Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Troubleshooting Eico HF-85
Troubleshooting Eico HF-85
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 9th March 2010, 01:30 AM   #1
neddyboy is offline neddyboy  United States
diyAudio Member
Join Date: Sep 2006
Location: SF Bay Area
Default Troubleshooting Eico HF-85

First, I'm one of those guys who knows enough to get into trouble. I picked up an Eico HF-85 preamp recently to rebuild.

It was all original and filthy -- after cleaning I replaced all the caps on the underside of the chassis, built a new filter cap set, and installed a three prong plug. I haven't been industrious enough to replace all the ceramic caps on the mode rotary switch yet.

After fixing a few mistakes or later mods, I have one fully functional channel and one with rolled off highs and lows as well as low in volume. You will have noticed that I have not mentioned any resistors -- except for two suspicious looking cardboard resistors, I've left all the original carbon comp resistors in place.

Knowing that, can anyone suggest my next steps? I've checked both channels against the schematic, compared them to each other, and swapped all the tubes around. I don't know enough to test voltages, but I can go in and replace resistors. Is there an area I should start with? Is it likely that this problem stems from drifting resistors?

There's a schematic at this URL: http://www.triodeel.com/hf85s.gif

I'd be forever grateful if anyone could give me some resistor numbers to change out or could let me know if I'm on the right track!
  Reply With Quote
Old 9th March 2010, 02:12 AM   #2
mashaffer is offline mashaffer  United States
diyAudio Member
Join Date: Jun 2004
Location: Indiana
I would check all the resistors I could get to just for completeness but if cathode or plate resistors are off it can negatively impact the bias and or gain so that is a good place to start. If you have a voltmeter with clip on leads go ahead and measure the cathode voltages and post the results. For safety my process is to turn the power off and allow caps to discharge every time I am going to move or touch a meter lead. It makes progress slower but I don't get any light shows either. If you want to measure plate voltages too that might also be instructive.
The only reason I drive the 91 Colony Park is that I can't afford a 65. Me old school? Couldn't be.
  Reply With Quote
Old 9th March 2010, 02:22 AM   #3
HollowState is offline HollowState  United States
diyAudio Member
HollowState's Avatar
Join Date: Aug 2006
Location: Taxland, New Jersey
Well, there are no such things as cardboard resistors. Perhaps you're refering to the two encapsulated circuits PC1 & PC2. These are resistor/capacitor networks used in the tone control circuit. Now, can you get normal and equal output from the line outputs J14 and J15? If so, the last stage is where the problem is. Normally PC1 &2 don't cause much trouble, but they can. At some point you will need to take some voltage measurments. It is not proper to begin changing resistors on a hit and miss basis.
There cannot, and never will be, any progress in society, vis-a-vis racism, so long as the concept of celebrating Diversity -- in it's current, politically-correct, deliberate, punitively extractive form -- continues.

Last edited by HollowState; 9th March 2010 at 02:31 AM.
  Reply With Quote
Old 9th March 2010, 03:26 AM   #4
neddyboy is offline neddyboy  United States
diyAudio Member
Join Date: Sep 2006
Location: SF Bay Area
Default Thanks Guys

Mashaffer, thanks for the tips. I use the "one hand behind my back" method! I'll see about checking the voltages this weekend.

Hollowstate, thanks for identifying those two components on the schematic. The PC1 label threw me -- I didn't think there were any printed circuits in the thing. Anyway, no, I'm not talking about those. There were two brown components marked "1/2 watt 100k ohm" that looked like cardboard with metal ends.
  Reply With Quote


Troubleshooting Eico HF-85Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
HF-85 Rebuild advice barchetta Tubes / Valves 52 25th May 2012 03:14 PM
FA: Eico HF-85 Tube Preamp & HFT-90 Tube Tuner joe-electro Swap Meet 0 31st May 2009 05:14 PM
FS Eico HF-85 tube pre-amp jdeitch11 Swap Meet 0 19th October 2008 01:27 AM
Any comments on Eico HF-14 and HF-85 percy Tubes / Valves 5 13th January 2005 01:11 AM

New To Site? Need Help?

All times are GMT. The time now is 10:25 AM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio