[Heathkit] Heathkit IM-18

[Garey Barrell]

Ron and others who replied -

I finally got back to the VTVM.  The problem was a short from the case to the 'hot' terminal of the 
DC CAL pot.  After you all calmed me down from my frustration I had to agree that the bridge was 
obviously unbalanced, and with the tube out, there was nowhere for the current to go other than 
through a pot or the switch.  As I said earlier, the original builder did a beautiful job, and there 
were NO solder bridges, left over lead clippings, etc. floating around in there.  I measured from 
one side of the bridge, (Pin 3,) and measured 47 ohms to ground.  Now I had something I could work 
with.!

So I did what they do on TV, I 'pulled the red wire' out of the PC board and measured from the pad 
to ground and sure enough, still 47 ohms to ground.  After some solder sucking (thank you Mr Hakko!) 
and isolating pins on the three CAL pots, I determined that the DC CAL pot was indeed shorted to 
it's case.  A little pot surgery showed no animal carcasses, solder blobs or other detritus floating 
inside, so I re-assembled and measured open from all pins to case on the pot.  So I re-installed it, 
replaced the leads I had to disconnect to get to the board, and fired it up.  Perfect!

So it's cooking in on the bench, and I'll CAL in the morning.

Thanks again for all the 'directed thinking' I needed.  It's so nice when one actually comes together!

73, Garey - K4OAH
Glen Allen, VA

Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>


Ron wrote:
>
> I would check/clean/replace the zero adjust pot R1. If OK, I would check R33
>
> (150K), R34 (220K) and R35 (150K). If OK, check R14 (10K), R15 (100) and
>
> R16 (27K). Statistically, it should be fixed by now. If not, measure the voltage
>
> across C1 (16uF/150V) - should be 65V-80V and clean the function switch
>
> (unlikely, but what the heck).  
>
> -- Ron
>
>
> ________________________________
>
>
>> ________________________________
>> From: Garey Barrell<k4oah at mindspring.com>
>> To: Heathkit Reflector<heathkit at mailman.qth.net>
>> Sent: Thursday, May 31, 2012 12:03 PM
>> Subject: [Heathkit] Heathkit IM-18
>>
>> OK, I give up.........
>>
>> I have an IM-18 that has been working for years.  Turned it on, and the meter pegs 'downscale' on
>> 'AC'.   On 'DC-' it pegs downscale, on 'DC+' it pegs upscale.  ZERO control has no visible effect.
>> No combination of the CAL controls makes any visible effect.  Interestingly, results are identical
>> with the 12AU7 removed.  DC voltages on the power  supply capacitor are nominal.  Whoever built it
>> did a VERY nice job, beautiful solder on board and switch.  Close visual examination shows not
>> obvious shorts, bridges, etc.
>>
>> Open to ideas or suggestions..
>>
>> 73, Garey - K4OAH
>> Glen Allen, VA
>>
>> Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
>> and TR-4/C Service Supplement CDs
>> <www.k4oah.com>
>>
>>