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Noob with a soldering iron

Started by Jungle-Jim, December 21, 2014, 09:01:50 PM

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Jungle-Jim

Hi all
I have a Session Rockette 30watt amp of mid-80s vintage. It's a good little amp, but it's started to get a hum/buzz, too noticeable to record with.

I am thinking that it's the electrolytic caps - is this a likely culprit? If so - is it good idea to replace all the electrolytic caps - there's only 12. This could be cheaper than going to an amp tech, and I am very experienced with a soldering iron so it wouldn't be a problem.

If 'no' - what is more likely the cause of the hum?

If 'yes' - is there a fav brand of caps to use - is it true that Elna Silmic II are the ones to go for? I have sourced them on ebay, though they are not cheap - am I going overboard here, or is there a cheaper brand of cap that will be just as good? For the power supply end I have found some Panasonic FC and FM pairs which are the right spec (note I tried to get all 12 from Panasonic, but there's a few which aren't available in their FC, FM ranges).

And just one more question: at the very input end of the amp circuit, it specs a 1uf 63v electrolytic, but I can't seem to get that as a Panasonic or Elna - so could I use a 2.2uf 63v (or a 1uf 50v) of either of those brands instead, or should I just get another one I found on the web which is this spec:

MCGPR63V105M5X11 Multicomp CAPACITOR, 1UF 63V
Capacitance: 1F
Capacitance Tolerance: 20%
Diameter: 5mm
Lead Spacing: 2mm
Operating Temperature Range: -40C to +85C
Voltage Rating: 63VDC
AC Ripple Current: 12mA
External Length / Height: 11mm
Lead Diameter: 0.5mm
Leakage Current: 16.86A
Operating Lifetime: 2000h
Operating Temperature Max: 85C
Operating Temperature Min: -40C
Package / Case: Radial
Ripple Current Frequency: 120Hz
Ripple Current Temperature: 85C
Terminal Type: Wire Ended

I have attached the schematic for this amp, which I found on the Session website. Note that at the input it has that 1uf 63v electr cap - but could that have been a 2.2uf 63v or a 1uf 50v?

Thanks I hope that's not too many questions at once.

Merry Xmas,
Jim

phatt

Replace ALL the caps,,, Yikes!!! like what for?  :loco

That is about the hardest, longest, most expensive way to go and you have no idea if that will fix it. The shoot anything that moves approach is not the way. xP

Get the circuit board in good light and just quietly peruse the real estate inside, First noting the Mains voltage wires, fuse, switch etc and keep a mental note of those mains wires and never touch them.
Now look for any bulging caps mainly the electrolytics but flip it over and With very good light and a magnifying glass check the solder joints of the big electros, you are looking for hair line cracks around the solder pads.
Re flow any suspect pads and try again, if you can't find any obvious faults then take some pictures of both sides of the boards and post them that will help us to help you.

Caps; a capacitor needs to be *Above* the working voltage of the circuit,,so if you have a 50 volt supply rail the filter caps will probably be 63VDC. Always add at least 10%~20% over the working voltage.

Re the brand name game;
For guitar amplifiers as long as they are the right value and voltage they will sound the same in a blind test.
Yes Yes I know some big name guitar player said blah bla bla about Bright Orange caps improving the tone mojo.

If His ears can hear it,, then it can be measured,,, but sadly most of this has been tested to death and never shown anything meaningful.
There are rare circumstances where a certain type of cap will improve something but you will find that will only help if you are building a space station.

BTW, Leo Fender purchased all the seconds from the factory to save a few bucks, yet some of his amps are legendary,,,go figure.
Phil.

Roly

Hi Jungle-Jim and welcome.


There are sundry bits of zombie mythology that just seem to keep coming back no matter how many times they get shot down, and your post contains two of the most common (which ain't your fault);

1) "whatever is wrong with your amp, changing all the caps will fix it"; and

2) "randomly changing components is a reasonable way to service an amp".

Caps certainly fail, but not nearly as often as the "experts" on some music forums seem to think.  The most common cause of amp problems is dirt, typically in connectors and associated switching contacts, followed by broken solder joints and the like.

