Soundcraftsmen A5002 Issue

[geode]

Well I finally got back on the SC amp I was working on in this thread http://www.audiokarma.org/forums/showthread.php?t=492078&highlight=geode I replaced the Bias transistors with 2n3417 which is listed in the schematic. It had 2n3415. The Darlingtons were also exchanged with new onsemis. NOw I can dial the bias in at 21.6 mv on both channels. Only problem now is the Variportional lights stay on for both channels. The heatsinks don't get hot. After letting it sit idling for about an hour they are slightly warm. The variportional lights staying lit apparently means the amp is stuck on the high voltage rails. Anybody know what can be causing this to happen. Thanks.

 

[goat67]

I would put the original Darlingtons back in and see what happens.
These control the switching between the low and high rails.
Are you sure they where installed correctly?

Well I can't tell about my model woth the documentation I have. But the model MA5002A shows Toshiba 2SB645 and 2SD665. The manuals I have covers the Models 5001, 5002, 5003, 5001A, 5002A, RA7500, and RA6501. I plan on uploading it all to the database as soon as I find a scanner.

Did you ever upload the schematic I would like to see what it looks like

 

[geode]

I would but the original Darlingtons back in and see what happens.
These control the switching between the low and high rails.
Are you sure they where installed correctly?

Did you ever upload the schematic I would like to see what it looks like

Hi Goat. I think the darlingtons are installed correctly. All the outputs and darlingtons are new onsemis. Recall before I couldn't get the bias to adjust. I don't know if the new darlingtons or the bias transistors did the trick. I also forgot to add I replaced R16 which read 330 ohms with a new metal film 270 ohm. The schematic shows 270 ohm. I have not uploaded those schematics, yet. I need to get to a copier.

 

[goat67]

Hi Goat. I think the darlingtons are installed correctly. All the outputs and darlingtons are new onsemis. Recall before I couldn't get the bias to adjust. I don't know if the new darlingtons or the bias transistors did the trick. I also forgot to add I replaced R16 which read 330 ohms with a new metal film 270 ohm. The schematic shows 270 ohm. I have not uploaded those schematics, yet. I need to get to a copier.

I am leaning towards the Bias Transistors and changing R16 to get the Bias to adjust.
R16 is this the one in series with the Bias adjust Pot? And did you replace them for both channels?
What was the markings for the original R16 was it 330 ohms or was it 270 ohms?
Did it look the same as all the other resistors?

I still think the Darlingtons are suspect did you install the correct one in the correct place NPN vs PNP?
Measure the voltages on the collectors of the Darlingtons to Ground.

 

[geode]

I am leaning towards the Bias Transistors and changing R16 to get the Bias to adjust.
R16 is this the one in series with the Bias adjust Pot? And did you replace them for both channels?
What was the markings for the original R16 was it 330 ohms or was it 270 ohms?
Did it look the same as all the other resistors?

I still think the Darlingtons are suspect did you install the correct one in the correct place NPN vs PNP?
Measure the voltages on the collectors of the Darlingtons to Ground.

R16 is tied to the bias trimmer. The color code is 330ohm and it measures 328 ohms. I thought that this was a previous repair where the tech didn't have a 2n3417 and substituted the 2n3415 for it. Also the 2n3415 only handles 25 volts where the 2n3417 handles 50 volts. I measure 93 volts on the darlingtons collectors as measured from ground to pins A5 and B5.

 

[goat67]

The Darlingtons seem to be shorted or the MR751 Diodes could be shorted.

The circuit consists of a CR21 1N3070 Q19 MPSL01 Q20 MPSA93 and the Darlington

I would check all these. But something is wrong it did not do this before you changed to the new Darlingtons.

Playing Seseame Street what does not belong here.

I would put the originals back in and see what happens

 

[geode]

The Darlingtons seem to be shorted or the MR751 Diodes could be shorted.

The circuit consists of a CR21 1N3070 Q19 MPSL01 Q20 MPSA93 and the Darlington

I would check all these. But something is wrong it did not do this before you changed to the new Darlingtons.

Playing Seseame Street what does not belong he

I would put the originals back in and see what happens

After the new darlingtons went in the Variportional light stays on. Weird that both channels do the same thing. I can put the originals back in.. More to come. Thanks.

 

[geode]

I put the original 330 ohm resistor back in. The result was the same problem of not being to adjust the bias. I put the new 270 ohm resistor back in and am able to adjust the bias again. The vaiportional lights are still on. Then I put the original motorola darlingtons back in. No change, lights are still on. I also did a transistor test on the darlingtons and they seem to be okay.

 

[Jon_Logan]

Do you have a scope to troubleshoot with?
Are the heatsinks getting hot??

I ask, because if either of the above questions have true answers, check the heatsink temperature and use the scope to look at the signal. Sometimes when devices are changed, there is a risk of the circuit going into oscillation. If the circuit oscillates, the VariPortional light(s) might stay on. and the amp will probably get hot.

 

[geode]

Do you have a scope to troubleshoot with?
Are the heatsinks getting hot??

I ask, because if either of the above questions have true answers, check the heatsink temperature and use the scope to look at the signal. Sometimes when devices are changed, there is a risk of the circuit going into oscillation. If the circuit oscillates, the VariPortional light(s) might stay on. and the amp will probably get hot.

I do have scope and signal generator. I'm not very proficient with it yet. I have it connected to the amp with dummy loads. The heatsinks got pretty hot when running a 1khz signal into it. It also clipped. The big MR751 diodes CR23 and CR24 both have a bad looking waveform.

