audio woes on the 38-5 - Printable Version
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audio woes on the 38-5 - NostalgiaRadioTime - 01-24-2018
Did the usual recap / checked & replaced resistors / using known good tubes. Was pretty straightforward, or so I thought. Just gave it a first try this evening, bringing it up on the variac. There's no excessive current draw, so all seems good there. I have not yet checked voltages, but the initial problem is lack of reception. There is no response from touching or clipping my long wire on the "RED" antenna terminal and the only way I can get the strong local station to come in...and play fairly strong...is to touch the gator clip of my antenna wire on the grid cap of the 6k7 tube. ON the tube itself. If I clip it to the grid cap wire attached to that tube, there's no reception.
The #66 electrolytic can was blown when I got the set....it was a replacement and the top was blown off of the can with a large puddle of dried molten wax all around the base of it.
I will dig into it deeper tomorrow evening but am wondering if, based on the symptoms I'm finding, if anyone has any thoughts on what to zero in on? Terry??
RE: audio woes on the 38-5 - morzh - 01-24-2018
Coils 1,2,7.
Does the oscillator oscillate? (Two radios howling test)?
RE: audio woes on the 38-5 - NostalgiaRadioTime - 01-25-2018
Thanks Mike. I spent the evening checking coils. I checked them all: # 1, 2, 7, 8, 13, 15, 23, 28 and they all checked good. None are open. I double checked the resistors too (some of the originals were within tolerance and I left them in... they weren't the old dogbone style). They are good too. So it's looking more and more like a missed connection, or a bad solder joint. That will be tomorrow night's project to check. I feel good that none of the coils are open. That would be a more major issue to overcome. .
RE: audio woes on the 38-5 - Radioroslyn - 01-26-2018
Check your shadow meter. Is it plugged in and is the coil good (abt 800-1200 ohms).
RE: audio woes on the 38-5 - morzh - 01-26-2018
Oh... they used one without shunt? Yep, very likely.
RE: audio woes on the 38-5 - NostalgiaRadioTime - 01-27-2018
Thanks Mike and Terry for the suggestions. Spent another evening gathering data. I started with measuring the shadow meter. I got 1400 ohms across the coil (reading across the pins with it disconnected). That seems within reason.
The voltage chart on the Philco service bulletin for the 38-4 / 38-5 is sparse. Mostly just plate voltages with some screen grids. Here's what I got: (all readings taken to chassis ground)
6F6 Plates read 274 VDC on each. For the screen grids I get 264 VDC on one and 285 VDC on the other
6K5 Plate reads 174 VDC
6J5 I get ZERO volts on the plate and on the screen grid. Just the 6.3 heater voltage
6K7 I get 267 VDC on the plate and 86 volts on the screen grid
6A8 I get 249 VDC on the plate / 86 VDC on the screen grid / 152 VDC on the OSC plate (as it's called on the voltage chart)
6U7 I get 276 VDC on the plate / 100 VDC on the screen grid
I also get 274 VDC on the primary's of the 1st and 2nd IF's
BUT, I get negative (-) 18 VDC on the negative side of the electrolytics (64, 64A, 66). This is from the center tap of the transformer to the negative common of the 3 electrolytics.
I measured the resistance of the transformer at the socket pins in the transformer for the 5Y4 rectifier and from the center tap of the transformer to the plates of the 5Y4 (transformer secondary windings) I get 106 ohms on one side, and 96 ohms on the other.
Based on this data, what could be killing the voltage to the 6J5 tube?
RE: audio woes on the 38-5 - Ron Ramirez - 01-27-2018
In this radio the 6J5 is used as a dual diode - the plate and cathode make up one of the diodes, and the grid and cathode is the other diode. One is used as the set's 2nd detector, the other for AVC. It is normal for the 6J5 plate to read 0 volts (or an extremely low voltage of 1 volt or less, likely less).
Also, a 6J5 has no screen grid, just a control grid. Again, in this application, the voltage will be zero, or very low.
Edit: As for the electrolytics...(64A) and (66) should read a negative voltage to ground (chassis). Ditto the positive lead of (64).
RE: audio woes on the 38-5 - Radioroslyn - 01-27-2018
All the voltage look good but check pin 5 of the 6a8. Should see a volt or three negative there. Also check the cathode voltages all should be low like 10v or less.
RE: audio woes on the 38-5 - NostalgiaRadioTime - 01-27-2018
Ah! Thanks, Ron. Didn't realize that about the 6J5 tube. Will check pin 5 of the 6A8 and the cathodes tomorrow.