Real techs (those of us who actually earn a living repairing amps) don't use the "blunderbuss" or "scattergun" technique of randomly replacing components because it is wasteful of time and components, but mainly because it is ineffective - bench time is valuable and you can run up a tidy sum in fitting new components and still have the fault :(, at which point you have to do what should have been done in the first place - diagnosis of the fault, then fixing the cause.









First up we feed a signal into Fx Return (e.g. just plug your guitar in there) and see if we still have the fault.  If so then we look to the power amp section, if not we look to the preamp.  That way we cut the whole amp in half and see which half is the source of the problem.

A very big thanks to Session UK for putting up the circuits of their amps, and yours seems to be this one (includes pre and power supply);

http://www.award-session.com/pdfs/manuals/sg30-sch.pdf


To answer your specific questions; C1 1uF/63V can be replaced by a 50V unit, but I seriously doubt that is the cause of your trouble. {what phatt said is generally true however this particular cap is an input signal coupling cap and is unlikely to be exposed to more than a few volts.}

If any caps need to be replaced they will most likely be the 2200uF/35V pair in the power supply, but do the Fx Return test before rushing in with your soldering iron.


Generally speaking there isn't much practical difference between caps of a reputable brand, such as Elna or Panasonic, and there is certainly no need to pay through the nose for "boo-teek" caps at several times the price.  I have been using Elna caps for around 50 years and found them perfectly satisfactory if properly specified (e.g. not over-volted etc).

Conduct the Fx Return test and post results.

Also, do you still get the buzz with all the volume controls at zero?
If you say theory and practice don't agree you haven't applied enough theory.

Jungle-Jim

#3
Hi phatt and Roly

Thanks for your sound advice to get the ball rolling...

Yep - Roly - that is the same amp - a Rockette 30. For several reasons, I want to keep this amp - it was made not far from here in the south-east of England, and is a relic from the days when things were made in small production facilities here (now it all comes from China). Apparently the guy who designed the Session amps is still around, and even though the company was bought-out then went out of business, they still have a factory where they rebuild and retro-modify these amps. Publishing the schematics is a good move - it helps people to keep the amps alive. I'd rather fix this than have a shiny new thing from China.

Anyway - here's the key things it's doing...
* When you switch it on with nothing in the jacks, there's a metallic, jagged hum, which is constant, and may be a 50hz hum. Note to answer your Q Roly - this hum remains when all knobs are turned to zero.
* When you put an instrument lead into the FX Return jack, the hum gets just a bit quieter. When you play an instrument through this jack, the tone is clean and good, it's just that there's a hum in the background.
* When you switch the amp off, it usually makes two sounds - first is a clicking or popping to accompany the switch, and after a short delay, another sound a bit like a wooden gate slamming shut.
* With Channel A, the channel sounds OK without overdrive, but if you wind the Overdrive knob up from zero, a white noise-hiss kicks in, on top of the metallic hum.
* With Channel B, the volume knob sounds poor (there is no ODrive knob), and white noise/hiss increases badly with volume (the Ch B 'volume' knob has a similar hiss to the 'overdrive knob' of Ch A).
* I noticed that when you switch the amp off with the instrument lead in the FX Return, it may have the first click/pop of the switch going, but if you take the instrument lead out within about 5 seconds, the second sound accompanies this moment. If you wait longer, you can take the lead out without this sound happening then.
* The background metallic hum is consistent, and the Ch A & B hisses are the same every time, but the sounds that the amp makes when you switch it off and on are not always consistent - sometimes they are louder or quieter.

I hope there's something in there I've said which sounds like hallmarks of common problems you've seen before.

Thanks
Jim

J M Fahey

1) don't replace parts at random, just because you read it somewhere
2) brand means zilch, just buy standard generic commercial products, whatever a reputed supplier carries (Mouser/Digikey/Farnell/etc.)
3) test what's suggested above: get a spare plug, short it (plan B is to get a regular guitar cable and wrap the free plug end with aluminum paper or similar, so you short tip to body) and in sert the other end in the FX return jack.
That kills all of the preamp, including Reverb, and shorts power amp input.

If nasty buzz stops, it comes from an earlier point (preamp/reverb/poorly grounded pots and jacks/etc.)

If it stays loud and annoying, it's in the power amp or supply area.

Big suspects are main filter caps, although we can not discard a poor (rusty/corroded/loose)  ground connection.

