08-25-2018, 10:25 PM
Today I got up early and worked all day on the 37-690.
Finished with Derusting
I used naval jelly on the few remaining stubborn spots on the chassis.
Removing the rust left behind some pitting where the rust was (expected). I don't really like to paint chassis - so I applied Boeshield to seal up the raw metal and let it dry for a couple of hours.
This is what the chassis looks like now:
[Image: https://farm2.staticflickr.com/1899/4245...d11c_z.jpg]
Grid Cap Clips
As you might recall from the eBay listing - whoever the seller bought this from had tossed the tube shields and clipped off a bunch of the grid clip connectors in their zeal to scrap it. I have no idea if the backstory is true - but whatever the case the clips are missing. While I was waiting for the Boeshield to dry I patched some new ones onto the main chassis. In all 6 clips were missing. I raided a junker 37-630 and 37-650 chassis for the clips.
One of the nuts is frozen solid on the IF cans - so for now I just patched on the clips. Hopefully I'll be able to replace the grid cap wire entirely once I start work on the main chassis in earnest.
Before;
[Image: https://farm2.staticflickr.com/1899/4245...ec8f_z.jpg]
After:
[Image: https://farm2.staticflickr.com/1871/4421...ef64_z.jpg]
Reinstall Transformers and Electrolytics
Once the chassis was sealed I remounted the chokes, output transformer and electrolytic capacitors.
[Image: https://farm2.staticflickr.com/1879/2932...4471_z.jpg]
[Image: https://farm2.staticflickr.com/1874/4421...ceea_z.jpg]
Reconnect Everything
Next I re-connected all of the parts I had previously removed.
[Image: https://farm2.staticflickr.com/1873/4245...3762_z.jpg]
Replace Candohm
The 2.2K and 3K 50W resistors arrived from Mouser. So the next step was to mount them.
Space is tight so I didn't really have much option except to mount them directly on the chassis. I applied some lithium grease underneath to help with heat transfer.
Old candohm:
[Image: https://farm2.staticflickr.com/1841/4421...2ede_n.jpg]
Replacements installed:
[Image: https://farm2.staticflickr.com/1846/4421...14fa_z.jpg]
Recap
Normally I would bring up a radio on the variac after replacing the filter capacitors. The 37-690 is such a big set that I decided it was safer to recap the amp chassis ahead of applying power.
Over the course of the afternoon I worked through the bakelite blocks and restuffed the two paper caps.
A few resistors were out of spec and were replaced.
After:
[Image: https://farm2.staticflickr.com/1870/4336...6384_z.jpg]
Clean Tube Sockets,
I used a small brush with deoxit on it to clean the socket pins on both the amp and main chassis. I don't always do this but some of the tubes had some sort of oxidation on them - so I figured it would be on the socket pins as well.
Install Tubes and Tube Shields
All of the tube shields were missing. On a 20 tube set... well that's a lot of tube shields! I had just enough shields in reasonable enough shape to use.
I had previously gone through and tested all of the tubes (replacing a few weak ones).
Ready for Bring Up
[Image: https://farm2.staticflickr.com/1854/4355...f539_z.jpg]
Well.... almost ready for bringup.
Woofer Problems
Now to tell you guys about something that I noticed last week. There's an issue with the woofer: the voice coil and connecting wires are damaged. I saw what looked like a dangling wire in the eBay photos so this wasn't a total surprise. Once I took a closer look after I had everything apart I confirmed the bad news:
[Image: https://farm2.staticflickr.com/1890/4338...1d69_z.jpg] [Image: https://farm2.staticflickr.com/1833/4319...7b86_z.jpg]
At first I thought I might be able to patch it (delicate!) but measuring the remaining bits of wire with the multimeter I couldn't get any resistance.
I had to decide between sending the speaker for repair (I don't want to learn voice coil repair on this set!) - and finding a replacement. Believe it or not, Gary from oldradioparts.com had a 37-690 woofer in good condition.
Gary's prices are very reasonable - cheaper than it would be to have a professional repair it even including shipping. So the new speaker is on it's way (arriving Tuesday).
----
I still plan to bring it up on the variac and check voltages since it's only the voice coil that's disconnected. The field coil and output transformer measures good - and the tweeters are intact.
