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SAE 2600 restoration/recap.....need assistance.

It's alive! I took a relay from a SAE 2201 I need to repair so now the right channel has a working relay but, still really low sound in the right channel. I started looking at the meter board which the pre-drivers go through. Although I originally had changed nothing, it appears the last guy who tried to repair it had the gain wires reversed. The right channel now sounds just as good as the left. :banana:

There are still a few things I need to do before it's done.
1. Order the correct diff pairs for the pre-drivers.
2. Once installed, set the DC offset for both channels.
3. Set the bias.
4. Double check the DC offset.
5. repair one bulb socket for the right meter.
6. Sit back and enjoy the music. :D

If I'm forgetting anything please let me know.

I have run both channels for 5 minutes or so and it seems to be running very cool. I don't feel much heat rising from anywhere although I haven't taken it up past 4-5w. For all I know the bias is way off from the last repair job so I don't want to run it any more until the bias and DC offset are dialed in.

So I've got to get through a few more steps and my first recap/repair will be complete.
 
ryuuoh said:
:thmbsp:
Sounds like you're almost there!
Click to expand...

Yes,and I really appreciate all the help from you...and others as well. It made my job go a lot easier, and fun.

I forgot to add another issue to the list. I get a pretty bad thump in the right channel when I shut the amp down weather the preamp is on or not. Could this be caused by bad diff pairs or do I need to look elsewhere?
 
I need to change them anyway for the DC offset. Maybe I'll get lucky and kill two birds with one stone.
 
When the members get interested in a project, it gets pretty good. It helps that you can handle a lot of the work yourself.
 
Rob41 said:
I forgot to add another issue to the list. I get a pretty bad thump in the right channel when I shut the amp down weather the preamp is on or not. Could this be caused by bad diff pairs or do I need to look elsewhere?
Click to expand...

Start with the diff pairs and see if that resolves it.
 
Rob-
Impressive. Well on your way now. You're getting lots of great advice. I bet that will be sweet once the DC offset (diff pairs replaced with matched) and bias are adjusted properly. More pics please! :thmbsp:
 
Haven’t checked this thread for a while, great job Rob. The driver and pre driver boards had no bad semiconductors? That is surprising.
 
New Pics!

FootFungus said:
Start with the diff pairs and see if that resolves it.
Click to expand...

Sounds like a good plan and I think it's my next step.

GoRNR said:
Rob-
Impressive. Well on your way now. You're getting lots of great advice. I bet that will be sweet once the DC offset (diff pairs replaced with matched) and bias are adjusted properly. More pics please! :thmbsp:
Click to expand...

Thanks. Below are a few pics.

grateful said:
Haven’t checked this thread for a while, great job Rob. The driver and pre driver boards had no bad semiconductors? That is surprising.
Click to expand...

Thank you. I did change out all 4 devices on each board if that's what you're referring to. The little black ones I did not change. Sorry I'm not up to speed yet but which ones do you mean?

----I thought the right channel now had the same level as the left but now that I have a second phone to RCA adapter I can hear the right channel is attenuated about -6 compared to the left. This amp has four attenuation buttons for each channel off, -6db, -3db, and -0db. The off works but there is no difference between -6, -3 and -0db. In any of the three settings they all sound like -6db. I'm not sure if the diff pairs could do this but it seems to me that there is an issue in the attenuation switches. They work as designed in the left channel. Both meters work as they should and I have repaired the bulb socket for the right meter but still need to get a bulb for it.


SDC11282_zps73413a2f.jpg


SDC11284_zpsdcd6c47e.jpg


SDC11291_zpsf068acf0.jpg
 
Oh so you did change all the drivers, I didn't see that it the thread. The diff pair won't have much effect on the gain. It sounds like a wiring issue in the attenuator switch. Those switches are in the feedback loop
 
grateful said:
Oh so you did change all the drivers, I didn't see that it the thread. The diff pair won't have much effect on the gain. It sounds like a wiring issue in the attenuator switch. Those switches are in the feedback loop
Click to expand...

I think so too. I've got the balance 3/4 to the right and the left channel attenuated by -6db on the amp just to balance things out.

Even though there is more to do yet, I'm surprised by the imaging I'm hearing. No doubt it will get even better once everything is adjusted and sorted out.
 
Think of gain as the signal multiplication from input to output. By reducing the gain more input signal is required to drive the amplifier to full output. Those switches control gain, as you go to a higher attenuation number the volume goes down.
 
FootFungus said:
I think those buttons are for reducing gain (max output), not attenuation.
Click to expand...

I stand corrected.

grateful said:
Think of gain as the signal multiplication from input to output. By reducing the gain more input signal is required to drive the amplifier to full output. Those switches control gain, as you go to a higher attenuation number the volume goes down.
Click to expand...

Thanks for the clarity about gain and attenuation guys. I always thought they were the same thing.
 
FootFungus said:
Bongiorno recommends replacing the MPSU06/U56 parts with 2sa1837/2sc4793 in a thread on SAE Talk.
Click to expand...

He's right the 2sa1837 (pnp) and the 2sc4793 (npn) are the direct replacements for the SAE parts but neither Mowser or Digikey have both in stock. Is there another option I can use?

edit: I'm open to suggestions if anyone knows a good reliable source for these. I've been looking on the net but most of what I'm finding seems like it may not be the best way to go.
 
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Quest is the only place I've seen with the 2SC4793 that I'd buy from. If you get desperate, I think I have a few hundred pairs of the 2SA1837/2SC4793.
 
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