I have part number labelled components, to the avoid vague descriptions .
If thare are silkscreened IDs on the PCBs, please correct the ones I assigned; if not use mine.

Measure and post:
+HT
+LT
-LT
-HT

At each of them, DC and AC voltage.

Warning: if using a cheap multimeter (the ones which have only 2 AC scales, typically 200VAC and 750VAC), it might give you impossibly high AC readings, such as 50 or 60 VAC .

Also measure VAC supplied by the transformer, you can measure at F1 and F2 .

Jungle-Jim

Hi all
Following instructions from JM Fahey, these are the results...

- Shorting the FX Return Jack:
* The metallic buzz remained unaffected.
* Even with all the knobs at zero, there is a fair bit of white noise from the amp when it's sitting idle, and shorting the FX Return takes 90% of this white noise out.

Conclusion? there's two problems - the metallic hum which is continuous no-matter what you do, and a definite level of white noise/hiss from the pre-amp.

Other info you requested:
(Using a little Circuitmate DMM)
Voltage from transformer at F1 and F2: 53.7VAC

You asked for AC and DC voltages: (to do this I was seeing what voltage was on either side of the two 680R 3W resistors)
+HT  / 78.1VAC / 35.7VDC
+LT  / 35.5VAC  / 16.4VDC
-LT  /  35.2VAC / -16.3VDC
-HT / 77.7VAC  / -35.5VDC

Are those VAC readings in error? I have another multimeter, a Holdpeak Automotive Multimeter, and that didn't give a result at all for the  AC measurements.

Am I right to conclude that perhaps the power supply caps are causing the metallic buzz, but that the white noise is coming from something to do with the jacks and knobs - because there is a lot of 1/4" jacks on that amp. If so what's some basic cleaning and testing I can do for all those?

Thanks, and merry Xmas.
Jim

J M Fahey

Thanks.
I'm chopping your answers to what I find significant to this problem.
Quote from: Jungle-Jim on December 23, 2014, 06:51:58 PM
- Shorting the FX Return Jack:
* The metallic buzz remained unaffected.
* Even with all the knobs at zero, there is a fair bit of white noise from the amp when it's sitting idle, and shorting the FX Return takes 90% of this white noise out.

Conclusion? there's two problems - the metallic hum which is continuous no-matter what you do, and a definite level of white noise/hiss from the pre-amp.
OK, the main problem is still the buzz.
The white noise may be addressed later.
That said, those amps were not considered noisy in a band environment, what makes me think that you are testing them in a quite silent place,maybe an appartment or bedroom.
Is that so?
Because that multiplies perceived noises by 20X , yet later in a garage/rehearsal studio/pub/stage they are not noticeable at all.
QuoteOther info you requested:
(Using a little Circuitmate DMM) <-- ouch!! ;)
Voltage from transformer at F1 and F2: 53.7VAC <--OK if measured from F1 to F2, that's some 27VAC to ground each, which is reasonable

You asked for AC and DC voltages: (to do this I was seeing what voltage was on either side of the two 680R 3W resistors)
+HT  / 78.1VAC / 35.7VDC
+LT  / 35.5VAC  / 16.4VDC
-LT  / 35.2VAC / -16.3VDC
-HT / 77.7VAC / -35.5VDC

Are those VAC readings in error? I have another multimeter, a Holdpeak Automotive Multimeter, and that didn't give a result at all for the  AC measurements.
They are wrong, but I had warned you:
QuoteWarning: if using a cheap multimeter (the ones which have only 2 AC scales, typically 200VAC and 750VAC), it might give you impossibly high AC readings, such as 50 or 60 VAC .
Obviously the holdpeak is better.
Does it have a sensitive AC scale?
Something like 200mV AC
If so, we can use it to measure noise.
Post the exact model to google its manual.
All 4 DC voltages look fine.

QuoteAm I right to conclude that perhaps the power supply caps are causing the metallic buzz,
Big filter caps seem to be working .
I'd expect more of a deep "hum" than a higher pitched "buzz" , which makes me suspect more grounding than anything else.
We'll see .
Quotebut that the white noise is coming from something to do with the jacks and knobs - because there is a lot of 1/4" jacks on that amp. If so what's some basic cleaning and testing I can do for all those?
Jacks and knobs do not create white noise, that's an active electronics job :(
In this amplifier, think Op Amps .
But please don't rush to replace them, let's go step by step.
For now, Merry Xmas too.

