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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 17th Nov 2018, 6:14 pm   #41
Raving Loony
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Default Re: Revox A77 Stops Playing (Transport Fault)

I guess this is where I start to struggle then. So near yet so far
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Old 17th Nov 2018, 6:22 pm   #42
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Default Re: Revox A77 Stops Playing (Transport Fault)

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Have enjoyed what I have done so far and couldn't have done any of it without the very kind help of yourself and all the others who have given their valuable time and assistance.
so my next question is how difficult is it to recap the power supply board and what do I need to purchase. Any info on this would be very much appreciated.
You are going to have to remove the power supply , take off the board , desolder the caps and solder new ones in . This is a bit tricky if you havent done it before because you have to remove the board from the supply and all the AC cables are on the rear of it . If you would like , you can send me the power supply , and I`ll recap it for you and I can change the rectifier if need be , (no charge just pay for the caps and the shipping both ways )

David
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Old 17th Nov 2018, 6:27 pm   #43
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Default Re: Revox A77 Stops Playing (Transport Fault)

If you want to do it yourself , you need :

Solder
Iron
flux

2500uf 35v cap
1000uf 35v cap
3.3uf 35v cap

Electrolytic capacitors ....
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Old 17th Nov 2018, 9:13 pm   #44
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Default Re: Revox A77 Stops Playing (Transport Fault)

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I guess this is where I start to struggle then. So near yet so far
No, this is where you learn a couple of new skills. Nothing difficult, just new to you. There's plenty of help and encouragement around on here.

Just changing a capacitor involves unsoldering the old one, buying a suitable replacement and soldering it in, taking care to get it the right way round. This may sound a lot to you taken as a whole, but each step is quite do-able. Do the steps and the whole thing looks after itself.

A bridge rectifier is just four diodes connected together and packaged. The circuitry they're connected to makes them difficult to test, so one way out is to unsolder the rectifier and just connect your multimeter to its legs free from the associated circuitry to test one diode at a time. If it's OK, you put it back in (taking care to get it the right way round) and re-soldering it. If it's not good, then you already have it out.

Your multimeter on its Ohms range can test diodes. Some even have a special diode testing range.

The part that needs a bit of skill is soldering. The answer to this is to cheat and do some practising on something sacrificial. You learn not only how to solder but to also look at a solder joint and judge whether it is OK or not. A bit of practice, some guidance and maybe some photos and we'll get you there.

It's very satisfying to teach someone and get them able to do things they didn't think they could. The people who really are impossible to teach are the ones who think they already know everything.

David
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Old 17th Nov 2018, 9:22 pm   #45
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Default Re: Revox A77 Stops Playing (Transport Fault)

agree with every thing David just said , this stuff is pretty simple to do once you grasp the practicalities . I learned most of what i know here in about 8 months , I`m still pretty new to it all but I`m processing somewhere between 8 and 10 machines a month and just loving every minute of it , I`m more than happy to sit and talk you through it at every step as people here did with me , its a great community here. Re-capping is just as it sounds , taking out the old caps , putting in new ones, its 90% of what i do to make sure these machines when i sell them on wont fail any time soon. IMO its imperative to longevity , there are people here that disagree with me , but the parts that fail consistently on these machines are the capacitors , because they are old . Transistor will fail with age , but not anything like capacitors , because of their make up. capacitors have oil in them that over the years dries out and end up either going open or short, neither is good . If you want to learn ....we are all here to teach you .... and believe me , once you get going , its impossible to stop ...hehe
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Old 17th Nov 2018, 10:42 pm   #46
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Default Re: Revox A77 Stops Playing (Transport Fault)

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Originally Posted by Raving Loony View Post
I guess this is where I start to struggle then. So near yet so far
No, this is where you learn a couple of new skills. Nothing difficult, just new to you. There's plenty of help and encouragement around on here.

Just changing a capacitor involves unsoldering the old one, buying a suitable replacement and soldering it in, taking care to get it the right way round. This may sound a lot to you taken as a whole, but each step is quite do-able. Do the steps and the whole thing looks after itself.

A bridge rectifier is just four diodes connected together and packaged. The circuitry they're connected to makes them difficult to test, so one way out is to unsolder the rectifier and just connect your multimeter to its legs free from the associated circuitry to test one diode at a time. If it's OK, you put it back in (taking care to get it the right way round) and re-soldering it. If it's not good, then you already have it out.

Your multimeter on its Ohms range can test diodes. Some even have a special diode testing range.

