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Field Change Notice FCN2-005

August 16, 1982

Effectivity: Chroma, Model 2103, Serial Numbers 0011 through 0106, 0108 through 0113, 0116, 0121, 0125, 0132, 0138, 0139, 0141, 0149 through 0151.

Reason for Change: To insure the Dual Channel Boards tune properly during auto tune, and to prevent Dual Channel Boards from drifting beyond normal tuning range. (See also Field Change Notice FCN2-001.)

Material Required

Parts Kit: Order Parts Kit KFC2-005.

Procedure

NOTE: The following information lists all known possible problems which may affect tuning quality. Call the Service Department if you are in doubt about the extent of the changes required.

1. The 10 OHM resistor and 100uf capacitor combine to filter the +5V analog reference voltage to the DAC circuit on the I/O Board. This effectively eliminates random changes in control voltage which occur whenever the 120Hz ripple coincides with a specific S & H refresh. Such a change in control voltage would negate the auto tune since the computer would issue a false correction to compensate for it.

On the I/O Board cut the trace between the junction of R42 and Z20, Pin 15 and the +5V analog bus from J1, Pin 4. Install R69, 10 OHM, 5% as shown on the I/O components drawing. Install C32, 100uf, 10V (also shown on the I/O components drawing). Be certain to solder the + lead of the capacitor to the junction of R42 and Z20, Pin 15.

2. Temperature compensating resistors are mounted on top of the 3086 oscillator chip on the Dual Channel Board to correct for thermal changes. On Chromas having the affected serial numbers, these resistors are from two vendors. Those resistors marked Tel Labs do not correct quickly enough and should be changed. Those resistors marked KRLP react faster and should be changed only if they are misaligned or do not have heat sink compound under them used for better thermal transfer. The resistors may be readily identified by their size. When mounted parallel against each other and placed directly on the 14 pin dip chip (3086, Z1) the KRLP resistors will not overhang but the Tel Labs resistors have a larger diameter and overhang the chip. This is readily apparent at a glance [see Voice Board Revisions: TEMPCO Resistors for pictures]. Note also the resistors should be aligned parallel to the edge of the chip so they do not overhang and must be down against the top of the chip evenly along their length.

If at this point you decide the resistors must be changed, carefully cut the glue bonding the resistors together and remove them by desoldering. The top of the chip must be free of any glue, if not, carefully scrape or sand the glue down to the plastic top. When mounting the new resistors (1.87K, 1W, temp. comp.), use a drop of cement (epoxy or silicon) to bond them mechanically together and use heat sink compound betwen the chip and the two resistors. Take care to align them over the chip and make sure they are pulled down against the top of the chip before soldering. IT IS VERY IMPORTANT THAT THE RESISTORS CONTACT THE TOP OF THE CHIP ALONG THEIR ENTIRE LENGTH. Refer to the Dual Channel Board drawing.

Do not confuse the oscilators with the filter chip. The oscillator chip is mounted beneath the two potentiometers on the right upper corner of the board with the components facing you. The filter chip Z13 is on the left. The T.C. resistors on the filter do not have to be changed.

3. Implementing the above changes will correct most tuning problems. There are a few other things you can do however if the tuning is still in question:

  1. Connect a digital D.C. voltmeter (100 microvolts resolution minimum) between TP 0 and TP 1 on the I/O Board (see components drawing). Do not reference the voltmeter inputs to ground. Set the adjacent potentiometer for 0 VDC. This pot, R1, adjusts the offset null to the output op amp of the 12 bit DAC and must be nulled for proper tuning.
  2. Set the pulse width of each oscillator on all Dual Channel Boards. The pulse width should be set to equal the pulse interval yielding a 50% duty cycle. Connect a scope between TP 1 of a Dual Channel Board and ground. Set up a scratch program and adjust the pulse width parameter to 32 (50% value). Initialize the Chroma with a set split "50" then adjust pot, R1 for a 50% duty cycle. Move the scope probe to TP 2, adjust R2, etc. for all 16 oscillators.
  3. Check that the 8 bit (Z20) and 12 bit (Z24) DACs are seated properly in their sockets on the I/O Board. These sockets have metal that contact the I.C. pins only on the inside. Squeeze the pins together carefully to ensure contact with the inside of the socket but do not round or curve the pins in, they must be straight.
  4. Check Z20, the 8 bit DAC on the I/O Board. This 7523 chip may be a T523J or a 7523L. The 7523L may produce slightly better results than the 7523J.
  5. Check sample and hold jitter/DMUX leakage.
  6. Certain Dual Channel Board may be contaminated. Contamination usuaully causes tuning problems under conditions of high humidity.

If you wish to check sample and hold jitter or suspect contamination, contact our service department in Woburn for exact procedures.

Bill Thomas
Rhodes Chroma National Service Manager