IMPROVING THE EICO HF-81 INTEGRATED AMP (We Hope)
PART ONE
Click here for larger image.
While preparing to work on this amp and write this article, I spent time on the web
seeking information on this 14 Watt, 6BQ5/EL84 based, integrated tube amp. Imagine
my surprise to find that it’s a pretty hot item at the moment.
Description
Just in case this amplifier is new to you, let me convey what I know/think I know about it
in general. My amp was built around 1960 (SN B11401) making the HF-81 an early
single-chassis stereo tube amplifier that I think would be safe to call a dual-mono
design. Rectification is via twin 6CA4 tubes. It has a phono section utilizing two 12AX7
tubes; the line stage uses two each 12AX7 and 12AU7s. Output is via a quartet of
6BQ5/EL 84 tubes. Rated power is 14 Watts. You can switch between normal stereo
amplification, stereo reverse, and separate right and left amplifiers. There is a stereo
phono input, two stereo line inputs, and inputs for mono AM, mono FM, multiplexed
FM, microphone, and stereo tape as well as a tape output. There are taps for 4, 8, 16,
and 32-Ohm speakers. Finally, there are bass, treble, and balance controls. I think
the words “full featured” apply pretty well when discussing this amp. You might want to
check out this link to an original advertisement: http://users.rcn.com/fiddler.
interport/HF81.HTM Check out the price.
The History
I purchased my amp about twelve years ago at a yard sale. I paid $4 for it, which also
included an Eico mono FM tuner screwed, via some homemade brackets, to the top of
the amp. I gave the tuner to my friend, Tom Boffa*, thinking he might be able to do
something with it. As found, all the tubes in the unit were US made RCAs and probably
original. I think my amp was built as a kit. It’s well constructed but doesn’t appear to
have been professionally done. I base this assessment on my own expertise in the
field of amateur soldering.
The first time I tried to use this amp, I couldn’t figure out how to turn it on. There is no
apparent on/off switch. I had to pull the top cover to find that the on/off switch was
built into the treble control. Who knew? The switch was “on” so I went ahead and
plugged the amp into my variac after attaching some dummy loads to the speaker
connections. (Never plug a tube amp in without a load on it.) As the voltage from the
variac neared 100V, the amp remained as lifeless as the conscience of a career
politician. This wasn’t good but before panicking, I cranked the variac down and put a
jumper across the AC switch. This time as the voltage went up, the red lamp on the
front of the unit started to glow as did the tubes. In my limited experience, bad AC
switches are pretty common in old equipment with accessory outlets, especially
preamps. Too many amps got plugged into the accessory outlet and the AC switch
just wasn’t designed to handle that much current. The accessory outlet on my amp
looks like new so the on/off switch probably failed for some other reason.
This HF-81 was in working condition when I found it but a lot of the caps and resistors
looked marginal. I replaced a number of each. At that time, I wasn’t interested in
modifying the unit, just getting it working safely and reliably. I put in the best quality
parts I could find at my local parts store that were the same values as the originals.
This sort of substitution makes it a fairly safe bet that the unit will still run when the
work is completed. My two “rules” of parts substitution are: get as close to the original
part value as possible and, second, match the parts (if there is more than one) to each
other as closely as possible.
While I was replacing parts, I also did the one thing I have some regrets about. I
installed an on/off switch in the front of the unit. I regret changing the appearance of
the façade, although I think the toggle switch I used looks all right as far as that goes.
(The irony is that I turn the amp on and off using the surge protector about 99% of the
time and could have gotten by without putting in any switch at all. Oh well. I don’t
expect to ever get rid of this HF-81 so the economic consequences of my actions
probably won’t ever be known. It’s not something I will do again.)
I ran this amp with its replaced parts and original tubes for a couple of years until it
started making a buzzing noise in one channel. At this point it went over to Tom for a
professional checkout. The problem was a bad connection in one of the tube sockets
but Tom also felt that the filter cap needed replacing, which he did. At this time, all the
original RCA 12AX7 tubes were replaced with new Sovtek 12AX7WXT+s and the two
12AU7s were replaced with a pair of Ei tubes. The 6CA4 rectifiers were replaced with
Sylvania NOS tubes. For at least ten years, I have used the amp this way. The only
change I have made in the interim was to install a new quad of matched Sovtek
6BQ5/EL84s about four years ago, finally replacing the original RCAs. While the
Sovteks lack the romance of NOS tubes, they have worked well and I have been very
happy with the sound.
