros at the end of the build, and all seemed well at the time, and there is nothing that shows up from a visual reinspection of the solder joints (so far), I think my problem is more likely to be in my tape deck connection. 

Would a frequency check help to show if the problem is actually in the Eros, and if so, what should I feed in?

 

[Paul Birkeland]

Grab an old phone with a headphone jack, use a 1/8" TRS to RCA adapter cable, then feed music into the Eros with the volume on the phone down one step up from the lowest setting, then have a listen. 

There will be too much bass and not enough treble, but otherwise everything will pass through.

 

[Bingtou]

Thanks Paul. As luck would have it, I have such a cable. I performed surgery on one of those to make my headblock to Eros cable, replacing the 1/8" TRS end with a six-pin male connector, for plugging into the female board connector of the cable that goes to the playback amp board.

I can use the spare cable and an iPad with a jack to feed out the music.

 

[Doc B]

What tape deck and how is the head output wired?

 

[Bingtou]

It's a Technics RS1500. Theres's one cable from the headblock switch to the playback amp. It's terminated with a four pin plug (female) at the head switch end and six pin plug (female) at the playback amp board end. The six-pin plug and the connection point on the board are labelled "V".

I thought I could just make an RCA cable with a male six-pin plug at the other end, and plug that into the female V connector on the cable coming from the head switch.  Then I could have a cable that can be unplugged easily: in particular, I wanted to test the setup first and be able to return to the original wiring if it didn't work out.

I have a photo of the plug I made, which I can post, but it'll be a day or two before I transfer it off my camera. I may have mis-soldered this plug, but as I understood it, I would need to connect a twisted length of the shield braid of the cable to the outer two pins on left and right (four pins total) to match the grounding shown in the schematic. Therefore I soldered the braid so that it crossed the outer pin to terminate on the next inward pin, and was soldered onto both. In other words, pins 5 and 6 are shorted together, and pins 1 and 2 are shorted together. 3 and 4 are the + pins for the left and right channel.

But it may be that I've read the pinout wrong, and shorted + to - ?

 

[Doc B]

Sounds like you did it right, but I have not attempted a mod like you are describing. We would simply cut the six pin connector off the end of the internal cables and reconnect them to some RCA jacks mounted on the back of the deck. I'm afraid I don't recall if we connected the shield to the signal cold at that point (we were doing this stuff about 10 or 12 years ago), but probably we did. My hunch would be that you might need to ground the chassis of the preamp to the chassis of the tape deck to kill the hum. But if you are not getting music signal there might be some other issue like a shorted hot signal leg.  First thing I would check is if the shield is actually disconnected from the signal cold when the internal six pin plug is not connected to anything.

 

[Bingtou]

Thanks for the advice. I'll take a look when I can set some time aside for it (still doing some work on the deck that might hold me up from getting to this right away).

I did later attempt a hookup from the internal cable to the back of  the RCA panel, but that didn't work out either - same hum issue. I didn't cut the plug off the cable, as I'm unwilling yet to do anything that can't be easily reversed, but I removed the wires of the cable from the V plug itself. It's a Molex or similar type, so the metal wire  terminations can be slid out of the plastic casing of the plug.