[HELP] Sansui AU-7500 dc offset sudden fluctuate

[ReisDaddy]

Hello AKers, i just bought AU-7500 and not recap yet, because i want to hear how it is before recap. And after checking all transistor i think its still original. Some transistor is not like in schematic, but its looks old.

While i test with speaker first day when i turn the power knob off, i hear a thump / thud / pop. Thats afraid me. Then i check the dc.

Here the list of transistor that is not as in schematic

F-2041 (protection booard)
TR901 Transistor 2SC1364 (6,7,8) i get : C945
TR902 Transistor 2SC1364 (6,7,8) i get: C945

F-1215-A
TR903 Transistor 2SC1364 (6,7,8) i get C634A (same as in service manual part list, but strange, look image below)

F-2013-1 Power Supply Block
And a bit crack on those plastic green transistor
TR002 Transistor 2SA678 i get: A678 (see on image red circled, crack?)




And what is that looks like dragon or root spread on the leg of transistor TR903 board F-1215-A (circled in blue)




I did set adjustment:
DC offset: (fluctuate) 0-20mV
Biass: 14mV (across final resistor).

And my main problem is, when i turn it off, it jump to 80-100mV right after relay click, it is same even replacing capacitor C902 from default 220/6.3 to 470/25 that delay the relay to 6-7 seconds.
And if i pull the phono board F-2014, after power off it jump to 30mV then decreasing to 0mV.

Then i see @Leestereo this thread i did put in driver board F-2034 these Panasonic FC 47/50 for C827, C828, C829, C830 and Rubycon 1/50 for C825, C826 at the back of pcb.
Sometimes its not fluctuate after relay click, but when i see the dmm, sometimes it fluctuate from 0-20mV then suddenly to 400mV, sometimes 800mV around 1 second.

What could be the problem? Please help where should i start to check?

PS: i never see this dc offset fluctuate so crazy 0-20mv (for me) because my previous Sansui is not like this, this is my oldest Sansui. Maybe old transistor just like this?


Looking forward to your advice, thanks in advance!

 

[stereofun]

I just solved a mystery where I had wild swings on the DC-offset in both channels on a 7500.

This would cause the relay to click off, then right back in. However, this only happened 30 seconds to 5 minutes after power-on - but it was intermittent, so a week of normal playing could go by before it began acting up again.

I have green 2SA678 transistors on the powerblock too.

On the protection block including 1215-a, you should have all 2SC634, but two of them may have been replaced with 2SC945 at some point.
I found repairs/replacements on my 7500, too, like replaced germanium diodes on the protection board and 4 transistors on the pre-amp.

Ok, in my case, the culprit was transistor 2SB507 - that regulates the voltage for both the preamp and driver boards. it gets quite hot per design - but after 50 years of heating-cooling cycles, it was beginning to fail.

Before you run off replacing that, do check if your DC-offset problem is in both channels!

Then you also want to check the transistor itself - Vdc emitter (or base) to chassis. See if you get erratic moves away from -44Vdc
Collector to chassis should be steady at 64V'ish Vdc ...if memory serves.

If you do and DC are in both channels, then you can replace it with this.

In general - do give all pots and switches a good cleaning. Also the DC-offset and bias pot. Spray some deoxit unto the pot and let it seep in, then them multiple times - set them back in the approximate position before powering on and re-calibrate. I prefer to set bias per Service manual - milliamp across the fuseholder. If you set bias via the output emitter resistors, you should measure the actual resistance first and use that for your ohms calculation. Unless you know what you are doing, then better to use the service manual because a full complementary amp will have a different calculation than a quasi-complementary amp.

You can also spray a bit of deoxit on the back side of prints, then use a toothbrush to carefully clean various residue away. Then, after that, give all prints a visual inspection with a magnifying glass and resolder/reflow anything suspicious.

If you only have DC-issues on one channel, then TR801/TR803 OR TR802/804 (differential pair) would be a candidate to pull and check in that channel.

 

[ReisDaddy]

Thank you so much for your time to reply. I did search in this forum somewhere that kevzep reply to someone that old transistor may get noisy. So i try replaced TR801 and TR803 with A992 with close hfe both channel, also all transistor in F-2041, F-1215-A except power supply F-2013-1.

It is more stable now, 0-8mv in terminal speaker. When power on its just jump to 30mv.
Im still waiting for the new transistor and caps, and cut cost for now, i just buy the audio path caps. For testing the right channel only, i want to know is there audible difference.

Cut cost because i plan to buy Sansui AU-999, im really curious of your mod and any other mods! I have friend that after he bought AU-999 (no mods) he really like them, that he will keep forever.

 

[stereofun]

My DC-Offset, jumps when power on, that's normal considering you have an unusual inrush of current charging up the large reservoir caps.

Yes the 999, is my overall favorite between sound, retro-look, controls, workability. There's probably a bunch of better amps that I just haven't listened too.as I hang around the early stuff.

Let us know how your 7500 comes along...

 

[ReisDaddy]

SOLUTION: Replace TR001 B507

When im working there, that greeny things also broken accidentaly pull them, so i replace them too (TR002 and TR003).
Thank you so much @stereofun . I think i found your posting about that Sansui AU-7500 with same problem. Funny is i was searching already in this forum, i cannot find, but after reading and reading, i found itm maybe because its is different title here.
Now it is very stable... 0-1 mv

Cant wait tonight to hear the difference between left (old cap) and right channel (recap audio path)
Again, Thank you so much....

 

[stereofun]

Very happy to hear this was helpful to you. This is what AK is all about - enjoy, Tom

SOLUTION: Replace TR001 B507

When im working there, that greeny things also broken accidentaly pull them, so i replace them too (TR002 and TR003).
Thank you so much @stereofun . I think i found your posting about that Sansui AU-7500 with same problem. Funny is i was searching already in this forum, i cannot find, but after reading and reading, i found itm maybe because its is different title here.
Now it is very stable... 0-1 mv

Cant wait tonight to hear the difference between left (old cap) and right channel (recap audio path)
Again, Thank you so much....