And rememberv to post multimeter specs or model.
Also some MP3 or YT showing amp noise might help.
Play something at a comfortable level (so we have a reference) and then put all guitar controls to 0, touching anything else.
Uso no pedals, just go straight from guitar to amp.

If you wish, play a little bit distorted, then a little bit clean, with and without reverb.
That should cover most (all?)  possibilities.

Jungle-Jim

To JM Fahey
Many thanks again for good advice.

I haven't made an audio recording yet - I can do that soon - but you asked what model the DMM was - it's a HoldPeak HP-760J - it was fairly cheap. I don't know why it didn't do those VAC readings - I tried it in the 200v and 750v ranges - and I'm sure the 20v range as well - blank screen. It did the VDC readings fine. It does however have a 200mV AC range. Unless I was supposed to take the red connector and put it in one of the other jacks - I just left it in the same jack it usually sits in.

What I plan to do is to unscrew the amp board off its mounting so I can see underneath - having a good look for bad solder joints or anything that doesn't look right. Also - I guess checking for earth problems I should run around the circuit board with the DMM checking all the 'ground' points for earth, and also checking the power plug as well as all the jacks and pots.

Also - there are four 8-pin ICs - I haven't got the amp unit open now, but I assume they are probably housing pairs of op-amps? What else could be they be? Anyway they are pressed into bases - perhaps I could carefully remove and reseat them (I have worked with ICs before this won't be a problem).

Using the DMM to test noise in the op-amps - I would be really interested to find out about how to do this - so if you do post info about this it will be greatly appreciated, and I will definitely not waste your time by not following up all your instructions.

Thanks again,
Jim

Roly

Meters can run into problems when trying to measure "mixed" voltages such as AC ripple on a DC supply.  You would think that a DMM would only respond to AC on the AC ranges but they also respond to DC and will give wildly high readings.

To measure the AC component on a DC rail (ripple voltage) most DMM's will require a small DC blocking cap in series with the probe - 0.1uF to 0.47uf poly, whatever you have to hand, not critical.


Your eyes are your best service instrument - and paying attention.


IC's; 1) supporting the board underneath, press each chip firmly down.  They will normally make a small click as they sit down 0.5 - 1mm; 2) gently lever each end of the IC up a couple of mm with a screwdriver, then reseat as above; 3) fully remove IC and replace.

If 1) doesn't do it, then 2) often does, progressing to 3) is not normally required.


DMM's and noise: yours is a good quality DMM but like most it has a limited AC frequency range and poor signal sensitivity, so it won't be suitable for wide band noise measurements.

A quiet preamp into a computer-based instrument would seem more hopeful.
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

Just checked the Holdpeak.

Looks like a cheap but somewhat above average (in that class)  meter.

It should measure AC without big trouble, there's one way to tell:
Get a (working ;) )  9V battery.
Set meter to 20V DC scale,
black probe on "Com T-  C-"
red probe on "V ohm Hz ... etc "
Measure 9V battery, it should somewhere around 9V

then set meter to 20VAC , it should measure close to 0 or maybe some initial value (going from 0 to 9VDC is AC) and in a couple seconds drift down to 0.
If so, it can be trusted measuring AC mixed with DC

If not, and it either displays some 20/25VAC it is not suitable.

Mind you, if it tries to display a value above nominal 20VAC (actually displayed as 19.99) it will show "overflow" , maybe by blanking the screen, or showing "+1---" (just the leading digit but blanking the following ones)
Unfortunately it seems to be doing this :(

But set to the 200VAC scale, that 9V battery should show some 20/25V "AC" .
Test it and confirm, so we can go on measuring knowing what to trust.

As of the noise measuring, it's basically not much more than measuring VAC at the output of the amp (that's why we need a sensitive AC scale here, usually 200 mVAC is fine, although I who use it often have an old and trusty Leader millivoltmeter with scales down to 1mVAC, full scale, and flat to 100kHz).
But of course in my case it's justified by somewhat regular use.

