A particular pin of an IC (Integrated Circuit) is denoted in the text by first the chip designation and then the pin number: e.g. U25-10 means pin 10 of 025.

Jumper patches W1 thru W12 are sections of traces on the audio board designed to be severed in order to isolate a section of circuitry for test purposes. Be sure to sever between the two eyelets, one round and the other square. The orientation of these pads is the same on the board as on the schematic diagram. Repair by adding a short jumper wire into the holes provided.

DO NOT repair any trace by adding a blob of solder at the sever. This will cause intermittent operation.

TEST PROCEDURE (refer to Sheet 6 of the Schematics)

To verify proper operation of D30,U40 and U50 disconnect the 16-pin ribbon cable at J2 and decrease the volume. Check U30-2, 4 and 6, U40-8, 10 and 12 with a logic probe, scope, or meter. They should read high or 2.4vdc - 5vdc (Logic 1).

Check for proper signal shaping as follows:

1 Wave form at U50-3.9, 11 and 13 should be normal square wave _| j_.

2 Wave form at U30-9, 11 and 13 should have longer leading edge __.

3 Wave form at U30-2, 4 and 6 should have shorter time duration and slightly lower amplitude:_n_This circuit is designed to filter out signal glitches but allow proper input signals to pass.

Next, with a test lead, ground U50-9. U50-2 should now measure low or 0 to 4 vdc. Follow this procedure for the other six pairs of Schmitt-Trigger circuits.

To verify proper operation of U49 and U39, Load all lows into their output ports by tying U50-1 to ground. Using another test lead connected to ground tap, U50-13 twenty times. Outputs QA-QH on both IC's should measure low.

Repeat the above procedure, but remove the grounded test lead from U50-1. This should load all highs into all the output ports.

To check U47, first, load 049 with all lows and tap pin 11 with the grounded test lead and measure low. Next, load U49 with all highs and again tap 047 pin 11 with the grounded test lead. The outputs should be high.

To check o's 48 and 38 load O's 49 and 39 with all lows and tap a grounded test lead to 045-10. Measure the outputs 1Q-8Q on each. They should all measure low. To load all highs into 048 and 038, load all highs into 049 and 039, and tap 045-10 with the grounded test lead. NOTE: if problems are encountered here the music gating circuit may be at fault. Refer to its troubleshooting procedure if necessary.

5MHZ CLOCK CIRCOIT (Test Procedure)

With a logic probe verify 02-12 is switching high and low or measure a 20MHz square-wave with a scope. 023-5 and

6 should measure 10MHz square-wave and 023-8 and 9 should measure 5MHz square-wave.

To verify proper operation of the Divide-by-Eleven circuit, tie one end of a test lead to ground and connect the other end to TP1. 024-6 should be high. Pins 11, 12, 13, 14 and 15 of 043 should all be toggling. Move the ground from TP1 to pin 9 of 043. Pins 11 and 13 should be low, pins 12 & 14 should be high. Remove the ground lead from Pin 9. Check the outputs of 033. Pins 2 and 3 should be toggling and measure 227KHz.

Was this article helpful?

0 0

Post a comment