Sunn 200s multicap can, etc. replaced but showstoping problem persists

[kupervaser]

Спасибо.

First of all after I changed some of those caps the amp worked. I Even gigged with it without problems. The first time it started blowing fuses when I put in the new power tubes and the rectifier tube. The fuses started to go. So I put in the old power tubes and the old GZ34 in it but it still blew fuses. I put the amp away for some time and than I still wanted to fix it. Took it out of the storage room and tried a new fuse. It didn't blow, but did blow after I took it to the rehearsal, and is blowing fuses right now all the time accept if when I remove all the tubes including the rectifier tube as well. I concentrated on the PT because as you see it blows fuses when using the mains switch a few times repeatedly. I didn't know about the thermistor.

[Enzo]

I don't see an inruch limiter, but the fuse is clearly labeled as 3A SLOW BLOW. If you are using plain old 3A fast fuses, I don't doubt they blow all the time.

[kupervaser]

I use 3A Slow Blo fuses. Still blowing, but the voltages on the secondary windings seem to be fine. Should I try 4A fuses maybe?

[Tom Phillips]

I use 3A Slow Blo fuses. Still blowing, but the voltages on the secondary windings seem to be fine. Should I try 4A fuses maybe?

No. Using a higher current rated fuse will not solve the problem. Something must be seriously wrong to blow a 3 A slo-blo fuse with all the tubes removed. There is either an internal transformer problem or there is an external problem causing an excessive current draw.

[kupervaser]

Ok what I will do now is I will wire the transformer back and start testing again, cause I start to forget when exactly the fuses die.

Another thing, when I got this amp the output transformer was disconnected and some wires were cut, anyway I sorted everything out, I also did some measurements on the transformer and I believe it is in a proper working order. So I wired it back on but instead of 4 and 8 ohm i need 8 and 16 ohm output.

Originally the wiring is 4 and 8 ohm and the 16 ohm tap goes to 2 resistors, 680 ohm and 1K.


THis is how I connected it:



Is this correct?

[Tom Phillips]

Ok what I will do now is I will wire the transformer back and start testing again...

I recommend that you go through the following sequence to help determine if the PT itself is OK.
1) Start with all the secondary windings open. Don’t connect them to anything.
2) With no primary voltage applied and the amp unplugged measure the resistance between each winding and the transformer core / mounting bolt. You should read near infinity (Very very high resistance. AKA ∞) OK? What do you measure?
3) Measure the resistance between one lead of each winding and one lead of each other winding. Again you should read near infinity.
4) Install the fuse and power the primary with 120 V and leave the transformer on for several hours. If the fuse blows or the transformer gets hot then you know that the transformer has internal problems and needs to be replaced. It is normal for a power transformer to get warm under these conditions. Typically the temp would rise ~20˚F over the room temp. If you have the capability to safely measure the primary current draw you can measure the transformer’s magnetization current. The exact value varies for each transformer size and design quality. If you measure more than ~250 mA then I’d say you have a bad transformer. You can do this measurement right after power on (with open secondaries). No need to wait. Let us know the results. Ask more questions if you need additional info. Don't do anything you don't feel comfortable and competent about.
5) If the PT passes the above tests then you can start hooking up the secondaries one at a time and testing. I’d start with the 5V, then add the 6.3 V, then the HV.

Regards,
Tom

[kupervaser]

Thanks for the walkthrough.
First of all I went through all the steps, and it looks fine.
I think i could have found a problem.

Let's look at the schematic on the HV secondary winding. There are 4 wires. I think some wires were mixed up. When I got this amp, I had to remove the transformer to clean it and to paint the transformer end bells. I made pictures before doing so and as far as I remember I connected everything the same way back. At this moment I think either I made a mistake or someone before me, here is why. Here are the measurements between the wires of the different taps of the HV secondary winnding:

6 (RED) - 4 (RED) - 751 VAC
6 (RED) - R-BL (going to 1N2070 diod) - 317 VAC
6 (RED) - R-Y (going to ground) - 374 VAC

As far as I understand it should be:
6 (RED) - R-BL (going to 1N2070 diod) - 374 VAC
6 (RED) - R-Y (going to ground) - 317 VAC

Is this correct?

