Revision Pioneer A-27

[hansu46]

Hello dear hifi friends,

I bought a Pioneer A-27 amplifier at the end of last year, it is a 100V Japan version.
The device is in pretty good condition for its age.

Then I bought a transformer 230V - 110V and connected the amplifier, the sound is not particularly good and the volume control scratches.

The first thing to do is clean, as there is a little dust inside.

Then all electrolytic capacitors are exchanged, have already been ordered.

I will also clean the volume potentiometer (I have not found a replacement) and the switches also need cleaning, I don't really trust myself.

I will contact you again when I have made progress.

Friendly greetings to everyone
Hansu

 

[hansu46]

Hello everybody,

I worked on my pioneer amplifier, unscrewed it and cleaned it.
Then I replaced the large filter capacitors, unfortunately I forgot to order the bypass film capacitors, I reinstalled the old ones, maybe I'll do that later.


I also removed the power transistors, cleaned everything, applied new thermal paste and reassembled with new mica discs.


This is the current status of the work carried out.

Best regards
Hansu

 

[MIJ_Pioneer]

Hi Hansu,

Welcome - the A-27 is a very nice piece.

1. Are you using a dumb bulb tester? If you do not have one make one before continuing service or powering on.

2. Your choice of thermal paste. You are using a thermal paste that is meant for a computer CPU. I also have done this. While they may be better or worse than what the audio industry uses for thermal coupling, there isn't too much data on how long it will last for. There are proven thermal compounds/greases that have been proven to last a very long time, like DOW 340. On the other hand, this grease would not be suitable for a computer processor.

I discussed the topic with others and they weren't too interested in wether my choice of Arctic Silver Ceramique was a good fit or not, they just didn't understand why I did not use what has been proven. If it were me I would not use MX-4 as being a computer tech for many years I have yet to see a thermal paste that didn't look questionable after 5 years. However on this vintage gear a lot of the times the thermal compound is still somewhat healthy (after nearly 40 years ).

 

[hansu46]

Hi MIJ_Pioneer,

you mean a light bulb in series in the supply line to the amplifier, I built something like that. But thanks anyway for your advice.


I didn't know about the thermal paste, I will probably order this DOW 340 and remove the large heat sinks again, then reassemble the power transistors with DOW 340.

Thank you very much for your help!
Best regards
Hansu

 

[markthefixer]

Yes, the thermal paste is CRITICAL.

Computer CPU type thermal paste may be conductive of electrical current (electrical conductivity of the thermal paste is not a problem in computer's CPU's) -
BUT the mounted power output transistors in the Pioneer depend upon the thermal paste being NON-CONDUCTIVE of electricity, while facilitating heat to flow through.

Do NOT power up the Pioneer until this is corrected. Thunder and lightning will occur. I have SEEN it happen!!

 

[hansu46]

Hi,

markthefixer thank you for your warning, it saves me a lot of trouble.

While I'm waiting for the DOW 340 and the MKP bypass capacitors, I have time to clean switches and potentiometers.



Best regards
Hansu

 

[rcs16]

Every time I hear about "Arctic Silver" I cringe and shake my head asking why when there are so may documented failed attempts. why people will just not use the Wakefield 120 or sub a high quality thermal pad, save yourself the mess. I do not think there is one mfg that still uses mica and paste in new designs.

 

[Japetus]

Hi Hansu,

nice that you've eventually started this thread - looks familiar, I'll follow with much interest! Make pictures!
I did not open volume Control while I did my A-27, no problems with it. Wasn't it sealed?

I wonder if high quality thermal pads are the better solution compared with mica and high quality thermal paste - in terms of heat conductivity or just in terms of mfg production cost reduction (while maintaining its purpose to an acceptable degree). Could be better - I just do not know.
However, I'd rely on the initial make design.

Best,
Frank

 

[hansu46]

Hi there,

With the thermal paste I have now started a war of faith, I did not want that.

Everyone thinks that his paste is the best, I don't know, I'm a beginner in this field.

I just meant my Arctic MX-4 was enough, because the manufacturer writes "non conducting / non capacitive"



Best regards
Hansu

 

[hansu46]

Hello Frank,

nice that you speak up to word, I have now ordered DOW 340 and new mica discs, I will do as suggested by MIJ_Pioneer.
I will also reapply the thermal grease on the small heat sinks on the various boards.

At the moment I am replacing the electrolytic capacitors, I will do all of them new.

I like your version of the MKP-bypass capacitors on the large filter capacitors, I will do that too.

I will get back to you later.

Best regards
Hansu

 

[markthefixer]

The applications for the paste are paramount in their choice, and a lot of the confusion comes from Gamer Computer building - where there are no voltage potentials to be isolated while shedding the heat from the chips that do the work. Perhaps sometimes stray capacitance might have an effect, but I am inferring that from statements made on marketing material, not engineering bulletins.

