There seem to be multiple issues 🙂
and yet we have:
Pulling R363 should turn off Q325 and that should give no volts across R367. Do you see... so that points to Q325 having an issue assuming all the component reference numbers are correct.
If you pull the base resistor to an output transistor it should be off at all times and no current should be in the 0.22 ohm to that transistor.
This stands out. Check check check... here's why.
and yet we have:
Pulling R363 should turn off Q325 and that should give no volts across R367. Do you see... so that points to Q325 having an issue assuming all the component reference numbers are correct.
If you pull the base resistor to an output transistor it should be off at all times and no current should be in the 0.22 ohm to that transistor.
8.82VDC to base of Q325 with chassis ground as a reference point. 61mv emitter to collector. R367 138mv confirmed.
With R363 lifted it should be non conducting and no voltage across the 0.22 ohm (R367). In other words the transistor seems to be conducting when it should be off. 137mv across 0.22 ohm is over 0.6 amp.
61mv 'emitter to collector'. If you 61mv across the transistor then it looks duff to me.
So based on the evidence either the transistor is duff or there is another undetected conductive path somewhere.
61mv 'emitter to collector'. If you 61mv across the transistor then it looks duff to me.
So based on the evidence either the transistor is duff or there is another undetected conductive path somewhere.
Sorry I have been down for the past several days trying to deal with the flu and covid. Hasnt been fun. So pull 325 and test it?
So I disconnected the base from the board and measured voltage at the base connector on the board and got 1mv. If something on the board was conducting a path to the base of Q325 would it still be measured with the base pin disconnected from the board?
With the base of the transistor isolated the transistor should not conduct.
If you measure the voltage from the emitter to the point on the board /print where the base connects you should see very little voltage. If voltage is present (say 400mv and higher) then that would turn the transistor on when the base is connected back up.
If you measure the voltage from the emitter to the point on the board /print where the base connects you should see very little voltage. If voltage is present (say 400mv and higher) then that would turn the transistor on when the base is connected back up.
Replaced Q325. .2 across .22 ohm (367) and R375. Bulb goes very dim to the point where it looks off
Hmm 🙂
We just don't seem to be homing in on any of these problems at all.
0.2 volts across 0.22 ohms... take a step back and think what that tells us.
When you turn the bias up on an amp with a DBT the bulb starts to brighten. I think you have seen that happen in the past. Even a couple of hundred milliamps of current in the output stage lights the bulb. 0.2 volts across 0.22 ohms is close to 1 amp. I=V/R which is 0.2/0.22 = 0.91 amps. That is huge in the scheme of things. The bulb should be quite brightly lit at that.
I feel you are going to have to accept repair this is outside your comfort zone as we are not really making progress on this.
We just don't seem to be homing in on any of these problems at all.
0.2 volts across 0.22 ohms... take a step back and think what that tells us.
When you turn the bias up on an amp with a DBT the bulb starts to brighten. I think you have seen that happen in the past. Even a couple of hundred milliamps of current in the output stage lights the bulb. 0.2 volts across 0.22 ohms is close to 1 amp. I=V/R which is 0.2/0.22 = 0.91 amps. That is huge in the scheme of things. The bulb should be quite brightly lit at that.
I feel you are going to have to accept repair this is outside your comfort zone as we are not really making progress on this.
It’s difficult to both teach circuit analysis and troubleshooting all in one go. I did say this could have all been done using the ohmmeter comparing the working channel to the faulty one but everyone has their own methods so too many cooks in the kitchen does not help and adds to the confusion. It’s a case of investing time to understand the theory of circuit op. in order to make sense of formulating a troubleshooting strategy. Being sick and foggy does not help in learning
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@Mooly I appreciate all the help you provided me. I did learn quite a few things on circuit analysis and for that I am grateful. I can always go back and reference this thread in the future.
@rsavas If it was you and you had this amp in the state it was in what would be there first thing you would do?
@rsavas If it was you and you had this amp in the state it was in what would be there first thing you would do?
I have given you my advise on a number of occasions. You even PM'd me and I said the same but we should keep it to this thread so others can read, learn and contribute.
I have a RX-900/U that I bought for $50 with a blown channel. I think the guys daughter spilt some perfume in it?
It was a fried output channel, o/p's and drivers, some resistors where changed. iirc I used ksa1220/ksc2690, ksa916/c2316 ( Now EOL) and some sanken o/p's I had lying around. I used my ohmmeter to determine what was blown or out of spec.
It did not see power until the two channels matched with the ohmmeter.
I have a RX-900/U that I bought for $50 with a blown channel. I think the guys daughter spilt some perfume in it?
