petehall347
the brandy coffee man
thermistor is fine by the looks of it . was just a be sure thing .
want to talk silly things see if ce9 is being a resistor
want to talk silly things see if ce9 is being a resistor
Bias problem on a KA-7100, new trimmer does nothing
- Thread starter tusntuk2
- Start date
ce9 test 0.L ohms after removing it from circuit.. I tried several of these, ce7, ce11, ce13, and all of them tested 0.L for ohms when on the workbench. My DMM has a capacitor tester function, but when I tried to test these little guys I got random wandering numbers that never made sense, never matched the markings, and were not consistent themselves. I don't have a clue what the DMM was trying to test. Maybe that's just for electrolytics? Anyway, that data was useless. I never got a resistance reading on a raw tiny ceramic cap. Once in the circuit I could get resistant readings, but they would typically wander and increase, so I figured that was useless too.
I'm puzzled by lack of voltage at VR3, the L bias trimmer. VR4 shows 1.23V and 1.16V, but VR3 shows about 340mv and -340mv. To me that means there is no voltage to send downline for bias, even if that's not quite how it works. If there is nothing at the control, the control has no influence, so testing the bias at the output emitters doesn't display anything or display any changes when you turn the trimmer. There are no changes. So where does the current come from that the L bias trimmer is supposed to control? I'm turning a water valve with no water...
dr*audio
Fish fingers and custard!
So this is what we have learned so far:
1. The voltages on the bases of the output transistors are low.
2. The voltages on the bases of the pre-drivers are low.
3. There is no evidence of oscillation, looking at the output with a scope.
4. The heat sink is getting hot on the bad channel.
5. There is no short between the output and ground.
Did you remove the outputs on that channel and install the 1K resistors as I suggested?
1. The voltages on the bases of the output transistors are low.
2. The voltages on the bases of the pre-drivers are low.
3. There is no evidence of oscillation, looking at the output with a scope.
4. The heat sink is getting hot on the bad channel.
5. There is no short between the output and ground.
Did you remove the outputs on that channel and install the 1K resistors as I suggested?
I pulled the main outputs, Qe27 + 29, and installed 1/2 w 1K resistors at Re57 + 59. I did not do anything on the right side. Original 57, 59 measured 327/330 ohms as shown on s/m.
When I power up, should I use the DBT? and where do I measure voltages?
Sure appreciate the help!
When I power up, should I use the DBT? and where do I measure voltages?
Sure appreciate the help!
dr*audio
Fish fingers and custard!
By Ohms Law, E / R = I (Voltage / Resistance = Current) so 4.7mV / 1K = 4.7uA and 7.2mV / 1K = 7.2uA. Next, connect your scope probe to the place where the 1K resistors connect together and set the scope on 1V / div. Turn the amp on normally with the volume at minimum. After the amp turns on, set the scope input to as low as it will go; 20mV / div, I think. Do you see any signal? Set the triggering to Auto, Ch A and see if you see any kind of sine wave or just noise. Just want to make sure it isn't oscillating before proceding. If whatever is there is 20mV or less, it should be ok.
Then, connect your meter again across one of the 1K resistors again and adjust the bias pot. Does the voltage change at all, and if so, what is the min and max?
Then, connect your meter again across one of the 1K resistors again and adjust the bias pot. Does the voltage change at all, and if so, what is the min and max?
OK, at the CP center point on the 20mV setting the noise filled about 3/4 of the first vertical division. If i understand this correctly that would indicate about 15mV on the scale. I left it on about 1 1/2 - 2 minutes, could feel a small bit of heat from common sources in PS and control board, but nothing radical, it seemed normal. Fuzzy line on screen of scope stayed constant, filling about 3/4 div.
Actually, after turning off the amp, I had left the OSC. probe connected while I wrote the last note. When I went to remove the probe I noticed the screen still showed some noise, perhaps 1/3 of the first division, so the reading might be even lower. I don't know, but I thought I would mention that.
