AR Amplifier bias pot replacements

[soundminded]

Has anyone got the link for the replacement of the bias control pots in an AR amplifier with fixed resistors? I just can't seem to find it. I think there were at least two versions depending on serial number. Thanks in advance.

[dynaco_dan]

Hi there;

I posted that information in the AR section.

I don't remember exactly when.

Just returned from searching for it, I wasn't successful, sorry.

These are archival copies from AR and this is what I was suggesting that should be in the library where it can't be buried.

Mark is going to do something with this idea in July, if he has the time.

[mrdb]

The bias info is in the AR Electronics Service Manual in the archives.

Bob

[mrdb]

>The bias info is in the AR Electronics Service Manual in the

>archives.

>

>

>Bob

I should have said "the AR Electronic Service manual in the Electronics section of the AR Library".

Bob

[dynaco_dan]

Hi again;

The manual that is here in the library does not have the detailed drawings on page 37 that I posted before for the bias mod.

Page 5 does have some written information.

There was a colour code dab on the transistors, which had some significant relationship with the resistor's value.

The drawings were quite helpful and detailed.

AR mailed me a copy and included the correct resistors for me to install.

The more I searched the AR forum, the more wrong choices I kept coming up with.

I wasn't able to pull up, AR bias.

I would suggest not proceeding until you can find those two or 4 pages I downloaded.

You'll see when you find them, the search is worth it.

[JKent]

AR electronics manuals also here:

http://www.hifiengine.com/manuals_type.php...h&mod=Amplifier

[dynaco_dan]

Hi Kent;

Thank you for that link.

It is the same or similar manual that I have been saying is not adequate enough for the mods.

The sheets that I posted here were dedicated for the bias mods only and was directly mailed to me from AR.

I am trying to remember details now.

I believe there was mention of a paint colour dab on the output or driver transistors.

The sheet listed the colour possibilities and the resistor value to compliment that mod.

If I knew how to view all of my postings since day one, I would search from day one.

If there is a way to reverse the view of the write-ups from the earliest date to the latest date, it might be a big help.

I've posted way too many to just pull up the correct one, sorry.

[roundhome]

>Hi Kent;

>

>Thank you for that link.

>

>It is the same or similar manual that I have been saying is

>not adequate enough for the mods.

>

>The sheets that I posted here were dedicated for the bias mods

>only and was directly mailed to me from AR.

>

>I am trying to remember details now.

>

>I believe there was mention of a paint colour dab on the

>output or driver transistors.

>

>The sheet listed the colour possibilities and the resistor

>value to compliment that mod.

>

>If I knew how to view all of my postings since day one, I

>would search from day one.

>

>If there is a way to reverse the view of the write-ups from

>the earliest date to the latest date, it might be a big help.

>

>I've posted way too many to just pull up the correct one,

>sorry.

were the files you were talking about ever found I have and amp that needs to be updated bad looks like some one started it but never finished.

Jim

[dynaco_dan]

Hi there;

I never tried looking for them again.

[roundhome]

Hello Vern thank you for your reply have you done much work on these I have another one that is mint late modle all the upgrade done buy the factory it has never been opened up before . at present one channel sounds great the other has reduce volume takes 1/4 turn on the balance to bring them to same out put but drops the volume alot. i'm going to change out the filter caps first there is a little hum.any ideas?

Thank you

JIm

[dynaco_dan]

Hi there;

I cannot be any further techncal help beyond advising reading the AR service manual in the library and definitely the hidden bias sheets in our site, somewhere.

There is a much clearer copy of the manual on the net, than the library and it is in .PDF format, I believe.

Perhaps they are cloaked.

I believe there should be, AR and bias, in the title description.

My servicing experiences were limited to speakers from, AR, Dynaco, and some misc..

Turntables from AR, Dual, Miracord, and misc..

Headphones from Koss primarily.

For the two year period of pleasure.

