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 Post subject: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Fri 17, 2011 3:10 am 
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Joined: Aug Thu 27, 2009 7:47 am
Posts: 2129
Location: Seattle
I'm trying to help identify this for a friend...he sent me these photos:

Image

Image

Image

tag says "mi-38618A s/n C1885"

Rubber stamped on chassis is 102661. Tube lineup is 6SL7 6SL7 6V6 6V6 5Y3

Anyone point me to a schematic for this guy, more info, etc.? Is the model MI-38618A? Grille Cloth DBs list a couple of Matco amps, but not any RCA, with this tube line-up.

I could really use a schematic (edited for clarity.)


Last edited by jkoebel on Jun Fri 17, 2011 3:50 am, edited 2 times in total.

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 Post subject: Re: Help identify an unknown RCA PA amp?
PostPosted: Jun Fri 17, 2011 3:46 am 
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Joined: Jan Mon 12, 2009 7:38 am
Posts: 885
Location: miami fl
you have a model number, the MI number and a brand. how much more ID do you need?

im not following what your after?


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 Post subject: Re: Help identify an unknown RCA PA amp?
PostPosted: Jun Fri 17, 2011 3:48 am 
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Joined: Aug Thu 27, 2009 7:47 am
Posts: 2129
Location: Seattle
Er, a schematic? I can't find one anywhere. Been searching every place I know to look.


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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Sat 18, 2011 3:02 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3059
Location: Boston, MA USA
It's going to be pretty simple. One 6SL7 is the microphone preamp, the other 6SL7 is the voltage amplifier/phase inverter, the two 6V6s are push-pull outputs, and the 5Y3 is the full-wave rectifier. You can probably find a very similar schematic in a mid-1950s RCA tube manual circuits section.

-David


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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Sat 18, 2011 3:39 am 
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Joined: Jun Sat 18, 2011 3:36 am
Posts: 730
Location: Milton, FL 32570
Actually tubes are 5y3gt
6sl7gt
6sj7
6v6gt
6v6gt
.

I was wondering if anyone had a rough estimate on the year. And what does MI correspond to inthe model number?

Thanks,
Jason


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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Sat 18, 2011 5:06 am 
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Joined: Aug Tue 24, 2010 8:56 pm
Posts: 3466
Location: Florida
If the unit is complete and unmolested, you could probaby get by without a schematic; these were not difficult to work on.

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William


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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Sat 18, 2011 5:20 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 12311
Location: Albuquerque, NM 87123
Many of RCA's amplifiers started with MI; I don't know what it means.

I checked Riders PA manual and it's not there; or in Sams.


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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Sat 18, 2011 6:30 am 
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Joined: Jan Mon 12, 2009 7:38 am
Posts: 885
Location: miami fl
id guesstimate your amp is from the early to mid 50's. you can rebuild it without a schematic. replace one part at a time. replace all the caps. take macro pictures before you start work as it will help you figure stuff out if you get in a bind.


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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Sat 18, 2011 1:55 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 6421
Location: Beaver Falls, PA. USA
RCA commercial equipment, including theatre and broadcast stuff, was all identified with the "MI" numbers,which stood for "Master Item". That equipment was covered in RCA's own service/instruction literature, which was in loose-leaf form. A copy, containing schematic, pictorial, and parts list, was included with each unit.

A few, but not many, MI-numbered amplifiers got covered in Rider's and Sams.

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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Sun 19, 2011 4:43 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3059
Location: Boston, MA USA
reeves03 wrote:
Actually tubes are 5y3gt
6sl7gt
6sj7
6v6gt
6v6gt
.

I was wondering if anyone had a rough estimate on the year. And what does MI correspond to inthe model number?

Thanks,
Jason

In that case the 6SJ7 (a pentode) is the microphone pre-amplifier and the 6SL7 is the voltage amplifier/phase inverter. The microphone input goes to the grid of the 6SJ7 (perhaps via a potentiometer) and the phono input probably goes to one of the grids of the 6SL7 (maybe via a potentiometer, likely not).

The MI number is the model number on RCA commercial equipment (not RCA Victor).

-David


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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Sun 19, 2011 4:51 am 
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Joined: Jun Sat 18, 2011 3:36 am
Posts: 730
Location: Milton, FL 32570
Cool appreciate all the info. I was just hoping for a schematic just like I look for a map before I go somewhere new.

