Yamaha CA-800 — Low frequencies distorted on left channel.

[Rob Jennings]

The bottom of the heat sinks on TR701 and TR704 were originally glued to the power supply board. What kind of glue should I use to do that? I see lots of guidance on gluing heat sinks to ICs but I'm not clear on what glue to use to attach heat sinks to circuit boards. These heat sinks are live and also get quite hot (obviously).

 

[avionic]

These heat sinks are live and also get quite hot (obviously).

If the sinks have screw fasteners , then they don't need any glue. If not. I use Permatex Ultra Black. Low odor , electronics safe gasket sealer.

 

[Rob Jennings]

The heatsinks are screwed to the transistor but I'm looking to glue the bottom of the heatsinks to the PCB to secure everything. That's how it came stock and without that glue the transistor / heatsink assemblies are floppy / unstable.

 

[avionic]

The heatsinks are screwed to the transistor but I'm looking to glue the bottom of the heatsinks to the PCB to secure everything. That's how it came stock and without that glue the transistor / heatsink assemblies are floppy / unstable.

Sometimes Yamaha screws the heatsinks to the circuit board as well as fastening the transistor to the heatsink.

 

[avionic]

The heatsinks are screwed to the transistor but I'm looking to glue the bottom of the heatsinks to the PCB to secure everything. That's how it came stock and without that glue the transistor / heatsink assemblies are floppy / unstable.

Just a little dab will do ya..

 

[Rob Jennings]

My Mouser package was delivered today so I replaced TR702 and TR703 this evening. I'm happy to report that the hum and intermittent static issue has now resolved. I very much appreciate everyone's help. To recap, I ended up replacing transistors TR701, TR701, TR703 and TR704 as well as fusible resistors FR703 and FR704.

I did test the B+ and B- voltage and found that B+ was at 52.3vdc. Is that an issue and if so, what could be the cause? B- was closer to spec at 50.2vdc.

If you recall, I originally set out to address some lower frequency distortion on the left channel. It was recommended that I replace the relay on the power circuit board, which I did but somehow managed to short something which ended up causing the issue above. With that issue addressed, I was able to run a test to see if replacing the relay resolved the lower frequency distortion. My kids are asleep so I couldn't test at my preferred volume but I do think the lower frequency distortion in the left channel has been resolved, which is also great. I'll test again tomorrow morning when I can turn the volume up much more and really make sure that the distortion is gone. Thanks again everyone, particularly avionic and mbz.

But before I put this amp back into daily use, I just want to check on a couple of other things:

1. I'm pretty sure the amp hiss at idle is louder than I remember or maybe I'm just noticing it for the first time. It's not a hum, just a louder version of the hiss you normally hear from speakers.
2. This is not new, but there is a channel imbalance. The left channel is louder than the right channel by 10 to 20%.

I'll be searching the forum but would appreciate any guidance on these items.

 

[mbz]

I did test the B+ and B- voltage and found that B+ was at 52.3vdc. Is that an issue and if so, what could be the cause? B- was closer to spec at 50.2vdc.

Assume you are talking about +/-B pins on the relay/psu board which connect to the +/-50 pins on the power amp boards. While the +/-B pins on the power amp boards come from the main caps (unregulated supply)

For me it's borderline, I'd simultaneously monitor both voltages to rule out any mains bounce.

Factors that affect the +B voltage are,
- FR704 resistance, ie I x R volt drop (measure resistance FR704, incircuit is ok, compare with FR703)
- TR703 base voltage set in part by zeners D704,6 (measure Vdc at D706 cathode, think I've got those voltages somewhere...)
- TR703Vbe slightly different to OEM?
- TR704Vbe slightly different to OEM?

There could also be some cap leakage(C710?), the psu/relay board should be recapped, Nichicon UPW(4x220uf/100V,,,), UEP gets the job done.

 

[mbz]

1. I'm pretty sure the amp hiss at idle is louder than I remember or maybe I'm just noticing it for the first time. It's not a hum, just a louder version of the hiss you normally hear from speakers.

When new the amps (CA-800/1000) had a very low noise floor possibly due to (low leakage) polar tantalums used in pre and phono audio path. I've worked on a few of these amps and they all seem to have some hiss though I sometimes play through moderately efficient (spl=102dB/1/1) spkrs. Some of the small signal transistors have some history of going noisy like 2SC1000 and 2SC1345 (depending on serial number) and a few others. If you don't hear the hiss when seated in your normal listening position then I'd let it be. If ever you plan to do a full rework then include pre/phono small signal transistors

2. This is not new, but there is a channel imbalance. The left channel is louder than the right channel by 10 to 20%.

Switches have been cleaned so possible non-linearity in volume pot?
With amp powered off/unplugged, volume set to normal listening position, measure the resistance between the ri pin on the volume board (red wrapped wire??) and the volume wiper(middle pin) for right channel, compare with li pin and left channel wiper.

