A77 Dead Recording Channel

[julie_m]

Ah. On this recorder you do actually need to swap over the inputs and re-set the levels during sound-on-sound recording, depending which track you are recording onto. So there isn't a combination of switch settings that would reverse the inputs after all.

In which case, check nobody has done something stupid like swap over the connections to the heads ..... easy mistake to make, when it's all plug-in modules.

[Michael Maurice]

Yes there is Julie.

I've owned an A77 from new (1976) and B77 from 1985.

What would be the point of lying to you

With BOTH record switches out, the left and right channels are reversed.

There are no relays in the signal path no switching IC's

Its all done through the two record switches next to the VU meters on the A and toggle switches on the B

[julie_m]

I'm not accusing anyone of lying. I just looked through the service and operator's manuals, diving straight into what I thought would be the important bits; circuit diagrams and "advanced recording techniques". And I didn't see any channel-swapping, because I was looking in the wrong place -- at the input selectors.

Now, reading back through the operator's manual from the beginning, I can see it right at the bottom of page 14:

If both record preselector buttons (32) and (35) are released, channel I and II are crossed over with the result that each meter will respond to the signals of the opposite channel.

You must agree, it is counter-intuitive, though!

[Panagiotis]

I realize I had convoluted my original post a bit but I did correct it farther down. I understand how to bounce tracks I just never had to use a single half-track machine to do it, my half tracks were for the occasional mix-down and originating live source material, I was minimalist mic'r. Anyway to clarify; with both record selectors depressed the ch. I level pot controls ch. II and the ch. II level pot controls ch. I (not depressed they correctly control their respective channel). This is wrong, I also have a B77 and it functions correctly. I like the ideal of sending separate and different signals simultaneously, that will allow me to compare left to right without much experience in signal tracing. Any advice on where I should I concentrate? Also, since my ch II meter is open circuit, may I simply jumper the connections or shall a place a resistor across the terminals?

[Panagiotis]

Originally Posted by julie_m

I'm not accusing anyone of lying. I just looked through the service and operator's manuals, diving straight into what I thought would be the important bits; circuit diagrams and "advanced recording techniques". And I didn't see any channel-swapping, because I was looking in the wrong place -- at the input selectors.

What in the world is the purpose of this?! And how do I quote the way you did?

[Station X]

Originally Posted by Panagiotis

And how do I quote the way you did?

Click the "QUOTE" button at the bottom right of the post you wish to reply to. Quoted test will appear. Edit as required, then add your reply.

[Panagiotis]

Got it, thank you.

I thought I may have lost my mind so I made a quick check of my B77, similar machine, high-speed half-track. I injected two different signals from two different signal generators and there is no channel swapping. I knew this already because I edited my own recordings and would have caught it straight off. This feature doesn't impress me at all and I apologize to Michael for insinuating that he was having fun with my inexperience (although I wouldn't blame him if he was). I should be able to easily reverse this and move on to my dead recording channel, no?

[julie_m]

Originally Posted by Panagiotis

What in the world is the purpose of this?! And how do I quote the way you did?

Perhaps it was an elaborate Revenue Protection scheme, to ensure anyone who obtained a used machine at least had to pay for a copy of the instruction manual, and could not just guess how to use it by looking at it? It must have made sense to somebody, I suppose.

You can create an "anonymous" quote by using just [quote] ..... [/quote] tags. And you can type [things in square brackets] by using [noparse] ..... [/noparse] tags around them.

Now let's get back on topic, before we incur the wrath of the moderators

[Michael Maurice]

No offence taken Panagiotis,

I try to be as an accurate and helpful as possible and this is a quirk of the Revox tape recorders.

[Panagiotis]

Perhaps it was an elaborate Revenue Protection scheme, to ensure anyone who obtained a used machine at least had to pay for a copy of the instruction manual, and could not just guess how to use it by looking at it? It must have made sense to somebody, I suppose.

LOL

They must have thought better of it with the B77 though, at least it gives the "Reverse" option on the Mode dial. It's a handy feature for checking true level balance, I used it all the time way back when..

Thank you Michael, you're very gracious.

