Orange instability - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

ITAM805 · 26th Apr 2014, 4:25 pm

Hi folks

well, I've had a breakthrough of sorts! After much ado, I pulled V6 and the dreaded scream ceased

V6, I thought, is purely responsible for the send/return of the reverb tank circuit and so it went untouched. I had the reverb return control (RV9) right down so I'm puzzled as to how it's been causing the preamp to go crazy?

julie_m · 27th Apr 2014, 5:15 am

If a valve is breaking into oscillation then, if the decoupling capacitors are ageing a little, it potentially can couple that oscillation into other valves via the HT line. If you can beg, borrow or steal an oscilloscope, then take look at the HT using a 10:1 probe set for AC coupling with V6 present and absent. Adjust the timebase in case it is oscillating in the ultrasound.

ITAM805 · 29th Apr 2014, 2:12 pm

Thanks AJS,

I tried fitting a 47uF to the top of R2 (anode V6-A), it's not HT decoupling that's causing the oscillation. But what is, I'm at a complete loss?

V6-A (reverb tx primary driver) seems to be at the heart of it, if this is removed then the channel works fine. The components associated with it all appear in spec. I tried replacing C6 (V7B) which feed g1 of V6A but still no difference.

To reiterate, when I increases the gain control (RV4) to around mid way, V6 suddenly bursts into oscillation, which in turn is getting back into the preamp.

Attached is a pic at the anode of V6-A, peak to peak is around 50V

Skywave · 29th Apr 2014, 4:04 pm

Try fitting ferrite beads on the wiring to the control grids of the valves in the suspect stages. Fit the beads as close as possible to the valve control grid pins. Plus the addition of 100 Ω 'grid stoppers'. Such a combination constitutes a low-pass filter.

Al.

ITAM805 · 29th Apr 2014, 5:05 pm

Thanks Al

tricky fitting ferrite's as it's pcb. As for a grid stopper, g1 is fed through a divider and thus has a 220k in place already. I just tried a 100nF on g1 to ground but it's still whistling away :

!

Just a thought, the tx on V6A measures 490R, so I tried (in desperation) a 470R resistor in place but with the same result as ever. So I was wondering if this values sound right for an ECC81 anode load, although it also has a 15k in line? If the tx maybe has a short in it, would that make it oscillate?

Aside from getting an exorcist in, I dont know what else to do with this damn thing?

jon_the_id · 29th Apr 2014, 8:52 pm

At the risk of stating the bleeding obvious, does this unit have all the shielding cans in place on the preamp valves?
If so, does moving them make any difference?

EDIT: Another thought - I know on older Oranges (OR120s) sometimes the path of the NFB wire is critical to stability.
You could try disconnecting it or moving it around and seeing if that alters anything.

ITAM805 · 29th Apr 2014, 9:11 pm

Hi Jon, all suggestion, obvious or otherwise are gratefully received!

When I got the amp the screening cans were missing, I thought 'there's yer problem' - wrong! The guy brought them round but they made no difference when fitted. Up to a point, if I move my hand near the board the oscillation stops, but this effect disappears as one increases the gain control.

The cans are the 'Chinese' type, they screw into the chassis, the holders are fixed to the board, which means when the board is out there's nowhere to fix the screens to. So what I've been doing it trying certain fixes then putting the board back in and trying them out, a right palaver believe me...

I'll try the NFB dodge tomorrow

McMurdo · 29th Apr 2014, 9:21 pm

is the reverb connected the right way round?

ITAM805 · 29th Apr 2014, 9:27 pm

it's not connected at all Kevin as the tank is screwed to the cabinet, although I did reconnect it at one stage, but it made no difference to the squeal.

ronbryan · 29th Apr 2014, 10:25 pm

Could the reverb tank driver transformer be magnetically coupling with an earlier stage - could it be rotated by up to 90 degrees as a test to see if that would reduce the coupling?

Ron

jon_the_id · 29th Apr 2014, 11:55 pm

I thought it had been tried using a resistive load instead of the reverb transformer and the problem persisted.

Another thought - you could try moving the grid stopper of the reverb driver valve to actually being on the socket itself, and see if that helps.

TrevorG3VLF · 30th Apr 2014, 10:35 am

If waving a hand near it affects the result, then there is RF instability. Bypassing grid to cathode with 1nF caps should find and perhaps cure the problem.

If this does not work, use it as a theromin.

ronbryan · 30th Apr 2014, 11:37 am

Quote:

Originally Posted by jon_the_id

I thought it had been tried using a resistive load instead of the reverb transformer and the problem persisted.

That's true, Jon, and I did overlook that post, but the AC load presented by the transformer will be nearer 10k compared to the 470 ohms used for the substitution test.

Ron

jon_the_id · 30th Apr 2014, 3:27 pm

That's true also Ron, but isn't the resistor doing a pretty good job of emulating the dc conditions - which, with no signal present and prior to the onset of oscillation is where you would be, isn't it?

ITAM805 · 30th Apr 2014, 3:56 pm

Hi all

well, I think I've cured the problem - ish?

Following Trevor's suggestion, I tried a 1nF on g1 which works but the thing still whistled at full tilt. So out it came again and I stuck a 10nF on, stood back, switched on.... no whistle

, but alas no top on the reverb either. That suggests I should try a value in between, which of course I haven't got but at last it's responding to treatment.

I also had a poke round with the wiring, as Jon (the id) mentioned, regarding Orange amps being fussy about layout. I discovered if I rerouted the o/p tx leads just a few mm the thing would screech a bit less.