This amp has been the heart of my living room system. Speakers, tuners, and CD
players have come and gone but the Eico marches on. The reason is simple: it
sounds great. I have had a lot of different equipment through the living room—it’s
where the garage sale finds go to try them out—and nothing has sounded as good. A
lot of other amps and receivers have sounded reasonably good on their own but fall
short when compared with the Eico. I honestly have never been tempted to replace
it.
Christmas 2005 we stayed at home for the first time in the past six years. The living
room system got a lot of use as a result. My CD source is a Philips DVD 963SA player
set in 96 KHz-upsampling mode. For the last three years, the Philips has pretty much
gathered dust since it didn’t work out as a transport in my main system. It feeds the
HF-81, which has been driving JBL L110 speakers. The JBLs were purchased at an
estate sale for $10 in the summer of 2004. They got new foam surrounds a little over
a year ago and are a workable match at 89dB efficiency to the Eico’s 14 rated Watts.
(At Christmas, this system was still situated on the window seat in the living room.
Since then, I have changed the room around and the JBLs have shown some
noticeable limitations, which I will be writing about.)
The Eico is reasonably quiet for a nearly 46-year old amp. When hot, there is some
chassis hum audible from about three feet away. It’s pretty low in level, but it is
definitely there. Through the speakers, there is also some noise audible from about a
foot away. It was these noises that got me thinking about going through the Eico again
given that twelve years have elapsed since I last worked on it. I thought I might see if
any of these noises could be eliminated or minimized and also to investigate any
improvements/upgrades that could be made via substituting parts.
Happily I found some help on the Internet. There appears to be a lot of information on
restoring and modifying the HF-81. I found the site http://home.earthlink.
net/~eico_hf81/links.htm to be a good starting point. I’m sure there are others.
The Plan
My research led to a list of items that I wanted to check/fix/replace or possibly modify:
My hope was that these changes would improve the sound or, at worst, not mess it up.
Click here for "before" image.
When I actually popped the bottom off the unit on my workbench, I was surprised to
see how many parts I had replaced originally. I counted at least 14 caps and about as
many resistors. There were a number of original resistors that didn’t look too good but
everything I had installed still looked fine. The ceramic caps all had a sort of “greasy”
quality to them like something was slowly seeping out of them. The scariest parts now
were the AC cord and the wiring to the on/off switch.
By dumb luck, my original work had replaced the tube coupling capacitors with
reasonable quality Sprague orange drops. (I can’t tell what type.) I had also used
Nichicon electrolytics in the power supply.
The ceramic caps I planned to replace share common solder points with a lot of
resistors. It was pretty obvious that in desoldering the caps, I was going to apply a lot
of heat to these resistors. Since saving them was pointless, and many of them looked
questionable, I made a list of all the resistors I expected to melt and bought anywhere
from five to ten of each value, depending on how many were in the amp, plus some
extras in case of mistakes. I purchased 1-Watt resistors, as they are no larger than
the old ½-Watt resistors in the amp. Every resistor I bought was also 5% tolerance.
While I was able to find the resistors I needed locally, I had no luck with any of the
caps. I ended up ordering all the replacement capacitors from Antique Electronics
Supply (www.tubesandmore.com). AES has only a ten-dollar minimum order. Their
handling and shipping costs are also very fair.
The ceramic caps in the amp were marked .01 microFarad, .02 microFarad, .025
microFarad, 225 picoFarad, and 150 picoFarad. I ordered Sprague 716 series .01 mF
and .02 mF 600V film caps. I sorted the Sprague .02mF caps using my capacitance
meter and used those caps that tested slightly higher in value to replace the .025 mF
ceramics. These actually tested at .022 to .023 mF. Silver-mica caps of 220 pF and
150 pF at 500V were available for the smaller values.
