CAV50 questions

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dprenerov
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CAV50 questions

Post by dprenerov » Sat Dec 24, 2016 6:52 pm

Hello,

I am new to the forum. I am from Bulgaria.
Honestly to say, I have never heard about Conrad Johnson before, until two days ago, when a friend of mine showed me his CAV50 and told me he has problem with it and asked me to help.
The problem is as he described it as something like pop or discharge noises. I didn't have opportunity to hear exactly what it is about. He believes the problem is due to capacitors and he asked me to replace all the capacitors of the amplifier.
As I have engineer education and I have experience with audio, the first thing I wanted to find is a schematic diagram, and possibly service manual in order to become more familiar with this amplifier.
So Google linked me here and I registered as I kindly have been asked to in order to download the documentation you have found so far.
Thank you for what you have done!
However I found almost nothing about CAV 50. The only document is about how to change from pentode to triode mode.
Also I found information that CAV 50 is PV10/MV55 combination in a single chassis. If this is the case, I should consider schematics of PV10 and MV55 for my purposes.
Unfortunately what I found here as schematic of MV55 is low resoultion/poor quality scan from paper. Not everything is readable.
No list of parts.

So can you please advise me if somebody is familiar with this amplifier and especially if the problem described is known issue, what should I do?
First of all, as far as I haven't heard personally the pops and discharge noises I can't have opinion about it's nature, however I doubt if it is really a capacitor problem although it is quite possible.

If this is the case what capacitors you would recommend me to buy. We think of electrolytic capacitors problem, but I also saw something about replacing all capacitors with teflon type on Conrad Johnson official web site.

I would appreciate any advice and/or information.
Thank you and Marry Christmas!

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Re: CAV50 questions

Post by admin » Mon Dec 26, 2016 10:44 am

I think you probably need some more info before starting a repair. Is this "pop discharge noise" on one channel or both. Is it with physically touching the unit or random? If you really think it's a capacitor issue the first step is to do a visual inspection of all the capacitors. Bad caps will often be bulging or discolored. Also, has he tried swapping the tubes? Any tube based equipment this would probably be one of the first steps as tubes are the components that will typically fail the fastest.

Unfortunately, we do not have the schematics for all CJ equipment in our database (as CJ does not publicly publish them). The ones we have have all been acquired by gracious members who have sent me copies for posting. We are the largest database of these schematics on the internet. If you do find any schematics that we do not have please email them to me and I can update the database.
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Current Conrad Johnson Equipment in System: Evolution 2000, PV-12L, and D/A-2b.

dprenerov
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Re: CAV50 questions

Post by dprenerov » Mon Dec 26, 2016 12:27 pm

Thank you for your reply.
You are right that I need more info. However I don't have the answers to your questions as I decided first to do some research on the internet before taking the amplifier at my place for test and repair.
May be the description of the static is not correct also.
I like your idea for swapping the tubes. However this will work only if the static is on one channel only.
I think I will replace all electrolytic capacitors or in all cases as the owner wants this and he believes that this will fix the problem without any doubt.
If after opening the unit there are visible signs on the electrolytic capacitors, of coarse this will be proof of damage.

If I find any schematics I will send it to you, but it is more probable that I will take the schematic from the physical device by myself and draw it on paper or using any software for this purposes.

I understand that CJ does not publicly publish schematics, but I think if any documentation exist, it will leak soon or late.
For example, I am a fan of the British company Quad Electroacoustics, manufacturing amplifiers and electrostatic speakers and other stuff like that also don't officially publish any schematics, however I have service manuals and schematics of many quads. Some of them is easy to find by google search, others I bought on ebay.
I will write here after having any progress, will much appreciate if somebody else can provide with more help.

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Re: CAV50 questions

Post by Big Dog RJ » Wed Dec 28, 2016 8:50 am

G'day mate from down under.

If you are based in Bulgaria, and there happens to be a CJ dealer somewhere there, I would try to contact that CJ dealer and request for the CAV50 service manual. At least that way it will be an official request for CJ HO to provide the schematic. CJ HO has every schematic of every product they have ever built to date, and I have requested many service manuals from them long time back.

As Admin correctly pointed out, those noises are typical of tubes, pops or clicks or even discharge can also be cap related no doubt. You need to also ask the owner whether he has ever changed those tubes? and when did he last change the smaller input and phase inverter tubes?

