9090db bias issue

ranukic

Active Member
I was able to zero the voltage but I cannot get the bias adjusted above 20mA.

I rebuilt the F-2624 board replacing
R33/34 = 180ohm 1/2w
R39/40 = 10ohm 1/2w
R41-44 = 150ohm 1/2w
R47-50 = 4.7ohm 1/2w

TR1-4 = KSC1845

C01/02 = 0.47/50V audio grade nichicon
C05/06 = 470/6.3V audio grade nichicon
C07/08 = 330/63V audio grade nichicon
C09/10 = 100/25V audio grade nichicon

D01-04 = 1N4148
D05/06 = 2X 1N4148 in series
D07-16 = 1N4148

Zd01/02 = 1N525B

STV-3H = 3X UF1004 in series

I verified the readings of the following resistors that I did not replace
R21/22
R25-28
R31/32
R35-38

I also have a spare F-2624 board that I am getting the same readings with so I have confidence that the issue is not on the boards.

Does anyone have any suggestions for where I should check? Did I make a similar substitute mistake on both boards?

Also, does anyone have what the readouts should be on pins 05-28 where the amp board plugs into the power supply? I have found a few but I would like to record all the readings onto the schematic I have.

Thanks
 
no DBT.

Pin DC voltage measurements from this morning
28, 58.4V
27, 61.8V
26, close to 0mV
25, 0.557V
24, close to 0mV
23, close to 0mV
22, -0.567V
21, -61.8V
20, -61.8V
19, -61.8V
18, -3.3mV
17, -3.1mV
16, -61.7V
15, -61.8V
14, -61.8V
13, -0.557V
12, close to 0mV
11, close to 0mV
10, 0.5625V
9, close to 0mV
8, close to 0mV
7, close to 0mV
6, 61.8V
5, 58.4V
 
those voltages look OK. There have been two instances that I know of in the last couple of years when guys have installed wrong values in positions R47 thru R50. In one case it was 4K7 and another 4M7. They couldn't get bias also. Just confirm you have 4.7 ohms in there...
 
Yeah I found those posted then double checked last night and triple checked again just now. All read 4.7 ohm.

As I mentioned I have (2) boards one that has good original TR09/10 installed and the other I had to replace the STV-3H. Both are reading the exact same so I was thinking it had to be on the power supply.
 
You do have the small input connector at the top of F2624 connected right? It needs the ground connection from the tone board to work properly.
 
Soooo it looks like I must not have tried the second board last night like I had thought. I must have put the same board in twice but in my defense I had a long busy day at work yesterday.

I placed my backup board (has the TR10/11) transistors mounted. This board dials right in on voltage and bias. So I am going to pull the 3X UF1004 in series from the board and tnsilver’s suggestion and find some STV-3H-Y to install.

I did a quick search on EBay but I didn’t see any offered from a German supplier.

Thanks again
 
This diode's cathode is marked with a dot. On the F2624 the dots are pointing inwards. It hooks up like this:

YxNM3aV.png


J8NnO8w.jpg


EDIT: It's cathode to base anode to collector.
 
Last edited:
On the board itself there are markings for a transistor (B, E & C) and that is where the diode gets attached.

The board I have has the Emitter through hole soldered closed and it appears to be factory done. That leaves the Base and Collector through holes open.

The diagram picture you just posted appears to indicate the Emitter is where the cathode should attach and the Collector is where the anode should attach. However looking at the actual board picture you just posted it looks as if both legs are attached very close in proximity indicating that the Base and Collector through holes are where that diode is attached. Would you have a top view photo showing the board markings that can be attached?

I appreciate your help and understanding that I am trying to avoid letting the mysterious smoke out.
 
This is the best I've got in terms of stock silkscreen... I think you're right - it's cathode to base.

YTKPQtc.jpg
 
Thanks. Let’s hope my gut is right. If it is I will post a picture for future reference. If not, well then I will be replacing more components.
 
Last edited:
No, you were right the first time, I've missed the first sketch (which I've fixed). The photos don't lie, that's how it is. Cathode to base, anode to collector.
 
Back
Top Bottom