I had no luck finding either the volume or balance control so I gave up on replacing
them. I found a 16-gauge AC cord at my local hardware store to replace the original
18-gauge cord.
Eventually, I assembled the parts I needed. I laid them out in little piles on my ping-
pong table, grouped by value. To avoid screwing up their installation, I took a number
of precautions.
With a slightly shaky hand, I began at one side of the unit and started to desolder.
The Execution
It took me about twelve hours over three days to complete the removal and
replacement of all the parts. I ultimately replaced 12 capacitors and 24 associated
resistors as well as the AC cord. This included time spent trying to figure out how I was
going to get the old parts out as well as make the new parts fit, grinding on the chassis
to fit in the grommet surrounding the AC cord, and cleaning the controls, sockets, and
tube pins. It did not include any testing.
One resistor fell apart as it was being unsoldered, another bubbled. The resistors that
looked worst actually measured fine once they were removed. It was the good-looking
ones that were wildly off.
The two 165-Ohm, 5-Watt bias resistors measured 174 and 192-Ohms respectively
once they were removed. I suspect this may have affected the amp’s channel
balance. To center the sound required setting the balance control to about the 10 o’
clock position. The Internet in formation I found suggested replacing the 165-Ohm
resistors with 180-Ohm. I had no luck finding any resistors that were 175 to 185-
Ohms. When I ordered from AES, I got some 200-Ohm resistors hoping a couple
would be off on the low side. They weren’t; they all measured 197 to 203-Ohms.
These may have worked fine, I really don’t know. I eventually ended up paralleling two
nominally 390-Ohm resistors for a net of about 189-Ohms per channel. I repositioned
one of these pairs from a ground on a tube socket to a new ground tab on the chassis
to make it fit.
The caps that came out of the amp were surprisingly close to their marked values with
the exception of a couple that were about half their marked value. Of course I was
measuring at room temperature. Who knows what their values were when the amp
was hot.
I was able to simplify things in a couple of places by cutting out the parts being
replaced where I had an unused portion of a tube pin or solder terminal to attach the
new parts to. There is one place in the amp where five parts go to ground in one big
solder lump. I added a second ground tab for my new parts rather than try to unsolder
this mess. I don’t think I could have desoldered it or soldered back together without
melting the chassis as well.
I had expected to replace some of the wiring but this did not prove to be necessary.
The few places where a wire end was fragile, I was lucky enough to have sufficient
slack to shorten the wire and continue to use it.
I used 96% tin, 4% silver solder everywhere and a 40-Watt soldering iron. I did have
to use my 100-Watt soldering gun to melt a few joints but it wasn’t needed for any
resoldering.
The reality of this amp is that everything is 46 years old. There is no clear-cut
stopping point when it comes to working on it. It would be possible to replace all the
electronic parts, the sockets, and all the wiring in the whole unit. I feel pretty good that
all of the caps and resistors in the line and output stages are now replaced. I did not
touch the phono section or any of the parts on the controls.
Click here for "after" image.
I added some additional rubber feet to the bottom of the unit to distribute the weight
more evenly. I also added some additional vent holes. Click here for photo.
Testing
I hooked up the amp to my sine wave generator and oscilloscope. I got very nicely
shaped, totally consistent sine waves out of the amp. They looked as good as the
ones going in. I tested at a variety of frequencies including going all the way down to
10Hz. Even that wave looked good and the voltage didn’t vary.
There is still a small variance in output voltage, channel to channel. The balance
control is now set just a hair to the left side instead of all the way to 10 o’clock. Since
there is no actual center detent on the balance control, it may actually be centered for
all I know. It wouldn’t be hard to replace the volume and balance controls, if exact
replacements were available. Modifying something to fit would be a nightmare,
however.
MEETING MY GOALS
I didn’t do any modifications to the amp, other than changing the bias resistors’ value.
The parts that went in, though, are much higher tolerance than the parts originally
used and extremely close to the values in the schematic. This should do some good. I
still want to try some tube rolling and a couple of other things once I get a handle on
the amp’s sound. Some of this will have to wait until I sort out my speaker situation,
which has gotten complicated.
