Magnavox 2st660s Console Restoration with David Gillespie's 9300 Modification (Project Thread)
- Thread starter blknd_vggs
- Start date
blknd_vggs
New Member
Yeahhh, such a silly oversight. For sure, I'll go ahead and check that pot. Luckily, I do have a spare on hand.
blknd_vggs
New Member
Hey all! Been a little while! I managed to get everything working as intended... at least it was and more on that later. Also, I'll post some pics at a later date when I'm not pressed for time...
TLDR:
I finished the 9302 amp and moved on to the 57 series tuner. For the tuner I replaced all electrolytics and resistors. I had to replace a couple RF tubes since I was getting wonky voltages around the IF circuit. Thankfully new tubes fixed the issue. Overall, I had a tough time getting the voltages correct, but I took gadget's advice on adding another cap and resistor combo to the amp so that I'm feeding the tuner with a lower voltage. After some math and trail and error, I found a combo that worked.
I was getting some pretty annoying hum from the tuner, so I tidied up the grounding scheme and that was successful. With that settled, I noticed I was getting some channel imbalance, so I took a look at the volume and tone control pots... Once I opened the bass pot, I realized what may be the issue. The carbon track had completely disintegrated and so that was replaced along with the treble pot. The only step left is to realign the tuner, but that's lower priority at this time. Overall, I was really happy with the outcome but the speakers on the console are certainly a limiting factor. Again, not a high prio right now, but will be fun to play with down the line.
Now I have another question for you all:
Everything as working just fine for about a week or so and then all of the sudden the SAME ISSUE that was plagueing this console popped up again. The issue is that once the amp/tuner warms up and sound is produced, one channel becomes nothing but a rolling static or crackle sound. The issue is independent of the volume and tone controls. I did my best to troubleshoot on my own, but I'm stumped.
Here is a list in order of what I've done to locate and toubleshoot the issue:
- I've swapped around output wires to the console speakers and the crackle is isolated to only one channel that's independent of the speakers.
- I tried swapping channels on the crossover (which is also with new caps and resistors) no effect.
- Tried different sources (AUX, FM, FM AFC, etc..) no effect
- Next I swapped the input tubes (the original 6eu7s) and the sound does not follow the tubes. I even rotated in the 6eu7 from the tuner to see what might happen... no change.
- I swapped the 6bq5 output tubes around - one pair flipped with the other and also one by one rotating. The crackles still remain on the same channel.
- I checked the output terminal itself and there was green corrosion on the wire for the problem channel, so the wire was replaced and then I went ahead and replaced the spade terminals with banana jacks. STILL GETTING STATIC IN THE SAME CHANNEL.
- I went back through my work and I cannot find anything out of order. I reflowed all solder joints. No effect.
- I checked voltages for the input stage... Pin 7 of the 6eu7 on the problem channel is hovering around 15 VDC (it should be at least 185 VDC).
- Checked cathode and grid voltages for the outptut stage (original 6bq5s) and all are within reason and similar to each other with the problem channel showing slightly lower voltages.
- I double checked resistors around the 6eu7 cathode pins and since I don't have a capacitor checking option, I tested the coupling caps with an ohm test and that looked normal to me... Resistance steadily rose to an open reading.
- I checked the resistance on the output transformer for the problem channel... all reading normal.
At this point, I'm leaning towards bad tube(s) on the 9302. I'm at least certain that the problem is isolated to the amp. Given that I'm experiencing a similar, if not the same issue, as before I did the entire refurb/modification, this likely has something to do with one of the original parts. The only original parts left in the amp are the 6eu7 and 6bq5 tubes and the transformers. Just in case you all had any other ideas, I figure I would at least ask here before committing to buying new tubes. Thanks for reading and I appreciate any suggestions you all make!
TLDR:
I finished the 9302 amp and moved on to the 57 series tuner. For the tuner I replaced all electrolytics and resistors. I had to replace a couple RF tubes since I was getting wonky voltages around the IF circuit. Thankfully new tubes fixed the issue. Overall, I had a tough time getting the voltages correct, but I took gadget's advice on adding another cap and resistor combo to the amp so that I'm feeding the tuner with a lower voltage. After some math and trail and error, I found a combo that worked.
I was getting some pretty annoying hum from the tuner, so I tidied up the grounding scheme and that was successful. With that settled, I noticed I was getting some channel imbalance, so I took a look at the volume and tone control pots... Once I opened the bass pot, I realized what may be the issue. The carbon track had completely disintegrated and so that was replaced along with the treble pot. The only step left is to realign the tuner, but that's lower priority at this time. Overall, I was really happy with the outcome but the speakers on the console are certainly a limiting factor. Again, not a high prio right now, but will be fun to play with down the line.
