Bogen RP-150BM

[Jdownj]

I just won an eBay auction for a Bogen RP-150BM. I was looking for a project, and(happily) won the auction for a low price without doing a whole lot of research. It has issues, and is missing parts(top of enclosure, etc) but appears to be complete electrically. This is a mono unit, AM/FM receiver and PA amp. The seller describes one of the EL84 finals as getting "too hot", probably red-plates and burned up.

Does anybody know how/where to get any service data on this? I see that radioera.com has a service manual for $25, is that my best bet, or is there another place I should be looking?

Nathan
KK4REY

[Jdownj]

Just adding some pictures:

[Jdownj]

Today it arrived, and the eBay seller included a photocopy of the original manual. I'm adding a picture of the schematic:


If anybody else works on one of these, I'm happy to scan the entire manual. Right now I'm putting together a Digi-Key order for the caps and resistors that I anticipate using. Need to see if I can find someone with a tube tester locally.

[init4fun]

Hi Nathan , Welcome to VK . That's a sweet looking radio / amp ya got there and bonus points for it coming with the schematic . You mention one of the EL84 outputs possibly red plating . As you know any of the resistors could and most likely have drifted in value and most if not all of the electrolytic and paper types of capacitors are suspect as well .

Something curious I noted on the schematic , the circuit calls for 415 volts on the plates of the EL84 outputs , and yet every tech sheet I have on those tubes lists a maximum plate voltage of 300 to 330 volts , depending on if it's RCA's specs (300V max) or Phillips or Mullard's (330V max) . Are the really standing THAT hard on those tubes , pushing them that far over their design rating , to get a maximum output level , tube life be damned ?

[Jeffhs]

Originally Posted by init4fun

Hi Nathan , Welcome to VK . That's a sweet looking radio / amp ya got there and bonus points for it coming with the schematic . You mention one of the EL84 outputs possibly red plating . As you know any of the resistors could and most likely have drifted in value and most if not all of the electrolytic and paper types of capacitors are suspect as well .

Something curious I noted on the schematic , the circuit calls for 415 volts on the plates of the EL84 outputs , and yet every tech sheet I have on those tubes lists a maximum plate voltage of 300 to 330 volts , depending on if it's RCA's specs (300V max) or Phillips or Mullard's (330V max) . Are the really standing THAT hard on those tubes , pushing them that far over their design rating , to get a maximum output level , tube life be damned ?

That doesn't seem right to me, either. If you are getting red plates on the EL84s, the tubes are definitely being overloaded. That Bogen system is likely old enough ('60s or perhaps late fifties, as evidenced by the Conelrad icons on the AM tuning scale) to have been designed with enough reserve as not to overdrive anything in the unit. I agree that the problem is almost certainly being caused by resistors drifting in value and/or weak or failing capacitors. I'd replace the wax-paper caps as a matter of routine, as they are almost certainly beginning to fail if they haven't done so already.

BTW, I would not expect a tuner/amp system such as your Bogen unit to have been purposely designed as to overdrive the output tubes; when this system was made, such cost-cutting measures, at the expense of tube life, simply were not employed, unlike many of today's radios, TVs, etc. in which every component in the set is being driven to within an inch of its life, if not severely overdriven to the point of early failure. Again, red plates on any tube means the tube is being severely overdriven and will fail in a very short time, so don't operate the chassis until the cause of the red-plating problem has been identified and the problem corrected.

[maxhifi]

Just change those 0.022uf coupling caps, and check voltages. I think the 200V voltage rating is too low, 400V would be better, also check the 150 ohm reaistor for value and that the cathode bypass cap C10 isn't shorted. I wouldn't be surprised if the electrolytics are fine but the coupling caps could be leaky.

The plate voltage is fine, just use good tubes..

[Jdownj]

Of course, I won't apply any power until the caps are changed, and resistors are checked or changed. I want to test tubes, at least the EL84s before power up as well. I have no idea how long the previous owner ran it red plated.

There are some who would fight harder to keep something more original, but I always test electrolytics by seeing what noise they make as they hit the trash can. This will probably be my intro to re-stuffing chassis mounted electrolytics, but I've been watching members of this forum on YouTube and I'm pretty sure I have a handle on it.

I won't be able to actually work on this for a few weeks, so I'm searching for an appropriate speaker to pass the time.

[maxhifi]

I usually don't bother to change the FP cans if they're still good, which seems to be most of the time in my limited experience, for quality equipment newer than mid 50s. Cardboard tubular electrolytic caps though are always bad. Those coupling caps put the EL84s in danger, which is why I would change them right away. Some of the others may be okay to leave, especially if they don't have any DC voltage on them in service.i guess it depends on how often you plan to use it and now reliable it needs to be, etc. I have original FP can electrolytics in both my mid sixties daily watcher RCA TV sets without any ill effects.

[old_coot88]

Be sure and check the 33K 4 watt resistor coming off the cathodes of (pin 3) of the EL84s. If it goes open, it will definitely increase plate current.*

*It'll make the cathodes less positive (more negative), which makes the (G1) grids more positive with respect to cathodes. Ergo, higher plate current.

[maxhifi]

I wonder why that reaistor is there. It looks like part of a voltage divider to stabilize the g2 voltage but why terminate it at the cathode and not at ground?

[Jdownj]

Just a quick update, tested all the tubes, all were good except the 6X4 rectifier that had a blown filament. I have sourced another tube, but I still haven't ordered the caps.