Summary
So far, so good. The real test now comes with the listening. Given that this little amp
worked as well as it did before, I have high expectations of improved sound quality and
quieter operation with the new parts. Whether or not this is the case will appear in Part
2.
I spent about $45 on everything that actually went into the Eico. I got a lot of resistors
that I did not end up using, which makes an exact figure hard to determine. I had also
bought some new ceramic caps just in case I could not find, or fit in, a better quality
replacement; these went unused. I think that this is a pretty reasonable expenditure
especially since there are much more expensive, exotic parts available that can be
installed. I may try some more audiophile-type parts in some locations in the future.
For now, I want to get started on the listening. Thanks for checking out this article.
Kent Johnson
March 5, 2006
*Tom repairs equipment for musicians in Arvada, Colorado (303) 467-0664. He no
longer does home audio repair but still, fortunately, allows me to consult with him over
the phone.
PART ONE
Click here for larger image.
While preparing to work on this amp and write this article, I spent time on the web
seeking information on this 14 Watt, 6BQ5/EL84 based, integrated tube amp. Imagine
my surprise to find that it’s a pretty hot item at the moment.
Description
Just in case this amplifier is new to you, let me convey what I know/think I know about it
in general. My amp was built around 1960 (SN B11401) making the HF-81 an early
single-chassis stereo tube amplifier that I think would be safe to call a dual-mono
design. Rectification is via twin 6CA4 tubes. It has a phono section utilizing two 12AX7
tubes; the line stage uses two each 12AX7 and 12AU7s. Output is via a quartet of
6BQ5/EL 84 tubes. Rated power is 14 Watts. You can switch between normal stereo
amplification, stereo reverse, and separate right and left amplifiers. There is a stereo
phono input, two stereo line inputs, and inputs for mono AM, mono FM, multiplexed
FM, microphone, and stereo tape as well as a tape output. There are taps for 4, 8, 16,
and 32-Ohm speakers. Finally, there are bass, treble, and balance controls. I think
the words “full featured” apply pretty well when discussing this amp. You might want to
check out this link to an original advertisement: http://users.rcn.com/fiddler.
interport/HF81.HTM Check out the price.
The History
I purchased my amp about twelve years ago at a yard sale. I paid $4 for it, which also
included an Eico mono FM tuner screwed, via some homemade brackets, to the top of
the amp. I gave the tuner to my friend, Tom Boffa*, thinking he might be able to do
something with it. As found, all the tubes in the unit were US made RCAs and probably
original. I think my amp was built as a kit. It’s well constructed but doesn’t appear to
have been professionally done. I base this assessment on my own expertise in the
field of amateur soldering.
The first time I tried to use this amp, I couldn’t figure out how to turn it on. There is no
apparent on/off switch. I had to pull the top cover to find that the on/off switch was
built into the treble control. Who knew? The switch was “on” so I went ahead and
plugged the amp into my variac after attaching some dummy loads to the speaker
connections. (Never plug a tube amp in without a load on it.) As the voltage from the
variac neared 100V, the amp remained as lifeless as the conscience of a career
politician. This wasn’t good but before panicking, I cranked the variac down and put a
jumper across the AC switch. This time as the voltage went up, the red lamp on the
front of the unit started to glow as did the tubes. In my limited experience, bad AC
switches are pretty common in old equipment with accessory outlets, especially
preamps. Too many amps got plugged into the accessory outlet and the AC switch
just wasn’t designed to handle that much current. The accessory outlet on my amp
looks like new so the on/off switch probably failed for some other reason.
This HF-81 was in working condition when I found it but a lot of the caps and resistors
looked marginal. I replaced a number of each. At that time, I wasn’t interested in
modifying the unit, just getting it working safely and reliably. I put in the best quality
parts I could find at my local parts store that were the same values as the originals.
This sort of substitution makes it a fairly safe bet that the unit will still run when the
work is completed. My two “rules” of parts substitution are: get as close to the original
part value as possible and, second, match the parts (if there is more than one) to each
other as closely as possible.