 

[zebulon1]

I mainly do the paste for authenticity. I find myself using the Wakefield products when doing repairs on the equipment not restorations.
Whats the difference? The customer doesn't want to spend on the looks.
Wakefield-Vette thermal pads:
TO-3, 2 pin config, CD-02-05-3-2
TO-220, w/ mounting hole, CD-02-05-220

 

[hansu46]

Hello everybody,

I removed the power transistors, cleaned and applied "Dow Corning 340", then reassembled. The second time it was a lot faster if you know how, ha ha! thanks again for your warning!


The MKP bypass capacitors are also installed.


The Power Supply Assembly (AWR-176) is also ready, all electrolytic capacitors have been replaced, new heat-conducting paste has been applied to the heat sinks, and for Q5, Q6, and Q20 I also screwed a heat sink onto the heat sinks.


Best regards
Hansu

 

[Japetus]

Hi Hansu,

this looks really familiar to me...
It is surely hard to quantify the effect of the additional heat sink wattage, but at least you know it is better than before - not much more one could want.
Keep in mind (I'm sure you do) the heat sinks might very well be alive, so prevent them from touching others.
Does the board still fit in after reinstalling the shielding?

Best,
Frank

 

[hansu46]

Hello Frank,

It was copied from you too, I just took it over without asking you, sorry!

I have not yet installed the cover, but should fit, I made sure that the electrolytic capacitors and the heat sinks are not too high.

Now I swap the electrolytic capacitors on the other boards, then we'll see.

Kind regards
Hansu

 

[Japetus]

Heck there is no need to ask me, by no means this is reserved information in any regard.
In contrary, the purpose of my thread is to help others and/or give ideas (without any warranties from my side of course).
Use whatever you like to, I hope that in the end you'll be as satisfied with the A-27 as I am.

Best,
Frank

 

[hansu46]

Hello Frank,

Thank you for your okay, I liked the arrangement of the bypass capacitors and the enlargement of the heat sink so much, I just had to do it.

The boards are installed, the voltages set according to the instructions. All voltages on the power supply board match the information in the diagram.

The settings for the idle current and the head amplifier are very difficult to set, I have seen that you have installed other trim potentiometers, it is certainly easier to set.
Maybe I'll catch up on that.

I then connected a music source to "aux", set the selector switch to "aux", switched on the amplifier, the relay clicks, the volume control slowly started up, but nothing happens, there is no sound from the speakers, not even a noise, everything quiet.

Somewhere the music signal has to be interrupted, if someone can give advice I am very grateful.

With best regards
Hansu

 

[MIJ_Pioneer]

Hi Hansu,

Correct voltages on the power board are very important. At this point we have many routes to go to troubleshoot the amp going into protection when set to aux with an input source.

Edit- you said when you turn on the aux input the relay clicks. When you turn on the a-27, after ~10 seconds does the relay click as well? I am thinking that relay clicking during the aux input selection is it going into protection but please verify if this is normal operation for an a-27. I haven't looked at the a-27 but it may have two relays that click when it's turned on.

Before someone more technical comes in here, try these few steps:

1. Have you adjusted the DC offset and bias?

2. Measure the DC voltage on the speaker output terminals. Left and right, record their reading, specifically we are looking for a spike (or lack of) of DC voltage right before going into protection.

3. Do you have a separate amplifier you could use that has a "power in" connector? To further troubleshoot we want to do the following:

A. Remove the jumper for pre-amp/power in. For test purposes attempt to plug in a source to aux and see if it still goes into protection.

B. Connect the pre-amp out into another amplifier. See how the new amplifier responds to the preamp of the a-27. Does it play normally? Does it clip into protection?

C. Connect a volume controlled source into the Power In connector on the a-27. With volume at the lowest level see how it responds. As you know the power in will play at Max volume so be sure to have the volume very low. Does the a-27 play music? Does it clip into protection?

Doing #3 helps further track down where the problem is, it may be in the preamp or power section.

 

[hansu46]

Hello MIJ_Pioneer,

Thank you for your quick response, I made the settings according to the service manual, direct current supply circuit, power amplifier, DC balance, idle current, MC amplifier, equalizer amplifier.

when i switch on the amplifier (without music source), the relay on the power transformer clicks and the lamp of the selected source lights up. Approximately 8 sec. later the relays click on the output terminal.

I have checked this so far, the next points that you have proposed will be checked if I have more time.
In any case, I will contact you again when I have tested it.

Thank you again !!!

With best regards
Hansu

 

[hansu46]

Hello everybody,

I can report a success, there seemed to be something wrong with my music source.

This morning I plugged a connection cable from my mac headphone output to the Pioneer tuner input, selected the tuner input, switched on the Pioneer and lo and behold, the amplifier is working.

So the whole thing is my mistake, why didn't I think of it beforehand.

MIJ_Pioneer thanks anyway for your help!

I have now measured the DC voltage at the speaker output terminals R = 0.010 VDC / L = 0.008 VDC

I will replace the trim potentiometers so that you can make the settings more precisely.

Then I have to test the different input sources, especially the phono section.

You will definitely hear from me again.

With best regards
Hansu