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SA-7800 Idle current one channel 400mv

camgilbert

camgilbert

Member
Hello all,

Iv'e just started working on a Pioneer SA-7800 which I acquired.

Generally the unit is very clean and fires up ok.

I did need to replace the power lamp but that's about it so far.

When checking the idle current as per the service manual I started with the right channel which I adjusted to 53 & 66mv across TP3 and TP4.

However when I test the left channel with V+ on TP1 and V- on TP2 I get about 380mv with both Vr7 and Vr5 set fully counter clockwise.

Before I dig really deep on this, I though maybe someone might have run in to this before.

Many thanks and Happy New Year to all.

Cheers

Cam
 
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I am working on a 7800 now too. What I have read is those trimmers should be replaced before touching them. They can take out you outputs and three or four resistors if they glitch. Get some new Bourns and swap them out

Turn the new ones all the way counter clockwise. You can measure them with your MM too for good measure. Then you can fire up on a DBT and test.
 
The amp is one of Pioneers NSA circuits. Its not for the faint of heart. Modern, high speed transistors can be difficult to find.
It is a good idea to get on top of this before it becomes a bigger issue.
For starters:
Checking all the resistors in the amp and solder connections is a must. The transistors, mainly the drivers can get hot enough to melt a poor trace connection. The advice from "loweran" is well placed as the trimmers can be a source/by product of the problem.
That's about all I can offer at this point but MTF has been offering advice lately. I'm sure this thread will spark his interest.

Read "Loweran's" restoration thread here.
 
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Hi guys,

Thanks so much for responding!

I ended up spotting the fact that the left channel output transistors pnp/npn were NOT the originals and had been replaced with slightly different units.

I managed to find an original pair on Ebay for 6 bucks, so i ordered them.

Hopefully that ends up being the right decision and was in fact the issue impacting the bias voltages.

Ill keep you informed.

FYI, you wouldn't know the part number of the Bourn's POT's you used by any chance?

Cheers

Cam
 
Trimmers from Mouser are:
100R,,,,,652-3296Y-1-101LF
68K,,,,50k,652-3296Y-1-503LF

You need two of each.

Should have let me know. I have two good original outputs that I pulled out of mine.
 
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One side was blown due to someone before me messing with those trimpots you are replacing. It seems to be fairly common and takes out the same resistors and outputs on the ones I have seen documented here. So, I replaced all four for symmetry with On-Semi MJL3281 and MJL1302.

You may want to inspect yours for visually damaged resistors. Also you will want to pull a leg on all the resistors and test them. It's not to difficult since most of them are in a line on the bottom of the amp board. Be sure you use your DBT when working on yours!

You can check out my thread here:
http://www.audiokarma.org/forums/index.php?threads/sa-7800-restore.797285/

Other threads to reference here. You should read them all:
http://audiokarma.org/forums/index.php?threads/sa-7800-another-one-bites-the-dust.751276/
http://audiokarma.org/forums/index.php?threads/pioneer-sa-7800-help-replacing-transistors.601238/
http://www.audiokarma.org/forums/index.php?threads/sa-7800-with-burnt-out-resistors.487176/
http://audiokarma.org/forums/index.php?threads/sa-7800-how-to-not-make-one-into-a-doorstop.752089/
 
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