I've been doing some more snooping and have additional information:
- The deck records both channels well in Mono I and II but in either case only the ch. II level pot affects the signal.
- It plays back both channels correctly.
- I removed the ch. II meter and probed the pads. I will try to attach the photos of the results.

I am unknowingly assigning this to DC leakage in that signal path and it does in fact measure .732Vdc. The ch I meter has a normal sine wave but still measures .209Vdc. I tested the diodes in the immediate area but all tested good. Where should I look from here and am I reading this correctly?

[Michael Maurice]

It would be helpful if you could have a working CHII VU meter but its not essential, nor is it necessary to put a resistor across the meter.

I've attached a block diagram for you of the record section.

I take it you've swapped the record cards?

Assuming you have and the fault remains the same, then the fault is with the wiring, probably around the record switch board

[Panagiotis]

Thank you very much, yes I've swapped the cards more times than I can count but I'll try again tonight, I may have been chasing my tail with that reversed channel "feature".

Thank you also for the highlighted diagram, it helps a lot.

[Panagiotis]

Well it appears that one of the boards does in fact have problems. I checked the voltages against the schematic and the +12v at junction R515 and R516 was low at 7.6v, the others were more or less correct. C515 turned out to be open and if I read it correctly, that's the signal to the head. I don't have an exact value replacement though, how far off can I be for a test and still discern if this is the issue, or at least part of the issue?

[Michael Maurice]

C515 is 3.3uF according to the service manual. Its probably a tantalum type. You could use 4.7uF to test it.

I'm surprised it affects the 12V at that junction, if it was short circuit, then yes I'd expect a problem.

You could of course remove C515 and see if that point returns to 12V if not then you may have another fault.

[Panagiotis]

I only had a moment to work on it tonight as my youngest was in town for a visit so I just removed the good cap from the other board and tested it on the bad board. It appears that it now records and the voltage has come back up. It was only tested with the one board of course but it definitely put the signal to tape so hopefully nothing else breaks when both boards are re-installed. If all is well here, I'll apply myself with a little more confidence to chasing down the EOT sensor problem which from what I've read should not be too hard.

Thank you once again, I'll post the results when both boards are back in their slots.

[Panagiotis]

Update: I had a few more minutes tonight to replace the donor cap and test both channels for recording and they now seem to work correctly. I don't pretend to understand this part but the channel swapping issue has also disappeared and that is a welcome consequence indeed. Now it's time to place an order with Nagravox and get on with that, I wanted so sort out the dead channel problem first.

I just noticed that it says Triode under my name, can someone please tell me what this tag-line references?

Thanks again everyone!

[Radio Wrangler]

It's just an indicator of the total number of posts you've made and counts up diode, triode, tetrode, pentode.... nonode but has to switch to a gas-filled valve for dekatron. Quaint and appropriate. It's a common feature in forum software, other fora have similar indicators but with words chosen to suit their special interest.

But quantity isn't a reliable indicator of quality.

David

[Michael Maurice]

I'm glad you've got it sorted, Before you change all the parts in the kit, beware that they also contain the presets for playback, record, bias etc levels and these will needd to be set up properly by someone with the required test gear. These presets do have a habit of breaking up which is why they are included in the kit.

If you dont have access to this test gear then as long as the presets are in good order and haven't been touched, dont change them.

May I also suggest you replace parts on one board at a time, and then test the machine so that if there is a new fault you can localise it to the board you last repaired.

Michael

[Panagiotis]

That's good advice about testing sequentially Michael, thank you.

I think I should be able to deal with the presets alright and I have read that they tend to break down so they should really be done. I have recapped a few machines before, mostly power supplies but also one cassette deck complete, it's a bit of a time commitment but the results have always been good. I'm much more comfortable with the mechanicals and have also worked over a few decks to improve the w&f, in one case to better than factory spec. Having a machine shop helps, there aren't too many parts I couldn't fabricate or improve upon when needed and I can machine most to very close tolerance. Fully conventional here as well, no CNC's in sight..

[Panagiotis]

Originally Posted by Radio Wrangler

It's just an indicator of the total number of posts you've made

Ah I get it, very clever. Some days I feel like a gas-filled valve myself..