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Old 13th April 2003, 05:24 PM   #1
zobsky is offline zobsky  India
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Default how do i figure out what's wrong with this (vintage) amp

managed to snag an identical pilot aa-903 integrated amp on e-bay to the one i own. small print, ....it doesn't work. i figured i'd take a chance and learn how to fix the thing, especially since i don't find this particular piece very ofen. ultimate goal, either run them as monobloc power amps or a dual mono integrated setup, because they do sound nice (esp. the midrange), despite a slightly flabby bass.
tube configuration
preamp section mostly 12ax7
amp section 6v6 x 2 (push pull)
my somewhat crude diagnostic procedure
1. visual inspection, ..all tubes light up
2. connect to el-cheapo speaker and cd-player, ...set correct connector,...no sound
3. replace all tubes with a working set from my working amp, .... no sound, ... therefore the tubes are not a problem, i re-install the orginal tubes that this amp came with
4. i clip the preamp - amp coupling cap just before the amp section and thereby seperate the pre-amp and amp+phase splitter section., .. i rig an RCA input wired to the the input of the amp + ground and attach this to a NAD pre-amp i have. i have sound, ..nice and clear, no hum or distortion, THE AMP SECTION WORKS !! , the only thing that bothers me is that i have to turn the pre-amp to 12 or 1 o' clock to get any decent voume out of it, which is not the case with (the NAD with any of my other amps) or (my working AA-903). maybe a tube pre-amp with higher gain will do the trick
5. to determine if my power / phase splitter tubes are weak, i replace them with the identical tubes from my working amp, ..... no real difference in sound vollume (maybe just a slight difference in sound resolution but nothing more)... so the power tubes are not weak.


based on this, .. i determine that something in the pre-amp section is wrong. the power supply seems fine
question, ..how do i debug the pre-amp section, ... it looks a bit like a mass of snakes. i know about discharging the power caps before poking around in the chassis and i can use a multimeter to check if the resistors are in spec, ...but
1. how do i test capacitors. do i just change them all out, ...and if so, what kind of caps do i replace them with?
2. is there any methodology to diagnosing the problem with this amp, or do i just
a. check caps and resistors first with the amp off and then
b. start from the input and measure volages (referenced to ground) to see where the circuit is broken


lastly, i have gathered that this amp is self biasing williamson ckt. and the tubes run in pentode mode
1. is there anything to be gained from
a. converting it to triode push pull
b. converting it to fixed biad, (i don't know how to bias amps yet, but i'll worry about that if/when i have to)


End Objective: ..if i can sonically improve this integrated amp by a noticeable amount (either as an integrated or as a monobloc power amp), .. i will then apply the same to my working AA-903. i came across this article on the subject williamson amp mods


thanks much
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Old 13th April 2003, 09:27 PM   #2
SY is offline SY  United States
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You won't like this answer.

If you're not using a scope and function generator (or other signal source), you're working totally blind.

But maybe you're lucky. Check the DC voltages at the tube pins and see if anything looks funny (e.g., no DC at the rectifier output, no DC on preamp plates, wrong cathode to grid voltages...). If you've got two channels dead, there's a good chance that one of the power supply voltages has gone MIA somewhere.
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Old 13th April 2003, 10:33 PM   #3
zobsky is offline zobsky  India
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thanks, ...
i think i'll start by

1. checking / replacing faulty resistors
2. bypassing the tone controls
3. replacing ALL the caps in the pre-amp section upto and including the couplingcap, ...there are only a handful anyway, ..

..which general purpose (read not high end) should i use, ..

film type, mica .... ???

i'll post back in a couple of days once i have time to get all the replacement caps.
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Old 14th April 2003, 12:12 AM   #4
SHiFTY is offline SHiFTY  New Zealand
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I definitely think you are on the right track, you might have a dud capacitor or resistor. Replacing them is a good idea. BTW Just use any new film caps (NOT ceramic) of the same value, the idea is to get it working first!

