RCA R8 radio

Good on the fact that the radio basically plays well but has the odd volume control problem. The table of voltages Frank mentions states that "Volume Control Does Not Affect Voltages" given in the chart. That fact may be helpful in finding the trouble. 

Well, from the schematic I am looking at (Rider's) there is a jumper shown between the cathode pin (4) and one of the filament pins-pin (5) as you have observed. Note that there is a separate 2.5V heater winding on the power transformer just for the 227 A.V.C. tube. So there will not be any difference of potential (measurable DC voltage) between pins (4) and (5) or for that matter between pins (1) and (4). Of course the AC heater voltage, around 2.5V, should be found between pin (1) and pins (4) or (5). I take it that the heater of the 227 tube is lighting up OK when the power is on. Did you also measure the dc voltage from the cathode pin (4) to chassis ground and got the minus 100 Volts? That is not given in the table of voltages for the model R-8. Note that the center tap of the power transformer is not grounded but goes to chassis ground thru L-9, the speaker field winding coil. This is not uncommon in the early radios as it often performs two or three functions: namely it provides the usual filtering action for the HV dc power supply, sets up the grid bias voltage for the 247 audio output stage and also is the source of a negative bias voltage to the cathode of the AVC stage. Looking at the voltage dividers (to chassis ground) set up by R-21 and R-22; and also R-17 and R-18 we can see how the proper negative voltage is obtained for the 247 control grid as well as the required negative voltage for the AVC stage, thru R-16, to the cathode (pin 4) of the 227 AVC tube. So there may very well be a highly negative voltage (-100V) from the 227 stage cathode pin (4) to chassis ground as you apparently measured. By the way, R-16 and C-22 looks to be a filter for the voltage obtained from the voltage divider R-17 and R-18.

I hope the points I made above will help to straighten out the volume control action problem. If the voltages you are getting for the 227 AVC stage are correct according to the table of voltages given for the set then I suggest you look elsewhere for the trouble.

73, Rod  WB6FBF

I took the voltages according to RCA's chart. All look good except the Heater, I get -100v. So took a jumper wire from pin 5 of the 47 tube and connected to pin 5 of the AVC tube , now I get 1.1 v on both pins. better but made no difference in radio performance . Let me say the radio plays great , just the weird volume problem. I did notice something , pins 4 and 5 of the AVC has a jumper across them. It looks factory but I don't see it in the schematic.
thanks for all of the suggestions.......

Rod- Yes I messed up on the pin connections. Took them directly from RCA schematic too. Thanks for catching that.

Hi, Thank you for all the reply's. I have swap the 27 tube with several others , all same results. All my voltages are to grd, I should have said that.  Yes I will follow the chart provided by RCA and what my voltages are then. This chassis is so darn tight. Oh yes this is an AC radio.

Good points and advice to Capt Drew, Frank but your pin out for the 227 tube is way off. Heater pins 1 & 5, plate 2, grid 3, cathode 4. I used to wonder about this same AVC circuit as used in the National NC-100 and NC-200 receivers that I had back in Lynchburg, VA and enjoyed so much. Not that I had a lot of problems with it in those sets. I'd say it is a type of DC amplifier and it gets its biasing (to cathode of the 227 AVC tube) from the most negative point of the power supply, namely at the center tap of the power transformer. This is not unlike the biasing used for CRTs in some oscilloscopes, etc.. Once again, good luck with your work there, Drew.
73, Rod  WB6FBF

Drew- It just hit me- Have you subbed out the [You must be registered and logged in to see this link.] tube with a spare?
 Also check the solder on the filament pins of it. Just re-solder anyway, come to think of it.
 It almost sounds like the tube heats up then something thermally fails. I have seen filament solder fail on tube pins and rectifiers ( Many!) too.
You could try swapping the AVC tube with the Detector [You must be registered and logged in to see this link.] to see if anything changes also.

Capt Drew- Where are you located?
Please add a little bit more to your profile information. Thanks!`

From RCA FACTORY literature- AKA Redbook for 1931/ 1932 RCA Service Notes: (Also Riders Vol 2 has the schematic

OK there are 2 R8 models lister in RCA service data. I will assume yours is the AC model. R 12 is the same as R8 AC model

What piece of equipment did you use to test the capacitors with? An ohmmeter test is useless on stuff this old. the capacitors need replacement. Also test the iron core interstage transformers. Frequent failure item, IIRC

Those voltage measurements  are way off, per factory data

 Were you measuring voltages to the chassis ground? 

NOTE WHERE THE BELOW MEASUREMENTS ARE FROM!!
 NONE ARE TO GROUND!!

Pinout for [You must be registered and logged in to see this link.] AVC tube

3 & 4 are fil
2 = plate
 1 = grid
3 = Cathode  

 Voltages should be :
 Fil = 2.66 V.
  H-K  = 0 V
  K to control grid = 0 V
  K to P = 25v

OK now you have some original factory data to go by.
 Yes, it is odd that they are measuring the voltages between elements and not to ground- but that is the RCA Way for this model.

So please re-check the voltages and post your findings. Then we can go further in assisting you.
 