[Mexicomike]

I've run into a similar problem trying to find info on adjusting bias WITH the pots. Further, I have the AR service manual which describes replacing the pots but it is not very helpful either for that modification - at least as far as I can tell.

In the mod section it says "...Connect one 12 or 13 ohm 1/2 watt resistor to each strip, leaving room for the shunt resistors which will be connected later"

I can find no further reference to these "shunt resistors" so it doesn't seem complete to me. Then again, I could be missing something obvious.

I didn't want to do the mod, preferring to find out how to set everything with the original pots but since I couldn't do that I thought I might perform the mod but I can't figure that out either!

[dynaco_dan]

>I've run into a similar problem trying to find info on

>adjusting bias WITH the pots. Further, I have the AR service

>manual which describes replacing the pots but it is not very

>helpful either for that modification - at least as far as I

>can tell.

You will need to find those 4 AR Bias modification sheets in the AR forum somewhere, probably cloaked by now.

Without these information sheets, there is no other posted information on bypassing the pots.

The driver transistors, I believe, had a colour dot of paint to identify them with.

All of my own drivers were identical colour, some may not be matched.

You cannot proceed without this information package!

>

>In the mod section it says "...Connect one 12 or 13 ohm

>1/2 watt resistor to each strip, leaving room for the shunt

>resistors which will be connected later"

>

>I can find no further reference to these "shunt

>resistors" so it doesn't seem complete to me. Then

>again, I could be missing something obvious.

>

>I didn't want to do the mod, preferring to find out how to set

>everything with the original pots but since I couldn't do that

>I thought I might perform the mod but I can't figure that out

>either!

You are wasting your time looking for the original spec's on the bias pots.

AR needed to do the bias pot replacements because the bias pots would change value in use, from the heat, I believe.

[Mexicomike]

I've pretty much given up on finding anything about adjusting the bias pots but SOMEBODY had to adjust them at the factory before they were shipped and there had to be a spec/procedure for that... Oh Well

Re these missing pages for the bias mod that were once posted and now can't be found...Could whoever posted them repost them or email them? I'd be glad to pay for the trouble.

[mrdb]

I would recommend replacing the bias pots with fixed resistors per the AR Service Manual listed on this site. The pots that were used were very unstable and any bias drift could be fatal to the output transistors. Replacing the pots with new ones will not improve the stability. The manual gives a biasing procedure on pages five and six, and a bias mod on page six. The biasing procedure is somewhat involved, and requires some test equipment to perform accurately, but is probably best followed. As the manual states, the output section is pretty basic and the two halves interact in determining the DC offset at the speaker terminals. The two halves must draw equal currents to maintain the offset as low as practical and must be biased on at a low level to minimize crossover distortion (hence the IM distortion spec). Too much bias current and the outputs run too hot, not enough and you get distortion. As a side note, the TO-66 cased driver transistors are hard to find. I think that they are 2N3054s. The outputs are most likely 2N3055s. The color dots on the transistors are probably an indication of the beta range of the devices.

I have two receivers and one amp and the amp (the oldest) needed the bias mod.

Bob

[mrdb]

In re-reading one of the previous posts, I hope that I can clarify the comment about shunt resistors. The 12 or 13 ohm resistors, which replace the pots, installed during the mod need to be trimmed to a value that meets the offset and distortion criteria. The shunt resistors are soldered in parallel with the 12 or 13 ohm resistors. As I stated in my previous post, everything interacts and the procedure requires patience. Pots are certainly easier to adjust, which is undoubtedly why the factory initially used them. Field failures dictated a change in the design to increase the reliability.

Bob

[Mexicomike]

As I mentioned in a much earlier post on this subject, there are other amps that use bias pots and offset pots for adjustment. I just went through these adjustments on my Moscode 600 which was exhibiting the same behavior as the old ar amps - overheating and shutting down.

It was exactly the same problem BUT, the procedure for cleaning the Moscode's pots, and adjusting the bias and offset is available and easily performed with a decent multimeter.