While I was replacing parts, I also did the one thing I have some regrets about. I
installed an on/off switch in the front of the unit. I regret changing the appearance of
the façade, although I think the toggle switch I used looks all right as far as that goes.
(The irony is that I turn the amp on and off using the surge protector about 99% of the
time and could have gotten by without putting in any switch at all. Oh well. I don’t
expect to ever get rid of this HF-81 so the economic consequences of my actions
probably won’t ever be known. It’s not something I will do again.)
I ran this amp with its replaced parts and original tubes for a couple of years until it
started making a buzzing noise in one channel. At this point it went over to Tom for a
professional checkout. The problem was a bad connection in one of the tube sockets
but Tom also felt that the filter cap needed replacing, which he did. At this time, all the
original RCA 12AX7 tubes were replaced with new Sovtek 12AX7WXT+s and the two
12AU7s were replaced with a pair of Ei tubes. The 6CA4 rectifiers were replaced with
Sylvania NOS tubes. For at least ten years, I have used the amp this way. The only
change I have made in the interim was to install a new quad of matched Sovtek
6BQ5/EL84s about four years ago, finally replacing the original RCAs. While the
Sovteks lack the romance of NOS tubes, they have worked well and I have been very
happy with the sound.
This amp has been the heart of my living room system. Speakers, tuners, and CD
players have come and gone but the Eico marches on. The reason is simple: it
sounds great. I have had a lot of different equipment through the living room—it’s
where the garage sale finds go to try them out—and nothing has sounded as good. A
lot of other amps and receivers have sounded reasonably good on their own but fall
short when compared with the Eico. I honestly have never been tempted to replace
it.
Christmas 2005 we stayed at home for the first time in the past six years. The living
room system got a lot of use as a result. My CD source is a Philips DVD 963SA player
set in 96 KHz-upsampling mode. For the last three years, the Philips has pretty much
gathered dust since it didn’t work out as a transport in my main system. It feeds the
HF-81, which has been driving JBL L110 speakers. The JBLs were purchased at an
estate sale for $10 in the summer of 2004. They got new foam surrounds a little over
a year ago and are a workable match at 89dB efficiency to the Eico’s 14 rated Watts.
(At Christmas, this system was still situated on the window seat in the living room.
Since then, I have changed the room around and the JBLs have shown some
noticeable limitations, which I will be writing about.)
The Eico is reasonably quiet for a nearly 46-year old amp. When hot, there is some
chassis hum audible from about three feet away. It’s pretty low in level, but it is
definitely there. Through the speakers, there is also some noise audible from about a
foot away. It was these noises that got me thinking about going through the Eico again
given that twelve years have elapsed since I last worked on it. I thought I might see if
any of these noises could be eliminated or minimized and also to investigate any
improvements/upgrades that could be made via substituting parts.
Happily I found some help on the Internet. There appears to be a lot of information on
restoring and modifying the HF-81. I found the site http://home.earthlink.
net/~eico_hf81/links.htm to be a good starting point. I’m sure there are others.
The Plan
My research led to a list of items that I wanted to check/fix/replace or possibly modify:
- Replace all the ceramic capacitors. This appears to be the major improvement
that I can make in this unit since I had already replaced some of the critical
coupling caps.
- Replace any other parts that appear to be burned/oozing/questionable since I
went through the amp originally. Use better parts where feasible. Redo any
solder joints that look dubious. Physically tighten anything that can be
tightened.
- Replace the bias resistors with higher values. My understanding is that this will
allow the amp to run cooler but with less power. Fortunately, it’s an easily
reversible modification if it affects the sound negatively.
- Replace the AC cord with a grounded three-wire cable. Replace the wiring to
the on/off switch with heavier wire. Remove the AC accessory outlet and the
capacitor that runs from the fuse to the chassis.
- Check the tubes; replace any that are questionable. Try some 5751s in place of
the 12AX7s and some 6189s in place of the 12AU7s. Possibly replace the
output tubes. Since the phono section isn’t being used, can the tube shields be
removed? What about the two phono section tubes, will the amp run without
them?
