Sansui G-7500 Trimmer Replacement and adjustments

[zelgy1]

Good day all! I've just finished replacing trimmers with Bournes 3296Y-1-101LF's, output transistors, emitter resistors, and a couple additional components on my Sansui G-7500. The moment of truth is upon me as I'm ready to fire this baby up for the first time using a DBT. I need a bit of assurance on initial trimmer adjustments before flipping the switch. I have adjusted VR03 and VR04 to minimum resistance. However, I tried to adjust VR01 and VR02 to midrange but they are maxing out at 33 ohms, so I am questioning if that is due to other components in the circuit. I have adjusted to 16 ohms for now, but I want to be sure I get these adjusted appropriately before flipping the switch. If there is anything else I need to verify before startup, please let me know. Please note I have referenced the attached document as far as adjustments. Any feedback would be appreciated!

 

[kevzep]

Set the Bias trimmers to minimum and set the DC Offset trimmers to 50%, then turn it on whilst monitoring Bias and Offset, I use 4 DMM's so I can see whats going on with all 4 adjustments.
Job done, easy.

 

[zelgy1]

Set the Bias trimmers to minimum and set the DC Offset trimmers to 50%, then turn it on whilst monitoring Bias and Offset, I use 4 DMM's so I can see whats going on with all 4 adjustments.
Job done, easy.

So the good news, the unit powers up and relay trips with no smoke. I was able to set DC Offset with no issue. However, I cannot seem to get any change when adjusting either VR03 or VR04 for the bias. They both read 0 mV and adjustments of the 2 trimmers doesn’t change the reading. I only turned them each 4 full turns and stopped there as I assumed I should have a reading change. I am reading from the rear of the emitter resistors as shown in the service manual and still using the DBT. Any thoughts would be appreciated!

 

[kevzep]

Check R35 and R36, also check R39 and R40, these should be all 150Ω, often Sansui used fusible resistors here, in fact I just had this very problem with a G9700 I have on the bench at the moment.
I think you will find they are reading very high.

This will stop the bias network functioning, you can use anything from 100-150Ω in this application, make sure they are metal film and set them up off the board a bit.

 

[zelgy1]

Check R35 and R36, also check R39 and R40, these should be all 150Ω, often Sansui used fusible resistors here, in fact I just had this very problem with a G9700 I have on the bench at the moment.
I think you will find they are reading very high.

This will stop the bias network functioning, you can use anything from 100-150Ω in this application, make sure they are metal film and set them up off the board a bit.

Thanks Kevin! I just checked all 4 of the noted resistors and all check good. I can see the light at the end of the tunnel. We’re getting close. Anything else I should check?

 

[zelgy1]

Check R35 and R36, also check R39 and R40, these should be all 150Ω, often Sansui used fusible resistors here, in fact I just had this very problem with a G9700 I have on the bench at the moment.
I think you will find they are reading very high.

This will stop the bias network functioning, you can use anything from 100-150Ω in this application, make sure they are metal film and set them up off the board a bit.

The suspense is killing me! ;-) Being the resistors all checked good, anything else I should be checking ?

 

[kevzep]

Sorry man, I am slammed at work, plus I have a half a dozen jobs I need to get on the bench....

You are going to have to measure the voltages around the Bias network transistors and work back from there, print the schematic and mark down the measured voltages and compare to what they should be, this will indicate where to look next, there is no magic bullet, it takes a good fault finding strategy to have success here.
Make sure you are measuring the actual bias, and not measuring over an O/C emitter resistor for example.

 

[zelgy1]

Sorry man, I am slammed at work, plus I have a half a dozen jobs I need to get on the bench....

You are going to have to measure the voltages around the Bias network transistors and work back from there, print the schematic and mark down the measured voltages and compare to what they should be, this will indicate where to look next, there is no magic bullet, it takes a good fault finding strategy to have success here.
Make sure you are measuring the actual bias, and not measuring over an O/C emitter resistor for example.

No worries at all Kevin! I was being a bit sarcastic. I’m on holiday, so have time to dig into this unit. Should I take the unit off the DBT at this point when starting to check voltages? Can you expand on the last point as far as making sure I’m testing actual bias and not measuring over an O/C emitter resistor? I am currently trying to adjust bias by reading off the emitter resistors as shown in the service manual. Is there a better point to take readings or am I reading into this wrong? I very much appreciate your input and knowledge on the forum!

 

[kevzep]

No worries at all Kevin! I was being a bit sarcastic. I’m on holiday, so have time to dig into this unit. Should I take the unit off the DBT at this point when starting to check voltages? Can you expand on the last point as far as making sure I’m testing actual bias and not measuring over an O/C emitter resistor? I am currently trying to adjust bias by reading off the emitter resistors as shown in the service manual. Is there a better point to take readings or am I reading into this wrong? I very much appreciate your input and knowledge on the forum!

Oh, make sure the resistor you are measuring on is not open circuit, if it is you wont get a reading.
Tonight I'll have a look at the schematic and see if I can give you a strategy to work through.....

Measuring voltages is a good idea, it can tell us a lot....so work on that in the meantime.

 

[Hyperion]

I only turned them each 4 full turns and stopped there as I assumed I should have a reading change. I am reading from the rear of the emitter resistors as shown in the service manual and still using the DBT. Any thoughts would be appreciated!

You might not be turning the trimmers enough, try turning them a few more turns, but DO make absolutely sure you are measuring across the emitter resistor(s) corresponding to the trimmer you are adjusting. Whatever you do, do not take the unit off DBT until you can successfully adjust the bias - you have an unknown problem at the moment - don't tempt fate by giving it full mains just yet.

 

[EastPoint]

Yeah, if you’re using 20-turn trimmers you may just need to keep going. Keep it on the DBT and you’ll be safe.

 

[zelgy1]

Hi all! Appreciate all the help! We have liftoff. As noted above, I simply needed to continue the turns on the VR03/VR04 trimmers. It took nearly 17 full turns to start getting a voltage reading on the DBT (40 watt bulb). I did turn them down to 4 mV’s before removing DBT. Once I fired the unit up w/o DBT, the reading on bias was 10 times and read 40 mV. For the amateurs like me, thought this would be of interest to show the difference w/ DBT and w/o. Again, I want to thank all who offered feedback on this post and those made elsewhere on the forum. Damn I LOVE this hobby!!!!!!

 

[kevzep]

Hi all! Appreciate all the help! We have liftoff. As noted above, I simply needed to continue the turns on the VR03/VR04 trimmers. It took nearly 17 full turns to start getting a voltage reading on the DBT (40 watt bulb). I did turn them down to 4 mV’s before removing DBT. Once I fired the unit up w/o DBT, the reading on bias was 10 times and read 40 mV. For the amateurs like me, thought this would be of interest to show the difference w/ DBT and w/o. Again, I want to thank all who offered feedback on this post and those made elsewhere on the forum. Damn I LOVE this hobby!!!!!!

Yeah, you only need 25 turn trimmers on the offset adjustment, I use 4 turn trimmers for bias otherwise you are winding all day for no good reason...

Good you had success, often these amps are not a success!!