This one is being especially stubborn in coming back from the dead. I'm not getting any response from the antenna (RED terminal) when I connect a long wire antenna to it (or even just touch it with my finger). Yet the antenna coil (and all other coils) ohm out to the values on the schematic.
The set howls and whistles, and at full volume it motorboats. I can only get the strong local station (somewhat) if I clip the antenna right to the tuning cap. I'm not sure the volume control is the original. It looks like it's been in there forever, but the shaft has a very narrow stem. So much so that I large shim is necessary to keep the knob on it. Wondering if that volume pot could be a contributing factor?
RE: audio woes on the 38-5 - NostalgiaRadioTime - 01-27-2018
Well here's where I stand tonight. I checked pin 5 of the 6a8 and get -1.3 VDC. I then checked all of the cathodes and each reads zero volts.
So I decided it's time to get out the signal generator and try to inject a signal. I was getting a signal to pass through the entire audio output section, moved over to the RF section and was poking around a bit. All at once it started to play! I was getting stations throughout the dial, albeit stronger / louder on the lower end of the dial. So, after letting it play a while I decided to upgrade a few of the tubes, as I had some that were Philco brand I wanted to use for originality. I replaced the 6K5, tried it, and it played fine. Moved over to the 6J5, replaced it, and, once again it reverted back to its old dead ways. So I put the original tube back in and STILL nothing!
I had it, and lost it
But, this tells me I have a bad connection or bad solder joint somewhere, and that's what I will concentrate on tomorrow.
So the status for tonight is close, but no cigar.
RE: audio woes on the 38-5 - morzh - 01-28-2018
Yes, it is pointing to a possible bad connection.
Take a plastic stick and poke around. Clean the sockets, rock the tubes.....the usual.
RE: audio woes on the 38-5 - Steve Chambers - 01-28-2018
Could be a loose or bad connection. Could also be a bad or weak 6A8 tube that refuses to oscillate or works intermittently. As suggested earlier in the thread have you tried listening for the oscillator signal on a known good radio that’s nearby?
Steve Chambers
RE: audio woes on the 38-5 - Electrothaumaturgist - 01-28-2018
I had a similar problem when I fired up the 38-1. Everytime I tried to put the RF cover on (with the chassis overturned), the sound would go out. Remove the cover, the reception would come back in. Went so far as to cut patterns of thin rubber and insulated the interior of the RF cavities, just in case there was a short. (Very tight tolerances there). Still had problems. Wound up being a solder connection between the rear of the tuning capacitor and the RF chassis. While your unit may not have the floating RF chassis as mine does, I'm sure it's something as simple as that! Poke around as Terry said, you'll find it!
RE: audio woes on the 38-5 - NostalgiaRadioTime - 01-28-2018
After 4 hours tiding up solder joints, changing a few resistors and micas in the area of the 6J5 I'm still no further ahead. No improvement. The set still howls and whistles and at full volume motorboats. I tried several different 6A8's with no improvement. I tried listening for the oscillator signal on a known good radio that’s nearby and that seemed to work. I was not able to get a strong station to come in very clear on the 38-5 but the one I was using was at 620 and I could hear the faint howling / whistling that was in the background at around 1100 on the dial on my freshly restored hippo I placed next to it on the workbench. When I turned off the 38-5, I could hear the whistling / howling go away on the hippo. So I think the OSC is working or trying to.
With my antenna wire clipped on to the grid of the 6K7, and my finger on the grid of the 6A8 tube, I can get that station AM 620 coming through loud and clear, sounding as it should. But that's the only station I can get. The local station (much stronger signal) on the upper end of the dial at 1470 doesn't come through with the same antenna / finger configuration.
Any thoughts on what to try next? I'm at a loss. I'm about ready to gator wire a new tube socket to the 6J5 and try that tube externally in a new socket to see if I'm not getting a good connection or have a damaged socket.
RE: audio woes on the 38-5 - NostalgiaRadioTime - 01-28-2018
Update: Just to eliminate the possibility of a bad tube socket, I gator wired in a new 8 pin socket to the pin connections of first the 6J5, then the 6A8, and tried the set with the tube in the new socket outside of the chassis (scary, but I made sure nothing was touching where it shouldn't be). In each case, it made no difference to the reception. It's as if the OSC coil, while it tests good, isn't working or something is fighting it keeping it from working. But why would it suddenly just quit working from simply trying a different 6J5 tube.