Now I have another question for you all:
Everything as working just fine for about a week or so and then all of the sudden the SAME ISSUE that was plagueing this console popped up again. The issue is that once the amp/tuner warms up and sound is produced, one channel becomes nothing but a rolling static or crackle sound. The issue is independent of the volume and tone controls. I did my best to troubleshoot on my own, but I'm stumped.
Here is a list in order of what I've done to locate and toubleshoot the issue:
- I've swapped around output wires to the console speakers and the crackle is isolated to only one channel that's independent of the speakers.
- I tried swapping channels on the crossover (which is also with new caps and resistors) no effect.
- Tried different sources (AUX, FM, FM AFC, etc..) no effect
- Next I swapped the input tubes (the original 6eu7s) and the sound does not follow the tubes. I even rotated in the 6eu7 from the tuner to see what might happen... no change.
- I swapped the 6bq5 output tubes around - one pair flipped with the other and also one by one rotating. The crackles still remain on the same channel.
- I checked the output terminal itself and there was green corrosion on the wire for the problem channel, so the wire was replaced and then I went ahead and replaced the spade terminals with banana jacks. STILL GETTING STATIC IN THE SAME CHANNEL.
- I went back through my work and I cannot find anything out of order. I reflowed all solder joints. No effect.
- I checked voltages for the input stage... Pin 7 of the 6eu7 on the problem channel is hovering around 15 VDC (it should be at least 185 VDC).
- Checked cathode and grid voltages for the outptut stage (original 6bq5s) and all are within reason and similar to each other with the problem channel showing slightly lower voltages.
- I double checked resistors around the 6eu7 cathode pins and since I don't have a capacitor checking option, I tested the coupling caps with an ohm test and that looked normal to me... Resistance steadily rose to an open reading.
- I checked the resistance on the output transformer for the problem channel... all reading normal.
At this point, I'm leaning towards bad tube(s) on the 9302. I'm at least certain that the problem is isolated to the amp. Given that I'm experiencing a similar, if not the same issue, as before I did the entire refurb/modification, this likely has something to do with one of the original parts. The only original parts left in the amp are the 6eu7 and 6bq5 tubes and the transformers. Just in case you all had any other ideas, I figure I would at least ask here before committing to buying new tubes. Thanks for reading and I appreciate any suggestions you all make!
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blknd_vggs
New Member
I replaced essentially everything, including all of the tube sockets and input/output jacks, with new components in the amplifier. The only original parts that remain are the transformers, tubes (except the rectifier), and most of the wiring related to the molex socket. You can see a current pic of the amp in a follow-up post below.
Yeah swapping the tubes didn't result in the static changing channels, but I just cannot think of anything else aside from the output transformer for that channel.
I hadn't actually tried leaving the RCA inputs unplugged... Oddly enough, the static switches channels with no input plugged in. Testing the voltages to the coupling cap from pin 7 of the 6eu7s are remaining the same channel to channel. Meaning, the low voltage reading I'm seeing on the problem channel remained put. Which seems odd to me but push-pull amps are odd to me in general. Reversing the RCA inputs made no change to the original behavior.
Fyi: At this time I have the amp seperate from the tuner. I'm running a 1kHz test signal from a smartphone to the input of amp. Also, just to explain thoroughly, I'm jumpering the molex and switching on the unit through my dim bulb tester that also has bypass mode. Voltages I'm reading are reading high, but that should be expected without the tuner connected to draw any current.
I'll also add that I went through both the amp and tuner (several times) and verfiied voltages to my schematic. These were all tested with everything hooked up as it would in the console (minus the crossover and with output channels running to a 4ohm dummy load). Once I verified everything was within tolerance, I plugged it in to a pair of 6ohm speakers at home and gave it some listening time for a couple of days before taking everything over to my partner's place to install in the console itself. Once installed, it ran just fine for about a week or so before we encountered the current static/non functional channel.
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blknd_vggs
New Member
Here's a current pic of the amp [hi-res copy]. I have those alligators on the input grounds to chassis because when paired with the tuner, the stock input cables are shielded and grounded to the tuner chassis. I found having only one or the other side grounded, reduced hum.
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that doesn't really make sense.
Mostly I was curious if it swapped to confirm the problem is in the amp, not just the amp getting whatever is happening upstream. If swapping the cables side to side doesn't change it, you've definitely got something in the amp happening.
also a bit strange, stock the input jacks would have been grounded right to the chassis where they mount. I wouldn't have expected a ground loop to form. Possibly that has something to do with he 470k resistors across the jack not being right at the jack like they were in stock form.
Looking at your pic here, I'm seeing something I don't entirely understand.
I get that the pink wires run down to what I'm assuming are the bias test points, but I don't see how this actually is meant to function. You should be measuring across the 10 ohm, the test point isn't just in series with it.
