This winter I repaired a similar A77 to yours. I had the same issues. The LDR resistor was too weak (high) the pick up reel motor had a bended shaft. And brakes need to be adjusted. Besides that I struggled a bit to learn to do a bias adjustment without having a calibration tape to start with. It was interesting to see your take on how to do the adjustment..
Doing bias adjustment doesn't require a test tape because it messes with the record side. The test tape is required for the playback side. If you don't have a test tape you can adjust the azimuth and the playback level by using an already known good recording as a test tape. Won't be accurate but may be OK. There is also a way to adjust azimuth by recording and flipping the tape on the back side. Google it. I have done many bias calibration by ear. If you get correct treble response it's good. But note, that is not a professional way.
Maybe I should. If the machine was mine I would, even if it wasn't necessary. The machine worked fine, that is why I did not go further. After all, doing more stuff I would risk damaging something that I couldn't find. This repair was a favor to a friend and I didn't wanna risk doing things maybe worse. Working in your living room is dangerous sometimes to do stupidities if you do more that absolutely necessary.
The last weeks I’ve made big service on my Revox A 77 mark III with all the parts from Bosch Trading Germany I ordered at. Mr. Erwin was also very helpfull in certain questions. A big adventure to do.., but now I need to calibrate it I’m desperate to find a service manual for this. Your movie of serving this machine had helped me a lot. Thank in advance.
i just picked up a mk2 it powered up,but i smelled burning.i replaced the 4 rifa caps everything else on the inside looks okay from i can see.my problem now is the motors spins ultra fast no matter what speed setting.should i do a full recap on the speed control board?
(scb=speed control board, I guess it was NOT replaced in the past and you have the non-IC type) Looks like the scb is not controlling the speed of the motor and the motor runs at its highest speed. A full recap of the scb is a first step (if you are lucky this might be enough). But it is not sure it will fix your problem. You will have to check if the tacho head is working. Put a scope on the leads (brown-blue) of the tacho head (on the scb) and see if it reads something. Check also the distance of the tacho head from the motor. Next step, check main transistor, check full bridge rectifier (have heard of some look like working but under load they don't, replace anyway. Replace the trimmer also. Check voltages on test points as per the service manual. The scb can be easy pulled out, check my other video where I do it on camera.
@@lito3481 If you have the IC version on a mk2, your board has been replaced in the past. Since the 555 IC has a base and can be easily replaced go on and replace that too (it is cheap).
@@dchatz i checked the tachometer with scope ,and dmm.readings are within spec of service manual.800hz & 1600hz , 500 ohms.i think i found the culprit ! the trimmer pot is very old ,the contact on the adjuster has broken off.i read on a forum that this trimmer pot will cause hyper speed on the reels if it is damaged.regardless i will recap,and install new trimmer pot thank you for you help.
Someone says digital osciloscope won't do the job on analog devices, specially tape decks, because it's not so fast enough as does an analog oscilospoce. Whats about this? Did you aproach nice results with digital osciloscope?
Maybe I have heard of that also but the thing is my oscilloscope is 100MHz capable which is way beyond the 20KHz upper level of human capability. Even bias signal is normally about 100~150KHz which is still low enough. So I cannot find any proper reason for a digital oscilloscope with proper frequency specification, not to be suitable for analog audio devices.
@@dchatz I do agree but they say the digital oscilospoce is slow. I don't have one, so I can't understand it. I would like get a chinese 2 channel osciloscope like yours.
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This winter I repaired a similar A77 to yours. I had the same issues. The LDR resistor was too weak (high) the pick up reel motor had a bended shaft. And brakes need to be adjusted. Besides that I struggled a bit to learn to do a bias adjustment without having a calibration tape to start with. It was interesting to see your take on how to do the adjustment..
Doing bias adjustment doesn't require a test tape because it messes with the record side. The test tape is required for the playback side. If you don't have a test tape you can adjust the azimuth and the playback level by using an already known good recording as a test tape. Won't be accurate but may be OK. There is also a way to adjust azimuth by recording and flipping the tape on the back side. Google it. I have done many bias calibration by ear. If you get correct treble response it's good. But note, that is not a professional way.
I have same braking squeal on my pr99. Was thinking it is revox in general 😂
It's Revox automatic information system for pointing out that brake tapes are rusty 😂
Why did you not change the electrolyte condensers and the tantalus when you changed all the trimmers?
Maybe I should. If the machine was mine I would, even if it wasn't necessary. The machine worked fine, that is why I did not go further. After all, doing more stuff I would risk damaging something that I couldn't find. This repair was a favor to a friend and I didn't wanna risk doing things maybe worse. Working in your living room is dangerous sometimes to do stupidities if you do more that absolutely necessary.
The last weeks I’ve made big service on my Revox A 77 mark III with all the parts from Bosch Trading Germany I ordered at. Mr. Erwin was also very helpfull in certain questions. A big adventure to do.., but now I need to calibrate it I’m desperate to find a service manual for this. Your movie of serving this machine had helped me a lot. Thank in advance.
@@ronaldbuskens3619 get the service manual from hifi engine. Other places too. You dont need to be desparate. Its easy to find.
i just picked up a mk2 it powered up,but i smelled burning.i replaced the 4 rifa caps everything else on the inside looks okay from i can see.my problem now is the motors spins ultra fast no matter what speed setting.should i do a full recap on the speed control board?
(scb=speed control board, I guess it was NOT replaced in the past and you have the non-IC type) Looks like the scb is not controlling the speed of the motor and the motor runs at its highest speed. A full recap of the scb is a first step (if you are lucky this might be enough). But it is not sure it will fix your problem. You will have to check if the tacho head is working. Put a scope on the leads (brown-blue) of the tacho head (on the scb) and see if it reads something. Check also the distance of the tacho head from the motor. Next step, check main transistor, check full bridge rectifier (have heard of some look like working but under load they don't, replace anyway. Replace the trimmer also. Check voltages on test points as per the service manual. The scb can be easy pulled out, check my other video where I do it on camera.
@@dchatz thanks i will test what you suggested. my board has ic's.
@@lito3481 If you have the IC version on a mk2, your board has been replaced in the past. Since the 555 IC has a base and can be easily replaced go on and replace that too (it is cheap).
@@dchatz i checked the tachometer with scope ,and dmm.readings are within spec of service manual.800hz & 1600hz , 500 ohms.i think i found the culprit ! the trimmer pot is very old ,the contact on the adjuster has broken off.i read on a forum that this trimmer pot will cause hyper speed on the reels if it is damaged.regardless i will recap,and install new trimmer pot thank you for you help.
Someone says digital osciloscope won't do the job on analog devices, specially tape decks, because it's not so fast enough as does an analog oscilospoce. Whats about this? Did you aproach nice results with digital osciloscope?
Maybe I have heard of that also but the thing is my oscilloscope is 100MHz capable which is way beyond the 20KHz upper level of human capability. Even bias signal is normally about 100~150KHz which is still low enough. So I cannot find any proper reason for a digital oscilloscope with proper frequency specification, not to be suitable for analog audio devices.
@@dchatz I do agree but they say the digital oscilospoce is slow. I don't have one, so I can't understand it. I would like get a chinese 2 channel osciloscope like yours.
@@MateusMachado go on and buy it. It's more than enough. The calibration process with it works without any problem.
@@dchatz alright, I will do it. Thanks for your advice.
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Αθηνα.