- Replace the stereo volume and balance controls. (I noticed over Christmas that
when the amp is turned down to background music levels, the left channel
disappears before the right. I am attributing this to poor tracking on the part of
the volume control. Maybe it is; maybe it isn’t.) Clean all controls and switches.
- Replace the feet with some that will raise the unit up for better ventilation. The
original feet are less than ¼ inch high and hard as rocks.
My hope was that these changes would improve the sound or, at worst, not mess it up.
Click here for "before" image.
When I actually popped the bottom off the unit on my workbench, I was surprised to
see how many parts I had replaced originally. I counted at least 14 caps and about as
many resistors. There were a number of original resistors that didn’t look too good but
everything I had installed still looked fine. The ceramic caps all had a sort of “greasy”
quality to them like something was slowly seeping out of them. The scariest parts now
were the AC cord and the wiring to the on/off switch.
By dumb luck, my original work had replaced the tube coupling capacitors with
reasonable quality Sprague orange drops. (I can’t tell what type.) I had also used
Nichicon electrolytics in the power supply.
The ceramic caps I planned to replace share common solder points with a lot of
resistors. It was pretty obvious that in desoldering the caps, I was going to apply a lot
of heat to these resistors. Since saving them was pointless, and many of them looked
questionable, I made a list of all the resistors I expected to melt and bought anywhere
from five to ten of each value, depending on how many were in the amp, plus some
extras in case of mistakes. I purchased 1-Watt resistors, as they are no larger than
the old ½-Watt resistors in the amp. Every resistor I bought was also 5% tolerance.
While I was able to find the resistors I needed locally, I had no luck with any of the
caps. I ended up ordering all the replacement capacitors from Antique Electronics
Supply (www.tubesandmore.com). AES has only a ten-dollar minimum order. Their
handling and shipping costs are also very fair.
The ceramic caps in the amp were marked .01 microFarad, .02 microFarad, .025
microFarad, 225 picoFarad, and 150 picoFarad. I ordered Sprague 716 series .01 mF
and .02 mF 600V film caps. I sorted the Sprague .02mF caps using my capacitance
meter and used those caps that tested slightly higher in value to replace the .025 mF
ceramics. These actually tested at .022 to .023 mF. Silver-mica caps of 220 pF and
150 pF at 500V were available for the smaller values.
I had no luck finding either the volume or balance control so I gave up on replacing
them. I found a 16-gauge AC cord at my local hardware store to replace the original
18-gauge cord.
Eventually, I assembled the parts I needed. I laid them out in little piles on my ping-
pong table, grouped by value. To avoid screwing up their installation, I took a number
of precautions.
- I tried to work off of only one tube pin at a time, although this wasn’t always
possible. Most of these pins had at least two or more parts attached to them,
usually going two or more places. I tried not to work any farther ahead than was
absolutely necessary.
- I drew a simple sketch of the area in which I was working so that I did not have to
rely on my memory once the old parts were removed.
- I only brought those parts to my workbench that I was actually replacing.
- I checked all parts values both with my meter and against the schematic. The
schematic check insured that if a wrong value part was already in the unit, I did
not duplicate the mistake. This was never a problem.
- I used a straight edge across the bottom of the chassis to check that the parts
would be below the level of the bottom of the case once installed. This is easy
to forget about while trying to fit larger replacement caps in what are fairly
confined spaces.
- Finally, I cleaned all the wire trimmings and other debris out of the chassis as I
went. I didn’t want a piece of overlooked scrap causing a short.
With a slightly shaky hand, I began at one side of the unit and started to desolder.
The Execution
It took me about twelve hours over three days to complete the removal and
replacement of all the parts. I ultimately replaced 12 capacitors and 24 associated
resistors as well as the AC cord. This included time spent trying to figure out how I was
going to get the old parts out as well as make the new parts fit, grinding on the chassis
to fit in the grommet surrounding the AC cord, and cleaning the controls, sockets, and
tube pins. It did not include any testing.
One resistor fell apart as it was being unsoldered, another bubbled. The resistors that
looked worst actually measured fine once they were removed. It was the good-looking
ones that were wildly off.
