Sorta newbie 38-690 low volume - Printable Version
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Sorta newbie 38-690 low volume - Hot Rod - 11-29-2017
Hello all, my first post!
I've had success with a handful of table tops but this bad boy is giving me a spanking! I have technical skills but am fairly new to toobs, mostly automotive stuff (hence the username). I own a DVM with cap tester and basic tube tester, that's all right now. The 690 receives ok on all bands (with my crap-o antenna). Somebody has been in there before me. Some lytics were replaced.
What I've replaced:
- All lytics and paper caps, excluding any mica caps and anything under the IF expander cans.
- All resistors except two buried in the RF chassis under the switch assembly, including the candohms with ceramics.
- Replaced the X-over 1mic cap with a 4.7 film cap.
- Changed polarity on one tweeter VC.
- Cleaned and lubed everything.
- Tubes test ok. Replaced both 6L6s with Tube Doctor units.
- The 3 xfmrs and 2 chokes on the lower chassis ohm pretty close. I have since read that I should have removed all tubes, which I did not.
- Speaker wiring, frayed but no shorts or opens.
- All 3 FCs and VCs ohm good.
- Volume pots ohm good.
- Tone controls pots ohm ok but treble does not go to zero on either end.
- Checked and double checked my work, but there's a lota stuff in there! I could have easily missed something.
- About 1/2 the volume that I would expect.
- If you remove either 6L6, no noticeable change in volume.
- As you increase treble (CW) the volume diminishes to near nothing.
- Bass control functions but will distort sound at full CW.
- One tweeter is lower volume than the other. The lower volume tweet FC is cold, louder one is lukewarm. This symptom follows wiring swap to the opposite tweeter.
Thanks!
RE: Sorta newbie 38-690 low volume - Nathan Slingerland - 11-29-2017
Congratulations on owning a great Philco.
I'll let more knowledgeable folks respond to your specific questions but welcome to the Phorum!
RE: Sorta newbie 38-690 low volume - klondike98 - 11-29-2017
Welcome to the phorum!
RE: Sorta newbie 38-690 low volume - morzh - 11-30-2017
1. Have you algned the radio? Especially IF.
2. The output stage is class AB amp so, yes, pull one tube and the other will keep the sound coming.
3. Have you checked the band coils?
4. Touch the antenna, see if the volume increases. If not, touch either 6A8 tubes' grid caps, see if it makes it louder.
5. Though the voltage chart is not available, see if your voltages make sense. At least look at the outputs of the tweeters' field coils.
RE: Sorta newbie 38-690 low volume - Radioroslyn - 11-30-2017
Hi and welcome,
I'd start w/ measuring the voltages around the 6L6 tubes. Pins 3,4, and 5 are key. 3 and 4 should be around +300vdc. Pin 5 abt -22vdc. The voltages you see on one tube you should see the same on the other.
RE: Sorta newbie 38-690 low volume - Hot Rod - 11-30-2017
Thank you all for the warm welcome!
- I aligned the radio "by ear". I know, I know, but I don't have the proper equipment. There was no appreciable improvement and I didn't move anything much, if at all. Just wanted to see if something was grossly out.
- I only checked the easy to get to band coils. I will check all this weekend.
- I'll check the tweeter VCs voltages too. That one was on my list.
- I didn't suspect the antenna itself because I have others that perform normally with the same antenna.
- I thought about 6L6 voltages but didn't know what to expect (lack experience). Thanks for the detailed advise!
RE: Sorta newbie 38-690 low volume - morzh - 11-30-2017
The voltages you measure at the speakers, - Field coil, not Voice Coil (you said VC). Measure relative to GND, on the pin that is not the outermost one - typically this is the pin that is the power supply output to the rest of the sch - to Amp and to the radio tuner.
Actually, measure both pins, inner and outer. This way we will know how much drop there is. The dropout should be around 40+V across the speaker coil that feeds the tuner and around 80V across the speaker coil that feeds the output amp (and a part of the tuner).
RE: Sorta newbie 38-690 low volume - Hot Rod - 11-30-2017
Oh yeah, FC not VC. Good catch. That's what I meant. I do understand the difference. And thanks for instructing me on how to check the voltage. I wasn't sure about that. I'll check those 6L6 voltages too and take notes on everything.
Any ideas about why the treble control diminishes the volume? That symptom is why I was going to dig into the IF expander. Very possible that I have more than one problem. But at least I have a working radio and somewhere to start.
I have yet to bench test anything powered up because I have to clear the table in my garage. Need lots of room to spread everything out. That'll be this weekend. I haven't removed speakers from the cabinet yet. I do know about the diffuser cone under the woofer. I've been researching this and other sites for quite some time.
RE: Sorta newbie 38-690 low volume - morzh - 12-01-2017
My treble ctl also diminishes volume. It is not just treble ctl, it is coupled with the IF coils mechanically and changes IF bandwidth as you move it. It opens up the bandwidth as the treble increases and closes it as it decreases.
RE: Sorta newbie 38-690 low volume - Radioroslyn - 12-01-2017
Did the '690 have a 2nd volume control attached to the treble/bandwidth control like the 37/38-116?
RE: Sorta newbie 38-690 low volume - morzh - 12-01-2017
Yes and no. It is a double pot. 37-116 does not have it, it only has one pot, controlling the driver 6F6.
The 38-690 (did not look in 38-116 but likely the same) has two pots, one is treble control and the other is 1st IF gain control.
My 37-116 does not have volume change when controlling treble/IF bandwidth.
RE: Sorta newbie 38-690 low volume - Hot Rod - 12-01-2017
Thank you morzh. I don't have the knowledge to explain the function, but yes, two treble pots and it also drives the three IF expanders.
But the volume should NOT decrease to almost inaudible as the treble control increases (CW), should it?
FYI, the volume control also has two pots and I'll be asking if/how/which one to connect for an ipod or iphone head phone jack cord.
RE: Sorta newbie 38-690 low volume - morzh - 12-01-2017
Well....I just went and checked mine. No it doesn't seem like the volume goes down when I rotate out of treble.
Check the integrity of the pot that goes to the K of the 1st IF tube. Better yet, check both pots.
RE: Sorta newbie 38-690 low volume - Hot Rod - 12-03-2017
So here's what I found:
- 6L6 voltages on pins 3, 4, and 5 are 324, 324, and -26 for both tubes.
- Tweeter FC volts are 332 and 254 (78v drop) for the "warm" FC that feeds the output amp.
- The other tweeter FC is 292 and 250 (42v drop) for the "cool" FC that feeds the tuner.
- The #83 front treble pot (nearest the shaft) goes to 6N7 2nd audio tube. Schematic says 1M ohm. It measures 1.4M ohm. At the center terminal it's 1K ohm CW and .72M ohm CCW.
- The #67 rear pot goes to the 1st IF 6K7 tube. Schematic says 2800 ohm. It measures 435 ohm. Center terminal goes up to 900 ohm, then drops to 600 ohm full CW. It's 29 ohm CCW.
RE: Sorta newbie 38-690 low volume - morzh - 12-03-2017
Your FC volages are good, they are supposed to be around 80V and 40V.
In addition, these pots have no way to influence the FC voltages.