2325 bias will not adjust

[oldguy68]

I am picked up this 2325 in working condition so I decided to restore it. First I went through all the boards and changed tantalums, diodes, electrolytics, the ly2 relay and cleaned the pots and checked solder joints. Then I set the voltage on P800 to 35v.
After trying to set dc offset I changed R713 on both P700 boards to Bourns multi turns. The offset for the left and right channel was set close to 0.
1 to 2 mv.
The bias on the left channel set to 14.7
The right channel bias is 1.3 and will not adjust when I turn R740. It will drop to zero when turned. I tried a multi turn here as well but did not change.
I have gone through the right board and changed resistors that were out of spec as well as new 1n4747 zeners at H716/717. I also changed the other diodes on the boards to 1n4148.
H701/702 were replaced with hfe matched KSA992's. Paste and heat shrink applied.
H703 was replaced with KSA992 hfe matched to 701/702.
After a slip of the meter probe checking voltages I shorted H708. (Not to smart)
Changed the following.
H706 to KSA1220 legs heat shrunk and crossed.
H707 to KSC2690 legs heat shrunk and crossed.
H708 to KSA940
H709 to KSC 2073
Outputs changed to MJ21195 and MJ21196
H710/713 changed to KSC 945 legs twisted with heat shrink applied.
H711/713 changed toKSA733c
H714/715 changed to 2n4923
Also changed 8pf and 100pf caps as well as replacement of the silver caps with polys.
Still can't adjust bias.
I did run it with this bias and it played ok for 12 hours.
From a previous post rcs16 replied the following.

Looks like you need some fine tuning in the bias generator ckt (H715) to increase the voltage drop across H706,707 collectors. The Vbe multiplier voltage drop is controlled by the current generated by the bias network connected to H706,707. Changing any comps will affect it.

Not sure what to do here so any help would be wonderful.

 

[catrafter]

I never twist the lead around to make 'wrong' parts fit.
You can get TO-202 parts (H706 2SA818) and (H707 2SC1628) but they can be expensive.
I am at a loss as to why you would need to twist the leads on the ASO transistors (H710 through H713) You can buy parts with the collector in the center like the originals, or you can safely test the amp into a resistive dummy load with these parts removed until you get it working.

Tom

 

[catrafter]

Did you make a typo or did you put these in backwards? C=NPN, A=PNP
H708 NPN KSC2073
H709 PNP KSA940
Tom

 

[oldguy68]

Correction
H709 KSA940
H708 KSC2073
Dick

 

[oldguy68]

Hi Tom
Thanks for the response.
Thank you for the info on transistor source. I used parts recommended on the posts in the Marantz section on AK. I did not have an issue with leg twisting as it was working after the changes but with 1.2 bias. I had been hunting for originals and was able to get them for H708/709.
Would the Subs cause no bias adjustment? From other posts others seem to make them work.
I will check the suppliers you mentioned and see what is available.
Thanks Dick

 

[oldguy68]

I was able to get some NOS transistors locally after some digging. Score!
H706 2sa818
h707 2sc1628
h708 2sc1669
h709 2sa839
h714 and h715 2sc496
h710 and h713 ksc945c (digikey)
h711 and h712 ksa733 (digikey)
After replacement I powered up on the DBT and relay will not engage.
Left channel powers no problem.
I removed the outputs and tested on a trans tester. All the transistors were checked.They all test good .
I have changed the diodes again and checked for solder and grounding.
All resistors have been checked.
h701/702 and 703 are hfe matched and all test good
h704 and 705 test good as well.
Relay will engage when I disconnect the output cable leaving the board.
I also replaced the c711 8pf and c710 100pf with cog/npo types.
Is it possible for a bjt to test good and not work in circuit?
Where do I look next?

Here are some pics of the test harness that I use.
Not pretty but it works.

 

[KutzlerTrans]

You could try carefully documenting in order, all of your base voltages on your transistors to see if its the chicken or the egg that is starting the issue.

Does it click with the outputs removed?

 

[oldguy68]

I was just going to check the voltages and document.
The relay clicks with the outputs removed.

 

[oldguy68]

The voltages are wonky.
All readings done with the output cable removed at J710. The relay clicks when I remove this cable.
H716 cathode +20.6
H717 Anode -20.6
Then it all goes downhill.
H701 B -2.7
E-11.8
C-21
H702 B-12.1
E-11.8
C-21
H703 B+19
E+21
C-11.8
H704 B+19.6
E+20.2
C+20
H705 B-20.8
E-21
C+20
H706 B+59.6
E+60.5
C-30.1
H707 B-59.1
E-59.1
C-31
H708 B-30.2
E-20.7
C+61.2
H709 B -31.3
E-30.9
C-61.2
H714 B-30.2
C-30.2
H715 B-30.4
E-31.3
C-30

What do I look for now?

 

[1978_DREAMER]

Wow this looks very similar to my issues on my 2285B. I've already learned one thing from Catrafter's post so I'll keep tabs on your progress. Good Luck with this project!
Jerry

 

[mbz]

Thinking H707 is not doing his job, Vbe=0, tested? replaced?
Also voltage at H702 base, check trimmer R713, has it gone open?

 

[oldguy68]

H707 tests good with DMM and also off the board when I removed it the other day. Will check R713.

 

[catrafter]

Do you have the 'input' cable to the amp connected (J 701 RCA jack)? The input and output 'ground' are separate and the output will 'float' without it.

Tom

 

[oldguy68]

I have been testing with the input cable attached.

 

[mbz]

Almost seems like there are 2 problems. -30V to the right of H706,7 and -12V around H702 base, ok they are connected by R729(100k)
but H704.5c=+20V. Will watch with interest.

 

[oldguy68]

R729 checks at 100k
R713 tests ok off the board.
Voltage at R711 to pin one is +2.675
Voltage at R712 sb neg but is +.464
Reading at R710 is .444 with trimmer turned all the way to the left.It will increase turning it to the right until it hits 2.675.
This is the original trimmer from the board.

I would like to do as catrafter suggested earlier and remove h710 through h713 and hook up a dummy load but I have no signal through AUX . I checked my hookup cables and they are good. What gives??
The plot thickens.

 

[oldguy68]

Ok so I moved the left board to the right side and have DC offset a 0v + - and bias at 14.7
Will also compare boards side by side and take some readings.

Hooked up iPod to AUX and no signal
Hooked cables to tape 1 and sound but reduced. Same for tape 2.
Loudness changes by moving stereo switch to reverse or left/right/
I have cleaned the switches a few times now so ???
I will try to trace AUX signal path further. Have signal at mode switch and not sure after that.

 

[oldguy68]

2325 problem solved! Now I know that I should not try to do things when tired. After laying both boards side by side and going through all components I found that I installed H703 backwards.
Not to smart but in the end after changing things around and adding new transistors H706 and H707 everything adjusts and works as it should. (Stupidity works in mysterious ways!)

 

[dr*audio]

The take-away here is to put tape on your meter probe so only the very tip is exposed. Helps prevent shorting things out.