The larger output tubes are easy to change when time is up, mainly by listening for distorted sound and when bias cannot be set properly (usually past three years of general usage). Smaller tubes should last longer but can also deteriorate due to other conditions. Again you need to find out from the owner whether he has changed any of those tubes and when? Are the output tubes biasing properly? EL34's are not finicky tubes to bias and will settle in well, therefore if a tube is bad, it should be quite obvious. Are any of the grids in the output tubes blood RED in colour?

Caps going bad is generally one major noise, pop loud bang and that's it! It will start to leak DC voltage and DC off set, causing the amplifier to shut down with a loud pop whenever the amp is switched off. This loud pop will be heard very noticeably on the speakers, and it is not pleasant.

I once experienced a similar problem with a PFR preamp. Whenever I pressed the mute button on the PFR, there was a massive pop noise, almost like a mini explosion each time mute was engaged. The amps were big 350 watt Manley monoblocks, I thought it was the tubes...
It turned out to be both! small driver tubes on both monoblocks and very leaky caps in the PFR preamp.

At the end, all caps were replaced in the PFR (CJ claimed this was due to excessive high humidity in tropical climate) and the driver tubes replaced in the Manley amps- due to old tubes. Problem solved!
Tubes amps are generally easy to repair and they go on forever, unlike solid state. I would definitely try to get hold of that service manual from CJ directly through the dealer if possible, this would be the safest and most reliable manual for servicing CJ gear.

I actually had the service manual for the CAV50 and many more amplifiers that we carried during that time, funny I thought I would never require these after the business was sold... sorry!
Try to get in touch with Jeff (head of tech/service at CJ);

I would think the owner would have bought his CAV50 from a reputed CJ dealer initially, therefore I am sure that dealer can help, unless they have shut down or don't offer the dealership anymore...
All the best, RJ

dprenerov
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Re: CAV50 questions

Post by dprenerov » Wed Dec 28, 2016 4:11 pm

Thank you everybody for your replies!
Today I picked up the amplifier and first opened it just to check for visible signs of something wrong before powering it up. It seemed OK, so I powered the amp and... surprise - the noise is not produced by loudspeakers, but by the amplifier itself! It sounds like transformer or something hum, but too loud to be normal transformer noise and also it is 50 Hz which is the normal mains frequency in Bulgaria. Normally transformers make noise with 100 Hz rather than 50 Hz. It like if you load a transformer by a single diode. Strange.
Sometimes the noise disappears randomly, and the starts again.

The bias of power tubes can be adjusted by trims, rotate clockwise until the LED lits, and than back counterclockwise just to LED goes off. This is on the all 4 power tubes.
I connected a sound source, and loudspeakers and it produces normal sound.

I don't know if the owner has ever replaced the tubes, but the case seems to be opened 1-2 times before.
I don't know where did he purchased the amplifier, may be he bought it not in another country.
So I think - is it possible that the amp is not for 240V AC power? How should I understand for what voltage is it?
I looked for labels around power connector showing the proper voltage but found nothing.
I am not sure if there is an official distributor/reseller of Conrad Johnson in Bulgaria, there is no such information at CJ official site, so I think it is better to contact CJ support directly.

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Re: CAV50 questions

Post by dprenerov » Fri Dec 30, 2016 3:00 pm

The troubleshooting is closed, the amplifier is fixed.
The strange noise came from the relays, which switch between different sources due to high level of ripple (about 1.3V AC) on their power supply.

Jeff from CJ support suggest to replace the 4 rectifier diodes, which are originally GI854 (3 amps according to specs) with 6 amp ones.
So I replaced the 4 diodes and voila no noise, everything is nice. I also replaced the filter capacitor 4700uF/35V.

Honestly to say, I don't understand why the diodes was the key in this case, however definitely replacing the diodes did the job mainly.
Thanks to everybody who helped me.

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Re: CAV50 questions

Post by Big Dog RJ » Sun Jan 01, 2017 1:14 pm

Jeff is the man! no doubt.

That was a bit worrisome- that the amp didn't state the operating voltage! There is always a label stating the required voltage, which is factory set for that particular country; this is located either below or at the top of the IEC power inlet.

Anyway, glad you managed to contact CJ and he was able to guide you in doing what was necessary. One great company to deal with, and who always find the time to listen and provide tech support even after warranty, allowing third parties to fix problems... very rare nowadays.
In general, most audio companies provide that kind of support to authorised service centres dealing with their own products; CJ, ARC, VTL and Manley are definitely above the rest when it comes to service and support.

Cheers and nice to see that it was fixed just before the new year!
RJ

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