Topics

DC-508 repair advice

radioconnection@...
 

Hello to the group--not exactly a Scope problem, but I was given a TM-503 that was full of plug ins. There is a 508 counter that does have the
preselector daughter board installed.

It is not working. When using the presecaler the input level led shows it is seeing a signal from my HP-8640 signal generator. The gate lamp
is flashing. The display leds are blanked until the counter sees a signal, and then the LEDs only show zeros, but no counts. On the direct
input the counter also does not count. Gate lamp is flashing. The PLL light goes out when I apply a 1 kHz signal. I am guessing there is
something wrong with the gate since the input and timebase seem to be working?

I do not have extender boards and I am very unfamiliar with this counter. I did download a PDF file of the manual, but it is rather cumbersome
to use on a computer. Any suggestions where to start?

Also, I've read there is an Option 7 that was intended to use the DC508 with a TR-503? What exactly does that show? The center scan
or the exact count as the SA is being swept? The 503 does have an aux. RF out that could be fed to the DC-508, will that do the same
function?

Pete

Jean-Paul
 

Pete, any old plug-in may have oxidized switch contacts.

My first guess is to exercise and lube all switches.

Often this solves the problem.

DC-508 is a great counter, but the many stacked boards make it difficult to repair.

Extender cables are one solution but you can access most of the plug-in by mounting in a TM 501,2,3 mainframe with covers removed

Good luck


Jon

 

Hi Pete,
I can answer your questions about what the Option 7 is for:
It is used with a TR-502 tracking generator and a TM-503 which has wired interconnections between these two plugins on the TM503 backplane. This DC508/TR502/TM503 combination will then connect to the 7L13 or 7L14 Spectrum Analyzers through a special 6 wire cable with LEMO connectors on each end to measure the exact frequency the 7L13 or 7L14 is tuned to and display it on the DC508 counter. As the spectrum analyzer's trace sweeps across the CRT, it stops momentarily at the midpoint of the CRT long enough for the DC508 to measure the frequency of the TR502 and display it with up to 1Hz precision. Once the DC508 makes the measurement it releases the sweep of the 7L13 or 7L14 to continue. The entire pause only lasts as long as the measurement period of the DC508 which can be anywhere from 1mS to 1S depending on how much resolution you want. The DC508 is capable of measurements up to 1.3GHz although these two SAs go up to 1.8GHz.

I think you are correct about connecting the TR502 Aux Out to the DC508. It's been a while since I used all of this and I am not certain.

It is a very clever combination of several instruments (tracking generator, spectrum analyzer, and frequency counter) making a very difficult and useful measurement if you are working with RF.

Dennis Tillman W7PF

-----Original Message-----
From: radioconnection@...
Sent: Sunday, December 08, 2019 6:29 AM
Subject: [TekScopes] DC-508 repair advice

Hello to the group--not exactly a Scope problem, but I was given a TM-503 that was full of plug ins. There is a 508 counter that does have the preselector daughter board installed.

It is not working. When using the presecaler the input level led shows it is seeing a signal from my HP-8640 signal generator. The gate lamp is flashing. The display leds are blanked until the counter sees a signal, and then the LEDs only show zeros, but no counts. On the direct input the counter also does not count. Gate lamp is flashing. The PLL light goes out when I apply a 1 kHz signal. I am guessing there is something wrong with the gate since the input and timebase seem to be working?

I do not have extender boards and I am very unfamiliar with this counter. I did download a PDF file of the manual, but it is rather cumbersome to use on a computer. Any suggestions where to start?

Also, I've read there is an Option 7 that was intended to use the DC508 with a TR-503? What exactly does that show? The center scan or the exact count as the SA is being swept? The 503 does have an aux. RF out that could be fed to the DC-508, will that do the same function?

Pete




--
Dennis Tillman W7PF
TekScopes Moderator

radioconnection@...
 

Looking at U1660 I am seeing a good gate signal on pin 5, good signal level on pin 9, but nothing on the Q output from pin 15... I don't have the truth table handy, but the R input on pin 4 is being held low. Q output pin 2 always high. Sound like a bad ECL chip?

radioconnection@...
 

Figured it out.... pin is only high when "totalize" is selected. Ordering a few replacements to verify the chip has croaked.

radioconnection@...
 

Q63 a 2N3906 level converter was bad...

