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looking for help diagnosing volume loss on tube amplified reel to reel player
looking for help diagnosing volume loss on tube amplified reel to reel player
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Old 21st May 2010, 12:08 PM   #11
Globulator is offline Globulator  United Kingdom
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Quote:
Originally Posted by diblet View Post
cripes, i guess people were tuffer in the 60s? is this something that should cause me to abandon to unit? that would be quite a shame, but i suppose it beats dying...
You just need a properly rated isolation transformer - that's all.

With that you could actually earth the chassis too. I certainly would not go near it without one, AC mains can give you a very effective heart attack - just check out transformers (maybe toroids) and ideally fit one inside to make it safe forever.
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Old 21st May 2010, 02:15 PM   #12
diblet is offline diblet  United States
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Quote:
Originally Posted by Globulator View Post
You just need a properly rated isolation transformer - that's all.

With that you could actually earth the chassis too. I certainly would not go near it without one, AC mains can give you a very effective heart attack - just check out transformers (maybe toroids) and ideally fit one inside to make it safe forever.
wow, how could something that is so dangerous ever be made then?

i will get an isolation transformer for it - does anyone have any suggestions on where/what i should be looking for? i agree getting one that would fit inside would be great.
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Old 21st May 2010, 06:43 PM   #13
diblet is offline diblet  United States
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how does this one look: Triad Magnetics - VPS230-350 - Transformers - Power Products - Allied Electronics

isolation transformer specs:
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the unit i am going to add the isolation transformer to reads on the side:
AC115V
60 C/S
60 VA

with an isolation transformer do you want the VA rating to be exactly what the amp needs, or would it benefit to go over? the closest value that i could find while still staying small enough to fit internally was the 80VA isolation transformer linked above


_

also it seems i'll need to deal with compensating for the "half-wave problem"

Fixing the half-wave rectifier problem

Click the image to open in full size.Click the image to open in full size.

where in the circuit should the solid state rectifier go in my case? i am just wondering how the fix might be different since there is a motor involved in my circuit before the mains hit the filaments or tube rectifier.

btw the motor is a simple pole motor...

again here is the schematic of my unit for reference:

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thanks so much for everyone's help - i am learning so much!

Last edited by diblet; 21st May 2010 at 06:53 PM.
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Old 21st May 2010, 09:29 PM   #14
Globulator is offline Globulator  United Kingdom
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If the motor is insulated OK then I'd be tempted to use the 80VA for everything except the motor, which gives it a little margin so it stays cool.

Option B looks good too - no need to load up the diodes for the AC heaters.
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Old 22nd May 2010, 03:00 AM   #15
diblet is offline diblet  United States
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Originally Posted by Globulator View Post
If the motor is insulated OK then I'd be tempted to use the 80VA for everything except the motor, which gives it a little margin so it stays cool.

Option B looks good too - no need to load up the diodes for the AC heaters.
yeah the motor has its own transformer - so i would run the mains directly into the motor as is, then through the isolation transformer into the rest of the circuit to keep from putting too much load on the isolation transformer?

also where would people suggest i add the earth ground when i tie in the isolation transformer? there are two ground connections, one on the amplifier chassis and one on the mechanical chassis connected by capacitors...
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Old 22nd May 2010, 09:29 AM   #16
Globulator is offline Globulator  United Kingdom
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Quote:
Originally Posted by diblet View Post
yeah the motor has its own transformer - so i would run the mains directly into the motor as is, then through the isolation transformer into the rest of the circuit to keep from putting too much load on the isolation transformer?
Sounds good to me!

Quote:
Originally Posted by diblet View Post
also where would people suggest i add the earth ground when i tie in the isolation transformer? there are two ground connections, one on the amplifier chassis and one on the mechanical chassis connected by capacitors...
Not sure about the two grounds but I would connect the ground from the plug to the chassis metalwork, and make sure a wire connects this to the ground on the input jack (if different). You do not want to create a loop but you need the metal touchy bits connected to the ground from the plug.
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Old 22nd May 2010, 04:41 PM   #17
lassoharp is offline lassoharp  United States
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Make sure it's not either the heads or the tape itself. Old tape can exhibit drop out that will produce the volume fluctuations you mentioned.

Check the bias, rec/palyback heads for hi oxide build up.

