Thanks Jthorusen, I'll get on it tonight.

Jthorusen, Thanks so much for the clear guidance. Well good news and bad news. I checked the continuity just as you describe above. I found a suspected cold solder joint and resoldered it. The whole power circuit checked good for continuity. So I fired it up through my dim bulb, I always do that when moving around a chassis or pcb. The display came on. I inserted a wire into bnc connector an turned on the Total count. It was reading to fast to read at all and the overflow light never came on. I hooked up my signal generator set at 455Kc. The counter was reading 68Kc. I switched the power cord to house power and the counter was still reading at 68Kc. After about 5 minutes the fuse blew.

A couple of things I noticed: This unit has been factory modified from the schematic. There is an inductor coil and a capacitor 47pf? on the top of the board and two capacitors on the underside.

I've never seen this kind of fuse before with a resistor inside. I believe the color code is Green-Violet-Gold, so that would be a 5.7 ohm. The parts list says it's a 1/4A slow-blow fuse. Where would I get such a beast or what would be an acceptable substitute. The markings on it said 1/4A 250V, which is different than the schematic says 1/4A 125V.

I'm not sure what cause it to blow. I do note that on my second unit that needs repair, does not have those additional capacitors and coil mentioned above and its fuse is blown also. In addition, D6 - a 4.7V zener diode and R35 - a 270 ohm resistor are burnt out. Could that be a weak point in the unit that I'm working on?

I do have a 6800uf 16V electrolytic cap to replace the existing 6000uf 16V cap. I did measure the AC voltage on C16 and it read .48Vac.

I appreciate the guidance.

Those little "resistor" things are pretty common to see in low current fuses. I may have some around here if you still need one. The higher voltage rating on the fuse simply means it was certified to be able to make a clean disconnect at the higher voltage. It's not uncommon to see 250V fuses in 120V service.

Greetings to jseabee and the Forum:

I second what hwhall said. The resistors are used to generate sufficient heat to melt the low melting point solder that holds the innards together. The spring then pulls the innards apart, breaking the circuit.

I would not replace the fuse with anything other than the specified type and rating. Not all slow-blow fuses have a resistor with the color code visible; there are different designs. As long as the rating is the same, the internal appearance may vary.

The most likely suspect for your fuse blowing is C16. After that, it's a toss-up between C17, one or more diodes in the bridge or the IC regulator. It shouldn't be anything after the regulator chip because (I am assuming it is a 7805 or equivalent) the regulator has built-in over-current shutdown. Even at 1 amp (the rated current for a 7805), the power consumption at 5 volts is only 5 watts plus losses.

One-quarter amp at 120 volts is 30 watts. If the regulator IC is operating properly, a dead short on its output should not blow the fuse.

One other place you might look is the leads from points AN and AS and the external power jack. Make sure nothing is able to cause a short there.

I'd suggest that you replace C16 and the fuse and try again.

Regards,

I see at least seven tantalum capacitors in your photos. It wouldn't hurt to check them for being shorted out. I've had that problem on some newer Heathkit things I own. I also have an IM 4100 counter that my father and I assembled new around 1978. I have the manual for it.

Thanks guys. New fuses on order (with spares). I'm replacing C16 in both units and going to de-flux the bottom of the pcb.

I'd second taking a close look at the tantalum capacitors. I had more than I'd like either go short or outright explode when I worked for an OEM in the early 80's. Exploded ones were easy to find, shorted not so much.

Michael

lasermike, I did do a aural continuity check across each capacitor. Did not find any shorts until I got the very last one. It was C15. However when I removed it, it measured .01uf and no short. What I found was that when switch 1 was in the 120V position I would get a continuity beep when I placed the probes on the two pads for C15, but when I threw the switch to 240V it would not.

Is the aural continuity check adequate for this testing?

Looking at the schematic, C15 sits across the power transformer selector switch. A meter connected to it's pads will show the two windings in parallel in the 120 V position and in series in the 240 V position. I don't know how low the resistance must be for your meter to beep but it appears that the windings in parallel is low enough while the windings in series is too high for it to beep. I suspect this is why you had continuity across C15 when you first checked it but it tested OK out of circuit. I would use the resistance range on your meter to check for actual resistances of the two windings. They should be close but will probably not match exactly.

It sounds like the tantalum capacitors might be OK. I'd continue with the plan to replace C16, the large electrolytic and looking at the components and values that Jthoruse listed above.

As for the "extra" capacitors and the inductor, they appear on an addendum to the manual. It looks like the TTL circuits may have been putting noise on the 5 volt rail that the impedance converter didn't like so extra filtering was added between them, in effect creating two 5 volt rails. Look for a page the says "IMPORTANT NOTICE" at the top.

The schematic does have a labeling error regarding the regulator IC, IC27. It shows the terminals as B, C and E left to right. It should be labeled INPUT (or 1), GROUND (or 2) and OUTPUT (or 3) since it's a standard LM7805 regulator. It's not a big issue but may lead to confusion down the road if someone asks you to measure the voltage on pin 3 or on the output pin. Also, the schematic shows that there should be 9.8 volts DC to ground on pin 1 of IC27 with the unit on and no input signal applied.

