Eico HF-85 preamp questions

[Sam Cogley]

I'm going to use this thread as a clearinghouse to deal with any questions that arise as I rebuild my HF-85.

First off (and I've run searches all over the place for this question), has anyone here modified their HF-85's selector switch to convert the Tape Head input into either a second phono input or a line level input? Such modifications are common on Dynaco gear, but I haven't found anything referencing the Eico switch design. If it's possible, that would let me turn the tape speed EQ switch into a tone in/out switch.

 

[GordonW]

If anyone had an HF-85 assembly manual, I'd think we could glean at least some of the info we need from there. I looked at the schematic, but the jack assignments (which input is tape, phono, etc) and switch functions aren't labeled very descriptively on there. It may well take someone just going in and tracing the wiring in the preamp itself, to determine what goes where, and does what with each selector switch position.

I don't see why it wouldn't be possible... but it would be more than moving one or two wires, probably...

Regards,
Gordon.

 

[dcgillespie]

It should be very doable to make the tape input a second phono input, but all but impossible to turn it into an additional high level input if the original selector switch is to be retained.

Assuming you want to retain the original selector switch, you would first remove all the wiring and components (two resistors and a cap) associated with the tape speed switch (R19, 20, 27 & 28, and C13 & 14) , and then:

1. Strap together the tape and phono output terminals on the selector switch (terminals# 1&2 on section 1D, and terminals# 2&3 on section 1E).

2. Strap together the tape and phono EQ input terminals on the selector switch (terminals# 3&4 on section 1B, and terminals# 4&5 on section 1G).

All component and terminal designations are based on the Eico schematic. Also, you would need to add a new phono cartridge load resistor (typically 47K) across the tape input jacks as well. And that should do it.

Good luck with your project!

Dave

 

[GordonW]

Speaking of someone tracing the wiring... it would figure Dave had already done that.

Regards,
Gordon.

 

[ferninando]

the shack sells a nice 2pole 6position rotary for replacement when removing old one. use for both selector and mode.
I'm choping on a 85 I want to clean up and make more for my wants with new pots/sws, etc. I have a download blk&wht copy of the mnl but just bought an original with the colored dwgs I hope will ease the rewireing when it comes. the b&w sux. theres info on eico yahoo group. you should join.

 

[jaymanaa]

It's been a very long time since I re-built mine Sam, but I seem to remember using the tape outs as a way to bypass the tone controls, or was that on a PAS?:scratch2: This is the same switch (I think) http://www.audiokarma.org/forums/showpost.php?p=5994609&postcount=17

The 81 has two aux, 1 phono, one tuner, and tape in which makes 5 total (4 with a phono). How many does the 85 have stock?:scratch2:

 

[Sam Cogley]

The 85 has seven positions (in order from left to right):
Aux A
Aux B
AM-FM
FM-Multi
Mic
Phono
Tape

 

[dcgillespie]

Replacing the selector switch does clean up the wiring, but also eliminates the ability to ground out the unselected inputs to eliminate potential feed through from unselected sources that are still hot. If that is not a concern, then replacing the selector switch is a viable option.

Dave

 

[Sam Cogley]

I haven't looked at the schematic for a few days. Is that MIC input low or high level?

 

[ferninando]

low if I recall rite from the specs in the mnl. mite be wrong. better check.


it's 1mv sensitivity. so that's low.

 

[Sam Cogley]

Grr. I finally finished stripping the chassis all the way down. I knew it had some gunk on it, but it turns out someone soaked the damned thing with Cosmoline. And it didn't react well to something else that got on the chassis later, specifically under the pots and switches. After I finish getting the gunk off, it looks like I'm going to have to paint this one, too.

 

[jaymanaa]

The 85 has seven positions (in order from left to right):
Aux A
Aux B
AM-FM
FM-Multi
Mic
Phono
Tape

Correction, this is the same as the 81. I'm using aux A, Aux B, phono, and FM+MP as Aux C or tuner as it would be.

 

[Sam Cogley]

Yep. When I get that second selector switch I'm going to rebuild it with two sets of the front two wafers, instead of having the FM input jack feeding two different positions. It'll give me four line-level inputs. I think I'm going to strap all of the low-level inputs together as described above to create three phono inputs. In the end, the resulting unit should have a lot more functionality for the modern world.

