KR6200 Repairs: Caps on Main Amp Board #X07-1110-10-------------------------------------------------------------------- Replaced Ce16 with 250 uF 50 v. It was leaking and legs broken off. 10/29/05 Replaced Ce18 with 100 uF 100v. It had an "open" connection internally (it acted as if the cap was not there). This cap suppresses the overload relay from oscillating. The relay oscillated at power up. Now there is a delay before the relay turns on. This is normal, but with the larger replacement cap, the delay is longer. 10/29/05 1/2/08 Replaced all remaining electrolytic caps on main amp board. C7,C8: 47uF,50v replaced with Sprague 50uF, 50v. C17: 47uF, 10v non-polar replaced with two 100uF, 35v, polar in series negative-to-negative. C3, C4, C15: 1uF, 50v replaced with 1uF, 50v non-polar. C5, C5, C11, C12: 100uF 6.3v replaced with 100uF 25v. Transistor replacements--------------------------------------------------------------------- As of 1/8/08, none have been replaced yet. Here are some substitutions: 2SA620WL5 (2SA620) --> 2N4058 2SC983 --> 2N2222A Signal Loss and Scratchiness------------------------------------------- Channels sound scratchy or drop out when first turned on. I cleaned the connectors and slipped in a piece of paper to tighten the fit. I also re-shaped the metal retainer/shield to put less stress on the board. Still occasionally scratches. 10/31/05 1/6/08: The scratchiness and outright signal loss is due to loss of bias current to the power transistors. On the main amp board, lost the -.6v to the base of Q9. It would end up at -.2v when the signal was lost. Was not able to locate the exact cause of the loss. Replaced Q1, Q3, and Q5 transistors, but they were not the problem so I put the old ones back in (I did not have exact replacements on hand). Next, I resoldered all the traces on the main amp board. This eliminated the problem. Somewhere there was a bad trace. Speaker Switch is very scratchy. May be cause of signal-loss problem. The signal out to tape monitors is fine. S7 Lo Filter switch was making poor contact. I rebuilt the switch. I cleaned all of the switches (tape mons, FM muting, Loudness, hi-low filters, selector, mode, speakers) and volume/balance control. Loudness switch still occasionally makes poor contact. 11/1/05 Signal path for Aux input--------------------------------- Aux in -> S1 selector -> Tape Mon A -> Tape Mon B -> S2 Mode -> S8 Hi filter -> S7 Lo Filter -> Tone Control -> Loudness -> Volume -> Balance -> Unknown Board X13-1380-00 -> Pre Out -> Main Amp Board -> Output Transistors -> Sub Main Board -> Overload Relay -> S10 Speaker Switch -> Speakers and Headphone Out See the drawing I created for clarity. Left - Right tracking:------------------------------------------------------------------------------- Tone-amp: Right channel has a little more gain at low frequencies (0~120hz). Tone-amp: Left channel has a little more gain at mid to high frequencies (240hz~16Khz). Tone-amp: both channels same above 16Khz. x13-1380-00 booster board (not documented in the manual) has a little more gain on the right channel. Volume knob allows more amplitude on the right channel at low volumes (0~3). Volume knob allows more amplitude on the left channel at higher volumes (7~10). Bias problem---------------------------------------- Voltage on Rq3 and Rq4 is supposed to be .02v . But the lowest possible on Rq3 is .05v (Rq4 will adjust below .02v). I set both channels to .05v (set with no signal). I could not get a setting that would cause a distortion in a sine wave. 11/1/05 1/6/08 In order to bring the bias current down, I replaced the 100-ohm R22 resistor with a 47-ohm resistor. The result was a minimum of about .03v on Rq3. I set both channels to .03v. Lights----------------------------------------------------------- "stereo" beacon: Original was small tic-tac size oval bulb. replaced with: 12v, 75ma, .5mscp, T-1 3/4, 1000 hrs, midget screw E-5 10/28/05 Frequency graph bulbs: Replaced with 12v automotive, 3.4w, mini-bayonet, same as gauges on motorcycle 10/31/05 Replaced fuse for lights. 2A, 250v. Not soldered in, but better to remove by un-soldering holders. The holders hold way too tight. 10/31/05 ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- KR6200 Service Manual Corrections: Page 38 of PDF-------------------------------------------------- Main Amp Board #X07-1110-10 Parts List Ce16 220 uF 50 v (not 1uF) page 15 of pdf------------------------------------------------- bias adjustments are done on VRe 1,2 on Main Amp Board #X07-1110-10 (page 37,38) BIAS ADJUSTMENT--------------------------------------------------- bias adjustment is on two pages. page 3 and 15 of pdf. Vre 1 and 2 are on the Main Amp Board X07-1110-10 Rq3 and Rq4 are on the Sub Main Board X13-1200-10 The manual is contradictory. The resistors Rq3, Rq4 are .47 ohm. It says the voltage on each resistor should be .02v (on page 3 of pdf), but it says the current through each resistor is .03a (on page 15 of pdf). Either the voltage should be .014v or the current should be .043a . Also, the negative side of the resistor goes to B-, not ground as shown on page 15. When using the method with the voltmeter or ammeter, that is with no signal. When using the waveform distortion method, I could not get the distortion to occur. Board X13-1380-00 is not mentioned in service manual. It is inserted after the volume knob, but before the "Pre-Out". It provides gain to the signal before going to the main amp board. It appears to be powered by a single 20v DC source.