So I'm going to tempt fate and say that it's on the road to recovery. Many thanks for everyone's help and perseverance with this enigma

26th Apr 2014, 4:25 pm	#21
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Orange instability Hi folks well, I've had a breakthrough of sorts! After much ado, I pulled V6 and the dreaded scream ceased V6, I thought, is purely responsible for the send/return of the reverb tank circuit and so it went untouched. I had the reverb return control (RV9) right down so I'm puzzled as to how it's been causing the preamp to go crazy?

27th Apr 2014, 5:15 am	#22
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Orange instability If a valve is breaking into oscillation then, if the decoupling capacitors are ageing a little, it potentially can couple that oscillation into other valves via the HT line. If you can beg, borrow or steal an oscilloscope, then take look at the HT using a 10:1 probe set for AC coupling with V6 present and absent. Adjust the timebase in case it is oscillating in the ultrasound. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

29th Apr 2014, 2:12 pm	#23
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Orange instability Thanks AJS, I tried fitting a 47uF to the top of R2 (anode V6-A), it's not HT decoupling that's causing the oscillation. But what is, I'm at a complete loss? V6-A (reverb tx primary driver) seems to be at the heart of it, if this is removed then the channel works fine. The components associated with it all appear in spec. I tried replacing C6 (V7B) which feed g1 of V6A but still no difference. To reiterate, when I increases the gain control (RV4) to around mid way, V6 suddenly bursts into oscillation, which in turn is getting back into the preamp. Attached is a pic at the anode of V6-A, peak to peak is around 50V Attached Thumbnails

29th Apr 2014, 4:04 pm	#24
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Orange instability Try fitting ferrite beads on the wiring to the control grids of the valves in the suspect stages. Fit the beads as close as possible to the valve control grid pins. Plus the addition of 100 Ω 'grid stoppers'. Such a combination constitutes a low-pass filter. Al.

29th Apr 2014, 5:05 pm	#25
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Orange instability Thanks Al tricky fitting ferrite's as it's pcb. As for a grid stopper, g1 is fed through a divider and thus has a 220k in place already. I just tried a 100nF on g1 to ground but it's still whistling away :! Just a thought, the tx on V6A measures 490R, so I tried (in desperation) a 470R resistor in place but with the same result as ever. So I was wondering if this values sound right for an ECC81 anode load, although it also has a 15k in line? If the tx maybe has a short in it, would that make it oscillate? Aside from getting an exorcist in, I dont know what else to do with this damn thing? Attached Thumbnails

29th Apr 2014, 8:52 pm	#26
jon_the_id Pentode Join Date: May 2007 Location: London, UK. Posts: 144	Re: Orange instability At the risk of stating the bleeding obvious, does this unit have all the shielding cans in place on the preamp valves? If so, does moving them make any difference? EDIT: Another thought - I know on older Oranges (OR120s) sometimes the path of the NFB wire is critical to stability. You could try disconnecting it or moving it around and seeing if that alters anything. Last edited by jon_the_id; 29th Apr 2014 at 8:58 pm.

29th Apr 2014, 9:11 pm	#27
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Orange instability Hi Jon, all suggestion, obvious or otherwise are gratefully received! When I got the amp the screening cans were missing, I thought 'there's yer problem' - wrong! The guy brought them round but they made no difference when fitted. Up to a point, if I move my hand near the board the oscillation stops, but this effect disappears as one increases the gain control. The cans are the 'Chinese' type, they screw into the chassis, the holders are fixed to the board, which means when the board is out there's nowhere to fix the screens to. So what I've been doing it trying certain fixes then putting the board back in and trying them out, a right palaver believe me... I'll try the NFB dodge tomorrow Attached Thumbnails

29th Apr 2014, 9:21 pm	#28
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,276	Re: Orange instability is the reverb connected the right way round? __________________ Kevin

29th Apr 2014, 9:27 pm	#29
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Orange instability it's not connected at all Kevin as the tank is screwed to the cabinet, although I did reconnect it at one stage, but it made no difference to the squeal.

29th Apr 2014, 10:25 pm	#30
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Orange instability Could the reverb tank driver transformer be magnetically coupling with an earlier stage - could it be rotated by up to 90 degrees as a test to see if that would reduce the coupling? Ron

29th Apr 2014, 11:55 pm	#31
jon_the_id Pentode Join Date: May 2007 Location: London, UK. Posts: 144	Re: Orange instability I thought it had been tried using a resistive load instead of the reverb transformer and the problem persisted. Another thought - you could try moving the grid stopper of the reverb driver valve to actually being on the socket itself, and see if that helps.

30th Apr 2014, 10:35 am	#32
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Orange instability If waving a hand near it affects the result, then there is RF instability. Bypassing grid to cathode with 1nF caps should find and perhaps cure the problem. If this does not work, use it as a theromin.

30th Apr 2014, 3:27 pm	#34
jon_the_id Pentode Join Date: May 2007 Location: London, UK. Posts: 144	Re: Orange instability That's true also Ron, but isn't the resistor doing a pretty good job of emulating the dc conditions - which, with no signal present and prior to the onset of oscillation is where you would be, isn't it?

30th Apr 2014, 3:56 pm	#35
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Orange instability Hi all well, I think I've cured the problem - ish? Following Trevor's suggestion, I tried a 1nF on g1 which works but the thing still whistled at full tilt. So out it came again and I stuck a 10nF on, stood back, switched on.... no whistle , but alas no top on the reverb either. That suggests I should try a value in between, which of course I haven't got but at last it's responding to treatment. I also had a poke round with the wiring, as Jon (the id) mentioned, regarding Orange amps being fussy about layout. I discovered if I rerouted the o/p tx leads just a few mm the thing would screech a bit less. So I'm going to tempt fate and say that it's on the road to recovery. Many thanks for everyone's help and perseverance with this enigma