The part that needs a bit of skill is soldering. The answer to this is to cheat and do some practising on something sacrificial. You learn not only how to solder but to also look at a solder joint and judge whether it is OK or not. A bit of practice, some guidance and maybe some photos and we'll get you there.

It's very satisfying to teach someone and get them able to do things they didn't think they could. The people who really are impossible to teach are the ones who think they already know everything.

David
I think the small amount I have done today has been a bit overwhelming . I am busy for the next couple of days so will have a chance to reflect on how I want to proceed. Your help and support has been great and you seem to be good at boosting my confidence. Will post if I am brave enough to try it or not.
Many Thanks
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Old 17th Nov 2018, 10:56 pm   #47
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Default Re: Revox A77 Stops Playing (Transport Fault)

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agree with every thing David just said , this stuff is pretty simple to do once you grasp the practicalities . I learned most of what i know here in about 8 months , I`m still pretty new to it all but I`m processing somewhere between 8 and 10 machines a month and just loving every minute of it , I`m more than happy to sit and talk you through it at every step as people here did with me , its a great community here. Re-capping is just as it sounds , taking out the old caps , putting in new ones, its 90% of what i do to make sure these machines when i sell them on wont fail any time soon. IMO its imperative to longevity , there are people here that disagree with me , but the parts that fail consistently on these machines are the capacitors , because they are old . Transistor will fail with age , but not anything like capacitors , because of their make up. capacitors have oil in them that over the years dries out and end up either going open or short, neither is good . If you want to learn ....we are all here to teach you .... and believe me , once you get going , its impossible to stop ...hehe
Many thanks for your kind support and offer of help. Will have a think and let you know.
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Old 17th Nov 2018, 11:49 pm   #48
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Default Re: Revox A77 Stops Playing (Transport Fault)

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There is a significant possibility that a bad capacitor takes out part of the bridge rectifier, or that a failure of a diode in the bridge rectifier damages the capacitor. I wouldn't replace either without checking the other.

David

Though usually if the bridge fails, it takes out the fuse.


The capacitor is 2500uF/35V but is stud mounted, I dont know where you'll find a replacement.
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Old 18th Nov 2018, 12:13 am   #49
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Default Re: Revox A77 Stops Playing (Transport Fault)

I use clip ins and glue them to the board , no problems so far.

2200uF 50v is the modern equivalent I`m using . Vishay are the brand I`m using I think .

You can use a 3300uF 35v if you want to go up rather than down but the 2200`s are close enough and work just fine . You can go to around double the capacitance of a electrolytic before problems start occurring but you dont want to go too far under .

Last edited by NorfolkDaveUK; 18th Nov 2018 at 12:27 am.
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Old 18th Nov 2018, 9:37 am   #50
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Default Re: Revox A77 Stops Playing (Transport Fault)

Yeah i just checked what I have and they are vishay 3300uF 35v I`m using . RS do a Nichicon too but they are twice the price . I think the ones i use are around 2 each inc the vat . something like that. They work just fine and with a but if super glue they stay put pretty good .
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Old 18th Nov 2018, 9:43 am   #51
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Default Re: Revox A77 Stops Playing (Transport Fault)

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Though usually if the bridge fails, it takes out the fuse.
That's a sign of good design. In the days at HP when we were using iron transformers in power supplies, there was a corporate-required test to short each secondary of the transformer in turn and check that the fuse blew each time. This was done primarily as a fire precaution because a short that didn't pull the fuse would leave a transformer getting hotter and hotter. It had the secondary advantage that it limited the extent of damage from any downstream failure.

Diodes in bridges sometimes fail open-circuit, which can leave only half-wave rectification and a supply which droops badly when loaded and doesn't pull a fuse. So it's worth checking if it's thought a failed capacitor may have given the bridge a hard time.

David
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Old 18th Nov 2018, 10:09 am   #52
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Default Re: Revox A77 Stops Playing (Transport Fault)

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Though usually if the bridge fails, it takes out the fuse.
That's a sign of good design. In the days at HP when we were using iron transformers in power supplies, there was a corporate-required test to short each secondary of the transformer in turn and check that the fuse blew each time. This was done primarily as a fire precaution because a short that didn't pull the fuse would leave a transformer getting hotter and hotter. It had the secondary advantage that it limited the extent of damage from any downstream failure.

Diodes in bridges sometimes fail open-circuit, which can leave only half-wave rectification and a supply which droops badly when loaded and doesn't pull a fuse. So it's worth checking if it's thought a failed capacitor may have given the bridge a hard time.