I absolutely do not disagree that fixed resistors is a better solution for production work but if one is willing to perform periodic maintanance, then cleaning/adjusting the pots is no big deal IF the procedure and specs are known.

My current AR amp works fine but it does get HOT - as in you CANNOT hold your fingers on the heat sink fins after it's been running for say, 30 minutes. However, it has never shut itself off. I can adjust the offset - currently 38/43Mv but I have no idea how to adjust the bias. The Moscode's offset, as a reference, is now 0. That seems to me to be the beauty of having pots IF you clean/maintain them.

[dynaco_dan]

Back again;

The 12 or 13 ohm resistor mentioned just above would be one value needed.

I believe you need to remove the metal covers below the amp and write down the colour dab on each transistor.

One or more of the cloaked sheets has the colour's listed and the appropriate value resistors to use.

When I had corresponded with Roy Allison many years ago, he mailed me the correct resistors for my mod, at no charge, along with the sheets and a good luck wish.

I have downloaded here page 5 and 6 from the AR Service Manual posted in the AR library.

It mentions another sheet or more, Part I.C. 2.

This is in all likelyhood the missing 4 pages, from the manual and cloaked on our site.

Perhaps someone here can go in, in stealth mode and find them.

I see this manual has a procedure for removing the pots and installing terminal strips even.

Those sheets that I posted a long time ago did not go into that detail, I am certain.

The Leach Double Barrel amp, for just one, and others had bios pots on their circuit boards.

Those sheets may be cloaked but in this universe, you are at least narrowed down to this website.

Have patience and they will show up.

I am going to HIFIengine to see if their manual has the sheets.

[mrdb]

Vern,

The manual section I.C.2 is the bias mod on page 6. While I agree that pots greatly simplify the setup of the amp, this design is very sensitive to resistance variations. I think that the pots used are wirewound and humidity and time take their toll. Just think about AR's speaker crossover pots. The pots could be cleaned and this might help for a while, but the best solution is still trimmed, fixed values. The amp that is too hot to touch after half an hour, assuming that it is not driving a heavy load at high power, is definitely overbiased. Try reducing the bias on each output section while keeping the offset less than 100mv and see if this helps. In looking at the schematic I see 0.33 ohm emitter resistors for each output transistor. Try setting the bias pots for 5 millivolts dc across the resistors, with no signal applied. This might not be quite enough bias but is unlikely to overdissipate the output transistors. Let the amp run with no signal for a while and see if it operates much cooler. Listen to the amp. If it sounds ok, this might be about right. If you hear low-level distortion, try increasing the bias to give 10 millivolts across the resistors and verify idling temperature.

I probably would not increase the bias too much more.

Also, verify that the output and driver transistors are firmly attached to their respective heatsinks and that the biasing diodes are in physical contact with the emitter resistors.

Of course, the above information assumes familiarity with working on live, ac line-operated equipment. One slip of a tool could destroy both output and both driver transistors, let alone greatly shortening the life of the tool wielder.

Bob

[dynaco_dan]

Hi Bob;

I would not suggest anyone on this site do any mods to the AR amp pots until the bias sheets are found.

Straying into cleaning and adjusting pots, at least with these AR bias pots, without technical data, is a leading cause for disaster.

Even a technician would probably decline repairs without valid service data.

To suggest that a novice or non-novice attempt this project without those sheets, I cannot do.

Do remember that each amp and receiver affected by this, is unique, in that the driver transistors were not identical for each.

Enough that they needed to be or just were identified.

Dynaco, for one, used close tolerance transistors, but not absolute precision, they did not need to identify their typical transistors, as did most manufacturers.

AR, for whatever reason used paint dabs to identify their driver transistors, even with the pots.

More to come out in the future.

At least with the Leach Double Barrel amp project, Marshall gave all the test information for grading the transistors, even.

This would have been an amp everyone, almost, could dive into.