It was a fried output channel, o/p's and drivers, some resistors where changed. iirc I used ksa1220/ksc2690, ksa916/c2316 ( Now EOL) and some sanken o/p's I had lying around. I used my ohmmeter to determine what was blown or out of spec.
It did not see power until the two channels matched with the ohmmeter.
You used the 1220/2690 pairs for the drivers? I bought up a supply of those before they went obsolete. I have another 900u that I use for parts, The right channel was blown so when I pull parts from I test them, obviously the right thing to do.
I will start ohming out everything comparing good channel to bad channel
I will start ohming out everything comparing good channel to bad channel
There is one unconventional step you could try but its at your own risk. Depending what faults there are a resistor might burn.
If you remove the outputs and drivers and then add say a 100 ohm resistor as shown here the amp (a working amp) should run with correct voltages. You must not connect a load with it like this. Keep the vbe multiplier linked out as well.
If it shows a DC offset fault then that should be fixable with it in this state.
If you remove the outputs and drivers and then add say a 100 ohm resistor as shown here the amp (a working amp) should run with correct voltages. You must not connect a load with it like this. Keep the vbe multiplier linked out as well.
If it shows a DC offset fault then that should be fixable with it in this state.
That’s not really “unconventional”, although with amps like this you have to add the resistor. The more conventional (but poorer performing) topology has R437 and 439 connected to the output, and R355 split and also connected to the output. You run off the predrivers to get the DC balance right, then off the drivers into a couple hundred ohm or few k ohm load. Once it passes that you install outputs.
In the days of TO-3’s they were all that way, with outputs mounted with screws. It was easy peasy.
In the days of TO-3’s they were all that way, with outputs mounted with screws. It was easy peasy.
Easy peasy for you super techs, for me thats foreign language although I am more than willing to learn and not afraid to muck something up, thats how you learn
OK, here is where I am at. I was totally frustrated with myself for not being able to totally grasp everything so I put everything back together. Q307 and Q309 was reinstalled with proper replacements as well as Q341, doing this from memory as I am not in my hobby room. Also re,oved the 10k jumper.Reinstalled all the 4.7 ohm emitter resistors with all new 4.7 ohm resistors.
I started comparing the good channel with the bad using my ohm meter. I found one decrepency and that was R327 which only measured 390 ohms instead of 560 out of circuit. Replaced the out of spec resistor.
Powered up the unit thru the DBT and much to my surprise came out of protection with a very dim bulb. Let is sit for about 10mins and nothing was burning up or getting hot.
I Then removed the zero bias jumper on Q311 and powered up again. Came out of protection and I was able to set the bias but it is very wonky and touchy. I can st it to 6mv but it will slowly creep up and then I will set it back to 6mv again. If I go to below 2mv it will hold steady and not move at all.
I wanted to take measurements across the .22ohm resistors but I haven't had a chance to due to life and other things I had to get done. My plan after I get off work is to take voltage measurements comparing good to now somewhat working.
I did hook up a source and was able to get good clear sound out of both channels.
I started comparing the good channel with the bad using my ohm meter. I found one decrepency and that was R327 which only measured 390 ohms instead of 560 out of circuit. Replaced the out of spec resistor.
Powered up the unit thru the DBT and much to my surprise came out of protection with a very dim bulb. Let is sit for about 10mins and nothing was burning up or getting hot.
I Then removed the zero bias jumper on Q311 and powered up again. Came out of protection and I was able to set the bias but it is very wonky and touchy. I can st it to 6mv but it will slowly creep up and then I will set it back to 6mv again. If I go to below 2mv it will hold steady and not move at all.
I wanted to take measurements across the .22ohm resistors but I haven't had a chance to due to life and other things I had to get done. My plan after I get off work is to take voltage measurements comparing good to now somewhat working.
I did hook up a source and was able to get good clear sound out of both channels.
Well its quite an adventure 🙂
When you 'set the bias' according to the procedure in the manual you are measuring across the 0.22 ohm resistors but only across one of them in each channel. In a good amp with closely matched transistors the voltage across all 0.22 ohm should be similar but in practice there will be differences. It is normal for bias to wander a little as the amp warms.
When you 'set the bias' according to the procedure in the manual you are measuring across the 0.22 ohm resistors but only across one of them in each channel. In a good amp with closely matched transistors the voltage across all 0.22 ohm should be similar but in practice there will be differences. It is normal for bias to wander a little as the amp warms.
6 mv is a 27 milliamp bias current (per output pair) which is below where theory suggests it should be. I would set it to the 6mv when its warm and leave it at that.
Lower increases crossover distortion, higher runs hotter although I doubt you would detect any audible difference even with it on zero.
Lower increases crossover distortion, higher runs hotter although I doubt you would detect any audible difference even with it on zero.