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DM502 with problems


Colin Herbert
 

I have a DM502 which isn't working correctly. I have checked the power supply voltages at +5V, and +/- 12V and they are within spec. I have also adjusted the clock frequency to 20.48 kHz and that is fine. On initial switch-on and warming-up, the display "jumps about" rather than staying stable as one would expect, even with only a battery connected to the test-terminals and a suitable DC voltage range selected or with a resistor across the test terminals and a suitable ohms range selected. After being powered on for a while, the display just shows a constant sequence of digits, whatever the position of the range switch, only the position of the decimal point changing. I haven't got around to checking anything else, yet, but after the plug-in has been on for a while,the transformer T410 is rather hot to the touch, so much so that I can't keep a finger-tip in contact with it. Is this correct behaviour, or should I check other components (don't forget that the voltages out of the transformer are within spec.)?
TIA, Colin.


Roger Evans
 

Colin,

Some of the TM500 plugins had notoriously unreliable IC sockets. I would certainly remove the socketed ICs, clean pins and see if anything improves. Several people have commented that the long term solution is to replace with decent IC sockets, I expect someone will know how to identify the unreliable originals. Have you checked for ripple on the +/-12V and 5V lines? Also it is worth exercising the range switch a few dozen times! Is the DM502 a new acquisition or one that was working OK but has now failed?

Regards,

Roger


Roger Evans
 

Correction, I don't think the cam switches improve with use, if they are dirty they need cleaning with IPA and strips of paper.

Roger


Colin Herbert
 

I've had a look at that possibility, Roger. I have cleaned the attenuator cam-switch contacts on a couple of 400-series scopes, but I can't see how to clean them on the DM502. I have taken the cover off the cam-switch assembly, but I simply can't see the gold-plated (?) contact fingers and pcb contacts. It seems more like the time-base switch contacts on 400-series scopes than the attenuators on the same scopes.
Correction: I have taken another look after removing the cover and I can see the contacts underneath the cam-switch. I think I'll have a go with the IPA and paper, though whether IPA is as required as it is with attenuator contacts, I'm not sure. The pcb material looks as though it is fairly ordinary, rather than the polyphenylene oxide used in scope attenuators, which can be damaged by some solvents.
Another fault is that sometimes one or more sections of the seven-segment led displays tend to flick on or off - maybe this will improve with switch-contact cleaning?
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Roger Evans via groups.io
Sent: 05 June 2020 11:55
To: TekScopes@groups.io
Subject: Re: [TekScopes] DM502 with problems

Correction, I don't think the cam switches improve with use, if they are dirty they need cleaning with IPA and strips of paper.

Roger


Roger Evans
 

Colin,

If there are intermittent segments on the LEDs then the 74(LS)47 7 segment decoder is the likely culprit and is probably socketed (it is on my DM501).

Roger


Dave Daniel
 

I believe that the paper-soaked IPA cleaning method has benn discussed before and there probably messages in the archive.

I do recall that I had decided that when I needed to clean any TM-500 series plug-cam switches I would try that method.

DaveD

On Jun 5, 2020, at 07:59, Colin Herbert via groups.io <colingherbert=blueyonder.co.uk@groups.io> wrote:

I've had a look at that possibility, Roger. I have cleaned the attenuator cam-switch contacts on a couple of 400-series scopes, but I can't see how to clean them on the DM502. I have taken the cover off the cam-switch assembly, but I simply can't see the gold-plated (?) contact fingers and pcb contacts. It seems more like the time-base switch contacts on 400-series scopes than the attenuators on the same scopes.
Correction: I have taken another look after removing the cover and I can see the contacts underneath the cam-switch. I think I'll have a go with the IPA and paper, though whether IPA is as required as it is with attenuator contacts, I'm not sure. The pcb material looks as though it is fairly ordinary, rather than the polyphenylene oxide used in scope attenuators, which can be damaged by some solvents.
Another fault is that sometimes one or more sections of the seven-segment led displays tend to flick on or off - maybe this will improve with switch-contact cleaning?
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Roger Evans via groups.io
Sent: 05 June 2020 11:55
To: TekScopes@groups.io
Subject: Re: [TekScopes] DM502 with problems

Correction, I don't think the cam switches improve with use, if they are dirty they need cleaning with IPA and strips of paper.

