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Old 9th March 2010, 12:30 AM   #1
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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!
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Old 9th March 2010, 01:12 AM   #2
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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.
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Old 9th March 2010, 01:22 AM   #3
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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.
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Last edited by HollowState; 9th March 2010 at 01:31 AM.
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Old 9th March 2010, 02:26 AM   #4
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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.
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