Hi folks,

I'm restoring a Heathkit IM-5228. The unit has been recapped & tested all the resistors and they are within the range. I think the person before me had also done some restoration on it. But I had the issue where the needle of the meter goes half way and then keeps shaking at that point. Like putting it to 1.5V range and then connecting a C type 1.5V battery it goes only half way. On ohms range it works sometimes and at other times no deflection at all. I changed the 12AU7 tube but no difference. The only remaining thing to test is the EB91 tube, which I'm waiting to get the replacement next week. Could this be the symptom of a bad tube?

Thanks.

Update: I have attached the schematics now.

 

Can you post the circuit schematic?

 

Can you post the circuit schematic?

Hi,
I've attached the schematics.

 

Read through this and see if it helps your situation:

http://www.antiqueradios.com/forums/viewtopic.php?t=149427

There is a better schematic here:
http://www.heathkit-museum.com/test/images/v-7aa.jpg

 

Hello,

Here I got a more complete manual.

Bertus

 

Could the meter sticking be due to a mechanical problem (needle touching dial, bearing friction) ?

 

Can you measure the anode, grid, and cathode cathode votages of both halves of the 12AU7 (1.5V range with 1.5V applied).

 

Could the meter sticking be due to a mechanical problem (needle touching dial, bearing friction) ?

And check for static buildup on the face of the meter.

 

And check for static buildup on the face of the meter.

I checked for static and it seems to be fine. Still the same thing.

 

Can you measure the anode, grid, and cathode cathode votages of both halves of the 12AU7 (1.5V range with 1.5V applied).

So, here are the measured values.
Input supply voltage: 106.8VAC

Values are entered in Section 1 & 2 of the tube respectively in DC,

Plate - Grid: 53.1 & 46V
Grid - Cathode: 1.73 & 1.99
Plate - Cathode: 68.4 & 69.2

Also something I forgot to check before was the calibration and AC balance pots. The value on the pot says 10k on the three of them. But the measure resistance on the two outer leads without the center post is like DC cal 6.8K, AC cal 11.2K and AC bal 7.8K.

 

Hello,

Here I got a more complete manual.

Bertus

Thank you very much.

 

Could the meter sticking be due to a mechanical problem (needle touching dial, bearing friction) ?

I opened and inspected the mechanism and it’s fine. I applied a small voltage on the meter terminals directly from a tiny watch battery and it was deflecting like normal and smoothly.

 

Read through this and see if it helps your situation:

http://www.antiqueradios.com/forums/viewtopic.php?t=149427

There is a better schematic here:
http://www.heathkit-museum.com/test/images/v-7aa.jpg

Maybe it’s the pot like in the post that says it’s faulty.

 

Values are entered in Section 1 & 2 of the tube respectively in DC,

Plate - Grid: 53.1 & 46V
Grid - Cathode: 1.73 & 1.99
Plate - Cathode: 68.4 & 69.2

Thise are distnctly kookie.
Can you repeat please, with the black lead connected to chassis and the red lead to each electrode in turn.

 

Thise are distnctly kookie.
Can you repeat please, with the black lead connected to chassis and the red lead to each electrode in turn.

With the 1.5V DC connection as before?

 

Thise are distnctly kookie.
Can you repeat please, with the black lead connected to chassis and the red lead to each electrode in turn.

Okay so with the 1.5V battery in 1.5V DC setting. SO, readings for section 1 & 2 are as follows with respect to ground:

Plate: 72 & 71.8VDC
Grid : 0.34 & 0 VDC
Cathode : 2.55 & 2.3 VDC

 

OK.
With DC+ and the 1.5V range selected the input should be connected directly to the left hand side of the 3M3 resistor connected to the grid of section 1 of the valve, yet you read only 0.34V on that grid. Check the voltage on the left side of that resistor. If it isn't 1.5V then check the path back through the switches to the input to find what isn't connected.
If the left side is 1.5V then the 3M3 is the chief suspect, but the capacitor to ground (4n7) may be leaky.

 

OK.
With DC+ and the 1.5V range selected the input should be connected directly to the left hand side of the 3M3 resistor connected to the grid of section 1 of the valve, yet you read only 0.34V on that grid. Check the voltage on the left side of that resistor. If it isn't 1.5V then check the path back through the switches to the input to find what isn't connected.
If the left side is 1.5V then the 3M3 is the chief suspect, but the capacitor to ground (4n7) may be leaky.

Okay so tested today. With DC+ mode slected and 1.5V battery. The input AC supply voltage was 108.2VAC today. The voltage with respect to ground and grid is 0.34V and to the other end of the 3.3M traitor was 1.30V.

I’ll try to clean the 10K pots and see if it comes back to specs and retake measurements and post it.

Also I’ve a question can I replace the EAA91 tube with a Soviet 6H2P? This is the only type I have on hand now. Have ordered a few EAA91 from US but they are still yet to arrive.

 

The voltage with respect to ground and grid is 0.34V and to the other end of the 3.3M traitor was 1.30V.

OK.
There is too much voltage drop across that 3M3.
It is possible that your meter is loading the circuit. To confirm this, connect across the 3M3. If you read something much less than 1V then it is your meter - we'll come back to that later.

If you read about 1V then:
Try this again with the valve removed.
If you still get 1V then:
Remove the 4n7 capacitor on the grid and read the voltage across the 3M3 again.
Still 1V?
Remove the 3M3 and measure its resistance.
If it reads 3M3 then you have some leakage somewhere - perhaps across the valve base or a tagstrip that the 3M3 is mounted on - clean everything.

 

Can you tell what kind of pots they are? Carbon composition pots, even now, have broad tolerance for the end-to-end resistance. ±20% to ±30% is not out of the question. Wirewound are usually better. Typically you can feel a bit of roughness when you turn a wirewound.

I'll leave it to you to verify, but it appears to me that the pots may be shunted by other resistances, depending on the function switch setting (I always do best with that sort of thing if I have a paper copy of the schematic and get out my crayons).