ca-1000 channel imbalance

[jkvw]

I have a ca-1000 in beautiful condition that I bought "for parts". Seller claimed that left channel didn't work. I've found and fixed a couple bad solder joints on the input board which seems to have fixed the complete drop-outs of one or both channels. However, a problem remains where at very low levels on the volume dial (0-2) the right channel is 30mV lower than the left channel. If I keep turning it up to > 4 the imbalance switches to the right channel being higher/louder than the left channel. My ears hear this and I can see it on an oscilliscope.

Yesterday I opened it up and cleaned out the relay. I also found a bad solder joint on the volume board plus sprayed the volume pot with D5. However the imbalance persists.

Now, if I turn the balance half a click to the right, things balance out at low-volumes so I could just try to ignore it (though it would irritate me). The problem is that if I turn up the volume while compensating with balance I'd have to remember to rotate balance to the left of center since the imbalance switches sides (which would really irritate me).

So... Any other ideas to check? I saw that avionic suggested to clean out the pre/power switch which I will try. But given that the balance changes with the volume I think the pot is showing it's age. Based on the service manual I think the volume knob is specific to the ca-1000. Does anybody know if it can be found in a cheap/easier to obtain model?

 

[jkvw]

On further inspection the right channel heatsink is warm compared to the left channel heatsink. TR604 is hot to the touch with just a light test tone playing. +50,-50 are correct and B+,B- are 51.8,-51.8. I've adjusted the main circuit board as described in the service manual. All voltages seem correct. When I switched from Normal to Class A for adjustment and then switched back there was some quiet crackling that eventually went away. Might still be something wrong with volume but seems like something is wrong in right channel.

 

[mbz]

Suggest using your oscilloscope a little more to track down the inbalance. Test at wires on the volume board. There are 4 sets of wires ie, 4x White(L), Black(GND) and Red(R).
eg, Yellow wrap set goes to filter board
Also test at the white wrapped set on the tone board that comes from the function board
Also test at Pre-OUT RCA's
EDIT: Expect the orange wrap set to be balanced, if the Red set is inbalanced then points to problem with Vol/Balance pots/wipers...

 

[jkvw]

Suggest using your oscilloscope a little more to track down the inbalance. Test at wires on the volume board. There are 4 sets of wires ie, 4x White(L), Black(GND) and Red(R).
eg, Yellow wrap set goes to filter board
Also test at the white wrapped set on the tone board that comes from the function board
Also test at Pre-OUT RCA's

Yeah good point, will try this. I was looking at the "In" on the power amp board too to test the incoming signal.

 

[mbz]

EDIT: Expect the orange wrap set to be balanced, if the Red set is inbalanced then points to problem with Vol/Balance pots/wipers...

Suggest start with this to confirm a pot/wiper issue. Yeah, dirty switches are an issue in vintage. The CA-1000 is worth spending some time on, nice amp though it's a PITA to work on.

 

[jkvw]

Suggest start with this to confirm a pot/wiper issue. Yeah, dirty switches are an issue in vintage. The CA-1000 is worth spending some time on, nice amp though it's a PITA to work on.

Seriously. The thing is like a transformer. I was going to do a quick recap to rule that out but other than the "electrolytic board" it doesn't seem that would be very easy.

 

[avionic]

When I switched from Normal to Class A for adjustment and then switched back there

Snap action switches may need replacing on the normal/class A switch assembly... CA- 800 & 1000 should not be switched " on the fly" , with power applied ,from normal to class A and back .It tends to pit the snap action switches.

 

[mbz]

There are many threads providing guidance of refurbishment of a CA-1000, because it's a PITA you should cover all problem areas in the one pass, eg, fusible resistors, tantalum caps,,, Removing the tone board (unsolder 4 sets of wires) makes things a little easier. Power amps, function and power supply boards are reasonably easy to do. I always accidentally break wires when doing the filter board.

 

[avionic]

Seriously. The thing is like a transformer. I was going to do a quick recap to rule that out but other than the "electrolytic board" it doesn't seem that would be very easy.

There built like a tank..

 

[jkvw]

Snap action switches may need replacing on the normal/class A switch assembly... CA- 800 & 1000 should not be switched " on the fly" , with power applied ,from normal to class A and back .It tends to pit the snap action switches.

Believe it or not I remember this discussion on my m-80 thread so I haven't been toggling back and forth frequently, but I guess you're saying never to switch modes while it is on? I just won't touch it for now.

