Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old January 17th 16, 11:56 PM posted to sci.electronics.repair
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On 18/01/2016 9:28 AM, Gareth Magennis wrote:


"Trevor Wilson" wrote in message ...

On 17/01/2016 3:04 PM, Phil Allison wrote:
Trevor Wilson wrote:



* I have a valve tester (AVO VCM163).


** The VCM163 ( Valve Characteristic Meter) is oddball among valve
testers, uses only AC voltages for ALL supplies - plates, screens and
grids !!

http://www.radiomuseum.org/forum/tub...principle.html


Expensive too, working examples go on Ebay for up to $3000.


**The transformers inside the thing are amazing things. They would be
very expensive to produce today. Mind-numbing numbers of taps.







I built this thing and modified it to suit my needs for basic valve
testing.
http://triodeel.com/tester.htm

I trust it more than anything I might buy off the Interweb that is going
to be decades old.


**Fair enough. However, the VCM163 is a VERY clever device, which could
be done more cleverly today. That said, it is very convenient and it
operating within specification. Except for the transformer switching, it
is a very simple device at it's core.


--
Trevor Wilson
www.rageaudio.com.au

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Old January 18th 16, 03:09 AM posted to sci.electronics.repair
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"With transistor amps, the same idle current always flows in both output devices cos they are connected in series. "

I have wanted to pound a couple of things into people's heads about this for some time.

What you say is true for idle current, the bias, but if there is an offset the current through the outputs is not equal. With no load it should be no matter what.

Anyway, the sledge hammer is to beat it in that :

1. Adjust the bias first, with no load.

2. Adjust the balance/offset next, also with no load.

Do this after having warmed it up a bit, if you can't, recheck it hot.

If the manufacturer starts this **** about taking out jumpers to measure bias current, **** all that. Just do the math and calculate it from the emitter resistors. Take my word for it.

In fact I can adjust it with no spec. II can just feed it with low level program material and watch the waveform at the collector(s) of the voltage amp(s). The ||_||_ in that waveform, produced when the feedback is taking care of the crossover distortion is the prime test point. Adjust bias to flatten that out, done.

However, going farther, that is biasing it harder, does have some merit. First of all it is not that much, and it also gives the amp more low power damping factor. Believe me I know the difference. I am not talking going into class A here, I think that is stupid. If you are going to go class A go all the way and go SE. Then for the most part you only got even order distortion.
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Old January 18th 16, 04:59 AM posted to sci.electronics.repair
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Trevor Wilson wrote:


** The VCM163 ( Valve Characteristic Meter) is oddball among valve
testers, uses only AC voltages for ALL supplies - plates, screens and
grids !!

http://www.radiomuseum.org/forum/tub...principle.html


Expensive too, working examples go on Ebay for up to $3000.


**The transformers inside the thing are amazing things. They would be
very expensive to produce today. Mind-numbing numbers of taps.



** However, the VCM163 is a VERY clever device, which could
be done more cleverly today. That said, it is very convenient and it
operating within specification. Except for the transformer switching, it
is a very simple device at it's core.


** Full article and internal pics he

http://schmid-mainz.de/Radio-Bygones_140.pdf

One of the C-Core transformers is used with a half wave load, which could make it growl a bit and get hot after a while.

BTW:

Can it be used with valves that have anode caps ?

Like the 6CM5, 6DQ6B, 807, 300B and 6146 types.


..... Phil
  #24   Report Post  
Old January 18th 16, 08:14 AM posted to sci.electronics.repair
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On 18/01/2016 3:59 PM, Phil Allison wrote:
Trevor Wilson wrote:


** The VCM163 ( Valve Characteristic Meter) is oddball among
valve testers, uses only AC voltages for ALL supplies - plates,
screens and grids !!

http://www.radiomuseum.org/forum/tub...principle.html




Expensive too, working examples go on Ebay for up to $3000.


**The transformers inside the thing are amazing things. They
would be very expensive to produce today. Mind-numbing numbers of
taps.



** However, the VCM163 is a VERY clever device, which could be done
more cleverly today. That said, it is very convenient and it
operating within specification. Except for the transformer
switching, it is a very simple device at it's core.


** Full article and internal pics he

http://schmid-mainz.de/Radio-Bygones_140.pdf

One of the C-Core transformers is used with a half wave load, which
could make it growl a bit and get hot after a while.

