diagram needed
 

I just picked up a portable rca model 6-bx-8 and need a diagram please. or the Sams set/folder number for it and my capehart by farnsworth 21n2, thanks.

In the Beitman file there are two models, the 6BX8A and the 6BX8B. I didn't look for the Capehart one but it may be in there too.

I got the Beitman files on line; Google them and download the whole thing.

I've never heard of beitman before but I will try it. I have a riders for the capehart but can't read all of it well. The rca I couldn't find a riders for from nostalgiaiair.

If you are unable to find Beitman, I can look for a link or, failing that, can convert to files which could be uploaded.

Okay try this:
http://makearadio.com/beitmans/index.php

I found it, thanks. I will download what I want tonight after I get home from work.

Beitman didn't have the diagram for the capehart, does anyone have a master index to look that sams up for it?

It's not in Sams.

thank you for checking

Is this it?

http://www.nostalgiaair.org/PagesByM...8/M0005788.pdf

For the capehart,yes.That is the only one I can find. I do not have riders at home to look at and nostalgiaair's copy is fuzzy on the diagram itself. A couple of the micas I need to replace are in the fm detector and osc. circuit. the fm staions from about the middle of the dial to the right float for about 20 minutes. ex. 107.5 starts at 106 something and drifts over to 108 and then stays there. I can't read all the values or the parts list number on the diagram well enough to be comfortable to change them . atleast one of them I can't tell the color code on the mica itself. Maybe my electronics cleaner took the paint off? So I'm not sure what caps to buy. My eyes aren't what they used to be.....

I looked and can read the mmf. values of the micas on the NostalgiaAir schematic. If you can identify the caps you want by where they are located in the actual set, i.e., going from terminal X to terminal Y, we could locate on the schematic and determine the value.

Ok Reece, lets see... on page 22 of the pdf which is the second part of the diagram the first cap is the one coming off of the grid of the fm osc. tube that is in parallel with the 33 k resistor. #2 is the cap going to pins 4 and 5 of the 6h6 disc. tube. #3 and 4 are the two caps in series from pin 6 and 3 of the same discriminator tube 6h6.

Oh, by the way what does" N-750 " below the cap on the diagram mean? There are a couple that have that written by them.

N750 means negative coefficient of capacitance with temperature, -750 ppm/C

So that would be a temperature stable cap? correct?

No, a temperature stable capacitor would have a zero temperature coefficient of capacitance. The -750 has a calibrated drift, decreasing in capacitance as temperature rises.

Was that the highest quality you could get back then or just what they wanted in the osc. circuit probably?

It's not a quality issue. Yes, the calibrated temperature coefficient components were used to reduce the temperature dependence of resonant circuits.

Capacitors have several purposes. When they are used in resonant or timing circuits, their capacitance stability is of importance. In filters they are less critical. For bypassing and for power supply ripple control they are not critical at all.

Is it going to be critical for me to replace a cap (n-750) with a temp stable or will new manufacture mica/ceramic be ok?

I have no idea what circuit uses that in your unit, but if they specified a coefficient, it's probably wise to stick with it. I don't know if you still can buy those but I see no reason why not.

Before you order anything, look on the data sheet for the part (yes, capacitors have data sheets too) and see what temperature coefficient of capacitance it has. Do a little survey, perhaps of the bigger manufacturers and see what they offer.

On the other hand, if you are restoring this and don't plan to use it much, it probably doesn't make a lot of difference.

from a previous post I was told to use temp stable in critical circuits but these calibrated temp coeffecient caps just kind of caught me off guard. I will do what I can to get similiar caps. This set is going to see a lot of use once I am done. Still need a couple mica/ceramic caps and then I will tackle the turntable. Its a webcor needs a little work which I will post later.

Why are you replacing the micas? Usually they are OK. I think it's best to replace the papers and electrolytics and then see how the set operates. If you shotgun the micas you may have it way off alignment in a complicated set.

Edit: whoops, I forgot in the beginning you said the set drifts. Pay no attention to my query!

Maybe its an alignment but also if I am not sitting perfectly on a station the set makes a thunk thunk thunk sound in a repeating pattern on fm.

It was earlier today brought to my attention that maybe my issue with fm drift is me not letting the set warm up enough. I turn it on fm, set it on a station, take a shower and I will need to retune it.From there it stays locked in period. if I turn it on am , take a shower and then switch to fm it stays locked in. Ok so once again maybe this is a learning issue......this is my first fm set. Opinions anyone?

It's just temperature. With the proper temperature compensation components it shouldn't drift so much that you ever have to retune. So that -750 part may be important after all.

Some engineers would cheat by adding a transformer to keep the oscillator tube warm but that only reduces drift caused by tube temperature variation. Another approach is to make the whole thing a temperature controlled oven, with a thermostat and heater/resistor so that there is essentially no warmup drift. Or there is the compensation approach. None of these is ideal, as they only reduce drift. It's impossible to eliminate it, short of synchronizing with an external or atomic standard.

Some modern radios use a synthesizer, with an accurate crystal oscillator as the standard. My ham radio transceivers work that way; they drift less than 0.5 ppm, something not achievable with an LC oscillator.

I really need to get more books on radios and start reading !