 

[jkvw]

Yeah good point, will try this. I was looking at the "In" on the power amp board too to test the incoming signal.

The orange are balanced. The red is also balanced down to 44mV (which is about position 3 on the knob). Below that I can't measure with my cheapo digital oscilloscope. But considering that at higher volume positions I see the opposite problem on the speaker outputs and I don't see an imbalance in the orange wrapped wires, maybe that rules out the volume knob that I suspected?

Edit : Testing in progress so sorry for the fluid results, but I am seeing a 20mV difference on red set (not orange as I originally wrote) when volume is around 6 position and up. Orange is balanced.

Time for dinner But it does appear that at higher volume it is balanced in favor of the right channel. I would bet with a better oscilloscope I'd see the imbalance switch to left channel below position 2.

 

[mbz]

Expect the orange wrap set to be balanced, if the Red set is inbalanced then points to problem with Vol/Balance pots/wipers...

My bad, there is an error in the schematic re:wire colours.
The correct sequence is ORange wrap RI/LI into volume/balance pot, BRown wrap RO/LO from balance/volume pot.
So if orange OK and brown notok then issue with VOL or Balance pot.

I am seeing a 20mV difference on red set (not orange as I originally wrote) when volume is around 6 position and up. Orange is balanced.

. Indicates mode switch is ok. Problem could be in the tone board or volume/balance pot. Need to test at brown wrap on volume board. If brown is not ok then test volume pot at the two solder joints marked in blue. If brown is OK then need to look at the tone board, it may use 2SC1345 transistors which some members think are problematic/candidates for replacement. I've had problems with the turnover switches, needed cleaning.

 

[mbz]

If volume board brown wires are balanced then test at (Red/White) marked solder joints on tone board. If these are ok then probably an issue with the turnover switches, certainly work the switches several times and note any effect when returned to bypass positions.

 

[jkvw]

Brown is not ok, it shows random variances as i increase and decrease volume. Do you mean to test the resistance of the pot at those two blue x's or the signal level at the x's.

 

[mbz]

Do you mean to test the resistance of the pot at those two blue x's or the signal level at the x's.

All above tests are for signal level, test points are for L&R channels so expect equal levels

 

[jkvw]

One more question, test between the blue x and the pin pointed to by the red arrow? Or between blue x and the grounded pin on the right (or black wire).

I tested between blue x and red arrow pin and they are not balanced. Shows the same 20mV difference. Interestingly it is backwards. At full left of the volume pot I'm getting near full output at that pin and full right I'm getting near 0 output. Which is interesting because I previously couldn't measure a signal at full-left because of the min voltage of my meter. If I measure from center to right pin I assume I'll see the regular signal?

Edit : Measured between left pin (blue x) and right pin (connected to black wire/pin) I'm seeing the same signal level on both channels. Measured between middle pin and right pin, I'm seeing 10-20mV difference between the two channels.

 

[mbz]

One more question, test between the blue x and the pin pointed to by the red arrow? Or between blue x and the grounded pin on the right (or black wire).

You are using some sort of oscilloscope, hopefully at least 2 channels. Standard practice is to connect probe 1 GND clip to chassis/GND of amp, then connect probe 1 to one blue x and probe 2 to second blue x. Should be ables to see any differences in amplitude. The red arrows is a note for me to signify audio flow.

The blue x's are inputs to the balance control. If they are not equal the problem is earlier on, ie, volume pot (since orange wire is balanced). If the blue x's are equal then problem is with the balance pot.

 

[jkvw]

OK, the blue x's were balanced. Balance pots were my conclusion too. I think my only choices are to risk opening and cleaning the pot myself or to try to find a pot from a donor amp then?

 

[mbz]

If you remove/unsolder the pots from the board then you should have good access to spray cleaner etc...

 

[jkvw]

If you remove/unsolder the pots from the board then you should have good access to spray cleaner etc...

OK, I actually detached the board and knobs etc for my last round of tests. One thing I noticed is that the balance "gear" that the knob attaches to has a break in it, this break is not centered when the pot "clicks". The balance ring was attached so that it looks like it's centered, but if the break in the gear is an indication then it is off-center at the click. I wondered if the two pots are physically out of sync. I also noticed (and can see) that one of the pots hits a stop with a physical clunk in one direction but not in the other. I haven't opened many pots so not very familiar, but from initial inspection it seemed unusual.