Michael

Thanks Mike. All the caps checked good on my second unit, but they don't have the mods from the "important notice" bulletin. What the heck are tantalum capacitors........ never heard of them before, though have found them on ebay. What voltage rating should I be looking for? Haven't found a 26uH inductor yet.......... any suggestions?

I'm awaiting the fuses, and I'll try again!

These are my, third and fourth frequency counters that I have not been able to work right (gotta admit two were given to me).

UPDATE: I finally got my fuses in the mail today. During this downtime I did replace C16 with a 6800uf electrolytic capacitor and defluxed the underside of the PCB. Fired up the unit through my dim bulb and all looked good, the display came back on. I switched to house power. The display was jumping all over the place. I tried the Total count with a wire in the BNC and it was working but the overflow light did not come on when it went past 99. I tried to read the frequency to my Signal Generator at 455Khz, but got no stable reading (jumped around say from 22Kc to 97Kc). I also noticed the overflow LED on. When I do this, I am using a BNC connector with gator clips and clip them to the leads from my RCA Signal Generator Kit. I have the AF MOD/INT Mod off and have the RF attenuation on but down low.

I did measure 120Vac to the board and had .62Vac on C16 and 7.9Vdc on C16 and C17. I measured 9.3V at the blue wires (AP - AR). AL-AF was 120V as was AE-AM. As Jthorusen suggested, I disconnected the blue wire from the regulator at point AG. I measured 5.26V at AR-AB and AP-AG.

I did put my Klein MM2000 multimeter on the Diodes it measured .45V on Diodes D7-D11.

I did notice that the transformer was very hot to touch and stayed hot for several minutes. This concerns me.

Any suggestions as to what to look for next??

Thanks,
Jay

Greetings to jseabee and the Forum:



I don't know where point AG is; I can't find it on my schematic. AB on my schematic is a logic line; I don't know why you would measure from a point with AC on it (AR) to a logic line (AB). I also don't know what you are measuring when you measure AP to AG since I can't find AG on my schematic.

However, it sounds as though you have restored the 5 volt power supply. Without any reference, I have no idea how hot your transformer is supposed to run; it should only be supplying 5 watts or so maximum, so I wouldn't expect it to be warm at all unless it was of marginal design, which some Heathkit transformers were. This is predicated on the assumption that the voltage regulator is a 7805; please supply us with the type number on the chip so that we can avoid invalid assumptions.



You need to turn your RF output up to maximum. You virtually can't overload the counter but if the level is too low, you will get very erratic counting.

Regards,

Jthorusen, you are correct and the voltage from AR-AB should have read AR to AG. Sorry for the lack of clarity, I believe that AG is the node where the input wire from the regulator IC meets C17, C16 and the output of the diode bridge. Earlier you suggested that I lift the input pin of the regulator and measure the voltage.

I will definitely take a picture of the regulator tonight and post it.

Thanks for the tip on turning up the RF output. I’ll try that again.

Jthorusen, the 5V regulator is supposed to be a LM7805 per the manual. But you were correct in that that is not what is in the circuit. It is a 7620. I measured the voltages on the regulator at Pin 1 I got 8.14Vdc; Pin 2 = 5.5mVdc; Pin 3= 5.2V. (all in reference to ground). A side note: My second one of these units has 7550 voltage regulator.

Tonight I resoldered the input wire to the regulator back to point AG. The unit did fire up and I put the RF sig Gen output on maximum at 455Khz. In the Khz range, it read 68Khz and the overflow light was on with Attenuation at X1. if I move to 10X or 100X attenuation, it jumped all over the place with the overflow light on.

When I shifted to the Mhz range, I got 455Khz at 1x attenuation!! I swept to a few other frequencies and it seemed to track pretty well. If I moved to 10X, it read .50 to .590 Mhz. At 100X, it jumped all over the place again. In any of the Mhz ranges, the overflow light was not on.

Oh ya, I did run the total count and while very fast, it was readable and the overflow light came on when it cross 99xx.

The transformer still gets hot.

So I think we are getting there. Where should I look to try to fix the Khz range?

Thanks to all for the guidance.

Those are LM7805 regulators with 442-54 Heathkit part numbers.

The 7520 and 7620 are probably date codes (1975, 20th week and 1976 20th week.)

Thanks Scott for the clarification. Definitely not very intuitive.

What kind of signal generator are you using and how is it setup?

RRM

It’s a RCA Signal Generator Kit. I have the AF Mod/Int Mod off and RF output near maximum.

if you're sure power is getting to all dips, it's time to drag out a scope and start following the signal path.

RRM

To fix the KHz range, read the instruction manual available at this site:
https://www.vintage-radio.info/heathkit/

The KHz range only reads up to 99.999 KHz. It can’t read 455 KHz. According to the manual, if you feed it more than 99.999 KHz, the overflow light will come on.

So your counter appears to be working correctly.