I'm a bit confused so I need wiring advice to bypass (using fixed value resistors) the volume and balance pots of my Revox A77.
I want to know if I've got it right that for fixed volume output to bypass pots P301,P302 I need to short blk/red/yel wires and then add resistor between shorted blk/red and grn for one channel and another resistor between shorted blk/yel and org for the other channel.
As for fixed channel balance do I have to short blu/grn and add resistor from shorted point to yel and for the other channel short red/org and add resistor from this to brn?
I want to know if I've got it right that for fixed volume output to bypass pots P301,P302 I need to short blk/red/yel wires and then add resistor between shorted blk/red and grn for one channel and another resistor between shorted blk/yel and org for the other channel.
As for fixed channel balance do I have to short blu/grn and add resistor from shorted point to yel and for the other channel short red/org and add resistor from this to brn?
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Whats the reason for modifying a great deck?
0,01% improoved sound quality, that you think its worth the effort?
0,01% improoved sound quality, that you think its worth the effort?
The reason is that the pots in all these decks are so old and of not high quality that after all these years rheostat inside pot makes bad contact leading to unstable output or sudden output level changing even with a slight finger touch on the pot.
After all I am not removing the pots, any changes made are 100% reversible.
After all I am not removing the pots, any changes made are 100% reversible.
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I had thought it all wrong, with a bit more caution I understood I have to make a divider by fitting a resistor between red/grn = out/in and another resistor between red/blk = out/grnd for one channel.
As for the other channel doing the same by fitting a resistor between yel/org=out/in and another resistor between yel/blk = out/grnd.
Fitted resistors pairs sum must be as close as possible to pot's 25K resistance.
Now I have to figure out how I'll make the balance pot of fixed value.
Any help is much appreciated.
As for the other channel doing the same by fitting a resistor between yel/org=out/in and another resistor between yel/blk = out/grnd.
Fitted resistors pairs sum must be as close as possible to pot's 25K resistance.
Now I have to figure out how I'll make the balance pot of fixed value.
Any help is much appreciated.
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Just finished both pots bypassing with success, same procedure as above (created a divider) using a pair of 13K resistors for each channel on the balance pot.
Results sounds fine and got rid of channel imbalance and nasty scratches when touching volume or balance knob.
For the record before trying this I had tried cleaning the pots and even disassembling volume pot to rebuild it which I succeed but the result was not as good as it should, after that I had bought a used pair from the bay market but these were in worse condition than mine.
Volume and balance pots on these tape decks become a big problem after so many years and as far as I know unfortunately there is no source to get new ones as replacements.
Results sounds fine and got rid of channel imbalance and nasty scratches when touching volume or balance knob.
For the record before trying this I had tried cleaning the pots and even disassembling volume pot to rebuild it which I succeed but the result was not as good as it should, after that I had bought a used pair from the bay market but these were in worse condition than mine.
Volume and balance pots on these tape decks become a big problem after so many years and as far as I know unfortunately there is no source to get new ones as replacements.
Many other persons did manage to restore those decks to 100% original condition without any modification, additional divider resistors etc. They just took the time to rebuild the whole unit. Of course, that takes time and effort. To install additional parts is a much faster operation. In fact, thats a mod. You are modding a famous design just because you'r too lazy or not competent enough to restore it.
And of course, everything can become a big problem after some 50 years of use.
You have to clean or change parts everywhere, thats not the exception, thats the rule with such old audio gears.
And of course, everything can become a big problem after some 50 years of use.
You have to clean or change parts everywhere, thats not the exception, thats the rule with such old audio gears.
I respect your opinion but my A77 is not an exhibitional piece in a museum, it's a machine I use from time to time and I need it to reproduce my tapes as best as possible in order to give me pleasure.
If you ask me I prefer a modded but perfectly working machine than a malfunctioning one in original condition.
After all even when recapping an A77 and replacing all the awful rusty trimpots with sealed modern ones it is not any more a machine in original state, so I think it is impossible to find a perfectly working A77 being in original state.
If by any chance you know a source to get new Revox pots I'll be happy to know.
I don't know/haven't read of someone having rebuilt a Revox A77 volume pot with 100% success.
If you ask me I prefer a modded but perfectly working machine than a malfunctioning one in original condition.
After all even when recapping an A77 and replacing all the awful rusty trimpots with sealed modern ones it is not any more a machine in original state, so I think it is impossible to find a perfectly working A77 being in original state.
If by any chance you know a source to get new Revox pots I'll be happy to know.
I don't know/haven't read of someone having rebuilt a Revox A77 volume pot with 100% success.
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Hmmmm.....what's so special about the ReVox A77 volume pot that it can't be replaced by a modern equivalent?
You put in a screw type instead, install a rod & a piece of tube for linking to the pot's shaft.
Yes, it takes time and patience. That's the fun of it.
Audio Potentiometers repair kit | Nagravox
Yes, it takes time and patience. That's the fun of it.
Audio Potentiometers repair kit | Nagravox
Yep, THAT is a damn long shaft! DIY Audio Guru Red Elliott has a solution---an extender shaft that will allow use of standard pots:
ESP - Extension Shaft Purchase Information
ESP - Extension Shaft Purchase Information
Thanks a lot both of you for the great info, I'll keep these options in mind in case I need to sell the deck (which I don't see happening).
For the time being I am very satisfied with the result of bypassing the pots.