David
Yeah I agree , my concern with Bill checking the rectifier at this point was purely because the a a swine to get to with the board still attached to the transformer and I myself have shorted the darn things trying to probe them when i first got going , so my plan is to have him get the board off and then diode test it rather than test the supply I think that`s going to be a lot easier/safer . It can be done of course , I now have a much better fluke meter with much better shielded probes so its a lot easier to do it without setting the house alight with all the sparks ..hehe .
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Old 18th Nov 2018, 11:02 am   #53
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Default Re: Revox A77 Stops Playing (Transport Fault)

You could just tack a capacitor of a lower value across the 0v and 27v tags to prove the point, If the original capacitor was pulling the 27v rail down by nearly 9volts due to excessive leaking the rectifier and fuse would have blown.

Lawrence.
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Old 18th Nov 2018, 11:09 am   #54
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Default Re: Revox A77 Stops Playing (Transport Fault)

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You could just tack a capacitor of a lower value across the 0v and 27v tags to prove the point, If the original capacitor was pulling the 27v rail down by nearly 9volts due to excessive leaking the rectifier and fuse would have blown.

Lawrence.
You could Lawrence , but I dont want to get into the realms of confusion for Bill . The psu needs to be recapped anyway so better to get that done and if that doesnt fix it , look else where . Otherwise he`s going to get confused and frustrated and put the thing back in the cupboard I would imagine, lol . (been there , more than I will admit...hehe) If the worst comes to the worst , I have a pile of PSU`s for A77`s if need be .
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Old 18th Nov 2018, 11:25 am   #55
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Default Re: Revox A77 Stops Playing (Transport Fault)

If the 27v rail is restored by tacking a capacitor across, one can go forth in confidence.

Test don't guess is the best way to learn this game.

Lawrence.
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Old 18th Nov 2018, 11:28 am   #56
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If the 27v rail is restored by tacking a capacitor across, one can go forth in confidence.

Test don't guess is the best way to learn this game.

Lawrence.
OK bud , over to you then mate . All yours .
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Old 18th Nov 2018, 11:45 am   #57
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Default Re: Revox A77 Stops Playing (Transport Fault)

I just re-read that and I didnt mean it how it sounded ..haha ... I`m not sure how to go about what you are saying so you are the best man to explain it is what I meant , I`ll follow with interest .
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Old 18th Nov 2018, 12:17 pm   #58
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Default Re: Revox A77 Stops Playing (Transport Fault)

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I just re-read that and I didnt mean it how it sounded ..haha ... I`m not sure how to go about what you are saying so you are the best man to explain it is what I meant , I`ll follow with interest .
Temporary bridging a capacitor with a known good one to prove the point is not a new trick, I don't think it needs an explanation, except to say that the capacitor in question is the reservoir capacitor, the purpose/theory of which you probably already know.

Lawrence.
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Old 18th Nov 2018, 12:19 pm   #59
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Default Re: Revox A77 Stops Playing (Transport Fault)

As always, there are several ways of doing anything. The art is in picking one suited to the repairer's experience and available test equipment.

A look at the capacitor voltage with a scope while it was under load would tell me if the voltage ripple frequency was full-wave or half wave and that would tell me if all diodes in the bridge were doing their work, and the depth of the ripple would tell me if the capacitor had gone low in value.

For someone just beginning, the most important thing is to avoid confusion. Take the slow route and point out the scenery. Calling out what they should see along the way helps build confidence that they're going the right way.

Revox-fixing is quite a well-trampled field. They are a good machine and well worth fixing. Also, plenty of them have been through the hands of experienced fixers, so all the usual faults are well known. There is plenty of good advice (and spares) available for those starting out.

David

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Old 18th Nov 2018, 1:09 pm   #60
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Default Re: Revox A77 Stops Playing (Transport Fault)

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As always, there are several ways of doing anything. The art is in picking one suited to the repairer's experience and available test equipment.

A look at the capacitor voltage with a scope while it was under load would tell me if the voltage ripple frequency was full-wave or half wave and that would tell me if all diodes in the bridge were doing their work, and the depth of the ripple would tell me if the capacitor had gone low in value.

For someone just beginning, the most important thing is to avoid confusion. Take the slow route and point out the scenery. Calling out what they should see along the way helps build confidence that they're going the right way.

Revox-fixing is quite a well-trampled field. They are a good machine and well worth fixing. Also, plenty of them have been through the hands of experienced fixers, so all the usual faults are well known. There is plenty of good advice (and spares) available for those starting out.

David

David
Very good advice. I'm just beginning so it's slow steady steps.
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