[mrdb]

Vern,

I agree with you that caution is recommended in terms of the bias mod/adjustment without some familiarity with solid state amps. I am an electrical engineer and many years ago was service manager of Tandberg in southern California so I have a fair amount of experience with audio equipment. I made the mod to my AR amp and it has worked fine for three years now. Like Mexicomike, mine ran very hot before the mod. Recently it has developed some low-level hum so my next repair will be power supply filter capacitor replacement.

Regarding the color coding of the driver transistors: this is probably a sorting for beta (gain) and made the biasing procedure easier and helped with keeping the distortion within spec. The biasing procedure in the manual is somewhat independent of transistor beta assuming a reasonably close match. Information that would be very useful is the actual idling current through the output section. The procedure that I suggested previously would be a good first approximation and would not be too likely to overdissipate the output transistors.

The AR amp circuit topology is a bit unusual in that it used driver transformers driven from a complementary symmetry predriver section. By the time the amp was designed (I'm guessing 1967 or so) many other amp designs has dispensed with the driver transformer and had gone to completely direct-coupled topology. There is nothing inherently wrong with transformers (many will disagree), especially in the driver section, but high quality transformers are expensive, bulky and heavy and introduce phase shift that must be accommodated in the overall design. In this same time frame JBL had the direct-coupled T-circuit amps designed by the late Bart Locanthi. They still have a good reputation and a loyal following.

Anyway, enough rambling for now. Hope that you can find your bias update info.

Bob

[Mexicomike]

Thanks guys for the continuing discussion and for the suggested bias adjustments. I'm one of those novices when it comes to electronics circuits. I can solder, replace components, make whatever measurements but I DON"T actually know much about electronic circuits. I can identify the various individual components in an ampllifier (or whatever) but I have no training - either school or self - in elec circuits.

I have repaired amps using the common sense method - finding a bad component in one channel by taking readings throughout the other channel but I don't actually know what I'm doing!

I REALLY would like to adjust the bias on this amp with the pots so I think I'll give the suggested settings a try and see what happens - I am capable of doing this, and understand the care needed with active circuits. I just don't know how everything interacts to do what it does!

I probably fit perfectly into the "He knows just enough to be dangerous" category when applied to this subject.

[dynaco_dan]

>I REALLY would like to adjust the bias on this amp with the

>pots so I think I'll give the suggested settings a try and see

>what happens - I am capable of doing this, and understand the

>care needed with active circuits. I just don't know how

>everything interacts to do what it does!

Sorry, but there is no suggested settings on this site for adjusting the AR bias pots.

When you remove the cover to the AR amp, or receiver, you are exposed to dangerous voltages.

One slip and you can hurt yourself, do property damage or damage irreplaceable parts or all three.

I've written all I can for this topic.

[Mexicomike]

Vern - thanks

By suggested settings I was referring to the numbers Bob suggested for bias not to any "official" settings.

I have found other posts on the internet re this subject; one where the individual states that his AR amp now runs cool to the touch - the transformers, he says, are warmer than the heat sinks. He performed the bias mod but unfortunately, he didn't post the specifics or any pictures.

So hopefully the missing pages you mentioned will show up eventually! I've become convinced that despite my interest in keeping the amp absolutely original, the mod is the way to go, especially since it was a factory mod anyway. So I guess I'll leave it alone until the mod specifics are available.

[Mexicomike]

Well, this isn't working for me!

One of my faults is my inability to leave something alone when I know it needs work. So, after further fruitless searching for weeks re AR amp Bias, and finding nothing directly related, I found this generic article: http://www.tnt-audio.com/clinica/bias_e.html

It seems to me to provide the info to undertake bias adjustments on the AR (or any other) amp with the pots and it's easy to understand. So I plan to give it a shot. We have houseguests at the moment so I don't have time to mess with it now but when I do this, I'll let you all know how it works.

However, if anyone sees an inherent fault with the article's suggestions re the settings, please say so! The suggested settings/range correspond with the bias/offset settings (per manufacturer) that I checked/set in my Moscode 600 so they seem reasonable to me.