ReVox B77 record mystery - UK Vintage Radio Repair and Restoration Discussion Forum

diefenbuck · 25th Jun 2011, 12:44 pm

Greetings!

I have a B77 4-track STD that will no go into record mode. After reading all relevent posts here I am still stuck. The problem is as follows: Power is on and transport is stopped. Flick Record-Right switch to ON and the right LED lights. Flicking Record-Left causes right LED to dim slightly, left LED is off. Tried to record anyway but no luck. Unplugging connector that feeds erase and record heads results in right LED not lighting, as expected.

I have swapped all PCBs, including the Audio Interconnect, Transport Control, and Speed Control boards, one by one, to no avail. Also tried all combinations of tape speed, reel size, and varispeed - no effect.

Just received another Oscillator card and swapped it in - no effect. Put original Oscillator PCB back and right LED does NOT come on with Record-Right is switched on. Removed card, unplugged head feed connector, then reconnected and inserted card. Right LED now lights as before.

Does anyone have any ideas as to what on earth is going on ? Any help would be greatly appreciated. This is driving me MAD!

MikeyPP · 25th Jun 2011, 10:03 pm

Have you checked the LED's themselves as they feed the record relays on the OSC board as far as I can see.?
They are the only two components external to the OSC board (which you've already swapped out) apart from the REC L & R switches

Michael Maurice · 27th Jun 2011, 11:18 am

Hi, I've been away for the weekend so only just seen your post, I will try later to get the manual out and take a look for you. One thing you should try is cleaning the contacts on the switches themselves. You have to dismantle the Audio Interconnect PCB.

diefenbuck · 27th Jun 2011, 1:31 pm

MikeyPP - I think you're onto something. Will grab an LED from another machine and check it out.

I dismantled the Audio Interconnect and cleaned all the gold contacts under each of the 6 switches and the contacts on the switches themselves before I swapped it in. Investigation continues...

Michael Maurice · 27th Jun 2011, 10:09 pm

Check C1 and C2 on the oscillator board they have been known to go leaky or short circuit. Check also R1 and R2 that they are 330 ohms

Assuming they are OK, switch the machine on and first LH REC switch and then RH record switch, load a tape (or cover the sensors) and go into record.

Check that you have 24 on pin 13 of the edge connector, this should be present when either or both record switches are in REC mode.

Now check pin 12 of the connector you should have 0V or very near 0V. If not then check/replace Q10 on the tape drive board.

Let us know the results of your checks.

Michael

MikeyPP · 27th Jun 2011, 10:12 pm

He's supposedly swapped out the OSC board ?

Michael Maurice · 27th Jun 2011, 10:40 pm

So he did.

I wonder why unplugging and replugging the head connector made any difference, it shouldn't have as its on the secondary side of the oscillator transformer. I've never known an LED go high resistance but there's always a first time.

Resistance and voltage checks are the only way to go forward.

MikeyPP · 27th Jun 2011, 10:42 pm

Seeing as the OSC board has been swapped out, One side working takes it down to the LED or the switch in my view at first glance

Unless there are physically stressed solder joints on these connectors maybe ? and on interboard connectors too?
worth examining

Quote:

I've never known an LED go high resistance but there's always a first time.

The relay's being fed the 24V via these LED's , I suppose if the relay or the relay diode (or even the cap near relay) went short it might pull excess current through the LED and kill it? or take it high resistance.
The OSC board swap keeps removing some components from the equation though

diefenbuck · 28th Jun 2011, 4:52 pm

Well chaps, meter to the dodgy LED shows 12.5 MegaOhms as opposed to no load on the good LED. Something really sinister is afoot. I replaced the faulty LED and swapped in a known good set of 5 PCBs and record does not work. Put the 5 cards back in the original machine (a Mk I) and now *it* does not record. I think the Mk II might be responsible for toasting some other cards as well. Any thoughts?

Michael Maurice · 28th Jun 2011, 11:42 pm

How do you know that the 'new' boards are OK? Board swapping is NOT really the answer,

May I suggest you refit the original boards, but replace C1 & C2 on the oscillator board and then take your voltage measurements.

diefenbuck · 2nd Jul 2011, 6:17 pm

Thanks for your help lads. We have builders coming on Monday so I will have to put off solving the mystery for a few months.