For the time being I am very satisfied with the result of bypassing the pots.
I just used Kontakt 60 to clean them and they worked fine.
Replacing the trimpots is a bit of a pain. Modern ones from Pihar seem to fit well enough. Starting with catalogue number PT10LH05 fit on the boards:
PT10LH05-501A2020 50 Ohm trimpot for record amplifier (2 in total)
PT10LH05-203A2020 20k Ohm trimpots for record amplifier and output amplifier (8 in total)
PT10LH05-503A2020 50k Ohm trimpots for Oscillator board (4 in total)
Then there are tons of little caps to replace... but after replacing everything (and taking the time to tune it) the sound is amazing. I have a High Speed machine with IEC equalization.
The only part that was a real pain was fixing the VU meters. There are replacements that can be bought but I was determined to make the originals work again... 😉
Replacing the trimpots is a bit of a pain. Modern ones from Pihar seem to fit well enough. Starting with catalogue number PT10LH05 fit on the boards:
PT10LH05-501A2020 50 Ohm trimpot for record amplifier (2 in total)
PT10LH05-203A2020 20k Ohm trimpots for record amplifier and output amplifier (8 in total)
PT10LH05-503A2020 50k Ohm trimpots for Oscillator board (4 in total)
Then there are tons of little caps to replace... but after replacing everything (and taking the time to tune it) the sound is amazing. I have a High Speed machine with IEC equalization.
The only part that was a real pain was fixing the VU meters. There are replacements that can be bought but I was determined to make the originals work again... 😉
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I re-read this thread - the 5k ohm recording level potentiometers are indeed quite impossible to repair, and the elongated shaft is quite narrow. This makes finding an alternative challenging.
Hi,
the pots are in good quality. Like all pots in this age, they had to cleaned up from inside. It's a difficult work but worthy. All contacts I had cleaned and afterwards sealed. Then all this remounted. With two copper ribbons (from a power cable) that I stuffed, the pots are solid mounted. They work fine again.
Vintage Pots should be handled carefully, in many cases you got no proper replacement.
the pots are in good quality. Like all pots in this age, they had to cleaned up from inside. It's a difficult work but worthy. All contacts I had cleaned and afterwards sealed. Then all this remounted. With two copper ribbons (from a power cable) that I stuffed, the pots are solid mounted. They work fine again.
Vintage Pots should be handled carefully, in many cases you got no proper replacement.
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The recording pots are also good to clean. I had them completed with a bronze feather for better handling in turn the knobs. (I don't know, why Revox this in the recording pots spared?) The a little drop of silicon grease for the shaft...
This long shaft pots were a special series exclusivly for Revox an you will find no replacement for this.
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Hi,
That is fine so long as the Wiper is not damaged. I had the two 5k recording level potentiometers with cracked/damaged wipers. Ok, they are 50 years old now...
Trying to solder the cracked wiper back together is VERY challenging. I managed to do it, but am not sure how long it will hold, so I sourced some used ones online. I am not sure if the wiper from a donor pot would fit... Others from this manufacturer seem to have thicker shafts, so didn't try it.
I also used wire to hold the assembly together after. Note: The little black (carbon?) plug on the wiper can fall off. It is important not to lose this tiny piece!
Ian
Yes, getting them back toe
That is fine so long as the Wiper is not damaged. I had the two 5k recording level potentiometers with cracked/damaged wipers. Ok, they are 50 years old now...
Trying to solder the cracked wiper back together is VERY challenging. I managed to do it, but am not sure how long it will hold, so I sourced some used ones online. I am not sure if the wiper from a donor pot would fit... Others from this manufacturer seem to have thicker shafts, so didn't try it.
I also used wire to hold the assembly together after. Note: The little black (carbon?) plug on the wiper can fall off. It is important not to lose this tiny piece!
Ian
Yes, getting them back toe
The only way is to get ones from some dealers which offers old replacement parts from disassambled A77...
I gather for the balance, you placed a 13K resistor between red and orange and a second one between orange and brown wires on channel I and between yellow and blue and blue and green wires for channel II. Did I read this correctly? Also, did you remove the wires from the pots (I would think you did but you didn't mention as otherwise you'd have parallel resistors)? I have "cleaned and lubed" the pots but haven't tried them yet and am anticipating not wonderful results but I am looking to your solution if it doesn't. Since I only use this for occassional playback, I am not worried about the record volume pots but I might have to do the playback volume. And you are right about basically just looking at the balance pot and it crackles! 🙂
Thanks!
Soulmerchant said
"Modern ones from Pihar seem to fit well enough. Starting with catalogue number PT10LH05 fit on the boards:"
All of the trimmers in an A77 are replaceable with Piher trimmers in the PT15LH series, except for the crosstalk control which is a horizontal PT15LV.
The PT10 range is smaller, and is used in B77 and PR99 machines.
For the volume and balance pots, spray some Deoxit F5 fader lube right inside and exercise the pot back and forward 20 times - never fails.
"Modern ones from Pihar seem to fit well enough. Starting with catalogue number PT10LH05 fit on the boards:"
All of the trimmers in an A77 are replaceable with Piher trimmers in the PT15LH series, except for the crosstalk control which is a horizontal PT15LV.
The PT10 range is smaller, and is used in B77 and PR99 machines.
For the volume and balance pots, spray some Deoxit F5 fader lube right inside and exercise the pot back and forward 20 times - never fails.