The heat generated by tubes over time also can crack and break solder joints, so I would re-solder just about every tube socket (basically just melt them and add a little more solder till the joints are nice and shiny.) This has cured many problems in my old amps.
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Old 14th April 2003, 12:22 AM   #5
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Default We're All Different.

Hi,

Quote:
basically just melt them and add a little more solder till the joints are nice and shiny
I'd use a desoldering pump to remove anything looking remotely suspicious but I guess someday, somehow I'll accept we all have different standards.

Cheers,
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Old 24th April 2003, 02:12 PM   #6
zobsky is offline zobsky  India
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I managed to get this amp fixed, ..here is a log of what I had to do, ... thanks for all the responses and great advice.


1. re-connected the pre-amp to the power amp section, ...bypassed the tone controls by snipping and shorting the wires to & from the bass/treble controls

2. tested, .. and then there was sound, but very weak ...., maxing out the volume control , i noticed tube break up distortion, although the overalll volume was relatively low. i know what the power amp section of this amp is capable of, so i deduced that the power amp section was not to be blamed...time to concentrate on the pre-amp.

3. i replaced all the electrolytic caps. for a few caps, i was not able to find the exact replacements, so i went with values of +/-5%, ...which is a reasonable assumption. .. at the end of this exercise which thoroughly taxed my caveman soldering skills. .... there was no major improvement, a slightly clearer sound maybe, ... but still the distorted volume.

4. distortion, ...thinking that maybe the pots were dirty, ... i sprayed contact cleaner into the volume and source selector switches and cleaned them, ..still no improvement,

5. tried swapping the tube set again, ... no improvement

6. maybe i have a boatanchor ....

7. i take another look at the amp, ..there are only two caps (apart from the big power supply cans) which are still stock. they are attached to the (bypassed) tone controls. i look more closely at the tone controls and notice a ground connection between the bass and treble pot conections.

8. hmm... my bypass method involves a direct short from the tone control input to the tone control output. ..that doesn't seem right. recalling the distortion which sounded like overdriven pre-amp tubes, .. i think that perhaps the only signal getting to the power amp section is a leaked signal. so, ... i replace the tone control caps, use contact cleaner on the pots and re-connect the tone controls to the circuit.

9. i fire up the amp again, ... eureka, ... it works, i insulate any of my solder joints which might possibly ground the chassis, put everything back, ..and embark on a re-arrangement of my audio furniture.


BTW, ... the big electrolytic cans seem fine, but i couldn't figure out how to replace them, ..they seem to have 3 contact points coming out of them, .... . how would i wire in conventional electrolytic caps as replacements ????
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Old 24th April 2003, 03:15 PM   #7
EC8010 is offline EC8010  United Kingdom
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Double electrolytics were quite common in the valve heyday. If you look at the can, you will find that the writing will say something along the lines of 50uF + 50uF 350V, or 32uF + 32uF 450V. The negative sections are commoned. Whip it out and fit replacements.

You haven't mentioned what sort of an HT rectifier your amplifier has. If it is a valve one, it won't like more than 47uF of capacitance. If it is selenium, then it's an explosion waiting to happen, and should be replaced before it takes out your mains transformer.
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Old 24th April 2003, 03:35 PM   #8
zobsky is offline zobsky  India
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it uses valve rectification

you are right about multiple cap. values noted on the electrolytics

so, ...do i try to find NOS replacements, , ..or are new double elctrolytics available.
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Old 24th April 2003, 03:51 PM   #9
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Default TWISTCANS

Hi,

For the caps you can try :

ANTIQUE ELECTRONIC SUPPLY

Cheers,
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Old 24th April 2003, 04:38 PM   #10
EC8010 is offline EC8010  United Kingdom
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Default The vintage panel-beaters will hate me...

NOS electrolytics will be poor. Use new components.
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