 

[goat67]

I think the reason it is running hot is due to it running on the 93 volt rails.
This amp was not designed to only run at 93 volts.

I am not saying it is not going into oscillation but it seems weird it started
to do this after you worked on it.

With the amp off check for a short between Emitter and Collector of the Darlingtons maybe a bad insulator but seems strange all would be bad.

Something is forcing it to switch full on to the high rail.

I just bumped a thread that I posted a schematic in.

R34 R35 R50 and R55 are changed for different amps I would like to see what
values you have in your amp. Since another resistor was changed who knows.

Check the +-62 volt power supply voltages.

Check the 10 ohm R122 it is common to both channels I just noticed it is missing on the schematic I created.
In my manual it is Tied to Ground and is labeled K1 it is tied to the output thru 5 ohm 5w then .1 uf to Ground

Look for a disconnected or broken wire something has changed that is common to both channels.

Then take a look at Q19 Q20 Q23 and Q24 get voltages for e b c

I am still thinking it is something simple.

 

[geode]

Simple you say Well, I had it running and was poking around with the scope probe and then the fuse blew. Previous to that it tripped the circuit breaker. I did check all the darlingtons and none were shorted. I put the new onsemis back in. The bias is pretty stable after it is set at 21.6mv. The schematic I have is different than the one in your thread. It is for the A5002. I will do my best to upload it tomorrow so we are looking at the same thing. Thanks.

 

[geode]

Here are the schematics for the A5002 that closely matches my amp.

 

[geode]

Hey Goat, I just noticed something while looking at the output board schematic. The output transistors that are NPN seem to be attached to a PNP darlington while the PNP outputs are connected to NPN darlingtons. Is this correct, because I have the PNPs with a PNP and NPN with NPN. I'm not sure how I screwed this up if this is the case.

 

[Jon_Logan]

The Darlingtons are the B+ and B- commutators.


It is correct that the PNP Darlingtons are on the positive rail and the NPN Darlingtons are on the negative rail. for this design.

 

[geode]

The Darlingtons are the B+ and B- commutators.


It is correct that the PNP Darlingtons are on the positive rail and the NPN Darlingtons are on the negative rail. for this design.

Thanks Jon. I'm not sure what that means. So, you're saying the darlingtons are installed correctly. Why does the schematic show the PNP with the NPN etc. Just trying to understand and learn something.

 

[Jon_Logan]

Thanks Jon. I'm not sure what that means. So, you're saying the darlingtons are installed correctly. Why does the schematic show the PNP with the NPN etc. Just trying to understand and learn something.

The Darlingtons are used as Commutators, or as power supply switches. They switch in the higher voltage supply (93V) and connect it to the Collectors of the output transistors, which normally operate on the 63V supply. That is the VariPortional circuit. It switches in the higher voltage (through the Darlingtons) when the signal demands it. ...under higher output conditions.

Soundcraftsmen has designed the circuit to use a PNP Darlington to switch in the positive higher voltage supply, that feeds the NPN output transistors.


....and Soundcraftsmen has designed it to use an NPN Darlington to switch in the negative higher voltage supply, that feeds the PNP output transistors.

It's completely correct to have the PNP Darlington to be "grouped" with the NPN outputs and the NPN Darlington to be "grouped" with the PNP outputs.

The Darlingtons are just switches in this case. Don't confuse them with outputs because they look like them.

 

[geode]

The Darlingtons are used as Commutators, or as power supply switches. They switch in the higher voltage supply (93V) and connect it to the Collectors of the output transistors, which normally operate on the 63V supply. That is the VariPortional circuit. It switches in the higher voltage (through the Darlingtons) when the signal demands it. ...under higher output conditions.

Soundcraftsmen has designed the circuit to use a PNP Darlington to switch in the positive higher voltage supply, that feeds the NPN output transistors.


....and Soundcraftsmen has designed it to use an NPN Darlington to switch in the negative higher voltage supply, that feeds the PNP output transistors.

It's completely correct to have the PNP Darlington to be "grouped" with the NPN outputs and the NPN Darlington to be "grouped" with the PNP outputs.

The Darlingtons are just switches in this case. Don't confuse them with outputs because they look like them.

I think I get what you're saying. So, in my case I have the PNP darlingtons installed along with the PNP output transistors and likewise with the NPN. This is wrong then..and is causing my problems. I should then install the PNP darlingtonn with the NPN transistors. Thanks again.

 

[Jon_Logan]

I think I get what you're saying. So, in my case I have the PNP darlingtons installed along with the PNP output transistors and likewise with the NPN. This is wrong then..and is causing my problems. I should then install the PNP darlingtonn with the NPN transistors. Thanks again.

It's hard to know what exactly that you're seeing, but you may want to confirm that the schematic is matching the physical placement.

If they are in the wrong positions, it could explain a lot. Check the Darlingtons when removed to make sure they are still functional.

 

[geode]

It's hard to know what exactly that you're seeing, but you may want to confirm that the schematic is matching the physical placement.

If they are in the wrong positions, it could explain a lot. Check the Darlingtons when removed to make sure they are still functional.

Well, I do know that each output board is divided into "A" and "B". I think the "A" is the + and "B" is -. According to the schematic I attached to this thread it shows the PNP darlington paired with NPN outputs on "A". And the same on "B" with NPN darlington paired with PNP transistors. It certainly seems like I have the darlingtons installed wrong and they should be switched. So the physical placement appears to be wrong. Jon, if you don't mind, take a look at the schematic in post 13 and what I'm trying to say might make sense.