MikeyPP · 2nd Jul 2011, 9:02 pm

Quote:

Originally Posted by diefenbuck

will have to put off solving the mystery for a few months.

grr

25th Jun 2011, 12:44 pm	#1
diefenbuck Retired Dormant Member Join Date: Dec 2008 Location: Guildford Posts: 4	ReVox B77 record mystery Greetings! I have a B77 4-track STD that will no go into record mode. After reading all relevent posts here I am still stuck. The problem is as follows: Power is on and transport is stopped. Flick Record-Right switch to ON and the right LED lights. Flicking Record-Left causes right LED to dim slightly, left LED is off. Tried to record anyway but no luck. Unplugging connector that feeds erase and record heads results in right LED not lighting, as expected. I have swapped all PCBs, including the Audio Interconnect, Transport Control, and Speed Control boards, one by one, to no avail. Also tried all combinations of tape speed, reel size, and varispeed - no effect. Just received another Oscillator card and swapped it in - no effect. Put original Oscillator PCB back and right LED does NOT come on with Record-Right is switched on. Removed card, unplugged head feed connector, then reconnected and inserted card. Right LED now lights as before. Does anyone have any ideas as to what on earth is going on ? Any help would be greatly appreciated. This is driving me MAD!

25th Jun 2011, 10:03 pm	#2
MikeyPP Retired Dormant Member Join Date: Jun 2011 Location: Berkshire Posts: 389	Re: ReVox B77 record mystery Have you checked the LED's themselves as they feed the record relays on the OSC board as far as I can see.? They are the only two components external to the OSC board (which you've already swapped out) apart from the REC L & R switches

27th Jun 2011, 11:18 am	#3
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,229	Re: ReVox B77 record mystery Hi, I've been away for the weekend so only just seen your post, I will try later to get the manual out and take a look for you. One thing you should try is cleaning the contacts on the switches themselves. You have to dismantle the Audio Interconnect PCB.

27th Jun 2011, 1:31 pm	#4
diefenbuck Retired Dormant Member Join Date: Dec 2008 Location: Guildford Posts: 4	Re: ReVox B77 record mystery MikeyPP - I think you're onto something. Will grab an LED from another machine and check it out. I dismantled the Audio Interconnect and cleaned all the gold contacts under each of the 6 switches and the contacts on the switches themselves before I swapped it in. Investigation continues...

27th Jun 2011, 10:09 pm	#5
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,229	Re: ReVox B77 record mystery Check C1 and C2 on the oscillator board they have been known to go leaky or short circuit. Check also R1 and R2 that they are 330 ohms Assuming they are OK, switch the machine on and first LH REC switch and then RH record switch, load a tape (or cover the sensors) and go into record. Check that you have 24 on pin 13 of the edge connector, this should be present when either or both record switches are in REC mode. Now check pin 12 of the connector you should have 0V or very near 0V. If not then check/replace Q10 on the tape drive board. Let us know the results of your checks. Michael

27th Jun 2011, 10:12 pm	#6
MikeyPP Retired Dormant Member Join Date: Jun 2011 Location: Berkshire Posts: 389	Re: ReVox B77 record mystery He's supposedly swapped out the OSC board ?

27th Jun 2011, 10:40 pm	#7
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,229	Re: ReVox B77 record mystery So he did. I wonder why unplugging and replugging the head connector made any difference, it shouldn't have as its on the secondary side of the oscillator transformer. I've never known an LED go high resistance but there's always a first time. Resistance and voltage checks are the only way to go forward.

28th Jun 2011, 4:52 pm	#9
diefenbuck Retired Dormant Member Join Date: Dec 2008 Location: Guildford Posts: 4	Re: ReVox B77 record mystery Well chaps, meter to the dodgy LED shows 12.5 MegaOhms as opposed to no load on the good LED. Something really sinister is afoot. I replaced the faulty LED and swapped in a known good set of 5 PCBs and record does not work. Put the 5 cards back in the original machine (a Mk I) and now it does not record. I think the Mk II might be responsible for toasting some other cards as well. Any thoughts?

28th Jun 2011, 11:42 pm	#10
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,229	Re: ReVox B77 record mystery How do you know that the 'new' boards are OK? Board swapping is NOT really the answer, May I suggest you refit the original boards, but replace C1 & C2 on the oscillator board and then take your voltage measurements.

2nd Jul 2011, 6:17 pm	#11
diefenbuck Retired Dormant Member Join Date: Dec 2008 Location: Guildford Posts: 4	Re: ReVox B77 record mystery Thanks for your help lads. We have builders coming on Monday so I will have to put off solving the mystery for a few months.