Federal AM864 troubleshooting help

[tubemonkey35]

NYDave, you don't have any more of those huh? It's hard to find anything over 1/2 watt these days so it seems.

 

[ToobieSnack]

hi everyone,
soundguy asked a question that I was interested to know also and has gone un-answered.

this (diodes) rectifier for the meter is very strange looking .... looks like a barrel ...kinda like a wire wound resistor...with 4 wires ... ?
has anyone ever seen these before?
are they reliable? should they be replaced?
(i think soundguy and i would prefer it...but hey... maybe these parts are actually better???...they really look strange....:shock: )

can anyone offer an opinion and maybe a substitute for these...?

Also: just curious let's say I wanted to DIY one of these...
what sould be a good replacement for the choke (L1)?
I assume that filter section could be updated and done with caps only. eliminating the choke.

As allways your kind comments, help and suggestions are ..much..appreciated
thanks
ts

PSsoundguy you think I could get a copy of that manual :grin: or maybe you could post some of the important info.. you have posted some of the voltage specs for the tubes (thanks it was extremely helpful) ...are there spec for the turret board junctions?

PPS thanks all for posting your experience with these myserious current and threshold adjustments...I have learned a lot about my unit and have done some initial testing using the info from here...thanks again

 

[NewYorkDave]

The "mystery component" is just a packaged diode bridge, probably made from copper oxide rectifiers. The closest available subsitute I can think of would be a bridge made from germanium diodes such as 1N34A.

As for the power supply: no, you can't just add more caps and get rid of the choke. The choke forms an inverted-L lowpass filter along with the filter cap that follows it. Eliminating it would result in much more ripple. Besides, filter chokes are not hard to find; Hammond makes ones that are reasonably priced. The value is right there on the schematic: 15H. The Hammond 158L, at about 17 bucks, looks like a suitable part. It's rated for 75mA and its DC resistance is 400 ohms.

It would be possible to use a resistor (of suitable power rating) instead if you increased the raw B+ by a suitable amount. But for the amount of resistance you would need to get the same effective filtering as given by the choke, it's pretty wasteful of power--power that's spent in heating up the resistor! The regulation would be pretty poor, too, which is an issue when you're feeding a circuit where the current draw goes up and down--such as a tube compressor!

As an illustration, the reactance of a 15H choke at the ripple frequency of 120Hz is 11.3Kohms. You'd have to use a resistor of 11.3K to get the same effectiveness in reducing ripple... and then you need to allow for the whopping voltage drop across that resistor, and the power dissipated by it.

This is why, despite their expense, you almost always see filter chokes used in tube circuits where appreciable current is being drawn.

 

[ToobieSnack]

thanks dave
as always excellent info

one question:

The "mystery component" is just a packaged diode bridge, probably made from copper oxide rectifiers. The closest available subsitute I can think of would be a bridge made from germanium diodes such as 1N34A.

these germanuims are they still in production?
NOS my only choice?
any silicon option?

AND while we are speaking of some DIY of this:
what would be suitable I/O and power trafo's for these
600/10,000...10,000/600 ....would these be similar to la2a iron listed in CJ's "transformer list for projects" ?
120/220v pri with taps for 5.0 2A/6.4 2A/175/CT/175.

i would really like to DIY this I think I have these exact meters (2)...I'll get them out and pic them and get the number off of them for verification.

thanks again
ts

 

[gyraf]

A selenium bridge rectifier would probably be the best option nowadays:

http://www.elfa.se/elfa-bin/setpage.pl?http://www.elfa.se/elfa-bin/dyndok.pl?dok=2009909.htm

Jakob E.

 

[ToobieSnack]

thanks jacob
CJ (an others) what about these trafo's
later
ts

 

[soundguy]

when I bring up that page in english, I dont get a price. I emailed them to get a price, Im likely going to order a few, if there's anyone in the US that wants to piggyback, I dont mind adding a few extra to my order. Speak up soon as I'll be ordering these as soon as I get the quote.

dave

 

[ToobieSnack]

i'll take three
ts

 

[gary o]

Hi guys, Hi Toobie, I DIYed one of these few months back, had bit of an issue with the H ladder attenuattor on the input, havent been messing with electronics for short time I quickly forget the little that I have learnt, il fire the thing up & check it out see if I can provide some info from experimenting, I have a Sowter input tran & Sowter BA6A output tran with homemade Daven T atten on output & my pots are all on front panel I just turn till I like the sound, FWIW I like this thing on vocals.

Cheers....................Gary O.