The two 165-Ohm, 5-Watt bias resistors measured 174 and 192-Ohms respectively
once they were removed. I suspect this may have affected the amp’s channel
balance. To center the sound required setting the balance control to about the 10 o’
clock position. The Internet in formation I found suggested replacing the 165-Ohm
resistors with 180-Ohm. I had no luck finding any resistors that were 175 to 185-
Ohms. When I ordered from AES, I got some 200-Ohm resistors hoping a couple
would be off on the low side. They weren’t; they all measured 197 to 203-Ohms.
These may have worked fine, I really don’t know. I eventually ended up paralleling two
nominally 390-Ohm resistors for a net of about 189-Ohms per channel. I repositioned
one of these pairs from a ground on a tube socket to a new ground tab on the chassis
to make it fit.
The caps that came out of the amp were surprisingly close to their marked values with
the exception of a couple that were about half their marked value. Of course I was
measuring at room temperature. Who knows what their values were when the amp
was hot.
I was able to simplify things in a couple of places by cutting out the parts being
replaced where I had an unused portion of a tube pin or solder terminal to attach the
new parts to. There is one place in the amp where five parts go to ground in one big
solder lump. I added a second ground tab for my new parts rather than try to unsolder
this mess. I don’t think I could have desoldered it or soldered back together without
melting the chassis as well.
I had expected to replace some of the wiring but this did not prove to be necessary.
The few places where a wire end was fragile, I was lucky enough to have sufficient
slack to shorten the wire and continue to use it.
I used 96% tin, 4% silver solder everywhere and a 40-Watt soldering iron. I did have
to use my 100-Watt soldering gun to melt a few joints but it wasn’t needed for any
resoldering.
The reality of this amp is that everything is 46 years old. There is no clear-cut
stopping point when it comes to working on it. It would be possible to replace all the
electronic parts, the sockets, and all the wiring in the whole unit. I feel pretty good that
all of the caps and resistors in the line and output stages are now replaced. I did not
touch the phono section or any of the parts on the controls.
Click here for "after" image.
I added some additional rubber feet to the bottom of the unit to distribute the weight
more evenly. I also added some additional vent holes. Click here for photo.
Testing
I hooked up the amp to my sine wave generator and oscilloscope. I got very nicely
shaped, totally consistent sine waves out of the amp. They looked as good as the
ones going in. I tested at a variety of frequencies including going all the way down to
10Hz. Even that wave looked good and the voltage didn’t vary.
There is still a small variance in output voltage, channel to channel. The balance
control is now set just a hair to the left side instead of all the way to 10 o’clock. Since
there is no actual center detent on the balance control, it may actually be centered for
all I know. It wouldn’t be hard to replace the volume and balance controls, if exact
replacements were available. Modifying something to fit would be a nightmare,
however.
MEETING MY GOALS
I didn’t do any modifications to the amp, other than changing the bias resistors’ value.
The parts that went in, though, are much higher tolerance than the parts originally
used and extremely close to the values in the schematic. This should do some good. I
still want to try some tube rolling and a couple of other things once I get a handle on
the amp’s sound. Some of this will have to wait until I sort out my speaker situation,
which has gotten complicated.
Summary
So far, so good. The real test now comes with the listening. Given that this little amp
worked as well as it did before, I have high expectations of improved sound quality and
quieter operation with the new parts. Whether or not this is the case will appear in Part
2.
I spent about $45 on everything that actually went into the Eico. I got a lot of resistors
that I did not end up using, which makes an exact figure hard to determine. I had also
bought some new ceramic caps just in case I could not find, or fit in, a better quality
replacement; these went unused. I think that this is a pretty reasonable expenditure
especially since there are much more expensive, exotic parts available that can be
installed. I may try some more audiophile-type parts in some locations in the future.
For now, I want to get started on the listening. Thanks for checking out this article.
Kent Johnson
March 5, 2006
*Tom repairs equipment for musicians in Arvada, Colorado (303) 467-0664. He no
longer does home audio repair but still, fortunately, allows me to consult with him over
the phone.
 |
| |
 |
| |
 |
| |
 |
