EICO HF-61

[jonny]

Hello all,

I've been reading this forum for a long time, but this is my first post. The knowledge I have picked up from you guys and gals has been unbelievable. I cannot thank you all enough.

Now to my post. I recently came across an old EICO HF-61. I found a copy of the schematic here:

http://users.rcn.com/fiddler.interport/HF65A.HTM

It is labeled as the HF-65a page, but the schem is for the HF-61. My question is this. Can I convert this into a mic preamp by bypassing the input selector switch? What type of transformers should I use to balance the unit? If I am able to convert it properly, will it sound decent, or should I use the parts to build something different?

Please forgive my remedial questions. I'm slowly getting my head above water with this stuff!

jonny

 

[CJ]

This would be a perfect box for getting off the ground with DIY. You can play around with it with out any transformers, install an input transformer, see how the sound changes, install an output transformer, see how the sound changes, study the power supply, tweak components, the whole nine yards. Then, after you have finished, you can either modify the thing to your own preferances and leave it in the box, or use your new modded circuit and rip the parts out and install into a 19 inch rack chassis to impress your friends.

You can an add XLR jack for a mic input. Did it come with the power supply?

Eico stuff was known mainly for low cost DIY er's , not state of the art fidelity and noise, but no big deal. That's where "the Lab" comes in handy.

Since it is fairly old, you might have some nice tubes in there. Any brand names on the tubes?

cj

 

[PRR]

> Can I convert this into a mic preamp by bypassing the input selector switch?

WHAT a selector switch!!!

The HF-65 has a mike input setting, so would be the simpler hack. Plug mike transformers into the mike inputs for balanced inputs.

The -61 has eleben different phono settings, nothing flat. If you want to give up the phono, hack-out the phono EQ part of the -61's selector switch and hard-wire a resistor:

This gives a gain of about 90, or 38dB. Use a $13 XLR-1/4" transformer, gain of 13dB, total gain of ~60dB. That goes to the Level control, your main gain set, then to V2A V3A for a further gain of about 20dB. Then a classic feedback tone control (probably the best part of the EICO), then a "Loudness" control (leave this full-up), then a cathode follower output.

Total gain is around 80dB, trimmable down to 40dB.

Later you can try more exotic transformers. But the $13 cans do a very good job, and already have the XLR on them. You can either cobble a 1/4"F-to-RCA cable, or gut the 1/4" plug on the $13 can and bring a shielded wire out.

Output can just about deliver +20dBu into 10K load. It really does not want to drive "Pro" inputs that might be true 600 ohms, and isn't really a transformer driver. You can connect it to balanced inputs, of course, and hum should not be a problem.

Phantom power is a problem. Nothing in here will give 48V at 10mA. The whole box may not be eating 10mA.

BTW: that schematic shows power coming from outside the box. Do you have a way to power it?

 

[fazeka]

I have an old (1960) Eico HF-81 integrated. 14 watts, but this puppy is scha-weet! :green: The word is out, though. Everyone and their brother have shot these prices up on eBone. I got mine 5 years ago for a song. :thumb:

 

[jonny]

Thanks CJ and PRR for the advice. I plan on hacking into this thing real soon. I'm sure I will have more questions when I get going.

Since it is fairly old, you might have some nice tubes in there. Any brand names on the tubes?

The unit has 1 BVA 12AX7 and 2 BVA 12AU7, with an EICO 6X4 on the output.

Then, after you have finished, you can either modify the thing to your own preferances and leave it in the box, or use your new modded circuit and rip the parts out and install into a 19 inch rack chassis to impress your friends.

I think I'm going to rack it up even if it sounds terrible. I have quite a few rack spaces to fill!

BTW: that schematic shows power coming from outside the box. Do you have a way to power it?

There is a power supply built into the unit. Everything lights up when I turn it on, so I guess that's a good sign.

It also has that great old gear smell!

jonny