BTW:

Can it be used with valves that have anode caps ?

Like the 6CM5, 6DQ6B, 807, 300B and 6146 types.


**No problems at all. Nuvistors are OK too. There is an adaptor, which
is designed to take some really odd-ball valves, but, unfortunately, I
didn't get one with my tester. I'll post some schematics to you tomorrow
or Wednesday. The power transformers are a joy to behold. BTW: I reckon
3 Grand is cheap. Not that you couldn't buy a better machine for that
kind of money, you can. It's just I scored a late model AVO 8 Mk7. Cost
me $140.00. The original price was more than 2 Grand! It is surprisingly
cheaply built. Based on what the AVO 8 originally cost, I reckon the
VCM163 would be more like $15k, if it was still in production. It is
very nicely hand built, in that typical, 1960s, Pommy way.



--
Trevor Wilson
www.rageaudio.com.au

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Old January 18th 16, 11:42 PM posted to sci.electronics.repair
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On Sunday, January 17, 2016 at 10:09:24 PM UTC-5, wrote:
"With transistor amps, the same idle current always flows in both output devices cos they are connected in series. "


I have wanted to pound a couple of things into people's heads about this for some time.

What you say is true for idle current, the bias, but if there is an offset the current through the outputs is not equal. With no load it should be no matter what.

Anyway, the sledge hammer is to beat it in that :

1. Adjust the bias first, with no load.

2. Adjust the balance/offset next, also with no load.

Do this after having warmed it up a bit, if you can't, recheck it hot.

If the manufacturer starts this **** about taking out jumpers to measure bias current, **** all that. Just do the math and calculate it from the emitter resistors. Take my word for it.

In fact I can adjust it with no spec. II can just feed it with low level program material and watch the waveform at the collector(s) of the voltage amp(s). The ||_||_ in that waveform, produced when the feedback is taking care of the crossover distortion is the prime test point. Adjust bias to flatten that out, done.

However, going farther, that is biasing it harder, does have some merit. First of all it is not that much, and it also gives the amp more low power damping factor. Believe me I know the difference. I am not talking going into class A here, I think that is stupid. If you are going to go class A go all the way and go SE. Then for the most part you only got even order distortion.


I have a digital AC power meter on my bench to monitor AC power consumption.. It has many uses that I did not think of when buying it. It is useful for eyeballing idling current to see if it is creeping up into thermal runaway. I found it can also be used to set idling current. Adjust the bias pot until ac power starts to change. Then back it off to match the idling power for that type of amp. This saves a lot of time trying to measure the voltage drop across output emitter resisters.


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Old January 19th 16, 12:18 AM posted to sci.electronics.repair
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On Sunday, January 17, 2016 at 11:59:22 PM UTC-5, Phil Allison wrote:
Trevor Wilson wrote:


** The VCM163 ( Valve Characteristic Meter) is oddball among valve
testers, uses only AC voltages for ALL supplies - plates, screens and
grids !!

http://www.radiomuseum.org/forum/tub...principle.html


Expensive too, working examples go on Ebay for up to $3000.


**The transformers inside the thing are amazing things. They would be
very expensive to produce today. Mind-numbing numbers of taps.



** However, the VCM163 is a VERY clever device, which could
be done more cleverly today. That said, it is very convenient and it
operating within specification. Except for the transformer switching, it
is a very simple device at it's core.


** Full article and internal pics he

http://schmid-mainz.de/Radio-Bygones_140.pdf

One of the C-Core transformers is used with a half wave load, which could make it growl a bit and get hot after a while.

BTW:

Can it be used with valves that have anode caps ?

Like the 6CM5, 6DQ6B, 807, 300B and 6146 types.


.... Phil


That looks to be a very well built tube tester. Leave it to England to raise the bar , Britannia rules the waves and all that. I suspect it comes with a plug in anode top cap lead. Someone on ebay is selling one for 57 eur. Who in their right mind would pay that much for a piece of wire ??

http://www.ebay.ie/itm/AVO-CT160-and...-/331110707340

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Old January 19th 16, 12:27 AM posted to sci.electronics.repair
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On 19/01/2016 11:18 AM, John Heath wrote:
On Sunday, January 17, 2016 at 11:59:22 PM UTC-5, Phil Allison
wrote:
Trevor Wilson wrote:


** The VCM163 ( Valve Characteristic Meter) is oddball among
valve testers, uses only AC voltages for ALL supplies -
plates, screens and grids !!

http://www.radiomuseum.org/forum/tub...principle.html




Expensive too, working examples go on Ebay for up to $3000.