 PS After 55+ years servicing- the oddball way RCA measures the voltages in this set  would have totally thrown me off too!

Please post your findings. We do want to help you get this going. If anything I posted does not make sense to you- let me know! The only stupid question is the unasked one. We all had to start somewhere. 

I check R 17, 18 and 23, all good. So I took some voltages of the 27 AVC tube. They look way off to me . I am only a hobbyist, so I could be wrong. They are Pin 2 =12mv, Pin 3 =-104v, Pin 4 = -103v , pin 5 = -105v. This makes no sense to me. So I am missing something. 

ugh

I overlooked R-17 and R-18. Check those too. They are connected to the center tap of the HV winding on the power transformer and the speaker field winding that provides grid bias to the 247 output stage. Also, you didn't mention if R-23 is OK. It is also part of the A.V.C. circuit. If all of these components check out then you will have to look elsewhere for the trouble. Good luck.
73, Rod  WB6FBF

Ok, another update. I check  R-1, R-4, R-9, R-10, R-11, R-16 . Allgood, check them all. I also check C-7, C-13, C-18, C-20 and C-22, all the values and wire correctly. Ugh...So the search is still on


thanks

Well, OK. I had to go back to your original post to check on what the problem was. That is that the volume control is not acting normally. Replacing the pot with another one didn't help. I have been thinking for a while now that the set has a problem connected with the A.V.C amplifier (UY-227) stage. At first glance it looks rather complicated. For reference, this set's AVC circuit reminds me of the AVC design used by the National Company in their late 1930s and early 1940s models of NC-100 and NC-200 communications receivers. OK on changing C-19, C-21 and C-25. I suggest to check all the other components in the AVC circuit. Here is a list of the remaining parts to check out: R-1, R-4, R-9, R-10, R-11, R-16 and R-23; C-7, C-13, C-18, C-20 and C-22. 
73, Rod WB6FBF

Ok, sorry for the last post. I found the problem. Broken wire. I must have broke it when replacing the capacitors. So I am back to the original problem.

Just an update. I change C 19, C 25 and C 21. There all the same type. No difference at first. Play the same. Next day I was listening to the radio, it plays very well. All of a sudden the volume slowly goes down to nothing. If I let the Radio cool down it works then same thing . Volume slowly goes down to nothing. I was thinking Tube. I check them, all check better then average. So now I have a new problem..yeah

A lot of times those were paper capacitors. They were frequently used for coupling from one stage to another. Good quality in their day. Such capacitors will surely be bad now.
73, Rod

Just FYI, C19 is a rectangular metal covered capacitor

Well, if C19 is bad and it sounds like it is-the AVC circuit will not work correctly and the overall gain of the receiver will be affected perhaps drastically. The diagram in Rider's shows it to be 9pF-a very small value but suitable for signal coupling in an AVC stage. What type or style of capacitor is C19? Is it a mica or ceramic? I take it from your measurement of .002uF that it is not just a twisted pair of insulated wires as sometimes is used to make very small value capacitors. RCA did make use their own special style of components in their radios (that no other manufacturer used) and sometimes it may be hard to tell what type it is from its general appearance, case style, etc.. Keep us posted on your progress.
73, Rod  WB6FBF

Hi , i check all of my tubes again. All tested very good. The 30k resistor, R12 reads 33k. Don't think that is the problem. R20 has always concern me on the schematic. Don't get it. One think I found , don't really think it would make much difference is C 19 schematic says .00001 {about}, I read .002. I also check some voltages, the AVC tube concerns me pins 2 and 5 read -112v. I think that is off. I will keep looking.

thanks

Note: This is a revision from my earlier post on your radio.
I took a look at the schematic diagram for your radio and I have to say that the volume control circuit used is the most unusual looking one I have ever seen. But then the RCA R-8 is an early 1930s model and it is not unusual to find non-standard circuitry in such sets. The 227 "Second Detector" tube is configured as a plate detector. The large cathode resistor (30K Ohm) indicates that it is of that type. The signal input is to the grid of the 227 tube. It appears that volume control action is achieved by way of C-24, L-12  and R-19. In addition, L-12 appears to be coupled to the iron core IF transformer L-10 & L-11. How exactly the circuit controls the gain of the set I can't say. I have had a few ideas about it though. When working I guess that it would be fine but since it is not working properly something is wrong in that area of the set. What R-20 is doing in there is a mystery too. It looks like one end of it isn't even connected to anything. If you can't find the trouble or should it turn out to be related to L-10, L-11 and L-12; or R-19 and R-20 turns out to be  some kind of odd-ball control (which in either case you probably couldn't get replacements for) then it might be possible to try out a more conventional audio volume control at the output of the 227 detector stage or at the grid input of the 247 audio output stage. Hope this helps.
73, Rod  WB6FBF

OK, thanks , I will look

One Idea is to Look to a defective tube. If, as it heats up the filament opens or shorts to the cathode, then cools down and the process repeats.
Also a defective resistor that opens up doing the same pattern.

Yes exactly

I am not sure what RadioMuseum.Org has. It is a RCA R8 Tombstone radio.

Is this the Radio you are referring to? 
Its posted at  RadioMuseum.Org
Click on the link below.
RCA R8