Roger







 

Only one company is associates with bad IC sockets - that is Texas Instruments. Their sockets are easy to find by removing the IC because the TI logo is stamped underneath the IC in the middle of the socket where it can be easily seen.

TI saw an opportunity to make money selling sockets when IC sales were booming in the late 1960s and early 1970s. Unfortunately for them other companies already had all the patents locked up. Other manufacturers sockets used two flat spring contacts (one on each side of each pin of the IC) to grip the pin from both sides of the pin's very wide flat dimension. TI could not find a way around these ironclad patents. The only way left to grip an IC pin was by the very thin thickness of the metal the pins were stamped from once the IC was finished in manufacturing. A metal die stamps away all of the metal from this metal frame except for the pins themselves and the die leaves behind very rough edges where it stamps the metal frame. As a result the flat side of the IC pin is smooth and an excellent place for a socket to grab the pin and the edge of the pin is thin and very rough. The worst place to grab the pin is on the thin rough edges. These rough edges gouge into the contacts in the TI socket and the result is they will eventually develop intermittent problems.

There is not much you can do except replace them which is a tedious job that can cause other problems without the proper equipment to do that with.

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via groups.io
Sent: Friday, June 05, 2020 5:00 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] DM502 with problems

I've had a look at that possibility, Roger. I have cleaned the attenuator cam-switch contacts on a couple of 400-series scopes, but I can't see how to clean them on the DM502. I have taken the cover off the cam-switch assembly, but I simply can't see the gold-plated (?) contact fingers and pcb contacts. It seems more like the time-base switch contacts on 400-series scopes than the attenuators on the same scopes.
Correction: I have taken another look after removing the cover and I can see the contacts underneath the cam-switch. I think I'll have a go with the IPA and paper, though whether IPA is as required as it is with attenuator contacts, I'm not sure. The pcb material looks as though it is fairly ordinary, rather than the polyphenylene oxide used in scope attenuators, which can be damaged by some solvents.
Another fault is that sometimes one or more sections of the seven-segment led displays tend to flick on or off - maybe this will improve with switch-contact cleaning?
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Roger Evans via groups.io
Sent: 05 June 2020 11:55
To: TekScopes@groups.io
Subject: Re: [TekScopes] DM502 with problems

Correction, I don't think the cam switches improve with use, if they are dirty they need cleaning with IPA and strips of paper.

Roger









--
Dennis Tillman W7pF
TekScopes Moderator


Colin Herbert
 

Hi Roger,
I haven't had a go at cleaning the cam-switches contacts, because I took your advice and re-seated and cleaned the pins of the ICs. The DM502 now seems good. There was a little hiccup when I replaced the temperature IC the wrong way around, but I have been soak-testing the device for some time now and it has been pretty much rock solid and no flicker/loss of display segments. I will probably leave well enough alone on the cam-switch contacts and have a bash at some kind of calibration, but it all looks fairly good so far.
Incidentally, the DM502 came to me with the initial condition - it was a cheap buy on eBay.
Many thanks to all who gave me advice.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Roger Evans via groups.io
Sent: 05 June 2020 11:53
To: TekScopes@groups.io
Subject: Re: [TekScopes] DM502 with problems

Colin,

Some of the TM500 plugins had notoriously unreliable IC sockets. I would certainly remove the socketed ICs, clean pins and see if anything improves. Several people have commented that the long term solution is to replace with decent IC sockets, I expect someone will know how to identify the unreliable originals. Have you checked for ripple on the +/-12V and 5V lines? Also it is worth exercising the range switch a few dozen times! Is the DM502 a new acquisition or one that was working OK but has now failed?

Regards,

Roger


Roger Evans
 

Colin,

Glad to hear it is working OK, if there is no need to clean the cam switches then don't rush into it, there is advice around that says energetic cleaning with the wrong type of paper can remove the gold plating. There are 40 odd segments to the cam switch so it could be a tedious job anyway!

Regards,

Roger