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iamdarren73 24th April 2013 01:29 PM

Issues with newly acquired Nakamichi PA-5 Amplifier
 
You know ho wit goes, you buy a mint, working PA-5 only to find when you receive it that there are 'issues' with it. I did not do my due diligence and purchase a mint PA-5 and CA-5 from someone on Craigslist without listening to them. Bad move. The problems are as follows:

CA-5 - sounds fine after cleaning the pots. I am missing the retainer ring for a knob so it does not 'click' into place when it is placed on the shaft.

PA-5 - here is where my issues are biggest. I was a broadcast engineer but have been out of the field for many years so I am at a bit of a loss on where to start. The amplifier sounds fine when driven with my McCormack and the CA-5 at low-levels of volume or with my iPad driven directly into the PA-5. The problems start when the volume is set over about 9AM on the CA-5 or 10AM on the McCormack and over 50% on the iPad driven directly. There is cracking, static and clipping with the sound reverting to 'thin' to muting to clipping on the RIGHT channel by way of the indicator LED. I am unsure the first step to take in diagnosing the issues. I also have a BUZZ near the power button that is not related to the physical power switch. It was initially not attached as the glue that originally held it had lost the ability to hold. I removed the front panel and after a bit of Gorilla Glue it is structurally solid. It seems the become louder (the buzz) after the amplifier is on longer and increases in level when more drive is provided to the amplifier.

I downloaded the schematics and I will look at it tonight when I put the PA-5 on the bench. I am going to start with cleaning the relay contacts (if they are not sealed) and remove a couple of input capacitors and check them on my Cricket capacitance checker. I am also going to check the FET's for issues short of removing them, although I know it is not ideal.

Any other thoughts? The buzz and clipping? Are they related? What would be the first thing to check?

Thank you in advance,

Darren

Noise 24th April 2013 04:05 PM

Hi
Well the first thing to check is the power supply. The description you made does sound like a weak power supply. Maybe the electrolytic caps are dry/defect. It could also be one of the rectifiers that have gone bad.
If your supply is not clean and stable under load, it will very often result in the problems you described.
The buzzing sound could come from the transformer, or it could be "arcing" inside the caps if they are broken.

hope you find the fault.

JamesBrennan4 25th April 2013 02:48 PM

Hi,

My PA-5 has problem of power switch loose from front panel. I was never successful in reattaching the switch. I have read of others having same problem with PA-5. But I don't have any buzzing from it or from near it.

Jim

iamdarren73 26th April 2013 12:58 PM

Update...
 
Well, I have the amplifier apart and while I have it disassembled I am going to re-wire it, place new binding posts and RCA connections as well as re-cap everything in the audio/power-supply path. I ordered a replacement HEXFRED rectifier and although I used Gorilla Glue to re-attached the power switch I removed it due to the buzz.

Unfortunately the buzz is not from the switch or the transformer. I have not had the chance to dive deeply into the problems, yet. I have a simple rectifier I made with diodes that I will use to test the PS until the HEXFRED comes in. I am going to check the capacitors with my cap checker and see if I can re-form them with my HP reformer.

Thank you for all the suggestions. While I am in the amplifier can anyone recommend modifications to the amplifier? I have read pages and pages but no real comprehensive layout of modifications. Can someone point me to a comprehensive list of mods and their impact? DC coupling sounds OK but that requires a fairly significant mod. I am more than capable of doing it but in the end do to the deliverables warrant the work?

I have the matching CA-5 with phono preamp and damn, that is a beautiful preamp. It compares favorably to my Rogue Audio preamp and the phono stage in the CA-5 kicks tubes off my Project twin 12AX7 Tube Box. I would say the CA-5 is the closest to my McCormack Live Drive Deluxe in that it has nearly zero coloration. I am extremely impressed with the preamp and especially the phono section. A steal for $100 that I paid for it.

Last question - anyone have a source for NOS parts like panels or face plates? I realize that is a tall order and likely they are not available but I thought I would ask.

The PA-5 should be a nice addition to my Jeff Rowland Model 5 mono-blocks that drive my Apogee Duetta Sig II's and Centarus.

This is a great resource. Thank you to all who support and run this forum. Hats off to a dedicated team.

Darren

JamesBrennan4 27th April 2013 02:22 PM

Read the thread "To improve the sound of Nakamichi PA-5 ?".

The value of the feedback capacitor is, to me, the single biggest improvement you could make.

Let me know what you think.

iamdarren73 27th April 2013 04:20 PM

Update - v.2
 
Caps - the buzzing is coming from the MAINS relay next to the power switch. The relays in the amplifier boards also clatter under load. I checked the caps and they are not close to spec. Rectifier checked OK but that will be replaced, anyway. When I first turn on the amplifier I see the relay ARC briefly so I traced it back to the caps as the only likely culprit. The voltage and capacitance drop so drastically that the amplifier thinks as though it is being powered on/off 3x a second. While I have it apart I am rewiring it, replacing the rectifiers, audio caps and PS caps. Loads of work and $$ for a 'mint and fully functional' amp. Word to the wise - always listed to audio gear before you buy it, regardless of your trust level in the seller. I paid a premium for the amplifier and dropped $280 on parts to make it 'mint and fully functional' not accounting for my time.

Thank you to everyone who helped. I will look into the bypass capacitor recommendations and move that way next.

Any thoughts of DC coupling and roll-off capacitor? Worth the work?

Darren

hvengel 17th May 2013 10:32 PM

It sounds like your PA-5 was abused at some point in it's life.

I have a PA-5 that I have owned since new that has been used almost every day for the last 20 years. Recently replaced all caps except power supply caps (they were in good shape) and have the bypass cap mod. Sounds better than new and looks almost like new other than a few very small scratches. Power switch is as solid as the day I purchased the amp.

Other than the bypass cap mod I would not change a thing from stock.

sjesus 20th May 2017 05:12 PM

PA-5 led does not goes off
 
Actually this is not a reply but a cry for help.
I'm new to Diy.
My PA-5 that I owned for the past ~30 years has one channel less. The start red led does not goes off and there is no signal out of it. Does anyone has clue what that might be ? Any common problem that I could solve at home (I hope) ?
Thanks for any help
sj

Mannegizen 23rd May 2017 08:00 PM

My first guess would be cracked/dry solders, mainly around the VAS transistors, that is causing a high dc-offset that stops the protection circuit from engaging the the speaker relay.
Especially if this happened after moving the amplifier.

Q108, Q109, Q110 and Q111 if it is the left channel that is silent.

Or Q208, Q209, Q210 and Q211 if it is the right channel.

All these should be accessible with the top cover off.

What you would need to find those bad solders is good light and a magnifying glass and a solder iron and some solder to reflow the them.

But if you don't feel uncomfortable doing this, you might find someone nearby that can lend you a hand.

Nelson Pass 23rd May 2017 09:19 PM

At least you have a working channel to compare it to.

With a schematic and enough documenting of what voltages are different,
the problem should reveal itself.


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