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The Phase Linear 4000 preamp with correlator plus peak unlimit, and standalone Phase Linear 1000 are great pieces of
gear designed to restore dynamic range and reduce tape and record hiss.
Both are cheap and available on eBay. Depending
on condition and if the wooden box is included, PL1000 runs between $20 to $50.
This article describes how to get your PL1000 working again due to a few common failures, and provides design modifications
that will have it sounding better than new. While this article is self contained, it's beneficial to buy a service manual
to fully understand how circuits work or debug serious problems. The PL4000 service manual has enough information to
understand either PL4000 or PL1000. This article focuses on series I.
Dynamic range recovery is 10dB.
Peak Unlimit 1.64dB
Downward Expander 3.84dB
Linear Expander 4.34dB
Noise reduction is 10dB with filters at
200 Hz
4 kHz
7 kHz
12 kHz
The PL1000 design flaw is a summing amplifier with too much gain that saturates during high input signal levels.
This
amp provides peak unlimit and correlator control signals. You don't hear distortion, but clipping reduces
peak unlimit
dynamic range recovery and introduces "noise swishing" in the correlator. Clipping causes square edges rich in harmonics
which open correlator bandpass gates even when there is no musical content at those frequencies. Hiss then swishes along
with the music.
This upgrade fixes the problem.
Your PL1000 may have one or both channels dead. The boards are not mechanically supported, and continuous swaying
causes hairline cracks in motherboard connector solder joints.
Below shows board location for these connectors. Label
and remove boards. Flip unit over and resolder motherboard connectors. Use solder braid to clean up accidental
solder bridges between pins.
If your soldering experience is confined to using a woodburning set, you're best off finding
a friend to help rather than destroying the PCB.
Now spray connector cleaner into pushbutton contacts. I like to use Puretronics contact cleaner and lubricant.
The boards have no alignment keys. Watch to make sure boards aren't mis-aligned by one pin when re-inserting.
Pieces of foam placed above the boards press against the case to hold boards firm, preventing future problems.
Your unit should now fully function. If not, there's too much to cover in this short article; you'll have to debug
yourself. However, debug is easy. Verify power supply provides a clean 32V. Check op amp outputs for biasing
and signals.
The diagram below shows the summing amp feeding both correlator and peak unlimit. The correlator/log amp combo
provide plenty of gain to allow reducing summing amp gain. The peak unlimit is mismatched in that it has much less gain
than the correlator. By increasing peak unlimit gain by 6dB, summing amp gain can be reduced by 2X. Our plan is
to double peak unlimit gain, reduce summing amp gain, and increase summing amp signal swing capability. The summing
amp uses a 20k emitter resistor which only allows a 16V P-P maximum swing due to AC load line constraints. Redesign
will drop gain by 8.6dB and flatten the AC load line to achieve a 26V P-P maximum swing.
Below is one PL1000 summing amp schematic. There are two summing amp designs in the PL1000. Lower serial
numbers have RB connected to the collector. Before you mod the summing amp, measure the high side of RB to ground.
If it's about 18V, you need to use RB = 680k.
Replace RB 2.2M (red, red, green) with 680K.
Replace RC 22K (red, red, orange) with 8.2k.
Replace RE 20K (red,
black, orange) with 4.7k.
With 2% metal film resistors available cheaply, you should use them.
When powered up, voltage across RE should
be around 18V. Boards need not be inserted for this test.
Below is the other PL1000 summing amp schematic. Higher serial numbers have RB connected to Vpp,
32V. Before you mod the summing amp, measure the high side of RB to ground. If it's about 32V, you need to use
RB = 1M.
Replace RB 2.2M (red, red, green) with 1M.
Replace RC 22K (red, red, orange) with 8.2k.
Replace RE 20K (red,
black, orange) with 4.7k.
With 2% metal film resistors available cheaply, you should use them.
When powered up, voltage across RE should
be around 16V. Boards need not be inserted for this test.
This mod is suitable for higher serial number PL4000 preamps, if RB is connected to 32V, and input resistors R21, R22
(unlabeled here) are 39k. Currently there is no mod for lower serial # PL4000.
Add a 39K resistor between center leg (yellow wire) and signal leg (gray wire) of the unlimit threshold pot.
This
gives more usable range to front panel unlimit threshold.
Below I've included a peak unlimit card schematic so you can see how we're doubling its gain.
Solder a 6.8k resistor across R39 120k (brown, red, yellow).
Solder a 1.5k resistor across R51 1.8k (brown, gray,
red).
Solder a 180k resistor across R50 150k (brown, green, yellow).
If you solder from top of board, a little resistor lead scraping will make joints tin.
R39 was formerly reducing peak unlimit gain by 2x. We parallel with 6.8k to limit transistor base current.
R51
controls downward and linear expander gain. We parallel with 1.5k to get 2x.
R50 controls transistor bias point for
maximum swing.
I paralleled resistors mostly out of laziness.
You can unsolder and replace R39 with 6.8k, R51 with 820
ohms, R50 with 82k.
Solder braid is your friend.
You can optionally increase 200Hz control filter gain. This gives back range to front panel correlator low
frequency calibration knob. Shown below is the log amp card.
Replace R2 820k (gray, red, yellow) with a 390k resistor.
Shown below are correlator bandpass filter cards.
These cards filter out 200Hz, 4kHz, 7kHz, 12kHz control signals
from the music.
There wasn't much around for op amps in 1975, so Carver used Norton LM3900 for non-audio quality signals.
These are current differencing op amps, and have trouble with negative input swings from capacitive coupling.
This is exactly
how the filters are configured.
I had trouble with oscillation with the later National Semiconductor chips, unlike my older PL4000. If you
have NS chips, socket them and replace with TI. Only replace the two chips pictured. The other LM3900 on bandpass
II card is not a filter configuration. If you replace these chips, cut them from their pins before unsoldering, thus
destroying them. (you are throwing them away, right?)
You're much less likely to cause PCB damage by removing one pin at
a time. (Did I say to use solder braid?)
I measured correlator control signals for the 4kHz, 7Khz, and 12kHz bands and was pleased to note that they open and
shut filters in unison when using cassette tape noise. This means the correlator will have a sharp knee where the background
noise is silenced.
Your Phase Linear is now upgraded.
You'll find peak unlimiting restores impact to flat sounding FM broadcasts and
records.
Correlation can reduce low level broadcast static, tape hiss, and record surface noise (not loud pops.)
My Phase Linear
4000 has had a similar upgrade for the last 25 years.
Here is a parts list.
Here's a sample resistor. You can substitute different values.
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