Set coupler switch to off and connect ipod at zero volume to main-in RCA's, slowly crank up ipod vol, still channel imbalance?
Do you have another amp to act as a power amp, ie, go from CA-800 pre-out to main-in of other power amp (imbalance?)
Same through headphones(issue with spkr selector)?

 

[Rob Jennings]

Assume you are talking about +/-B pins on the relay/psu board ...

Yup. I've tested several times and B+ is consistently about 2vdc over.

FR704 resistance

I measured and compared to FR703. In circuit, both measured about 46ohm. (They are 47ohm resistors.)

- TR703 base voltage set in part by zeners D704,6 (measure Vdc at D706 cathode, think I've got those voltages somewhere...)

Measured at cathode for each:
- D704: 20.8vdc
- D706: 52.8vdc

(I don't know what those should be.)

- TR703Vbe slightly different to OEM?
- TR704Vbe slightly different to OEM?

For TR703, OEM is 2SC1213S. The datasheet says the Vbe is typically 0.64.
Replacement was KSC2383. Datasheet gives a range from 0.45 to 0.75.

As for TR704, OEM is 2SC1061. The datasheet says maximum Vbe is 1.5.
Replacement was TIP31CG. Datasheet says max Vbe is 1.8.

Listening more closely last night, it's more than your typical amp hiss I'm hearing as well. It's a bit louder like I mentioned before but there's some very slight static in there too. I'm also sure there's some slight high frequency distortion, noticeable on things like cymbals where they sound a little more sibilant or static-like. So I'd like to address these items and the channel imbalance—I'll be reviewing your other message to get to the bottom of that too.

I prepared to spend the time I need to get this amp where it needs to be. I moved some of my office gear (Schiit Sys / Mani / Vali) to the main floor so I can connect my souces (turntable and streamer) and bought an Aiyama A07 Max to drive my Klipsch Heresy in the meantime so I've still got music where I need it.

 

[Rob Jennings]

There could also be some cap leakage(C710?), the psu/relay board should be recapped, Nichicon UPW(4x220uf/100V,,,), UEP gets the job done.

Yes, that's a good idea. I noticed you suggested 100v instead of 80v which is stock for those 4 capacitors. What about the 2 x 100uf caps, would you also recommend bumping those up? Stock is 63v but 100v is available.

 

[mbz]

What about the 2 x 100uf caps, would you also recommend bumping those up? Stock is 63v but 100v is available.

They don't see a lot of ripple, anywhere 63V-100V is fine

Measured at cathode for each:
- D704: 20.8vdc
- D706: 52.8vdc

(I don't know what those should be.)

D704 is a nominal 21V zener, so 20.8Vdc looks fine.
D706 is a 31V nominal zener over D704 so expect 20.8 + 31(?) = 52V
I'd expect D706 to be actual anywhere 30-32V, need to check datasheets, step out first.

 

[mbz]

I've tested several times and B+ is consistently about 2vdc over.

Out of interest, my CA-800 was +52.1/-49.5
CA-1000 +51.3/-49.5

Also, the function, tone and filter boards are +50(B) supplied, only the power amps are +/-50 supplied.
As the volume is increased expect higher current into function/tone/filter and the +/-50 difference to reduce

Measured at cathode for each:
- D704: 20.8vdc
- D706: 52.8vdc

With D706 at 52.8V I'd expect 51.6 at +B, ie, 0.6V drop at Vbe of TR703 and another Vbe at TR704.
I assume you got the pinout correct on the new transistors, KSC2383(ecb), TIP31CG(bce)

 

[Rob Jennings]

Out of interest, my CA-800 was +52.1/-49.5

So today I checked and B+ was +50.6. I don't think I'm going to worry about B+ voltage anymore.

Thanks everyone for your help so far. This is where I am now at:

Main amp sounds awesome when decoupled from the preamp!

When the main amp and preamp are coupled, things aren't so good. I think it's something to do with the function board and / or rear panel board. I think there's a grounding issue somewhere but I don't know if that's the only problem. Here are the symptoms:

1. There is a loud hiss that is louder in the left channel. This hiss persists regardless of which input is selected (all inputs including TAPE A and B playback) and whether or not a source is connected. The hiss becomes very noticeable at about 40% volume.

2. There is a hum over top of the hiss when on all inputs (AUX 1, AUX 2, TUNER, PHONO 1, PHONO 2 and MIC) that is much worse on PHONO 1 and PHONO 2. This hum doesn't occur on TAPE A or B playback. Touching the input selector switch (the actual metal switch inside, not the long shaft or outer switch) makes the hum incredibly loud. The hum can be alleviated but not eliminated by touching the metal panel that the tape and input switches attach to. There is a lot of static when you touch that panel as well. Also touching the input jacks on the rear panel causes static.