The white wire at the terminal strips appears to be the feed from the EFB regulator. I see a 10 ohm between the cathode (pin 3) and the EFB supply on the two left tubes on each channel, and that makes sense. I see a black wire run down to the test points from the EFB source, that would be for meter negative. The two cathodes are tied together which is fine for doing per-channel test points. Measuring from black to red test jack should give a meaningful number but measuring to the blue one doesn't really make sense. It will work but its not going to tell you what the current in the right tube on each channel is doing.
If you want to do that, you'd have to do this
the set value changes too. Per-channel I believe its 0.44 volts. Per-tube it would be 0.22 volts.
None of that should cause weird static problems, but just a mention.
I get that the pink wires run down to what I'm assuming are the bias test points, but I don't see how this actually is meant to function. You should be measuring across the 10 ohm, the test point isn't just in series with it.
The white wire at the terminal strips appears to be the feed from the EFB regulator. I see a 10 ohm between the cathode (pin 3) and the EFB supply on the two left tubes on each channel, and that makes sense. I see a black wire run down to the test points from the EFB source, that would be for meter negative. The two cathodes are tied together which is fine for doing per-channel test points. Measuring from black to red test jack should give a meaningful number but measuring to the blue one doesn't really make sense. It will work but its not going to tell you what the current in the right tube on each channel is doing.
If you want to do that, you'd have to do this
the set value changes too. Per-channel I believe its 0.44 volts. Per-tube it would be 0.22 volts.
None of that should cause weird static problems, but just a mention.
blknd_vggs
New Member
I think I see what you mean there. Those "extra" light pink wires running to 10 ohm resistors were done in an effort to take easy readings of each tube's cathode voltage and not to get bias readings at each tube. Not a bad idea to adjust it to your suggestion. From what I'm understanding, I'm decoupling the bias test points so that can see what each tube is doing with any bias adjustment?
Also, any suggestions on how I verify if the output transformers are not acting up in any way? Like I've mentioned, I did measure resistance between the taps and accross the primary to ground. All measurements were within spec.
blknd_vggs
New Member
I thought I was losing it when the static flipped to the other channel... It's all a bit strange! This amp has been less than straightforward to say the least. To triple-check, I'll give it another go when I get home this evening, but I was not expecting that at all.
I originally had those inputs grounded but found reduced hum when I desoldered them. Mind you that the hum was fairly minor and I was being a bit picky. With the tuner, amp, and turntable all hooked up and installed, the hum diminshed even further to almost null. I was picking up some RF hum on the tuner before making those changes. So perhaps I could solder those back down later to see what happens with everything hooked up in its place.
As-wired there won't be any effective difference measuring between the black and red vs the black and blue test points. The two cathodes are wired together with that pink wire that goes from pin 3 to pin 3 and the high impedance of the meter won't drop any real difference across that 10 ohm. You could just eliminate that resistor and the blue test point and it would give you the per-channel readings like the original schematic indicates.
Or modify it for one 10 ohm per-tube so you can see what each does. Still a single adjustment so it doesn't really make much difference other than the target bias voltage. You can use the individual sampling to shuffle the tubes around for best matching within each channel though. Thats how mine is wired.
blknd_vggs
New Member
I think in general my next plan of attack will be to simplify my lead dressing. I got a little focused on grounding scheme and maybe I inadvertently created some weirdness. I'll revert things back to the original modified design and test as I go to see if things change. The low voltage I'm reading between pin 7 of that 6eu7 and the coupling cap connected to pin 2 of the 6bq5 is stumping me. Resistors surrounding that were testing fine, but I'll double check as I do some rewiring.
I assume this is what you're referencing?
If you're reading 15 volts at the 6EU7 plate where it should be ~185, I'd be checking the resistor to make sure it is in fact a 220K or 270K, depending which one is low. If its only one triode plate, the 340v supply must be OK since it looks like all 4 resistors land on the same terminal.
If you're reading 15 volts at the 6EU7 plate where it should be ~185, I'd be checking the resistor to make sure it is in fact a 220K or 270K, depending which one is low. If its only one triode plate, the 340v supply must be OK since it looks like all 4 resistors land on the same terminal.
blknd_vggs
New Member
After messing around for a while testing all the resistors to make sure they were good... Since it was the only component left, I decided to swap the the 390pF mica caps channel to channel. Guess what? Well I have a bad silver mica cap. It was not what I'd expect at all, but I'm at least relieved to figure out what was causing my low voltage off that 6eu7 plate. Too bad I don't have a replacement at the moment, but it will be added to the order list for another project I have in the works.
thats an oddity for sure, especially a new one. Good job chasing that out. Honestly I would have assumed a wrong or bad resistor first, SM caps are usually very reliable. I've replaced maybe a handful in a bunch of years, and all of those were off-value, never had a shorted one.