My Aleph is freezing

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Well , no luck on my first turn on. I am getting 35v output on the speakers , 70v between GD and SD on the bottom half mosfets and 1.5v between GD and SD on the upper. I am not getting any voltage on Z5 or on the two ZTX450 ,even on the source resistors or anywhere else that is noted in the service manual. I have checked transistor orientation. I still am not sure on zener orientation. I understand the black band is at the anode or not ? Everything is quite cold despite the voltages above. I guess there must be a serious mistake somewhere. I thought that I would get part of the circuit at least working ,the 1st and output stage are reasonably independent or not? Shouldn't Z5 measure correctly independently of everything else if it is orientated properly ? Shouldn't the output mosfets warm up?
I guess there must be a serious mistake somewhere.
Clearly this must be so .... says he sounding like a "smart-****".

Actually, this is easier to do than you think. I am currently building an Aleph5 to replace my A4, given the later was built "on the cheap" and looks a little ordinary.

Fired up the modules on Sunday and Vgs across the current source FETs = 0.

Answer .... I had misread R19 as 221 instead of 221k ..... DOH!! :eek:

Go back and carefully recheck all component values, the circuit diagram etc. If all appears OK, then find a friend to check this for you ..... it can at times be hard to see your own error, when another will see it in 10 foot neon!



PS: See -
Man, Ifeel really bad. I got both channels working and then...... I blew both of them up!!
Here is what happened: I had the first channel working properly for about 15 minutes and then it failed.I decided to set up the other channel to see if something was wrong with that as well. It worked and measured perfectly electrically and temperature wise. In fact I connected it to my main system for about 20 minutes and it sounded great.I switched it off to listen to my resident amp for a quick a/b and when I switched it on again it blew! I can't tell you how frustrated I felt.
After speaking with Kostas Vazakas whose design I copied (its in the Pass diy gallery) it seems that there may have been a problem with the following : The board I made has provision for extra protection in the form of two fuses on each supply rail.I had put two 2.5A fuses there and in both cases one of them blew at some stage and then after replacing them it seemed the amp had failed completely. This might be a case of too much protection damaging the amp. If the fuses were at their limits (I am using a X4 type output mosfet configuration keeping all other values as in the schematic) and the amp drew a little more on one rail than 2.5A -like on switch on- the fuse would blow putting more strain on the other side mosfets and damaging them. Anyway this the theory because at the moment there doesn't seem to be any other explanation of why both channels were working properly and then decided to leave this world!
Any other ideas ?
It appears that you have a different problem for each channel. You did not describe in more detail the problem for the 1st channel but you did describe briefly for the 2nd channel. I'm not sure exactly sure what's the sequence of problem but let me see if I can provide some ideas for troubleshooting.

The 2nd time you turn it on, one of the 2.5A rail fuse blew. You then replace it with a new rail fuse but by then you found out that the amp went completely dead. My guess - and this might be too simple of an explanation - is that the 4A line fuse has blown (due to extra load from just one 2.5A rail fuse?). Also, did you use slow blow 4A line fuse?
I guess too much protections such as many fuses are not so convenient,unless you calculate values large enough to keep little margin for overbias times:(

And remember that the active current source might get AC current draw up to 4 A ,so might be better to put some 4 A fuses for the rails and some 5A on the mains ;)

Moreover the inrush suppressors on mains is also very useful to avoid fuse blowing problems:p



PS my Aleph 4 has no fuse on mains but power thermistors ,and the V+ rail has only a 70°C breaker,mounted on the current source heasink:eek:
Fuses on rails might be actually dangerous for the speakers in some cases (when one rail is off). From the schematics I've seen so far, nobody's really using fuses in that place anymore. They also create bottleneck at that point, which is really not desirable from sound quality perspective.
I only use one fuse on AC connection to an amp, plus inrush thermistor (although I was not always happy with thermistors performance), and never had any real problems. The fuse on transformer's primary is enough to protect your whole amp. Remove the rail fuses and maybe your amp will come to life again.;)
No the line fuse is fine its one of the rail fuses that blew and the amp is outputing 32v dc and is quite cold. One of the ztx450s is not at the right voltage anymore. I had a similar problem on the first amp although there is no guarantee that it is exactly the same. One of the rail fuses blew while I was connecting a speaker although I am pretty sure that I didn,t short it. Afterwards I replaced the fuse with a higher one and I got a spark from one of the output fets. I repkaced the faulty fet and that side seemed to be getting hot but not the other side and the other also did not have any voltage on the ztx 450 (q4). So I then replaced the ztx but no change no real voltage etc. However I noticed that only one of the fets seemed to get a bit warm out of the four on that side. The other side seemed to be generally quite hot with about 660mv on the 1 ohm resistors. Anyway I was a bit stumped so I decided to fix up the other channel. It worked really well with about 480mv on the output source resistors and all the other voltages were according to the manual. The temperatures seamed quite evenly spread around and the heat sinks were not above 55C. I measured the fets on the metal part and the highest reading was on the drain leg with about 80-85C while the heat sink right next to it was 65C. Is this the correct junction to heat sink difference? Output was 29mv dc offset. As I said it worked quite well under testing and sounded great playing music. But after the switch on the rail fuse blew -not the mains 2.5A 220V fuse and now it seems to be completely dead even after replacing the fuse as described above. I guess I have to take out all the power fets and check them. I don't have enough matched ones to replace them properly. I don't think the problem is with the ztx450 because I understand that it doesn't work correctly if one of the output fets is blown .As I said I have replaced one already and it doesn't seem to make any difference.How do you check the fets by the way? Do you put them in the Pass matching circuit and check voltages or do you compare ohmic values?

Looking at the schematic, it appears that if the +ve rail (Current Source) fuse blew, the Gain Source circuit would still work without having the whole amp going into dead mode.

So, in your case, I'm pretty sure that it's your -ve rail (Gain Source Circuit) that's causing the problem (hopefully your Current Source circuiting is o.k.)

May I suggest you troubleshoot the following way:
1. Check your Gain Source circuiting such as capacitor and diode polarity, Q1, Q2, Q4 "orientation", and double check the wiring that goes to the output gain transistors.
2. Correct the improper installation, if any, and then start troubleshooting the components itself. Replace all if you want to make life easier ...... since there is not too many components anyway to start with. (That's one thing I like Nelson Pass design ... there is hardly any parts!!).
3. When you're ready to power up again and if you don't have a Variac, hook up a 100W light bulb in series with the main AC lines. The light bulb may not tell you if the amp is working or not but it will allow you to take some meaurements on some critical points as indicated on the service manual. As I have indicated elsewhere on this forum, with the light bulb in series, you'll not get 34V on the two rails but you should be able to get a reading of ~4V between the Drain of Q3 and the Source of Q1.
4. If the voltage reading is not even close, do more troubleshooting. There is a possibility that there is a short on the Current Source too. So, do some troubleshooting there too.
5. Do not disconnect the light bulb until you're comfortable with the measurements.

Please keep us informed on your progress.
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