If its the amp - check circuit voltages while the tape is playing if possible. Does it do the same thing when the transport is not engaged? It may be dragging down the PS due to worn out caps.
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Old 23rd May 2010, 03:34 AM   #18
diblet is offline diblet  United States
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Quote:
Originally Posted by Globulator View Post
Not sure about the two grounds but I would connect the ground from the plug to the chassis metalwork, and make sure a wire connects this to the ground on the input jack (if different). You do not want to create a loop but you need the metal touchy bits connected to the ground from the plug.
sounds great - i think i am going to come up with a schematic for the changes on this thing to run by everyone before i break out the soldering iron and go to town just in case...

Quote:
Originally Posted by lassoharp View Post
Make sure it's not either the heads or the tape itself. Old tape can exhibit drop out that will produce the volume fluctuations you mentioned.

Check the bias, rec/palyback heads for hi oxide build up.

If its the amp - check circuit voltages while the tape is playing if possible. Does it do the same thing when the transport is not engaged? It may be dragging down the PS due to worn out caps.
all good suggestions - honestly i am afraid to do anything inside with the unit plugged in until i get an isolation transformer for it. but a recap job is on the way. so checking the voltage will have to wait i am afraid. but the play/record heads seem to be in okay shape and all - the volume drop was sudden and has been consistent - which leads me to agree with you on the caps...

i have parts ordered for the isolation transformer i'll be working in, as well as a whole new set of caps - really excited to get going on this project, its turning out to be a lot of fun - and i am learning a ton (thanks to everyone here!). i can see building some tube amps of my own in the future...

Last edited by diblet; 23rd May 2010 at 03:37 AM.
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Old 7th June 2010, 06:08 PM   #19
diblet is offline diblet  United States
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Originally Posted by bob91343 View Post
Check that you don't have an open .005 coupling capacitor.
^^^^
ding ding, we got a winner!

it wasn't completely open, but must have been leaking when voltage was applied - once the coupling capacitor was changed between the 12av6 and the 30a5 the unit came back to life.

i have now changed all the caps i had replacements for on hand and its working great.

i still have to figure out how i am going to deal with the 3x 40mfd/150v filter can... can you buy empty cans to stuff with caps or would you have to make one yourself? as i have a couple of 40mfd/160v caps on hand...

also would replacing the 5mfd/150v cap with a 6.8mfd/160v be okay? or should i look around for a 5mfd cap?

thanks again for all the help everyone getting this guy back up and running - i really appreciate it!!!

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Old 7th June 2010, 08:05 PM   #20
lassoharp is offline lassoharp  United States
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Quote:
Originally Posted by diblet View Post
^^^^
ding ding, we got a winner!

it wasn't completely open, but must have been leaking when voltage was applied - once the coupling capacitor was changed between the 12av6 and the 30a5 the unit came back to life.

i have now changed all the caps i had replacements for on hand and its working great.

i still have to figure out how i am going to deal with the 3x 40mfd/150v filter can... can you buy empty cans to stuff with caps or would you have to make one yourself? as i have a couple of 40mfd/160v caps on hand...

also would replacing the 5mfd/150v cap with a 6.8mfd/160v be okay? or should i look around for a 5mfd cap?

thanks again for all the help everyone getting this guy back up and running - i really appreciate it!!!

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You could take the existing can and gut it - very messy job. Those are all PS caps. Cheapest way to go for cans would be the Weber or JJ dual 50/500 cans. The problem you might run into there is the hole for the original 40/150s may be a tad small. Easy enough to ream it out with a uni bit though.

The 5uF cap is a PS decoupling cap as well. They're likely just using the smallest value needed for that stage's filtering without causing a noticeable bump in the low freq response. A 50uf cap there would be a little overkill but may solve mounting problems. 2 dual cans would cover all the PS caps.

As another option you can always leave the old can(s) in place and mount small axials under the chassis if there's space.

The cathode bypass caps may need to replaced too. For the 30uF/6V you can use a 33uF/25,35 or 50V. The 25uF/25V is fairly common.

Using a higher voltage in these apps is fine. You may very well need a bit higher than 150V on the PS caps anyway with the addition of the diode bridge rectifier.

You see those 25-30uF/6Vs alot on gear from that era. A 20uF/25V cap would have been a lot bigger and cost a lot more than a modern cap of same capacitance and voltage rating. Higher voltage ratings per uF = a physically larger cap size. The 25/6 bypass cap was smaller and more economical.
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