Model 18 Scope Troubleshooting
- Thread starter Fhamre
- Start date
Fhamre
Super Member
Well I put the new tranny's in, made sure they were insulated. Triple checked my work resoldered a couple of suspect joints and put things back in place.
Hooked up to dbt,prayed,ducked and flicked. No smoke no sparks, a good start.
Couldn't see the scope so I took it to a dark room . My shop get natural light I can't control. Tried to see it I could see the dot on power down.
Not there! Dam! Then I tried playing with the controls. What is this green light I see?
So I took it back to the shop,hooked up a source and we have a working scope.Yes! There is an Audio god!
Not sure if it was the new transistors as the original tested fine or a bad solder joint. No matter I don't need a new scope tube.
I thank you for your advice and patience.
Normally I would recap a unit but at this point I am hesitant. I likely have most of the axial and big electrolytic s but as always have to order the odd one.
Its just from what I read the quality of these caps are such that there are rarely failures.
Would welcome suggestions?
I still have to try and clean a yellowed faceplate, fix the pushed in/damaged phono inputs ,clean controls and switches and adjust the FM/dial indicator as it is off by about an inch.
May have to physically move it unless I can't trim it with the oscillator.
Any and all advice welcome.
Hooked up to dbt,prayed,ducked and flicked. No smoke no sparks, a good start.
Couldn't see the scope so I took it to a dark room . My shop get natural light I can't control. Tried to see it I could see the dot on power down.
Not there! Dam! Then I tried playing with the controls. What is this green light I see?
So I took it back to the shop,hooked up a source and we have a working scope.Yes! There is an Audio god!
Not sure if it was the new transistors as the original tested fine or a bad solder joint. No matter I don't need a new scope tube.
I thank you for your advice and patience.
Normally I would recap a unit but at this point I am hesitant. I likely have most of the axial and big electrolytic s but as always have to order the odd one.
Its just from what I read the quality of these caps are such that there are rarely failures.
Would welcome suggestions?
I still have to try and clean a yellowed faceplate, fix the pushed in/damaged phono inputs ,clean controls and switches and adjust the FM/dial indicator as it is off by about an inch.
May have to physically move it unless I can't trim it with the oscillator.
Any and all advice welcome.
bktheking
Gitter Done!
One of those transistors is likely leaky causing the voltage to be incorrect to center the scope, those transistor/board don't see direct heat from each other because of the remote wires used to connect so I doubt a bad solder joint was the culprit. Glad it's working. Mine isn't recapped, all original and it still performs without issue- recapping an 18 is more of a personal decision , I haven't done mine as I don't see a need to, it doesn't get used so I didn't bother. I own it cause it's a collectors item and not a daily driver.
Fhamre
Super Member
Hi Brad,thx for all your help.
Is there a way to turn off the scope display,totally. It seems to remain on although I should do some more adjusting. Just want to minimize wear.
Yes I share your sentiment about recapping the 18. Those Spragues look like great caps and cool looking.
Would it be a good assumption that since this unit appears to be made in 1970 that all the service bulletin updates from 1969 would have been done?
Is there a way to turn off the scope display,totally. It seems to remain on although I should do some more adjusting. Just want to minimize wear.
Yes I share your sentiment about recapping the 18. Those Spragues look like great caps and cool looking.
Would it be a good assumption that since this unit appears to be made in 1970 that all the service bulletin updates from 1969 would have been done?
bktheking
Gitter Done!
No worries, I know the 18 well so I'm happy to lend a hand. Not on the 18's unfortunately however you could just move the display off center which is what turning off the scope does on units that have that feature. It's about saving the face of the tube and preventing burn in, the tube will last a long long time. You can also turn down the intensity which will also save the display.
RV59
Active Member
Same for me, these Sprague capacitors are really good quality.
Personally I use the Clay Stone to clean facade: effective and not aggressive:
Fhamre
Super Member
I set the dc offset and bias as well as the phono bias adjustment. That went well.
Got the dial indicator where it should be. Sounds great. At some point the previous owner cracked the bakelite phono input plate for phono 1 and 2. My guess he tried putting very tight rca's on.
This morning I could get all functions working except phono 2. I had a good look at phono 2 to ensure all the connections were fine.
Now I still don't have phono 2 but I get a loud pop through both speakers when I move the selector from FM to Phono and back. No pop when moving the selector from tape to aux.
The pop is there even with no volume.Hmm?
Not sure what I did to create the pop. It wasn't there this am.
Got the dial indicator where it should be. Sounds great. At some point the previous owner cracked the bakelite phono input plate for phono 1 and 2. My guess he tried putting very tight rca's on.
This morning I could get all functions working except phono 2. I had a good look at phono 2 to ensure all the connections were fine.
Now I still don't have phono 2 but I get a loud pop through both speakers when I move the selector from FM to Phono and back. No pop when moving the selector from tape to aux.
The pop is there even with no volume.Hmm?
Not sure what I did to create the pop. It wasn't there this am.
monkboughtlunch
Super Member
What's the Mouser or Digikey part number for the correct replacement scope transistors? My scope is intermittent. When the scope trace disappears it shows up briefly when I power down the unit. Does this indicate a bad scope transistor?
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monkboughtlunch
Super Member
Did you change any components since this morning when the pop was not present?
I have the same exact mystery popping issue my 18. Only pops when selector is turned to fm (even when volume is down). No pop when other inputs are selected. Does anyone have a fix? Is the issue an out of spec cap on the FM board?
monkboughtlunch
Super Member
Mine had 47 year old electrolytics and sounded "tired" before the recap. After the recap the sound opened up (lower lows and more air around the highs). The original Spragues were top quality but 50 year old caps may not deliver full intended performance due to electrolyte degradation over time. I was pleased with the improvement the full recap delivered.
Fhamre
Super Member
Mouser part #511-TIP47
No components changed. But I did tweak the IF front end oscillators in order to get the dial indicator aligned.
The biggest challenge to replacing the transistors is insulating the 220 package. Just made sure with continuity testing that I was good to go before flipping the switch.
No components changed. But I did tweak the IF front end oscillators in order to get the dial indicator aligned.
The biggest challenge to replacing the transistors is insulating the 220 package. Just made sure with continuity testing that I was good to go before flipping the switch.
Fhamre
Super Member
Continuity seems fine to the phono 2 switch. I also tried to determine if there was continuity to the chassis etc that shouldn't be there. Seems to be fine.
The pop can be heard without speakers connected. I can't tell where it is coming from. Possibly the MPX Oscillator board,not sure. The pop occurs when the stereo light comes on and when that muting indicator lamp on the MPX board comes on.
The pop can be heard without speakers connected. I can't tell where it is coming from. Possibly the MPX Oscillator board,not sure. The pop occurs when the stereo light comes on and when that muting indicator lamp on the MPX board comes on.