Now I just need to get that damned Cosmoline off of the chassis.

 

[Sam Cogley]

Just for posterity: there is a serial number stamped into the chassis. E 2809.

I need to make a new input jack plate to replace the original that's set up for seven RCA jacks. Need one with eight. More to do...

 

[jaymanaa]

Sometimes, like on this 81 I'm doing, I will build a totally new piece for the input jacks so that nice cables can be used. Eico isn't "too" bad, but some manufacturers really put them close together. I think they had one RCA cable back then and it was small. Where did the cosmoline come from? WD40 is the best thing I've found for removing it.

 

[Sam Cogley]

I have no idea why it was coated in cosmoline, but it was done pretty evenly. Unfortunately some other stuff over the years actually reacted with the cosmoline and did weird things to the finish. Mineral Spirits got rid of it...which reminds me, I need to take my Mosin-Nagan apart and get the 60 year old cosmoline out of it...

I'm planning to re-space all of the RCA jacks. For whatever reason, Eico made the cutouts much wider than the standard jack plates they were using, so there is room to move them apart a bit further.

I'm going to try Rustoleum's metallic "satin nickel" paint for this one. The stainless-look stuff looks good on the SCA-35, but ACE didn't have any and I had a $5 rewards card to use up there.

 

[Sam Cogley]

It should be very doable to make the tape input a second phono input, but all but impossible to turn it into an additional high level input if the original selector switch is to be retained.

Assuming you want to retain the original selector switch, you would first remove all the wiring and components (two resistors and a cap) associated with the tape speed switch (R19, 20, 27 & 28, and C13 & 14) , and then:

1. Strap together the tape and phono output terminals on the selector switch (terminals# 1&2 on section 1D, and terminals# 2&3 on section 1E).

2. Strap together the tape and phono EQ input terminals on the selector switch (terminals# 3&4 on section 1B, and terminals# 4&5 on section 1G).

All component and terminal designations are based on the Eico schematic. Also, you would need to add a new phono cartridge load resistor (typically 47K) across the tape input jacks as well. And that should do it.

Good luck with your project!

Dave

Dave, further looking at the selector switch diagrams reveals that it should be quite easy to convert any of the low-level inputs to high-level. Currently the low level inputs run to layers 1A (1G) and 1B (1H), then through 1C (1F), back through the 12AX7 for that channel, back through a different trace on 1B (1H), then finally to 1D (1E) and out to the main channel amp. Instead of all of that rigamarole with 1A (1G) and 1B (1H), just take that input to it's appropriate points on 1D (1E) and 1C (1F). In other words, to make the MIC input a line-level input, just run it to point 12 on 1C/1D and point 1 on 1E/1F.

I'm also scratching my head as to why the output terminals should be strapped together, as described in point 1 above. Point 2 makes perfect sense, that's necessary to get the phono EQ connected due the really odd way the equalization is handled on that switch.

Eico apparently used 100K load resistors. I take it switching to 47k is desirable?

As noted above, I'm planning to build a switch using two of the 1A/1B and 1C/1D wafers. That will let me convert the now-useless AM-FM/FM-Multi input setup into a pair of separate line-level inputs. Unless I'm really losing my mind with the notes above about converting the low-level inputs to high-level, I should end up with two phono inputs and five line-level. Quite nice.

Edit: on the "duh" frontier, it might be more worthy to convert the "Tape Head" input to line level, as it can then be used as a regular tape input, and the totally useless "MIC" input can be turned into the second phono input.

 

[jaymanaa]

Yes, I change the 100k carbons to 47k 2% metal films.:yes:

 

[ferninando]

I am doing about the same thing on a hf85 that was given to me in great need of rebuilding.. I'd sure like to see pics of yours as the work progresses. Any re-wireing diags. too if you make any. I bought an "orig" manual and so have clear wireing diagrams of it for reference. I was thinking of using 2 pol 6 pos rotarys for select and mode(maybe). the kind RS sells.
I've already done an 85 for a friend for just aux and phono and he likes the heck out of it.
Says it sounds better that some other big $$ preamps he deals with.

 

[ferninando]

heres mine as received. I have all the tubes.