Here is where a can of freeze spray can sometimes serve to help with diagnostics. I serviced a Collins 516F2 power supply used with a KWM-2 and found that the molded socket for one of the rectifiers had an internal short that could not be seen from the outside.
-----Original Message----- From: Richard Knoppow <[email protected]> To: Randy WB4SPB <[email protected]>; drakelist <[email protected]> Sent: Fri, Feb 10, 2012 12:27 am Subject: Re: [Drakelist] R4B audio distortion A look at the schematic indicates that you have hit all the usual suspects. So, what else could cause the control grid voltage to go positive? I wonder if the tube socket is leaky. Even if the tube is on an extender it might be the socket. It might be something on the surface of the socket between pins. Pin 6 is the screen grid so leakage from that could cause the problem. Check the socket for carbon tracks or anything on the surface. Clean it thoroughly with dry alcohol. Temperature might make a difference if there is either a carbon track or something else there. Perhaps far fetched but easy enough to do and worth a try. If cleaning makes a difference but does not cure the problem a new socket might be in order. -- Richard Knoppow Los Angeles WB6KBL [email protected] ----- Original Message ----- From: "Randy WB4SPB" <[email protected]> To: <[email protected]> Sent: Thursday, February 09, 2012 8:53 PM Subject: [Drakelist] R4B audio distortion > Hi folks, > > I've owned my beloved R4B since the mid-70's. From about > 1985 until 2000, > it was stored while I got busy with job/family. I used it > again for several > months in 2002 and 2003, and I didn't notice anything > unusual. Now I am > active again and want to use it full time, but I am > hearing audio distortion > after about 5 minutes of operation. > > Poking around, I observe the cathode of V7 is at higher > voltage than > expected, and the grid has more than 2 volts, where 0 is > expected. Coupling > cap C185? No, lift one side, and no change. R154 has the > correct value. > Cathode resistor? R44 has the correct value, and I tried > a different one > anyway, in case it was thermally unstable, with no change. > I've replaced > C176 (along with the other small electrolytics in the RX), > no difference. > > I haven't tackled replacing the big electrolytic can yet, > but voltages and > ripple seem pretty much OK on all terminals of that unit > (e.g. 7mv at C90A, > and 35mv at C90D, 75mv at C90C). > > At power up, the V7 grid is slightly more than 0V, cathode > voltage nominal. > As time goes on, these creep slowly upward. After 5-6 > minutes, the grid is > approaching 3V. > > 30 seconds after power up: > > V7 pin 1: 4.8 > pin 2,5: .006 > pin 6: 134 > pin 7: 146 > > 5-6 minutes after power up: > > V7 pin 1: 6.2 > pin 2,5: 2.9 > pin 6: 129 > pin 7: 139 > > I note screen and plate voltages have fallen off a little. > If they were a > little high to begin with, it may be because my line > voltage is sometimes a > bit high: I measured 122VAC today. I presume the drop in > these values > reflects increased current through V7. I get the same > basic behavior when > I have the receiver on a variac and control the line > voltage to something > less than 120VAC. > > The audio sounds good for the first several minutes, but > it becomes harsh > sounding at some point. I don't know what the output > waveform looks like. > > So I could feel better about V7 running hard while I poked > around, I put a > fan on it, above the chassis ... and lo, the grid/cathode > voltages started > to drop. The grid stabilizes at about 0.3V with a healthy > amount of forced > air on the 6EH5, and the audio sounds good. Conductive > socket material when > hot? I don't think so: I put the tube on a 1.5-inch > extender to reduce > heat at the socket, with no change in behavior. Bad tube? > No, two others > behave the same way. > > I've researched other R4B audio distortion discussions, > some on this list. > None seem helpful. I'm stumped... any ideas? > > Thanks and 73, > > Randy WB4SPB > > > > _______________________________________________ > Drakelist mailing list > [email protected] > http://mailman.zerobeat.net/mailman/listinfo/drakelist _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
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