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January 20, 2021, 09:08:06 PM

Author Topic: CAS voltage not the same  (Read 2883 times)

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Offline bqc

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Re: CAS voltage not the same
« Reply #15 on: September 14, 2016, 07:48:53 PM »
I checked the jumpers for the 8 ohm secondary and they are correct. There are no tabs touching ground. I measured the secondary winding to be 0.6 ohm for the left channel and 1.1 ohm for the right. Is that significant?
The primary winding for the left is 218 Ohm and 215 for the right.
Shall I try to rewire the secondaries as 4 ohm and see if they work any better?
If the output transformer is bad, what is the spec for this transformer and do you know of a compatible transformer model that I can order from places like Digikey and mouser etc...

Thanks

Offline Doc B.

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Re: CAS voltage not the same
« Reply #16 on: September 15, 2016, 05:07:29 AM »
The secondary measurements are a bit of a concern - if they are accurate. It is pretty difficult to get good readings of resistances down around 0 to 1 ohm because oxidation or other contaminants on the tips of the test probes or the test point can throw off the reading. I usually burnish the tip of the probe against the test point to try to get through any schmutz.

If you continue to get those different readings with the probe tip and test point making excellent contact it's time to look very closely at your secondary connections and speaker binding post connections. The likelihood of the output transformer being defective is very low. I can't recall a bad output transformer from our winder in the past 15 years. So it would be prudent to inspect everything connecting to the transformer secondary very carefully before making that judgment.
Dan "Doc B." Schmalle
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Bottlehead Corp.

Offline bqc

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Re: CAS voltage not the same
« Reply #17 on: September 15, 2016, 11:14:55 AM »
I made sure the tip had good contact. It started out to be 3 ohms or so and slowly settled to the values reported. What is the expected normal range for the 8 ohm secondaries?

Offline Paul Birkeland

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Re: CAS voltage not the same
« Reply #18 on: September 16, 2016, 06:28:42 PM »
Normal DC resistance across the secondaries will be so low that it's difficult to measure reliably.

It sure would be helpful to see photos of the build, if at all possible, especially of the output transformers and speaker posts.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline bqc

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Re: CAS voltage not the same
« Reply #19 on: September 18, 2016, 12:58:43 PM »
here are the pictures.

I also rewire the left channel secondary as 16 Ohm but the voltage measurements are the same as before.

Offline Paul Birkeland

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Re: CAS voltage not the same
« Reply #20 on: September 19, 2016, 05:31:27 AM »
How is your supply of clip leads?
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline bqc

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Re: CAS voltage not the same
« Reply #21 on: September 19, 2016, 06:37:42 PM »
not too many, why, what do you have in mind

Offline Paul Birkeland

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Re: CAS voltage not the same
« Reply #22 on: September 23, 2016, 06:10:12 PM »
I'm thinking that you could remove the secondary wires from both output transformers (just take all of them off, the secondary wires are the jumpers and the twisted pair that heads back to the speaker posts).

Next, recheck your AC voltage measurements.  If they are still off, power down the amp, then remove the red and black wires at output transformer terminals 5 and 10 on both transformers.  Take a pair of clip leads and clip lead each output transformer into the other channel, then repeat the measurements.

Just as a reminder, the AC voltage measurements should be taken at terminals 5 and 11.

-PB
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline bqc

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Re: CAS voltage not the same
« Reply #23 on: December 27, 2020, 08:19:34 PM »
I finally got around to do this output transformer swap, the left output transformer is wired to the right channel and vice versa, and found that the issues is now reverse.  While a 60hz signal is fed to the input, terminal 11 (left channel) AC voltage now is twice the AC voltage of terminal 5. So it looks like the left channel output transformer is bad as wiring it to the right channel cause the AC voltage of the right channel to drop to half of the left channel.  So it looks like I need to replace the left channel output transformer. I am also thinking of converting this Stereomour to use type 45 tubes. I read that a 5K primary OT will be a better match to the 45 . In that case what type of OT do you recommend that would fit in the space of the old OT.

Thanks

Offline Paul Birkeland

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Re: CAS voltage not the same
« Reply #24 on: December 28, 2020, 05:45:11 AM »
Can you post some photos of your build?

Your first hypothesis was a bad plate choke, and now you've concluded that you have a bad OT. 

Could you put things back and take the measurements I requested? 

For what it's wroth, we have never had a defective Stereomour output transformer, other than a few that were put in the channel frames upside down about ten years ago.  The only plate choke I ever had to replace was due to a wire touching the chassis elsewhere that caused damage to the plate choke. 

I'm not aware of a suitable 5K OT that will drop in, but 4K works nicely.  4K is generally not recommended because there are so few 4K output transformer options out there.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline bqc

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Re: CAS voltage not the same
« Reply #25 on: December 28, 2020, 05:51:01 AM »
The pictures of the build are in post 19. If you needed more please let me know. The measurements you requested are:
AC voltage at K terminal for left channel is 0.763V  and right channel is 0.722V . Voltage at 20 is 0.763 V  and at 16 is 0.722 V

Offline Paul Birkeland

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Re: CAS voltage not the same
« Reply #26 on: December 28, 2020, 06:26:54 AM »
Plate choke DC resistances?
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Paul Birkeland

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Re: CAS voltage not the same
« Reply #27 on: December 28, 2020, 06:34:00 AM »
From the photos:

2U looks like it could use a little soldering help.

Terminal 3 on one output transformer looks to possibly not be soldered.

1L looks like it needs some more heat/solder.

The cathode bypass caps mounted above the board won't do you many favors down the road.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline bqc

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Re: CAS voltage not the same
« Reply #28 on: December 28, 2020, 07:17:54 AM »
Plate choke DC resistances?
Left channel. 556 ohms, right channel 553

Offline Paul Birkeland

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Re: CAS voltage not the same
« Reply #29 on: December 28, 2020, 07:18:41 AM »
That demonstrates that the plate chokes are not damaged and don't need to be replaced. 

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man