And the test is basically measuring VAC at the amp output, say you have 25mV noise (which is quite annoying in a silent room) and then, after replacing filter caps it's still heard, but now , say, 8 mVAC , which shows us a measurable improvement ........ or not ....  same as before ... in which case we know the problem is somewhere else.

Trying to attach some numbers to problems is much better than plain labels, such as annoying / bearable / etc.

Jungle-Jim

Hope you all had a great xmas,

With this Session Rockette 30 amp:
Today I replaced the 4 power supply electrolytic caps with Panasonic units I got off the web. I also went around the circuit board with a soldering iron resoldering any joint which looked dull or not right (don't worry - I used to work as an electronics assembler - I am a very good, neat solderer). I also re-seated the 4 DIP8 IC's which are all Motorola TL072CP K8733.

Anyway - after replacing the Caps, re-soldering and reseating the IC's - I can tell you that the hum/buzz is down to an acceptable level. With the jack in the FX Return, there is a tiny bit of hum, it's not dead-silent, but I'm sure the replacement caps have solved this problem.

However, the pre-amp hiss/white noise continues, more-or-less exactly as it was.

So I would be interested to see if there's any tests I can do regarding this pre-amp hiss? I was reading on another web forum, and people were talking about Session Rockette 30s and one person was saying their's was very quiet (not hiss/hum) - so these amps can be quiet. (Another person reported a hiss/white noise like mine.)

Also, using my DMM I checked earth/ground at many places around the circuit - I couldn't find an earth/ground point which was above the 0.1ohm the DMM was giving.

Regarding my Holdpeak DMM:
I did the 9v battery test - and yes, on the 20VAC range the DMM registered a voltage at first, which dropped to zero fairly quickly. Does that mean it works OK for op-amp testing purposes?

Thanks again,
Jim

Roly

Quote from: Jungle-JimI'm sure the replacement caps have solved this problem.

Yep, in this case it looks like the main filter caps were down on spec.   :dbtu:  ( :lmao:)

Hum
These supplies are very simple, and while normally adequate for stage use, could be improved on for session work at the expense of slightly reduce maximum power by adding extra filtering and or regulation.  The only practical way you are going to get close to zero hum is to have a regulator in each of the main supplies. 

Hiss
If there is no hiss with the main amp input shorted (at Fx Return, no audible hiss generated in the power stages), then we can assume that it its accumulating through the preamps op-amps.  {the dominant noise source at these levels are the active components, transistors/op-amps.}

The TL07n series aren't too bad, but things can be taken too far, then there are quieter ones.

The very first front end ones are the really critical ones, IC1(a,b) and if you were to replace any TL072 with, say LM833's, then the very front end one(s) would be the first choice, but I'd be inclined to do them all with LM833's, or better ('tho drive capacity might be more important for IC4(a) and 2(b).


We test amps, preamps or power amps, for residual hum and noise with their respective inputs shorted.  This means that any signal appearing at the output is internally generated (or coupled) and not coming in the input.  Hiss mainly comes from the transistors in the op-amps, but op-amps are very hardened against hum or noise getting in from their supplies.  Hum in preamps is either due to earth loops, or single-ended stages such as transistor buffers that have no supply rejection (however preamp supplies are typically regulated, at least with zener diodes as here.


Noises off
Mains-borne fridge pops, drill whine, etc.  One thing I'd do to a session amp is fit a mains line filter scrounged out of a computer power supply or monitor.


http://www.altronics.com.au/p/p8352a-c14-male-socket-chassis-mount-emi-filter-6a-iec/
If you say theory and practice don't agree you haven't applied enough theory.

Jungle-Jim

Thanks Roly

OK - with the op-amps - because they are in DIP8 sockets, and relatively cheap, and I don't have the skills to test them individually for noise, it makes sense to just buy some and spend 5 minutes swapping them over.

In which case which ones should I get? I am not worried about losing a bit of volume - it'd be mainly for practising and recording, and if there's a gig it'll just go through the desk. So low noise would be a priority. However maintaining the same amp tone is also important.

For pin-compatible low-noise replacements for TL072, you mentioned LM833N - is this the one to use? Googling the topic and reading a few articles I see people saying things which might be BS, but one person was saying that TLE2072 were low noise, and had a similar audio characteristics.

But one other question about this pre-amp noise: I can't believe that an amp would be released on the market with the hiss this one's got, so have the components deteriorated?

