Sansui G-7500 Fried Resistors F2980 Board

Having an auto-ranging DMM is always a big help, but even then it does take a little background knowledge to know which function to choose with the selector switch.
 
I don't know the best way to do this, but I feel like we need to know the health of the P/S.

It looks like the transformer in yours is set up the same as my G-6700. When you look at the transformer do you see a row of output studs with color jumper wires?
Are the jumper wires in this order: Red,Gray,Red; Yellow,Black,Yellow; Purple,Purple? If yes, here is what I want you to do -
1. Set your DMM for AC Volts.
2. Connect mini-grabbers to the two Red jumpers. Turn power on, read meter, turn power off. Reading should be ~43VAC, or less.
3. Connect mini-grabbers to the two Yellow jumpers. Turn power on, read meter, turn power off. Reading should be ~41VAC, or less.
4. Connect mini-grabbers to the two Purple jumpers. Turn power on, read meter, turn power off. Reading should be ~7.5VAC, or less.

I’d also like to know the voltage reading after the 43VAC has been through the bridge rectifier and converted to VDC. This can best be accessed at the underside of the P/S filter caps – the large black canister caps next to the xfmr. Be very careful as these points carry a lot of current and are potentially deadly. With the power off, attach one mini-grabber to ground and use the other to determine which lead of each cap goes to ground and then attach the other mini-grabber to the “not-grounded” terminal of the cap. Now set your DMM to VDC, turn power on long enough to get a reading and turn the power off. Repeat for the other filter cap. Always record the wattage of the DBT bulb when you take readings so you can account for any differences.

Might as well read the rectified 41VAC. Junction of R49 & R50 S/B ~ -59VDC, and the inboard side of R44 (right above R50) S/B ~ +57. (readings will be lower # with DBT)

Some of this may sound like 3rd grade, but I always try to be overly cautious when dealing with unknown voltage and current that could be lethal. Each of us must develop protocols for safely handling these situations, and this is part of mine.

PLEASE query me (or others) if any of my questions or instructions are ever unclear, or you don’t understand. Your silence tells me that you understand.
 
Using a 53W and 72W Halogen bulbs in my DBT I measured the voltages at the points you refer to above. All measurements are with my Fluke 117 True RMS auto ranging DMM. For the measurements referred to below I had the dial on AC volts. I used two different halogen bulbs, a 53W and a 72W.

53W bulb readings
Two red jumpers voltage measured 46.05VAC
Two yellow jumpers voltage measured 43.01VAC
Two purple jumpers voltage measured 4VAC

72W bulb readings
Two red jumpers voltage measured 52.24VAC
Two yellow jumpers voltage measured 48.60VAC
Two purple jumpers voltage measured 4.5VAC

I don't know why but he new right channel driver transistors TR16, and TR18 were shorted so I pulled them out. The resistor R52 was also bad and I pulled it too. I don't have any replacements so I will have to wait until I get them to measure the other voltages unless someone can tell me if its ok to do with the components out.

I would like to disregard all the measurements taking prior to this point if that is ok with everyone who is helping me. I think ghazzer you have been trying to steer me all along to start with the power supply and work my way across the board. So I would like to do that with the above being the first measurements of record. With my limited knowledge of not only some of the terminology, but also of electricity, I believe I must be confusing most of you. I will do my best to ask if I don't understand something. By the same token, please do the same for me. I'm just an old hobbyist trying to keep an old receiver alive.

Thanks again for all your help!
 
I am a wee bit surprised that the 'red' and 'yellow' readings are so high. The schematic says these should be 43VAC and 41VAC w/o the DBT, and should be lower with the DBT. The DBT is going in the right direction - note that the lower readings are with the lower wattage bulb.

Please measure the rectified voltages described above. You can just use the "72W equivalent" bulb for now.

If R72 & R78 are not getting hot you can repeat the jumper pin readings you took before. Skip pins 1-18 for now. 19 & above should all be VDC and your meter will tell you if + or -.
 
Voltage readings at the big caps was +25..67vdc, -25.05vdc. -25.67vdc at the junction of R49 and R50. +25.05vdc at R44. No other measurements were taken. The emitter resistors were getting hot.
 
The DC voltage readings ‘after’ the rectifiers are a uniform 43.7 ±0.2% of their schematic value. This consistency is good, but I would have expected more like 80% if I were using a 60W incandescent bulb in a DBT.

Please take readings of the other voltages in the P/S, just to be complete. (jumper pins 26 = -34vdc; pin 27 = +33vdc; pin 30 = +12.5vdc)

Next we need to consider what could be trying to bake those emitter resistors.
 
Across R72 and R78 (one lead of R72 to one lead of R78) I read -41.84vdc. Across R72 only I read -17.35vdc. Across R78 only I read +24.45vdc.
Pins 58 to 60 (one lead of dmm on pin 58, and the other on pin 60) I read -41.73vdc. BIack lead to chassis ground, red lead to pin 58 I read +14.56vdc. Black lead to chassis ground, red lead to pin 60 I read -27.28vdc. I also read pin 26 and got -23.48vdc, pin 27 and got +23.80vdc, and pin 30 I got +8.21vdc. R

R72 and R78 get hot. R78 gets hotter than R72. All the measurements were take with a 72W bulb. The emitter resistors are 100ohm (I need to get some symbols) 5W. I will reinstall 100ohm resistors on the left channel tomorrow.
 
It should mean that the base of the OP TRs in this channel are receiving different signals, and they are not zero, or minimum.

Bias is set up in a receiver channel such that the output transistors will be “biased” just above the ”on” state. Transistors are controlled by the signal on their base which tells them when, and how much of the voltage on their collector is to be ‘connected’ to their emmiter. When the signal at the base is very low the transistor is consider to be in the “off” state and zero voltage from the collector is passed to the emitter. As the base signal increases the transistor will transition to the “on” state and collector voltage will begin passed to the emitter. The transition is not always a specific “on” or “off” and the signal on the base can vary. This results in distortion of the transistor’s output during the transition.

This distortion is minimized or eliminated by “biasing” the transistor to its “on” state, above the region of distortion.

In general, the bias process that we use in our receivers measures the current flow in the small resistor beyond the emitter and we adjust the bias trimmer until the voltage drop is measureable, but less than a specified value. The “bias voltage” is established by multiplying the value of the resistor by the target current when the transistor is turned on. “Bias” adjustments MUST always be done with the volume at minimum and no input connected to the selected function so that the signal on the base of the transistor is at minimum value.

How to make some useful symbols on most keyboards:
You must be using a keyboard with a numeric pad, or be able to switch to a numeric function using regular keys.
Hold down the Alt key, type the 3 numbers on the key pad, then release the Alt key.

155 = ¢
159 = ƒ
171 = ½
172 = ¼
227 = π
230 = µ
234 = Ω
248 = °
241 = ±

There are other symbols available but you must know the ASCII table that your system and keyboard is using,
 
Would having those transistors along with R52 out of the circuit have an effect on the readings I posted? I should have the new ones by Monday or Tuesday of next week. Thanks.
 
You are correct. The readings should be the same if it was just missing the two transistors, so your readings are indicating additional problem(s).
 
Well I got the parts and installed the transistors. Also installed the temporary 100ohm emitter resistors. Misplaced resistors...reordered and am waiting for them. I’ll be ready to go again probably by the middle of next week. Taking a break and going to Mardi Gras.
 
Sorry for the delay but I have the new resistor in and have taken voltage readings. If I've made any errors, please let me know. Thank you. I have attached a .pdf file with the readings on it.
 

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