**The transformers inside the thing are amazing things. They
would be very expensive to produce today. Mind-numbing numbers
of taps.



** However, the VCM163 is a VERY clever device, which could be
done more cleverly today. That said, it is very convenient and
it operating within specification. Except for the transformer
switching, it is a very simple device at it's core.


** Full article and internal pics he

http://schmid-mainz.de/Radio-Bygones_140.pdf

One of the C-Core transformers is used with a half wave load, which
could make it growl a bit and get hot after a while.

BTW:

Can it be used with valves that have anode caps ?

Like the 6CM5, 6DQ6B, 807, 300B and 6146 types.


.... Phil


That looks to be a very well built tube tester.


**It's better than "well built".


Leave it to England
to raise the bar , Britannia rules the waves and all that. I suspect
it comes with a plug in anode top cap lead. Someone on ebay is
selling one for 57 eur. Who in their right mind would pay that much
for a piece of wire ??

http://www.ebay.ie/itm/AVO-CT160-and...-/331110707340



**LOL. It's still probably less than what AVO used to sell them for.

--
Trevor Wilson
www.rageaudio.com.au

---
This email has been checked for viruses by Avast antivirus software.
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Old January 19th 16, 02:00 AM posted to sci.electronics.repair
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John Heath wrote:


I have a digital AC power meter on my bench to monitor AC power
consumption. It has many uses that I did not think of when buying
it. It is useful for eyeballing idling current to see if it is
creeping up into thermal runaway. I found it can also be used to
set idling current. Adjust the bias pot until ac power starts to
change. Then back it off to match the idling power for that type of
amp. This saves a lot of time trying to measure the voltage drop
across output emitter resisters.


** I guess your meter reads in 1 watt increments ?

I have been doing the same jobs with a 3.5 digit LED current meter that reads in 1mA or 10mA increments with a maximum display of 20amps in two ranges. It couples to the AC line via a Hall effect sensor that also provides a waveform output for a scope.

It gets used with every repair job and is indispensable when working on valve or solid state amps. Makes it possible to diagnose some faults using only the readings and scope display - like high ESR main filter electros.

There is also a safety bonus, a glance at the meter tells you if the AC power is really switched off.


..... Phil
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Old January 19th 16, 03:16 AM posted to sci.electronics.repair
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My power meter had a red 3 1/2 digital display with .1 watt to 199.9 watt and a switch for 1 to 1999 watt. It was directly connected to the mains through a current transformer , no hall effects. With .1 watt sensitivity I could tell if a VRC carriage was sticking when going up or down or if the tape guild pins had too much friction when going to play mode. Your idea of monitoring current on a scope opens up new possibilities. I could see how the ESR of the main filters condenser could be guessed at by monitoring the inrush current from a cold start.

Interesting side note. Most smart phones have a 3D hall sensor to sense the orientation of the phone relative to north south poles. If you move a magnet over the back of the phone you can find the sweet spot where the 3D hall IC is located. Put an X on that spot and you have a DC current meter for free. Not the best current sensor but in a pinch it gets the job done.
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Old January 19th 16, 03:55 AM posted to sci.electronics.repair
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On 18/01/2016 3:59 PM, Phil Allison wrote:
Trevor Wilson wrote:


** The VCM163 ( Valve Characteristic Meter) is oddball among
valve testers, uses only AC voltages for ALL supplies - plates,
screens and grids !!

http://www.radiomuseum.org/forum/tub...principle.html




Expensive too, working examples go on Ebay for up to $3000.


**The transformers inside the thing are amazing things. They
would be very expensive to produce today. Mind-numbing numbers of
taps.



** However, the VCM163 is a VERY clever device, which could be done
more cleverly today. That said, it is very convenient and it
operating within specification. Except for the transformer
switching, it is a very simple device at it's core.


** Full article and internal pics he

http://schmid-mainz.de/Radio-Bygones_140.pdf

One of the C-Core transformers is used with a half wave load, which
could make it growl a bit and get hot after a while.

BTW:

Can it be used with valves that have anode caps ?

Like the 6CM5, 6DQ6B, 807, 300B and 6146 types.


**Schematic sent.


--
Trevor Wilson
www.rageaudio.com.au

---
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