 

[ToobieSnack]

WOW!very cool!
looking forward to pics/info
I have a ton of daven attenuators laying around here.... maybe I have the righ ones. :grin: H type huh? hmmmm.... I know I got some T's.
thanks
ts

BTW sowter ba6a input AND sowter ba6a output? correct?
this MUST sound nice...the trafo's on the originals have some frequency limitations...

 

[NewYorkDave]

Why f*ck around with ordering selenium bridges from Europe when four 17-cent 1N34A germanium diodes from most any US supplier will work just as well if not better? The forward voltage drop of selenium rectifiers may be too high for this application anyway.

And yes, Toobie, they are still made and still available. If you punch 1N34A into Google you'll find several places to buy them. And the same thing with the transformers: you can find several choices just by letting your fingers do the walking.

 

[PRR]

> Why f*ck around with ordering selenium bridges from Europe when four 17-cent 1N34A germanium diodes

Yeah.

> these germanuims are they still in production?

Who cares? Millions have been made. While some suppliers price them like gold, others have bucketfuls and will sell you a lifetime supply for $20.

Circuit Specialists

Same place, non-US customers, but minimum $200.

American Microsemiconductor, won't sell less than 64 at a time.

V/I curve, these guys will sell you 10 for $14.50

SPICE model

 

[soundguy]

thats a much better idea than ordering a single part from overseas if it will work!

Ok, so the update, previous owner had bypassed the filter can with 47u/450 caps, I took THOSE out, reattached the can and the hum in limit mode is now gone. Hooray. Got some power supply hum so Ill put in new filter caps, but will try to use closer to the values in the schematic.

With the old cap can, b+ is 190v, 10 volts less than the 200 spec'd in the schem. I *STILL* get this wierd off scale meter thing in gain reduction mode. Removed and checked R1, its good. Im not gonna waste too much time until I put the new caps in, but I'll be suprised if that fixes itself. In circuit, I have very close R values on both units, I'll be glad when the hum is gone but would love to understand wtf is going on with the meter.

I'll post more when I install the caps.

dave

 

[NewYorkDave]

That's funny: the replacement caps became worse than what they replaced! Then again, once they were added, they were handling the lion's share of the ripple current and "working" harder.

I was pretty sure the filter caps were at least partly to blame, but naturally I figured the older caps as most likely suspects.

I'd use 22uF caps for the replacements, 350V or greater. That'll give you a bit better filtering than stock but won't be so much extra capacitance that it'll stress your rectifier or filter choke. Get 105-degree C caps if you can find them in suitable values and voltage ratings. I think you can even still buy new multi-section cans in a limited selection of values if you absolutely MUST have a can.

 

[ToobieSnack]

I'm sure most know this but...FWIW

you could also empy the can and fit the new caps inside it .....since the new ones will probable be more modern.. thus smaller.....this is done in a lot of vintage radio repair where the asthetics of the original electronics is desired...

that's what I probably do with mine...if needed

later
ts

 

[NewYorkDave]

Dave, are you coming to AES on Tuesday night? I can bring you some germanium diodes. It'd save you the trouble of ordering them.

 

[soundguy]

dave, thanks for the offer, I dont think I can make it on tuesday.

dave

 

[ToobieSnack]

Dave I am eager to hear of your progress...
have you installed the new caps yet?
I look forward to you voltage measurements after this cap replacement.
Also would it be possible to get a copy of your user manual? please?
I have schematics and there are many posts on the operation...
I need specs the most....some you have posted ...some you have not...
can you help me?
I am eager to post measurements for my unit but...
for better continuity of this thread I am waiting for you to finish up..
I would like to be ready ...
thanksfor all the valuasbel info you (and others) have posted.
I can't wait to hear this unit in proper operational condition.

your help will be sincerely appreciated.
thanks
ts

 

[soundguy]

caps came today.

With 47u in place of A and 47u in place of B+C for C4, the limiter wasnt working right. I removed those, and with the old cap can, everything was working fine, albeit a little noisy. the voltages I had were 192 for A and 185 for b+c.

Today I installed 22u for A and two 10u for B and C. Unit is real quiet and works fine however the voltages I have now are 194 for A and 187 or b+c. I was hoping for 200v.

Any thoughts on this? Im pretty much ready to put it in the rack, it sounds good, it works, I think I want to use it for a while before tweaking more. The meter still pisses me off but I'll live with it for now.

One question- Is there any benefit for using 2 10u caps in parallel (as shown on the schem with 2- 12u schems) vs. using one single 22u cap for b+c? I know I should try and see for myself but I dont have an extra 22u cap today. boo.

dave

 

[ToobieSnack]

!!!great news!!!... dave
congrats! :grin:
can't wait to start posting data on my unit..
I'll need it for these ribbon mics I have now. :grin:

later
ts