Thanksfor all the help.

jason


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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Sun 19, 2011 5:00 am 
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Joined: Jun Sat 18, 2011 3:36 am
Posts: 730
Location: Milton, FL 32570
I know this may be old hat to many of you, but I'm trying to break into this. And was wondering if anyone has any tips or reference to drawing out circuits by looking at the chassis wiring. Forgive me if this sounds dumb but I think it'll help me understand more of what I'm doing.

thanks,
jason


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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Wed 22, 2011 1:56 am 
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Joined: Dec Mon 20, 2010 7:23 am
Posts: 483
Location: Minneapolis, MN
When "reverse engineering" something I usually start with the active elements....tubes...transistors. Get the pin out for the tube. Find each component from there. Then from there you can trace out the power supply.

For example lets say you identify a tube's plate load resistor. Most of the time the other end of that is going to wind up at a power supply voltage.


Work slowly and carefully and make your drawing. It doesn't have to be that neat yet.

Stop when you get tired or frustrated and come back to it later.


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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Wed 29, 2011 4:14 pm 
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Joined: Jun Sat 18, 2011 3:36 am
Posts: 730
Location: Milton, FL 32570
I would like to replace the multistage capacitor can on this amp(individual caps are fine).

Here is the info on the cap can

CDE
DD0185A
30 to 40 MFD 500 W.V. with half moon identifier
30 to 40 MFD 500 W.V. with square identifier
15 to 20 MFD 500 W.V with triangle identifier
15 to 20 MFD 500 W.V with no identifier
UPN 10690


to replace this I would assume getting 2 x 47uf 500v and 2 x 22uf 500v caps and wire them in or should I get a lower value? Final usage would hopefully be a guitar amplifier.

Under chassis I have 2 x 50 MFD 50 WVDC and 2 x 10 MFD 25WVDC
can I replace these with 47uf 50v and 10uf 25v?

thanks,
Jason


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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Wed 29, 2011 4:18 pm 
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Joined: Aug Thu 27, 2009 7:47 am
Posts: 2129
Location: Seattle
Those substitutions all seem pretty reasonable to me, that's exactly what I'd do if I were repairing it.


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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Wed 29, 2011 6:26 pm 
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User avatar

Joined: Jan Sun 24, 2010 7:59 am
Posts: 6171
Location: Pro Tech, Philadelphia Pa.
reeves03 wrote:
I would like to replace the multistage capacitor can on this amp(individual caps are fine).

Here is the info on the cap can

CDE
DD0185A
30 to 40 MFD 500 W.V. with half moon identifier
30 to 40 MFD 500 W.V. with square identifier
15 to 20 MFD 500 W.V with triangle identifier
15 to 20 MFD 500 W.V with no identifier
UPN 10690


to replace this I would assume getting 2 x 47uf 500v and 2 x 22uf 500v caps and wire them in or should I get a lower value? Final usage would hopefully be a guitar amplifier.

Under chassis I have 2 x 50 MFD 50 WVDC and 2 x 10 MFD 25WVDC
can I replace these with 47uf 50v and 10uf 25v?

thanks,
Jason


I dunno, that stinkin' little 3uf difference might be an issue.... it seems that tiny amounts of difference of capacitance cause lots of confusion on here.

Amateur tinkerers simply don''t know how to round things off and call it a day.

_________________
"Accept the fact that some days you are the pigeon, and some days you are the statue."


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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Wed 29, 2011 8:26 pm 
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Joined: Jun Sat 18, 2011 3:36 am
Posts: 730
Location: Milton, FL 32570
What about the multi-stage being listed as 30-40MFD and 15-20MFD?

Is this just a funny way of saying its tolerance factor?

Jason


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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Thu 30, 2011 1:06 am 
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Joined: Jan Sun 24, 2010 7:59 am
Posts: 6171
Location: Pro Tech, Philadelphia Pa.
Tube - equipment is very tolerable and forgiving when it comes to tolerances.

_________________
"Accept the fact that some days you are the pigeon, and some days you are the statue."


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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jun Thu 30, 2011 1:24 am 
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Joined: Jun Sat 18, 2011 3:36 am
Posts: 730
Location: Milton, FL 32570
Maybe so but the question is still there.

jason


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 Post subject: Re: Schematic for unknown(?) RCA MI-38618A amp?
PostPosted: Jul Sat 02, 2011 4:00 pm 
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Joined: Nov Sat 27, 2010 6:15 pm
Posts: 4859
To answer your question, electrolytic capacitors made 50 years ago had a pretty large variance from the nominal value 20% more or less was common. Therefore the range rather than nominal value was sometimes stamped on the parts. They all drift up in value and leak after decades, or short, or become completly open and should be replaced. Plus 20% or minus 10% is close enough for any audio application. I would use 450 VDC electrolytics and 630 VDC mylar caps in all cases just so I don't have to keep so many different parts on hand. Always OK to use higher rated parts.

While you at it, you could check the carbon resistors, and you will find most of them have drifted way up. So cheap and easy to replace them while you have the set apart, and can make a big difference in performance.


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