AU-999 Restoration & Modification

[leray1]

I got the ZTX694Bs in today and installed them, but I'm getting bright light on the DBT still. I looked over each board and noticed on one I had TR803 and TR807 reversed. I corrected that but still get the bright light. I didn't put the new trim pots in, but I turned the two originals both counterclockwise all the way and still get the bright light.

My time is limited this evening, but I'm open to suggestions.

 

[Hipocrates]

Double check, and tell us with detail what you got there. Remember the pinout.


TR801 --> ZTX795A PNP
TR803 ---> ZTX795A PNP
TR805 ---> ZTX694B NPN
TR807 ---> ZTX694B NPN
TR809 ---> leave alone
TR811 ---> ZTX694B NPN
TR813 ---> ZTX795A PNP

 

[leray1]

Here's my list of what went where and my crude reference drawing.

 

[Hipocrates]

 

[leray1]

I have them all wrong?!!! I do!

I used this drawing. I completely misread it as if I was looking at it from above. Well, that would explain it. What a dumb mistake.

I feel like an idiot but happy. Thanks!

Does soldering and desoldering transistors tend to ruin them?

 

[Hipocrates]

Does soldering and desoldering transistors tend to ruin them?

nah, those transistors are very resilient., still if I have new one I use them and save the ''used ones"

 

[slimecity]

Does soldering and desoldering transistors tend to ruin them?

It depends on how much heat you put on them. The aim is to minimise cooking them

 

[leray1]

It was hard to fall asleep. And I woke up an hour and a half earlier than normal and couldn't go back to sleep thinking about this. But if this is what keeps me up at night, I'd say life is good. So....

I corrected the orientation of the transistors on one board, bright light.

For the other board I corrected the orientation and put new transistors. The 100W bulb pulsated at first and gradually shone not real dim, but not full bright. I changed out one complimentary pair on the first board with new transistors, tested it, and got the same, pulsing to gradually shining dimmer than full bright. I got the same thing with both boards installed.

Is this ok? Should I put in the new trim pots and set bias and offset?

 

[Hipocrates]

Should I put in the new trim pots and set bias and offset?

It is a must on this units.

 

[leray1]

On my lunch hour, I went home and installed the new trimpots and roughly adjusted the offset and bias. The bulb stopped pulsing and it shined dimly. At least, dimmer than full bright that it was doing when I had the transistors installed incorrectly. Am I good to go?

Should I fine tune the bias and offset with or without the DBT?

 

[Hipocrates]

Should I fine tune the bias and offset with or without the DBT?

I do it with the DBT first, a let it for a while to see if is stable... after that, I adjust it with out, specially on this unit without protection circuit.
Others might do it without the DBT.

 

[leray1]

DC offset is steady ~10mV on both sides. But, with the VR803 bottomed out, the BIAS is ~52mA and ~56mA. Won't go lower. ????

BTW, amp's been on for about 25 minutes, bulb pulses were due to high bias. Bulb is steady with the bias lowered, but can't get lower than ~50mA.

 

[Hipocrates]

How are you testing BIAS? On the emitter resistors or as the manual states?

 

[leray1]

Emitter Resistors

 

[Hipocrates]

OK take a look of this picture by pete_mac on his restoration.

you should aim for 14mv (millivolts) not mA


Thread:

http://audiokarma.org/forums/index.php?threads/another-au-999-restoration.628891/

 

[leray1]

I can't get one lower than ~190mV and the other is at ~120mV. What's weird is that I adjusted the one that's currently at 190mV to 14.2mV, moved to the other one, couldn't get it lower than 130, went back to the first one and it was up to ~190 and it's not going down. I'm holding the probes on the leads (I have clips but would have to trim the sleeves).

Any ideas?

 

[Hipocrates]

Any ideas?

Ok, so we have this situation, just to be sure.

- DBT with 60W Oscillates
- Bias can't be Adjusted.

Check TR809, for broken pads, cold solders or similar stuff. if everything if correct, I would test those To-66 transistors, be sure they are isolated from chasis.
post nice resolution pictures, I use imgur or I can give you my email via PM

 

[leray1]

TO-66 tranistors are definitely isolated on the left. on the right, if I place the shaft of the probe on the bare section of the heat sink (where it was cut) I can get continuity, ohms stays at 0.000 but different values flash for less than a second sometimes. I don't get it anywhere on the chassis.

 

[kevzep]

Wrong transistors, wrong orientation, power supply polarity wrong...
Couple of things that come to mind...

 

[AnalogueBen]

The other guys will offer you the technical help you need more than me, I can however share things I'd look for..
Use a magnifying glass and go over your boards to look for and solder bridges or cold solders etc. And I'm going to assume you used braid to desolder, sometimes this breaks and leaves very tiny strands stuck in the solder and cause potential shorts, look for that. Follow the tracks with a bright light looking for damage, check top of board too for debris. Broken or eathed wire maybe.
Go back over the photos you've taken from the beginning as you may stumble upon something amis.
I've been sure on things before to then find out that I didnt evaluate reliably, so even try drawing what you see with your transistor arrangement as a problem may literally just appear.
Good luck