3. The inputs are "leaking". If I attach a source to AUX 1, I can faintly hear that source on all of the other inputs, including TAPE A and B. But I don't know if this was happening before or if it's something that happened after I removed the input selector switch this morning and disassembled it for cleaning. (That is NOT fun by the way.)

I've been cleaning switches (including the input selector) all morning. There are still a few to go, including the coupler switch.

Other than switch cleaning, does anyone have any other suggestions?

 

[Rob Jennings]

Argh. I’m an idiot.

I did not replace the nut and washer that mates the input selector to the metal plate attached to the function board. Clearly this grounds the switch’s

Replaced it and testing now to see where I am.

 

[Rob Jennings]

Alright. That cleared up some of the noise.

Now, the noise floor is reasonable when you are on an input with a source connected. If you are on an input without a source, the amplifier hiss is much louder and there is a loud hum. Amplifier hiss on TAPE A and B playback is very quiet.

Inputs are still bleeding to other inputs though.

 

[mbz]

1. Hiss most likely due to a dying transistor on the tone or filter boards.
An oscilloscope would be handy otherwise you could build an audio probe

2. On the CA-800/1000 a GND track sometimes fails, this is unually on the function board but can be on the output/rca board depending on serial numbers. Will try and track down a pix.

3. Did you correctly reassemble the function selector after cleaning?

 

[mbz]

pix from posts by saabracer23, output/rca and power supply/relay boards.

 

[Rob Jennings]

I have been disassembling and cleaning all kinds of switches. I have not figured out how to completely disassemble the multi gang pots though. Best I can do is desolder them and spray into from the large opening where the leads come out. I almost ruined the bass tone control pot trying to get the shaft out of the wiper in the second section. They do not want to separate. Not sure if anyone else is doing these differently but desoldering and liberally spraying into them is the best I could do.

I restored class AB mode to this amplifier. A previous owner connected the low voltage wires from the transformer directly to the capacitor board, bypassing the operation board completely—probably because they couldn't find replacements for the snap action switches. I was able to replace the snap action switches and restored the original wiring from the transformer to the operation board and then to the capacitor board so the amp can be switched between class A and class AB mode again. This amp gets crazy hot in class A so it will be good to have class AB as an option again for longer listening sessions. Thank you Avionic for the link to the switches on Mouser.

I also learned not to test for results without replacing all nuts and screws in the section you're working on because ground continuity often requires them and you'll get hums and buzzes that go away when things are properly reassembled.

I'm ordering some capacitors for the power board and main amp boards. On the power board, there are two smaller capacitors:

- 100uf / 6.3v
- 1uf / 50v

I can't tell from the markings or board if these are two are polarized or non polarized. Can someone else please confirm for me?

When the pre and main are decoupled and the main is tested, it sounds fantastic. No noise of any kind. But when the pre is coupled, there are minor issues:

- There is white noise on the left channel. The noise increases with volume. It is not affected by balance until you get about 3/4 of the way over to fully switching to the right channel and then it starts to quiet. When the balance is switched fully to the right, the noise is almost gone from the left channel but there is a little bit of noice on the right then.
- There is significantly less noise if a source is connected but it is still there on the left channel. (If I short the R and L RCA inputs on AUX 1, the noise goes away.) With a source connected, the noise occurs mostly at between 60 and 80% volume. It almost goes away completely above and below that.
- The left channel behaves this way on AUX 1, AUX 2 and Tuner. There is more white noise on both channels on PHONO 1 and PHONO 2. There is also more white noise on both channels on MIC but less than on PHONO 1 or PHONO 2.
- I am still confident that the left channel is 10 to 20% louder than the right right channel but I'll test that more thoroughly later.

Any help on this white noise on the left channel issue would be appreciated.

Thanks!

 

[mbz]

I'm ordering some capacitors for the power board and main amp boards. On the power board, there are two smaller capacitors:

- 100uf / 6.3v
- 1uf / 50v

I can't tell from the markings or board if these are two are polarized or non polarized. Can someone else please confirm for me?

C711 1uf/50V and C713 100uf/6.3V both are bipolar. Go with Nichicon UEP or similiar, this is for the protection circuit so no benifit with going with the nice green (UES) ones. Also I'd up the voltage eg, 6.3V to 16V???

 

[mbz]

The audio path is from rear RCA's to rear panel board, from there via all those top (backplane) pins to the function board
and from there to the function selector. Selected L&R outputs is presented at that front connector on the function board.

This means the following possibilities,
- dirty RCA's, clean both inner/female and outer/GND
- check solder connections on RCA's
- clean AUX1,2 TUNER backplane pins and corresponding connectors on the function board (use dental pixters???)
- clean function selector

Next would be a noisy transistor on the tone board or...

I'd test for noise at the function board front connector, an oscilloscope would be handy/preferred. An audio probe
would also do. If no noise at that point then you know it's not pins etc but a dying transistor, and you've saved
yourself some work...