Author Topic: Dana 5500 digital voltmeter: need help repairing ohms board FIXED (Read 4170 times)

lowimpedance · « **Reply #25 on:** February 18, 2019, 11:59:59 pm »

When you say shorted, you mean 0 ohms or not ~0.65V both directions on your DMM diode test ?. Have you lifted one end to check if one or both are short cct ?. These diodes are for protection and should conduct enough to 'hopefully' pop F2 before to much damage is done !. Was F2 intact still ?.

valley001 · « **Reply #26 on:** February 19, 2019, 12:09:30 am »

Quote from: lowimpedance on February 18, 2019, 11:59:59 pm

When you say shorted, you mean 0 ohms or not ~0.65V both directions on your DMM diode test ?. Have you lifted one end to check if one or both are short cct ?. These diodes are for protection and should conduct enough to 'hopefully' pop F2 before to much damage is done !. Was F2 intact still ?.

One of the fuses was not intact, I dont remember which and did not write it down unfortunately.

With the meter in diode check mode it displays as if I short the leads together (.000v) when I test across these diodes, in either direction. In ohms mode they read 0 ohms both directions. These tests are with the diodes in circuit.

floobydust · « **Reply #27 on:** February 19, 2019, 01:31:36 am »

This is with the PC board pulled? That looks like a dead short there- the two-diode clamp should read around 0.65V either polarity.
Lift an end or remove each diode CR19, CR20 and if the short is still there, it might be Q8 Q6 B1-B2 shorted.
If this (short) is with the card in, I would check C7, something with the range relay shielding can to ANA COMMON (ground), or maybe there is a switch that intentionally shorts the ohms-converter inputs when not used.

edit: Q6 not Q8 fuzzy text

valley001 · « **Reply #28 on:** February 19, 2019, 01:36:25 am »

Quote from: floobydust on February 19, 2019, 01:31:36 am

This is with the PC board pulled? That looks like a dead short there- the two-diode clamp should read around 0.65V either polarity.
Lift an end or remove each diode CR19, CR20 and if the short is still there, it might be Q8 B1-B2 shorted.
If this (short) is with the card in, I would check C7, something with the range relay shielding can to ANA COMMON (ground), or maybe there is a switch that intentionally shorts the ohms-converter inputs when not used.

These measurements are with the card removed. Ill lift these out of circuit and test. Hopefully Q8 is not toast, I think its custom Dana part. I do have a parts mule but it does not have the ohms converter option installed.

valley001 · « **Reply #29 on:** February 19, 2019, 01:47:11 am »

OK, diodes lifted out of circuit:

CR20- Reads .65v
~~CR21~~ CR19- Reads 0 ohms both directions

Vgkid · « **Reply #30 on:** February 19, 2019, 01:51:43 am »

Cr21 is toast. Lets hope that is the only thing fried.

valley001 · « **Reply #31 on:** February 19, 2019, 02:05:37 am »

Q8 tested in circuit with diode checker. Both sides read.65v ish B-E and B-C with positive DMM lead on base.

Is it possible the fried ~~CR21~~ CR19 is the cause of the problems with this board?

floobydust · « **Reply #32 on:** February 19, 2019, 02:14:58 am »

Dana 211236 diode "007" I got to a cross being 1N3595 200mA 150V low leakage. FDH300 looks like a possible alternate. It's not a tough diode, to clear the fuse if a big overload.
Dana 211083 diode "018" I got to a cross being 1N916. The 1N914 or 1N4148 seem fine as an alternate.
The Dana diodes are from 1967-68.

CR21 is a zener on the schematic, so I'm confused about CR19, CR20 being the clamp diodes?

valley001 · « **Reply #33 on:** February 19, 2019, 02:17:43 am »

Quote from: floobydust on February 19, 2019, 02:14:58 am

Dana 211236 diode "007" I got to a cross being 1N3595 200mA 150V low leakage. FDH300 looks like a possible alternate. It's not a tough diode, to clear the fuse if a big overload.
Dana 211083 diode "018" I got to a cross being 1N916. The 1N914 or 1N4148 seem fine as an alternate.
The Dana diodes are from 1967-68.

CR21 is a zener on the schematic, so I'm confused about CR19, CR20 being the clamp diodes?

That was me being in a rush, it is CR19 that reads 0 ohms both directions, not CR21.

valley001 · « **Reply #34 on:** February 19, 2019, 03:02:51 pm »

Could this diode be the cause of the problems?

lowimpedance · « **Reply #35 on:** February 20, 2019, 01:44:38 am »

As Flooby suggests, try a pair of 1n4148 and then retest the function and see if it works. Hopefully yes , if not then further repair is needed.

valley001 · « **Reply #36 on:** February 20, 2019, 02:25:51 am »

The diode was the Dana 211236 "007," I have a bunch on order. Unfortunately I do not have a large stock of diodes on hand. Would it be best to also change the one that reads correctly, CR20?

lowimpedance · « **Reply #37 on:** February 20, 2019, 03:50:35 am »

Yes. If one was dead the other would always be suspect so best to do both in this instance. With a bit of luck Q8 was spared and the diode kind of did its job

.

valley001 · « **Reply #38 on:** February 20, 2019, 08:24:37 pm »

Q8- Are we looking at the dual transistor directly down stream of the diodes? My photo is grainy and the number is not clear for you all, I apologize. This would be Q6. Just want to be sure I am testing the appropriate transistor. In the pic of the board Q6 is the transistor that shares the binding post with the two diodes bottom right.

lowimpedance · « **Reply #39 on:** February 20, 2019, 11:08:53 pm »

Ahh ok Q6 , my aging tired old eyes couldn't make it out even with glasses and magnifier

.
Yes with a bit of luck the dual transistor is still functional. Change the diodes and test the meter again.

valley001 · « **Reply #40 on:** February 21, 2019, 03:55:32 pm »

Completely my fault with the crappy image of the schematic.

Q6 tests OK with .63v/.66v B-C/B-E on both sides. Now waiting on diodes, should be Saturday. Fingers crossed.

valley001 · « **Reply #41 on:** February 22, 2019, 05:31:30 pm »

Well my diodes are stuck on the east coast and I am impatient.

I did manage to find a bunch of 1N914 lying around, which as noted seem to cross to the dana 018 diode (though as a novice I am unsure what I should be looking for in the datasheet). Is there any danger using these in place of the Dana 007 diode? They are in the small glass envelope (not sure that matters).

Kleinstein · « **Reply #42 on:** February 22, 2019, 05:53:01 pm »

The 1N914 should be OK for the "018" ones, not much difference there.
For the "018" type which is supposed to be more like the 1N3595 it would be a poor replacement, as it may not provide full protection and maybe more leakage. Probably a plain 1N4003/5 could be a better match, though maybe a little more leakage.

valley001 · « **Reply #43 on:** February 23, 2019, 07:30:53 pm »

I got it working this morning. Replacing the defective diode did the trick, and thankfully none of the custom transistors took a dump. I used two 007 diodes from a parts unit, since my new replacements are stuck in the mail somewhere.

I can now zero voltage and current per the manual. Now to get some precision resistors to zero each range.

Thanks all for your help with this.


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Author Topic: Dana 5500 digital voltmeter: need help repairing ohms board FIXED (Read 4170 times)

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