Revox B77 MKII - UK Vintage Radio Repair and Restoration Discussion Forum

Redders03 · 25th Jun 2012, 4:27 pm

Hi
recently Revox B77 has developed a problem such that the bulbs lighting the VU meters are alot dimmer than usual, measuring on J12 and J13 is 21 v on the Audio Interconnection PCB ?

Any Ideas of the Problem ?

Vicboduk · 25th Jun 2012, 5:59 pm

21 volts is correct. Check for bad joints on the crimp terminals and measure the voltage at the bulbs. It could be silvering of the bulbs over time - but this would be very gradual not something that happens suddenly. There’s also the possibility that your 21 volt rail has been high and has now returned to what it should be. The 21 volt supply is a very straight forward circuit using either a 78M20 or LM317 linear regulator (later versions) and a couple of resistors.

Vic

Redders03 · 25th Jun 2012, 6:18 pm

I am getting around 10 v at each bulb, the connectors are fine and i have recently replaced, D1,D2 R2,R3 and IC1 on Power Supply PCB, this was done in an effort to fix this problem and this made no difference.

mintras · 25th Jun 2012, 6:30 pm

I have one of these, but I think it will be some time before I look at it as it sparked, and fummed when I last plugged it in.

Vicboduk · 25th Jun 2012, 6:53 pm

If you are still getting 21v at J12/P12 & J13/P13 and only 10v at the bulbs then it must be a high resistance connection between the two. Measure the voltage from J13 (Red wire) to the red wire end of the bulb socket. Do the same for the Black wire on J12/P12. You should get zero volts in both cases – if not then there is a high resistance between the ends. My guess would be at the wire entry on the crimp terminals or on the bulb sockets.

Vic

Redders03 · 25th Jun 2012, 7:06 pm

I have checked the Cables and connectors and their is no resistance

Alex

Vicboduk · 25th Jun 2012, 7:15 pm

And you still get 10v at the bulbs? - I will have to take a look am my B77 later tonight...
A quick thought... you haven’t accidentally wired your bulbs in series have you? That would result in approximately 10v across each bulb.

Vic

Redders03 · 25th Jun 2012, 7:21 pm

I will have to check this, I gained a complete new Vu Assembly from ebay to replace the broken Vu meters and thought it was wired correctly.

Alex

Nickthedentist · 25th Jun 2012, 7:37 pm

Quote:

Originally Posted by Redders03

II gained a complete new Vu Assembly from ebay to replace the broken Vu meters

Ah! That's the kind of thing we need to know at the outset, Alex!

I'm certain you'll find Vic is right.

Redders03 · 25th Jun 2012, 7:38 pm

This has fixed the problem
Thank you

Alex

Vicboduk · 26th Jun 2012, 12:52 pm

I’m glad it’s sorted – I was running out of ideas!

Vic

25th Jun 2012, 4:27 pm	#1
Redders03 Pentode Join Date: Apr 2012 Location: Bromley UK. Posts: 161	Revox B77 MKII Hi recently Revox B77 has developed a problem such that the bulbs lighting the VU meters are alot dimmer than usual, measuring on J12 and J13 is 21 v on the Audio Interconnection PCB ? Any Ideas of the Problem ?

25th Jun 2012, 5:59 pm	#2
Vicboduk Hexode Join Date: Aug 2007 Location: S.W. London, UK. Posts: 416	Re: Revox B77 MKII 21 volts is correct. Check for bad joints on the crimp terminals and measure the voltage at the bulbs. It could be silvering of the bulbs over time - but this would be very gradual not something that happens suddenly. There’s also the possibility that your 21 volt rail has been high and has now returned to what it should be. The 21 volt supply is a very straight forward circuit using either a 78M20 or LM317 linear regulator (later versions) and a couple of resistors. Vic

25th Jun 2012, 6:18 pm	#3
Redders03 Pentode Join Date: Apr 2012 Location: Bromley UK. Posts: 161	Re: Revox B77 MKII I am getting around 10 v at each bulb, the connectors are fine and i have recently replaced, D1,D2 R2,R3 and IC1 on Power Supply PCB, this was done in an effort to fix this problem and this made no difference.

25th Jun 2012, 6:30 pm	#4
mintras Retired Dormant Member Join Date: Jun 2012 Location: Manchester, UK. Posts: 40	Re: Revox B77 MKII I have one of these, but I think it will be some time before I look at it as it sparked, and fummed when I last plugged it in.

25th Jun 2012, 6:53 pm	#5
Vicboduk Hexode Join Date: Aug 2007 Location: S.W. London, UK. Posts: 416	Re: Revox B77 MKII If you are still getting 21v at J12/P12 & J13/P13 and only 10v at the bulbs then it must be a high resistance connection between the two. Measure the voltage from J13 (Red wire) to the red wire end of the bulb socket. Do the same for the Black wire on J12/P12. You should get zero volts in both cases – if not then there is a high resistance between the ends. My guess would be at the wire entry on the crimp terminals or on the bulb sockets. Vic Last edited by Vicboduk; 25th Jun 2012 at 7:02 pm.

25th Jun 2012, 7:06 pm	#6
Redders03 Pentode Join Date: Apr 2012 Location: Bromley UK. Posts: 161	Re: Revox B77 MKII I have checked the Cables and connectors and their is no resistance Alex

25th Jun 2012, 7:15 pm	#7
Vicboduk Hexode Join Date: Aug 2007 Location: S.W. London, UK. Posts: 416	Re: Revox B77 MKII And you still get 10v at the bulbs? - I will have to take a look am my B77 later tonight... A quick thought... you haven’t accidentally wired your bulbs in series have you? That would result in approximately 10v across each bulb. Vic

25th Jun 2012, 7:21 pm	#8
Redders03 Pentode Join Date: Apr 2012 Location: Bromley UK. Posts: 161	Re: Revox B77 MKII I will have to check this, I gained a complete new Vu Assembly from ebay to replace the broken Vu meters and thought it was wired correctly. Alex

25th Jun 2012, 7:38 pm	#10
Redders03 Pentode Join Date: Apr 2012 Location: Bromley UK. Posts: 161	Re: Revox B77 MKII This has fixed the problem Thank you Alex

26th Jun 2012, 12:52 pm	#11
Vicboduk Hexode Join Date: Aug 2007 Location: S.W. London, UK. Posts: 416	Re: Revox B77 MKII I’m glad it’s sorted – I was running out of ideas! Vic