05-18-2014, 08:31 PM
I've had a 38-10 on and off my workbench quite a bit over the past 6 weeks. I think I stumped most of the guys over on ARF with this one too. Hoping maybe someone who is over here might have some ideas.
I went through it, did a full re-cap and replaced all resistors. Upon firing it up, I find I can pick up stations either with a 10' antenna, or touching the 6A8 grid. Stations even as far away as Chicago (I'm just west of Milwaukee). But the volume on them is very low with the radio volume maxed out.
I've gone through the alignment process multiple times. I'm pretty confident the IF's are set correctly. But something weird is going on in the front end. I had a heck of a time with aligning the front. No matter how many times I went back and forth between the 580 and 1500 adjustment, I couldn't get the dial to track. After some luck, I have the dial tracking for the most part (+/- 5kHz depending on the location). Here is the weird part. Nether adjustment 7 or 9 is peaked though. If I go and peak either of them, my dial tracking goes way off again, and I wind up bouncing back and forth because every time I peak one end, it completely screws up the other (and the dial tracking).
I cleared the audio section by injecting an audio tone at the top of the volume wiper. Depending on the volume of the gen and the radio, it got quite loud. So I know the radio is capable of putting out a much louder sound than it is.
The AVC seems to be working normally. So I think the radio is receiving the signal. but something is messing with it in circuit. For example, i was able to get these AVC readings:
Normal AVC, no station tuned = -2.53vdc
620: A very strong station in Milwaukee
10' Antenna connected normally = -4.55vdc
Touching the 6A8 grid wire = -6.1vdc
Antenna connected normally, plus touching grid = -6.25vdc
10' Antenna through .001 cap to 6A8 grid = -6.5vdc
1080: My SSTRAN in the room above my workbench
10' Antenna connected normally = -5.05vdc
Touching the 6A8 grid wire = -5.4vdc
Antenna connected normally, plus touching grid = -5.76vdc
10' Antenna through .001 cap to 6A8 grid = -5.2vdc
I ran through all the voltages on the tubes. Everything is within the expected range that is noted on the schematic, although some of these voltages are not listed (radio was powered off a variac @ 116vac):
6A8 - plate = 163.1, screen = 93.8, osc grid = -15.45, osc screen 137.7, grid cap = -2.64
6K7 - plate = 162.9, screen 93.9, grid cap = -2.36
6Q7 - plate 162.9, pin 4 diode = -2.54, pin 5 diode = -1.40, grid cap = -1.9
6F6 - plate - 233.5, screen 249.2
The sets original 6K7 caused the screen voltage to drop the longer the radio was on. After a couple minutes, it would cut out all audio. So I think it was gassy. I replaced it with a NOS 6K7 and it seems to have corrected the issue. I've run all tubes through my 533A TT and they all check out.
I'm not really sure what else to do at this point. At my disposal I have a VTVM, and Eico 324 sig gen, and a B&K 1470 scope. (although, I'm not good with scopes, so I'm not entirely sure how to set them up, or interpret the results). I'm open to suggestions.
Thanks!
I went through it, did a full re-cap and replaced all resistors. Upon firing it up, I find I can pick up stations either with a 10' antenna, or touching the 6A8 grid. Stations even as far away as Chicago (I'm just west of Milwaukee). But the volume on them is very low with the radio volume maxed out.
I've gone through the alignment process multiple times. I'm pretty confident the IF's are set correctly. But something weird is going on in the front end. I had a heck of a time with aligning the front. No matter how many times I went back and forth between the 580 and 1500 adjustment, I couldn't get the dial to track. After some luck, I have the dial tracking for the most part (+/- 5kHz depending on the location). Here is the weird part. Nether adjustment 7 or 9 is peaked though. If I go and peak either of them, my dial tracking goes way off again, and I wind up bouncing back and forth because every time I peak one end, it completely screws up the other (and the dial tracking).
I cleared the audio section by injecting an audio tone at the top of the volume wiper. Depending on the volume of the gen and the radio, it got quite loud. So I know the radio is capable of putting out a much louder sound than it is.
The AVC seems to be working normally. So I think the radio is receiving the signal. but something is messing with it in circuit. For example, i was able to get these AVC readings:
Normal AVC, no station tuned = -2.53vdc
620: A very strong station in Milwaukee
10' Antenna connected normally = -4.55vdc
Touching the 6A8 grid wire = -6.1vdc
Antenna connected normally, plus touching grid = -6.25vdc
10' Antenna through .001 cap to 6A8 grid = -6.5vdc
1080: My SSTRAN in the room above my workbench
10' Antenna connected normally = -5.05vdc
Touching the 6A8 grid wire = -5.4vdc
Antenna connected normally, plus touching grid = -5.76vdc
10' Antenna through .001 cap to 6A8 grid = -5.2vdc
I ran through all the voltages on the tubes. Everything is within the expected range that is noted on the schematic, although some of these voltages are not listed (radio was powered off a variac @ 116vac):
6A8 - plate = 163.1, screen = 93.8, osc grid = -15.45, osc screen 137.7, grid cap = -2.64
6K7 - plate = 162.9, screen 93.9, grid cap = -2.36
6Q7 - plate 162.9, pin 4 diode = -2.54, pin 5 diode = -1.40, grid cap = -1.9
6F6 - plate - 233.5, screen 249.2
The sets original 6K7 caused the screen voltage to drop the longer the radio was on. After a couple minutes, it would cut out all audio. So I think it was gassy. I replaced it with a NOS 6K7 and it seems to have corrected the issue. I've run all tubes through my 533A TT and they all check out.
I'm not really sure what else to do at this point. At my disposal I have a VTVM, and Eico 324 sig gen, and a B&K 1470 scope. (although, I'm not good with scopes, so I'm not entirely sure how to set them up, or interpret the results). I'm open to suggestions.
Thanks!