Mlynch001
 

Sometimes the very simple things are the issue. My TYPE576 was knocked out buy a single 2N3906 that failed. Good luck with your repair.

--
Michael Lynch
Dardanelle, AR

Ed
 

Make sure that Switch S1380 (section 13) is turning ON when selecting the prescaler. Measure 5V on U1570B pins 6 and 7. I just repaired a DC502 that had the prescaler select switch switch stuck in the ON position (which disabled the prescaler), maybe the lever arm for this one is stuck OFF.

radioconnection@...
 

It's working fine now... the transistor fixed it; but I was a few bucks on the ECL chip replacement.

Next project is a TM-504 auction site special.... BOTH 18,000 uF filter caps are open! ESR=infinity.
508 is in a good TM-503 for testing.

Mlynch001
 

On the TM503 check your pass transistors. My TM506 had 4 bays out of 6 that had very weak or outright "bad" pass transistors. They will wreak havoc with your plugins.

--
Michael Lynch
Dardanelle, AR

Mlynch001
 

TM504. excuse me. Anyway, check them out, they can cause all sorts of issues.

--
Michael Lynch
Dardanelle, AR

Carsten Bormann
 

On Dec 14, 2019, at 16:14, Mlynch001 <@mlynch001> wrote:

On the TM503 check your pass transistors. My TM506 had 4 bays out of 6 that had very weak or outright "bad" pass transistors. They will wreak havoc with your plugins.
And one problem is that some plugins are damaged by bad pass transistors, and these very plugins then proceed to break the pass transistors of the next slot they are in.

These things need condoms.

Grüße, Carsten

radioconnection@...
 

Well, thanks to the advice, I did find bad pass transistors, and a few plug ins seem to have some issues. This is turning into a larger than expected project. Those mainframes are miserable to work on!

Mlynch001
 

Check your fuses on your plugins. They are prone to open when the pass transistors fail. Also check your voltages as well. Plugin crowbars are also prone to fail. Pay special attention to the complementary transistor in the plugin I.e, the one that’s mated to the failed pass transistor. For example, in a TG501 on the 15v Supply, this would be Q610.

--
Michael Lynch
Dardanelle, AR

radioconnection@...
 

Thanks... I have manuals on order for most of them. Will check to make sure.

I managed to blow up the DM-501 at 2AM playing around (installed it upside down, since I had
the slides removed for servicing.) Another project. I need to go to bed instead of doing bench work at 2AM half asleep.

The only plug in I care about is the tracking generator which is mated with the 497P. The others were free for the taking.
I ended up with the 504 since the 503 was getting filled up rather quickly. But, I am anal and will get them all working
eventually.

Mlynch001
 

The hard part is finding replacement transistors for the pass transistors. I blew one up by plugging one of my plugins onto my extension cable upside down. The plugin extension did not have an indexing pin, so it would fit either way.
--
Michael Lynch
Dardanelle, AR

 

On Sun, Dec 15, 2019 at 09:16 AM, Mlynch001 wrote:


Check your fuses on your plugins. They are prone to open when the pass
transistors fail. Also check your voltages as well. Plugin crowbars are also
prone to fail. Pay special attention to the complementary transistor in the
plugin I.e, the one that’s mated to the failed pass transistor. For
example, in a TG501 on the 15v Supply, this would be Q610.

--
Michael Lynch
Dardanelle, AR
There ares several manuals for the 067-1201-99 TM500 power supply tester (I don't think it was ever commercially available) page on the Tekwiki web site at http://w140.com/tekwiki/wiki/067-1201-99. The manuals contain construction details for the tester, to be built into a 040-0652-02 blank plug-in chassis. I have one of the blank chassis kits, and have been wondering what kind of project I could build into it. These questions reminded me of the tester and I think it would be worthwhile, since I have several of the TM500 power supply units.

After revisiting the parts list for the tester, I think the 4pole 6position meter switch would be the most hard-to-find, but I found several of them available on Ebay. Not cheap, though. I think a 1pole 6 position switch could be used to switch in relays without much trouble. Small relays could be used, since none of the switched contacts seem to be in high current circuits.

Has anyone in the group built one of the testers? If so, did you make a PCB for it?

Cheers,
Dave M

Mlynch001
 

This sounds interesting! I need to look into this further.

--
Michael Lynch
Dardanelle, AR