Voltage Regulation
What would be involved in putting more voltage regulation into this amp? Are there voltage regulator ICs available? The two power supply levels are + & - 16VDC AND 35VDC. I'll also see if I've got a mains filter from a computer - I've got plenty of those around.

Thanks
Jim

Roly

#13
There is aging, product spread, and the input stage of anything can have an "interesting" life that it might find tiring, a FET input op-amp such as the TL07n-series in particular.

These are all "low noise" these days, so you have to actually dig into the datasheets to get the noise numbers.
Here's a quick noise reality check.

Whatca got;
TL072 - Vn = 18nV/rootHz Typ at f = 1 kHz
zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz


("The typical noise voltage is 18 nanovolts per square-root of the bandwidth, at 1kHz")

{For reference, the LM747, which is similar to a LM741, has a noise of 2uV/rootHz, about 100 times greater.
zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz {4000 z's} zzzzzzzzzzzzzzzzzzzzz> (2000nV)
}


TLE2072 - 17nV/rootHz (and the "experts" lose again)
zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz


LM4558 - 12nV/rootHz (cork sniffers favorite)
zzzzzzzzzzzzzzzzzzzzzzz


OPA2134 - 8nV/rootHz (Hi-Fi corksniffers)
zzzzzzzzzzzzzzz


My personal favorite is;

LM833 - 4.5nV/rootHz (about $3 a dual, IMO the best noise and bandwidth value for money)
zzzzzzzz


{Then we really go up market (but singles not pin-compatable)...

NE5534N - 4.0nV/rootHz
zzzzzzz


NE5534AN - 3.5nV/rootHz (Texas Instruments)}
zzzzzz



There is a chance this hiss could be due to something else, particularly if it's "rough", variable, or like scrunching paper, that's more likely to be an electro in the signal path or board contamination (sugardrink, beer, wine, coffee, rodent urine), but the input op-amps are my first obvious target, cheap and easy try.  If it's something else the upgrade won't will do some good anyway.  And no, you won't hear any tonal difference, just less hiss.


This amp has been built down to a price.  The minimal chassis provides little screening for the circuitry on the board, and has no protection at all against mains-born noise, so I think that's a must for a recording amp.

You can give it the post design tweeking it needs to shut it up and make it insensitive to external fields, etc., some screening/goundplane, get the power cabling dressed away from the signal wiring and circuits, (is the input socket of the shorting type, are its solder joints solid or wrenched and cracked?)


Voltage regulation

For the preamp a pair of three-pin regulators of the 78vv and 79vv families will perform better than the current zener regulators.

The main amp supplies are a bit more difficult, but similar, perhaps again using three-pin regulators as the essential element, but with current booster transistors so they can handle the current - but tractable at this power level.

A half-way since you have a bit of spare space between the amp board and the reverb line, would be to add some extra large value electro can caps, and feed them from the existing supply via a couple of power resistors, mebby a few ohms, 5W, making a second L-section filter for each main supply; might be easier if supply ripple turns out to be your problem, and you can then add regulators to this supply later if needs be, but it might be enough alone.


{ed: braino)
If you say theory and practice don't agree you haven't applied enough theory.

Jungle-Jim

Thanks Roly
That's good advice about those op-amps - I bought some LM833N dirt cheap on ebay - it'll take 5 minutes to put them in. Will post about the results when they arrive.

Back to the other topic of improving noise levels - following on from your last post, two q's come to mind:
* Shielding the amp from interference, and keeping the power leads away from the signal leads. I guess I could look at putting a shielding over it - what would be a good way to do this - what about having a plastic cover over the amp (inside the cabinet of course), which has al-foil stuck to it which is earthed? And then there's the EMI filter plug.

* Improving the power supply: the pre-amp spec is + and - 16vdc - yet 78/79** controllers only do 15vdc (12, 18.., not 16). I don't know if that's ok. Could you possibly do a little sketch or drawing giving me an idea of an improved power supply I could give the pre-amp? The power amp is + & - 35vdc - perhaps leave that one for the time being, pre-amp is probably more sensitive to supply.

Thanks very much for your help - I have learned a considerable amount already during this - much appreciation to Roly and JM Fahey who have both given a lot of great advice. Thanks guys.

Jim