CAS voltage not the same

[bqc]

I have measured that OA is 202 V  and OB is 215 V. The manual says they should be about 230 V.  The terminal 2 also is at 202 and terminal 14 is at 215 V. Could this be causing the volume imbalance issue on my Stereomour?

 

[Paul Birkeland]

Nope, these voltages will vary between 190 and 230V.

One easy way to get channel imbalance in your Stereomour is if the volume pot and balance pot are switched during installation.

-PB

 

[fullheadofnothing]

This is a Stereomour I, so no balance pot...

 

[Paul Birkeland]

Ah, yes, Stereomour 1. 

Another easy place to get an imbalance is in the wiring of the output transformers. 

If you are unable to resolve the imbalance when checking there, we can follow a 60Hz tone through your amplifier to see where the signal is getting attenuated.

-PB

 

[bqc]

Everything was working great for 3.5 years and all of a sudden, the right channel is louder than the left. I have swapped tube position, I have replaced both output tube. I have replaced the input tube. I have bypassed the volume knob, all to no avail. It seems to me that for something to go wrong like that without me changing anything, there must be some parts that go bad. I was told to make voltage measurements so here they are. I dont have a scope so would a DMV work for finding where the 60hz signal is attenuated? Where should I try to follow the 60hz signal? I have the supplied schematic

Voltage measurement with power applied to amp. No input signal

Term  Measured in V DC
Hv+        384
1            356
2            203
3            0
4            0
5            58
6            0
7            0
8            0
9            0
10          0
11          57
12          0
13          0
14          215.7
15          358
16          58
17          386
18          0
19          386
20          57

A1          59
A2          357
A3          8 mV
A4          59
C1          57.6
C2          358
C3          9 mV
C4          57.5

IA          386
IB          386
OA          202.5
OB          215.8

Kreg (both)    2.49
-reg  (both)    0

 

[Paul Birkeland]

One other element that I have seen before is that the cathode bypass capacitor gets disconnected over time, which will diminish gain on that channel. These caps are on the power supply board, and feed the circuit through the "K" pad on each side.

Otherwise, download a 60Hz test tone and feed it into input one of your amplifier with the selector set to that input and the level control all the way up. 

Now measure the AC voltage between ground and:
7 and 9 (2V scale)
2 and 12 (20V scale)
5 and 11 (200V scale)
16 and 20 (200V scale)
each red binding post (20V scale)

-PB

 

[bqc]

Ok here are the voltage measurements in AC for these terminals
Terminal    V
4              12.3
5              48
7                0.29
9                0.29
11            25.4
12              13.9
16                0.3
20                0.7

L binding post 0.54
R binding post 2.35
So it looks like terminal 5 and 11 are way off, and I trace them back to the output transformer. Could the left output transformer going bad be the cause of the volume imbalance? If so what part # can I replace it with?
Thanks

 

[Paul Birkeland]

5              48
11            25.4

You very likely do not have the cathode bypass capacitor in the circuit on terminal 11 side.  This the 220uF cap on the front of the board that feeds to the "K" pad and connects to terminal 20.  Be 100% sure that this wire is well soldered on both ends and not broken.  Also check the solder joint on the 220uF capacitor itself.

Could the left output transformer going bad be the cause of the volume imbalance? If so what part # can I replace it with?

This isn't very likely.  With the amp off and the high voltage drained off, you can measure the DC resistance between terminals 5 and 10 on each output transformer. 

What I have seen happen occasionally is that the center lug of one of the hum pots will get smooshed down into the chassis, which sets a very high bias current for the 2A3.  If this happens for long enough, the plate choke will get damaged, and gain and power will decrease.  To test for this, you will need to measure from HV+ to pin 2 on the 4 pin socket on each side of the amp.  (So DC resistance between HV+ and pin 2 on the left side of the power supply board/amp, then DC resistance between HV+ and pin 2 on the other side of the board/amp)  A damaged plate choke will have a significantly reduced DC resistance compared to a healthy plate choke. 

If the plate choke impedances are off by ten or twenty Ohms, that's nothing to worry about.  If one is a hundred Ohms less than the other, then you'll want to look at those hum pots and at the orientation of your 220uF/250V caps on the board to be sure everything is in order, then replace the choke.

-PB

 

[bqc]

So I checked the cable to goes from the K pad to term 20 and it is not broken and I resoldered  its 2 ends. The cap is also soldered well, but the problem persists. What else could be wrong? could that 220uf cap itself gone bad? there is no visible burn marks or any thing unusual on that cap

 

[bqc]

I swapped the 220 uf caps that go to the K pad between the Left and right channel and the problem is still there. I checked the choke resistance between the left and right channel and they are within 10 ohms of each other. So now what ??

 

[Paul Birkeland]

It's probably time for photos of the underside!

Another thing to measure is A3 and C3 for 60Hz AC voltage, as occasionally a carbon comp resistor can crack and fail.  Also, double check that the 249K resistors are actually 249K.  (not 249K and 2.49K, we have a 2.49K in the inventory that looks distressingly similar to the 249K)

-PB

 

[bqc]

Ok, I measure A3 and C3 for 60hz AC Voltage:
A3  13 VAC  ( right channel)
C3  14.9 VAC (left channel)
the left channel is the one that sounds weak. Is the voltage differential significant?

I also remeasure the DC voltage at V1 in the Schematic (the output of CAS)
right channel is 201 VDC
Left channel is 215 VDC,

Seems like there is a link between a larger DC voltage (215 VDC) on the left channel at V1 to a larger AC voltage at C3. Is there a link or it is just a coincidence and these differential AC voltage at A3 and C3 are not significant?

I also checked the 249 K Resistor Rg and it is indeed 249K for both channel.

 

[Paul Birkeland]

That's about 1dB of difference, not completely the end of the world.

The signal level difference coming out of the 2A3 is more like 5.5dB, which is quite bothersome.

Can you disconnect the red wires at 5 and 11, then recheck the AC voltage?  When you lift those wires, bend them so they are poking straight up, as we don't want them touching the chassis plate.

 

[bqc]

So I disconnect the red wires at 5 and 11 and here are the measurements of AC voltages:
A3  13  VAC
C3 14.5 VAC
5    50 VAC
11  59 VAC

So A3 and C3 has same voltage as when the red wires were connected.
Terminal 5 has same as before, terminal 11 (left channel) was 25 VAC and now 59 VAC when the red wire is disconnected from 11. Does this point to something wrong with the output transformer?

Thanks

 

[Paul Birkeland]

This points to the output transformer loading down the output stage.  This can happen if the jumpers on that channel are not properly installed, or if one tab is touching ground.  You can also check resistances of one transformer's windings against the other, though you would be the first person to have that particular issue.

 

[bqc]

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

 

[Doc B]

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.

 

[bqc]

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?

 

[Paul Birkeland]

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.

 

[bqc]

here are the pictures.

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