I also measured the voltages on my working Sunn Solarus head and these measurements back up my way of thinking.

[Tom Phillips]

Good job posting the schematic. Very helpful to have it handy to support the discussion.

I don’t understand what you are describing as a problem or what it is that you changed or propose to change. Your description of the HV wiring is just from a different perspective than the way I think about it which is:
The red wires are the ends of the HV winding. The R-Y wire is the center tape of the HV winding. The R-Y wire is grounded in a full wave rectifier circuit like the one used in the 200S. The R-BL wire is most commonly referred to as the “Bias Tap.” It is around 55 to 60 volts AC with respect to the R-Y center tap and is there specifically to feed a bias power supply circuit. If you measure the AC voltage between the R-Y and the R-BL wires you should see about 60 V plus or minus a bit. The diagram for the transformer secondary in the 200S schematic is not drawn proportionally correct so it is harder to understand the situation.

As long as the R-Y wire is grounded and the R-BL wire is connected to the cathode of the bias supply rectifier diode things should work fine unless there is some other problem. If you are wondering it does not matter if you swapped the red wire connections one for the other.

HTH,
Tom

[kupervaser]

Ha ok, I understand now. Than it has been connected correctly. Well at this moment with all the secondaries disconnected the 3A fuse does not blow, even if I repeatedly switch on and off. I measured all the secondary windings:

pin 2 - pin 8 of the rectifier tube - 4,64 VAC
Heater wires - 6,0 VAC
HV winding - 751 VAC

One more little detail. At this moment with all the secondary wiring disconnected the 3A fuse does not blow, but If I put in a 1A fuse, it sometimes does blow. It happens randomly. Sometimes it blows directly when I flip the mains switch, sometimes it doesn't blow directly, but will than blow if I repeatedly switch on and off a few times.

Should I start connecting the wiring back again, starting with the 5 VAC on the GZ34 than see if the fuse blows and than start connecting the heaters, etc.

BTW: just checked and indeed there is around 57 VAC between R-Y and R-BL wires.

[Tom Phillips]

Did you measure the magnetization current as described a few posts ago?

[kupervaser]

So by this you mean I should connect my DMM in series with the mains and measure the current draw?

[Tom Phillips]

So by this you mean I should connect my DMM in series with the mains and measure the current draw?

Basically yes. When I do this I bring the voltage up slowly with a variac or use a light bulb limiter if I'm concerned about a large current surge. For known good transformers I just switch it on. I'm just sayin' what I would do and trying to keep you and your equipment safe. It's hard to strike a balance between those two situations. Just don't do anything you aren't comfortable with.
If the amp were on my bench I'd want to know that the magnetization current was below an acceptable value before I started hooking the secondaries back up as discussed in post 21 of this thread.

If the transformer didn't get overly warm (Also discussed in post 21) then it should pass this test.

Cheers,
Tom.

[kupervaser]

Tom. I don't own a variac, I do have the bulb lamp current limiter. I can try to use it, but than, I eventually I would have to test it without the bulb lamp to find out the real current.

As for the DMM, will I not damage my DMM with this test? I am comfortable with doing any tests, I am extremely cautious and I always make sure to do everything correctly.

P.S, the inrush current is larger than 1A I suppose, because all of my 1A fuses blow. Does this say anything about the PT?

[Tom Phillips]

My comments:

A PT of that size will blow a 1A fast blow fuse during the turn on surge. Multiple ON-OFF-ON cycles timed close together will stress and then blow a 1A Slo-Blow fuse so that is normal behavior.

This type of work hasn’t damaged my DMMs. Mine have internal fuses in the current measuring circuit and I have blown those when I made careless mistakes such as forgetting that the leads were in the current position and then trying to make a voltage measurement.

[Tom Phillips]

Jeff,
Given all the evidence. It seems as if the PT is OK so I wouldn't worry about doing the magnetization current test. That's just a little extra curiosity now. The fact that the PT doesn't heat up when left on for a long time tells us that there are no internal shorts.
Tom