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

Member
2005-12-08 12:57 pm
Denmark
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.
 
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
 
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

Member
2013-01-19 12:22 am
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

Member
2017-05-20 4:40 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

Member
2013-12-10 7:45 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.
 

sjesus

Member
2017-05-20 4:40 pm
more info

Thanks Mannegizen and Nelson,
The new information is that leaving the PA-5 some time on, the speaker breaker circuit goes eventually on, so the red led goes off and it sounds perfect (as before). No hum, no noise, nothing. I didn't switch it off for the last 3/4 days and it works and sounds beautifully. If I switch it off and immediately on again the led goes off as normal; if I wait a few hours before switching back on, it takes sometime (10min ?) again (I tried it once). I didn't move the PA-5 recently but I also didn't use it for quite a while (before noticing the issue).
Temperature problem ?
Best
sergio
 

sjesus

Member
2017-05-20 4:40 pm
narrowing it down

Thanks a lot. It looks like things are narrowing down.
Huum,... if I understand correctly, comparing the voltage on the positive side of C110 with that of C210, I would see C110 voltage increasing faster than that of C210, after switch on, right ?
And that would mean change C210.
I will try that over the weekend.
Thanks
sj
 
Any progress?

I remember someone had a similar problem with a PA-7.
I can't find that thread now but if my memory serves me correctly, I think he narrowed it down to Q137 (left channel) or Q237 (right channel) in the protection circuit.

Nor do I remember what transistor he used to replace it with or if it actually was the culprit.

But if replacing C210 doesn't help it might be worth looking into.
 

Kokanee

Member
Paid Member
2010-09-24 10:00 pm
Puerto Vallarta/Kootenays
Mannegizzen,

Parts Connection has a sale on Nakamichi products. I was searching the PA-5, and came across this review below. Just wondering if it is the same with your amp???

Submitted by Mike a Audiophile

Date Reviewed: August 12, 2001

Bottom Line:
When I first took the PA-5 home I thought it airy and very nice..until I played Judy Collins and other high-pitched singers. They sounded truly bad,like early solid-state recievers. I put the amp on the test bench and found that it easily met all its specs. So what was wrong? Well,when I put square waves into the amp (a way of testing the transient and phase behavior of an amplifier)they did not come out the way they went in...this is never a good thing. Upon analysis I found that the feed-back loop was not configured to properly address the low midrange/upper bass. Attempting to find out what was up I called Nakamichi. Everyone I spoke to refused to talk to me about it...until I got to one brave tech who told me he knew what I was talking about but would be fired if he said anything!
Well,it turned out the solution was to examine a REAL Stasis amp and incorporate the componant feedback values into the PA-5. What a difference! The amp was warmer with better dynamics and the voices were now coherent. It seems that Nakamichi set the feedback values to make the amp look good on paper at the sacrifice of its real world
performance! At any rate the total cost of parts was $5.00
so I would not hesitate to buy this amp on the used market.
Now to the sound...The PA-5 has a warm softly detailed quality,never bright, and is very gentle on the ears. It produces a COHERENT soundstage. Coherence here means that all parts of a voice (for instance) are located in the same part of the image,that there is no tendency for various parts of an instrument to wander around the sound stage. This is a rare quality in a solid-state amplifier of this vintage. The bass is warm but lacking in slam compared to the Bryston,SAE, and the Dreadnaught (the Dreadnaught can push a speaker right out of its basket!). The midrange/highs are warm but lacking in detail and focus compared to the Sonographe and the Bryston (both newer designs),and the Fet-Valves midrange is much more lush and natural sounding, but the PA-5 holds its character into more difficult loads than the Sonographe and Fet-Valve can.The PA-5 has a very forgiving sound, almost never going harsh or hard.The PA-5 is also very easy for a preamp to drive due to its high imput impedance...I have even let the CD player drive it directly with no bass loss. Even into the Accustat speaker the amp still sounds warm and forgiving. This is one of the best amps for electrostats that I have found of that vintage (it easily out-performed the largest Hafler on the 'Stats. The over-all sound can best be described as coming from behind a velvet curtain...very much like the more expensive Threshold Stasis amps (but without Thresholds power,nearly holographic imaging, and detail),it is a good approximation of the Threshold at not nearly the price. The best statement that anyone can make is that,tho I can find flaws in its performance,I can,and have, listened to the PA-5 for hours without fatigue.

Myles
 

sjesus

Member
2017-05-20 4:40 pm
sticky power transistors

I know this is a cold thread. But I'm "back to business" after a long time. The problem I have is to reach the back of PCB and for that I have to detach the power transistors from the heatsink which, at start, seemed easier than unsolder 3x12 power transistor pins. In my old days they used to have mica + white grease. In the PA-5 they are posed on a gray sheet and apparently no grease. I took out the holding (and electrical contact) screws on all 12 transistors but those just won't come off. I'm a bit worried of forcing with a screw driver and damaging the whole thing. Sorry for the basic question: is there a trick here or should this be done otherwise ?
sjesus
 

sjesus

Member
2017-05-20 4:40 pm
Uau... that was fast ! I'm not very good with pictures but here it goes
IMG_20200209_194712.jpg
(I also hope the link works!)