Reconnecting to R59 I measured 6.0 mV this time. Turning the bias pot several turns in either direction produced no correlated changes in voltage. The meter did fluctuate a bit between about 5.8mV and 6.1mV, but not when I was turning, it was just a bit unsteady.
Reconnecting to R59 I measured 6.0 mV this time. Turning the bias pot several turns in either direction produced no correlated changes in voltage. The meter did fluctuate a bit between about 5.8mV and 6.1mV, but not when I was turning, it was just a bit unsteady.
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dr*audio
Fish fingers and custard!
I think what you are seeing is just noise and probably normal.
Did you test or replace Q15, Q17, Q19 and Q21?
Also please verify the basing of Q9, Q11, Q13, Q23 and Q25, vs the silk screen on the board and check the silk screen against the actual connections on the board. Checking 2 leads of each part will be sufficient. There could be an error in the silk screen or you could have mounted a part wrong.
Did you test or replace Q15, Q17, Q19 and Q21?
Also please verify the basing of Q9, Q11, Q13, Q23 and Q25, vs the silk screen on the board and check the silk screen against the actual connections on the board. Checking 2 leads of each part will be sufficient. There could be an error in the silk screen or you could have mounted a part wrong.
petehall347
the brandy coffee man
just remembered about the driver voltages . one seemed fine and the other reversed vbe . which i could not believe .. was this anomaly sorted ? sorry i cannot remember without reading through again .
Yes, I have tested and replaced every transistor you mentioned. I wanted to rebuild this amp the best i could, so this was planned. As noted earlier, I saw the 000. bias problem before doing the work, because the amp played I went ahead and changed the parts, figuring the new parts (and trimmer) would solve the problem. My bad!
My method is to check s/m, then check cross reference to new transistor. I use a lot of AK info for replaacements. I desolder the old and test it in a cheap (chinese) transistor checker. It tells me pinout and Hfe. I then test the replacement, and compare the pinout. I draw diagrams and labels on the packages. I use a magnifying glass to read. I check the silk screen on both sides, and follow the traces to adjacent parts. I check the parts to the schematic. When I am satisfied that the transistor is correct, I'll solder it in. I do one part at a time. I compare it to the right side. I often fired up the amp with a DBT after a section, say, the relay circuit, before continuing. I don't know electronics well, I'm a mechanic, so I'm as careful as I know how to be about getting things straight.
All that said, I know I will find something wrong in what I did, so I am doing the best I can. I use Mouser for parts. And i WANT to find a mistake in what I did! HaHaHa.
I'll get new base voltages soon. the only silk screen error spotted so far was the control board pins 7,8 underneath (B+/-) they are labeled backwards.
My method is to check s/m, then check cross reference to new transistor. I use a lot of AK info for replaacements. I desolder the old and test it in a cheap (chinese) transistor checker. It tells me pinout and Hfe. I then test the replacement, and compare the pinout. I draw diagrams and labels on the packages. I use a magnifying glass to read. I check the silk screen on both sides, and follow the traces to adjacent parts. I check the parts to the schematic. When I am satisfied that the transistor is correct, I'll solder it in. I do one part at a time. I compare it to the right side. I often fired up the amp with a DBT after a section, say, the relay circuit, before continuing. I don't know electronics well, I'm a mechanic, so I'm as careful as I know how to be about getting things straight.
All that said, I know I will find something wrong in what I did, so I am doing the best I can. I use Mouser for parts. And i WANT to find a mistake in what I did! HaHaHa.
I'll get new base voltages soon. the only silk screen error spotted so far was the control board pins 7,8 underneath (B+/-) they are labeled backwards.
petehall347
the brandy coffee man
take a closer look of bias test points .. i just head straight for emitter resistors if present . i do check both if fitted just see how well balanced things are . transistors should not warm up without any current .or high frequency that you might not notice .
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