Heathkit IM5228 Valve Voltmeter - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

TNC · 21st Jul 2006, 12:26 am

A final postcript! The AC measurement was helped by me not using the DC probe - which of course incorporates that 1meg resistance!!

In the end I decided to attempt the mod. mentioned on the Heathkit forum file which places the 1meg resistance in the machine rather than in the probe. Despite the switch numbering differing, some logical thinking worked through the possibilities and very quickly it was done. Now only one probe is required for any or all of the testing options - it makes it easier to operate and crucially, impossible to end up with the wrong probe.

Re-fitting the valves also required the re-installation of the two 22meg resistances from pin 7 of the EB91 - not a problem!

The machine set up fairly easily...and whilst not quite as accurate as my Korean (!) solid-state meter, at least its exceptional impedence means that I am able to measure virtually everything without unwanted effects.
Meter re-zeroing at the most sensitive measures (1.5v - 15v) is generally required but that is not really a problem as the adjustment is close to hand.

All in all a very nice piece of kit - typically Heathkit in it quality of make and design.

Trevor.

igranic · 21st Jul 2006, 11:04 am

Heathkit placed the 1Meg resistor in the probe in order to decrease the loading on RF circuits so that a relatively true reading of dynamic DC potential could be made.

With the resistor removed, the probe and screened cable can have a significant capacitive loading effect at RF, and thus skew measurements, particularly when setting up discriminators, or checking dc levels in RF stages.
This loading should be borne in mind and the instrument not relied upon for dynamic DC measurements in RF circuits.

Heathkit_Nut · 29th Aug 2006, 9:43 pm

Hi,

I'm new to this forum.....I have a Heathkit IM-5228 that I will be restoring soon.

The two tubes in question;

1. 12AU7
http://www.mclink.it/com/audiomatica/tubes/12au7.htm

2. 6AL5
http://www.kbapps.com/audio/tubemanual/112.html

HTH

John

TNC · 30th Aug 2006, 12:17 am

Hi John,

Welcome to the Forum...and thanks for the posting on the IM5228.

As you may have read I had some fun finding out that my IM5228 had been fitted with "Fetrons" - in the event one or other of them failed within a few minutes of powering-up so I had no real choice but to revert to the original valves (tubes) spec. to which you have just referred. The meter now works so well that I have accorded it the honour of being wall-mounted in the workshop!!

One last point - re. the mod. whereby the 1meg R is placed inside the meter rather than in the test-probe.....having read the good advice from a Forum member to ensure optimum accuracy, I have reversed the mod. and put the resistance back in the probe.
Having done that the only issue is remembering to use the right test-probe for the right job!!

It's still a great piece of kit....and from your Forum ID I see you must be a real Hearthkit enthusiast...I have a couple of other Heathkit units including a FM sweep generator and wobbulator...Heathkit stuff is generally of such excellent quality and reliability...keep us posted on your restoration of the IM 5228.

Trevor.

21st Jul 2006, 12:26 am	#21
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex, UK. Posts: 386	Re: Heathkit IM5228 Valve Voltmeter A final postcript! The AC measurement was helped by me not using the DC probe - which of course incorporates that 1meg resistance!! In the end I decided to attempt the mod. mentioned on the Heathkit forum file which places the 1meg resistance in the machine rather than in the probe. Despite the switch numbering differing, some logical thinking worked through the possibilities and very quickly it was done. Now only one probe is required for any or all of the testing options - it makes it easier to operate and crucially, impossible to end up with the wrong probe. Re-fitting the valves also required the re-installation of the two 22meg resistances from pin 7 of the EB91 - not a problem! The machine set up fairly easily...and whilst not quite as accurate as my Korean (!) solid-state meter, at least its exceptional impedence means that I am able to measure virtually everything without unwanted effects. Meter re-zeroing at the most sensitive measures (1.5v - 15v) is generally required but that is not really a problem as the adjustment is close to hand. All in all a very nice piece of kit - typically Heathkit in it quality of make and design. Trevor.

21st Jul 2006, 11:04 am	#22
igranic Rest in Peace Join Date: Jan 2005 Location: North Bedfordshire, UK. Posts: 395	Re: Heathkit IM5228 Valve Voltmeter Heathkit placed the 1Meg resistor in the probe in order to decrease the loading on RF circuits so that a relatively true reading of dynamic DC potential could be made. With the resistor removed, the probe and screened cable can have a significant capacitive loading effect at RF, and thus skew measurements, particularly when setting up discriminators, or checking dc levels in RF stages. This loading should be borne in mind and the instrument not relied upon for dynamic DC measurements in RF circuits.

29th Aug 2006, 9:43 pm	#23
Heathkit_Nut Retired Dormant Member Join Date: Aug 2006 Location: Seattle, Washington (USA) Posts: 1	Re: Heathkit IM5228 Valve Voltmeter Hi, I'm new to this forum.....I have a Heathkit IM-5228 that I will be restoring soon. The two tubes in question; 1. 12AU7 http://www.mclink.it/com/audiomatica/tubes/12au7.htm 2. 6AL5 http://www.kbapps.com/audio/tubemanual/112.html HTH John

30th Aug 2006, 12:17 am	#24
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex, UK. Posts: 386	Re: Heathkit IM5228 Valve Voltmeter Hi John, Welcome to the Forum...and thanks for the posting on the IM5228. As you may have read I had some fun finding out that my IM5228 had been fitted with "Fetrons" - in the event one or other of them failed within a few minutes of powering-up so I had no real choice but to revert to the original valves (tubes) spec. to which you have just referred. The meter now works so well that I have accorded it the honour of being wall-mounted in the workshop!! One last point - re. the mod. whereby the 1meg R is placed inside the meter rather than in the test-probe.....having read the good advice from a Forum member to ensure optimum accuracy, I have reversed the mod. and put the resistance back in the probe. Having done that the only issue is remembering to use the right test-probe for the right job!! It's still a great piece of kit....and from your Forum ID I see you must be a real Hearthkit enthusiast...I have a couple of other Heathkit units including a FM sweep generator and wobbulator...Heathkit stuff is generally of such excellent quality and reliability...keep us posted on your restoration of the IM 5228. Trevor.