How much volume is there out of the tweeters? Should I be able to get something out of them if things are more or less working?
Finished with Derusting
I used naval jelly on the few remaining stubborn spots on the chassis.
Removing the rust left behind some pitting where the rust was (expected). I don't really like to paint chassis - so I applied Boeshield to seal up the raw metal and let it dry for a couple of hours.
This is what the chassis looks like now:
[Image: https://farm2.staticflickr.com/1899/4245...d11c_z.jpg]
Grid Cap Clips
As you might recall from the eBay listing - whoever the seller bought this from had tossed the tube shields and clipped off a bunch of the grid clip connectors in their zeal to scrap it. I have no idea if the backstory is true - but whatever the case the clips are missing. While I was waiting for the Boeshield to dry I patched some new ones onto the main chassis. In all 6 clips were missing. I raided a junker 37-630 and 37-650 chassis for the clips.
One of the nuts is frozen solid on the IF cans - so for now I just patched on the clips. Hopefully I'll be able to replace the grid cap wire entirely once I start work on the main chassis in earnest.
Before;
[Image: https://farm2.staticflickr.com/1899/4245...ec8f_z.jpg]
After:
[Image: https://farm2.staticflickr.com/1871/4421...ef64_z.jpg]
Reinstall Transformers and Electrolytics
Once the chassis was sealed I remounted the chokes, output transformer and electrolytic capacitors.
[Image: https://farm2.staticflickr.com/1879/2932...4471_z.jpg]
[Image: https://farm2.staticflickr.com/1874/4421...ceea_z.jpg]
Reconnect Everything
Next I re-connected all of the parts I had previously removed.
[Image: https://farm2.staticflickr.com/1873/4245...3762_z.jpg]
Replace Candohm
The 2.2K and 3K 50W resistors arrived from Mouser. So the next step was to mount them.
Space is tight so I didn't really have much option except to mount them directly on the chassis. I applied some lithium grease underneath to help with heat transfer.
Old candohm:
[Image: https://farm2.staticflickr.com/1841/4421...2ede_n.jpg]
Replacements installed:
[Image: https://farm2.staticflickr.com/1846/4421...14fa_z.jpg]
Recap
Normally I would bring up a radio on the variac after replacing the filter capacitors. The 37-690 is such a big set that I decided it was safer to recap the amp chassis ahead of applying power.
Over the course of the afternoon I worked through the bakelite blocks and restuffed the two paper caps.
A few resistors were out of spec and were replaced.
After:
[Image: https://farm2.staticflickr.com/1870/4336...6384_z.jpg]
Clean Tube Sockets,
I used a small brush with deoxit on it to clean the socket pins on both the amp and main chassis. I don't always do this but some of the tubes had some sort of oxidation on them - so I figured it would be on the socket pins as well.
Install Tubes and Tube Shields
All of the tube shields were missing. On a 20 tube set... well that's a lot of tube shields! I had just enough shields in reasonable enough shape to use.
I had previously gone through and tested all of the tubes (replacing a few weak ones).
Ready for Bring Up
[Image: https://farm2.staticflickr.com/1854/4355...f539_z.jpg]
Well.... almost ready for bringup.
Woofer Problems
Now to tell you guys about something that I noticed last week. There's an issue with the woofer: the voice coil and connecting wires are damaged. I saw what looked like a dangling wire in the eBay photos so this wasn't a total surprise. Once I took a closer look after I had everything apart I confirmed the bad news:
[Image: https://farm2.staticflickr.com/1890/4338...1d69_z.jpg] [Image: https://farm2.staticflickr.com/1833/4319...7b86_z.jpg]
At first I thought I might be able to patch it (delicate!) but measuring the remaining bits of wire with the multimeter I couldn't get any resistance.
I had to decide between sending the speaker for repair (I don't want to learn voice coil repair on this set!) - and finding a replacement. Believe it or not, Gary from oldradioparts.com had a 37-690 woofer in good condition.
Gary's prices are very reasonable - cheaper than it would be to have a professional repair it even including shipping. So the new speaker is on it's way (arriving Tuesday).
----
I still plan to bring it up on the variac and check voltages since it's only the voice coil that's disconnected. The field coil and output transformer measures good - and the tweeters are intact.
How much volume is there out of the tweeters? Should I be able to get something out of them if things are more or less working?
