HF-85 Rebuild advice

[barchetta]

Well Anatech has reminded me that my original 1959 vintage Eico HF-85 preamp is running on borrowed time. I'm wondering which components I should be replacing and with what? I have heard that component selection is critical on this pre-amp to maintain the original sound..

I have also heard that the original carbon resistors are critical to the sound.. so if that is true (is it?) can I leave those? Should I measure them and see if they are in tolerance?

Which cap types can stay?

As you can tell I am a beginner.. so please try and respond so a beginner can understand.

The current condition of the unit is a very small amount of hum and amout 50mv of DC if I remember right.. it sounds great but lacks in detail a little..


thanks in advance for any advice you can offer.

[pchw]

I have 2 of these. One thing I recommend to do asap is to replace the power cord with a 3 prong cord. Both zapped me ocasionally :-) Also, try using the tape out as pre out, this has less gain but more natural and less distorted sound. In my experience, the coupling caps changed the sound more than the resistors. If you want to maintain the "original" sound, then try retaining as much original components as possible; however, most of the original components are likely drift off the tolerance .....

[SY]

Strip out and rebuild.

[frank754]

Yes, strip & rebuild is good advice for something that old. When I got an old 1948 Radio Craftsmen 6V6GT push pull amp last month off Ebay for $115, I had to order the Sams Photofact schematic off the web, and found out that someone had also completely rewired the thing as well and made a few mistakes, probably failed and gave up, because the one metal tube's filament power was not hooked up so that it would light up, and you can't see that by looking. The OPT was new (an old 60's Triad universal), but the power trans and all the tubes were original.
I completely gutted & rewired the underside only, bypassed the 2 metal cans with new electrolytics, and the whole job only took me the better part of a Saturday morning and afternoon. Some of the resistors were original, but on measuring them, several were off by more than 20%, so that alone merited a complete rewiring, besides the caps & messed up circuit. Works fine now with the same original tubes.

[kstagger]

back in the day - the problem I had with my HF-85 was that volume pot mistracking Left/Right and having severe balance problems. If that's in good working order - I would 'blueprint' the unit if you want to keep the vintage tone.

It's possible to get new carbon comp resistors (or use Kiwame carbon films). Use metal films in the power supply section. Use new oil capacitors (russians or other) to keep that pyramid 'tone'. I would ditch the ceramic caps for something a little more modern though JJ sells a multi-section electrolytic that could be used to replace the original.

The other option would be all metal film resistors and modern film capacitors.

I really liked the sound of my HF-85, especially using the tape-outs. I thought it sounded better than the PAS - but it's been awhile since I've compared the two.

[barchetta]

Originally posted by kstagger
back in the day - the problem I had with my HF-85 was that volume pot mistracking Left/Right and having severe balance problems. If that's in good working order - I would 'blueprint' the unit if you want to keep the vintage tone.

It's possible to get new carbon comp resistors (or use Kiwame carbon films). Use metal films in the power supply section. Use new oil capacitors (russians or other) to keep that pyramid 'tone'. I would ditch the ceramic caps for something a little more modern though JJ sells a multi-section electrolytic that could be used to replace the original.

The other option would be all metal film resistors and modern film capacitors.

I really liked the sound of my HF-85, especially using the tape-outs. I thought it sounded better than the PAS - but it's been awhile since I've compared the two.

WHich ones are the ceramic? And by oil caps does that mean electrolytic? Sorry warned you I was a beginner.

[pchw]

Those flat ones are ceramic. Oil referred to Paper In Oil (PIO) caps, they are the coupling caps - those 4 black pyramid and 4 paper caps.
Another mod that I recommend to do - the loudness control. In the original form, it can max out most of the amp's with less than 1/4 turn!!! Rewire the loudness will allow you to turn the volumne control higher which provides more balance tracking.

[BHD]

Here's a well done site where a guy details his rebuild of an HF-81. Not an HF-85, but the parts he replaced should still apply.

http://home.earthlink.net/~eico_hf81/

Hope you find it useful.

[barchetta]

Originally posted by BHD
Here's a well done site where a guy details his rebuild of an HF-81. Not an HF-85, but the parts he replaced should still apply.

http://home.earthlink.net/~eico_hf81/

Hope you find it useful.

This page you sent answers all my questions and MORE! THANKS!!

[BHD]

This page you sent answers all my questions and MORE! THANKS!!

Cool! Have fun and let us know how it turns out!