As I work toward resurrecting a dead TR-7A, I've started with the power supply board. After replacing all electrolytics and the tantalum, I found the voltages to be a bit low, at +23.4, +8, +5, and -3.8. By adjusting R2103, the +10V line has come up to spec, but it is near its physical limit; the pot was close to mid-range before. The -5V line remains at -3.8. The low-voltage output of the transformer looks a bit low, at about 3.8VAC, but the voltage feeding the bridge is at about 35VAC, which seems about right.
Any thoughts on bringing up the values on the +24V and -5v lines? (The schematic says it's a 26V line, but the text says 24V, and the regulator is a 78L24).

Thanks,

Bob
K7CY

开云体育

Tantalum Capacitors have a pretty high rate of infant mortality relative to other capacitor types. They almost always fail shorted or leaky.

On 1/19/20 10:47 AM, Gary Follett wrote:

Electrolytic caps worked in there for 40 years, why change
to Tantalum?

Another bad idea like the Sprague Black Beauties.



--
"I am a river to my people."
Jeff-1.0
WA6FWI
www.foxsmercantile.com

I agree with Gary on this one. With the high quality electrolytics out there nowdays there is no need to replace with Tantalum. Indiscriminately changing types can sometimes lead to unintended consequences. Replace electrolytics with good quality 105C modern parts. Replace tantalum with tantalum IF THEY ARE BAD.

-3.8 is typical for the -5V line. The +24 value is determined by the three terminal regulator. If your input voltage is OK and has low ripple the regulator IC may be out of spec. The single most critical cap (assuming old version PS board) is C2108 and should always be replaced.

Your +10 adjustment is a little more interesting. You may have created another problem when you changed caps, or the new caps are just pulling the circuit away from design spec. Check the outputs of all sources with a scope for oscillation.

If you really want to get off into the weeds, dive into the data sheets and app notes for the 723 regulator IC. There are some circuit tweaks that you can make that should make the +10 a bit quieter according to the designers. Overkill for most practical purposes and Drake engineers obviously did not think they needed to go to the extremes but fun to try at least. I did one not too long ago and I honestly can't say if it improved anything but I got warm fuzzies thinking about it. There's also a QST tech note out there from some years ago about adding bypass caps to the regulators to reduce internal noise. Basically the PS is a solid design as it is and you don't really need to change things up unless you really want to push the envelope.

73

-Jim
NU0C


On Sun, 19 Jan 2020 10:47:34 -0600
"Gary Follett" <dukeshifi@...> wrote:

Tantalum Capacitors have a pretty high rate of infant mortality relative to other capacitor types. They almost always fail shorted or leaky.

These caps are very intolerant of excess soldiering heat, and even less tolerant of rapid thermal shock so low temperature soldering with a low power iron (to reduce rate of heating) makes things much more reliable.

When we used Tantalum caps in our products, we always used devices with really high voltage headroom (double) and went through a proprietary “recovery” process if any failed after installation.

Electrolytic caps worked in there for 40 years, why change to Tantalum?

Gary

W0DVN

PS: From your voltage readings, I would have a look at the caps on the 5 volt line.

On Jan 19, 2020, at 12:25 AM, Robert Berg <Bob.B@...> wrote:

As I work toward resurrecting a dead TR-7A, I've started with the power supply board. After replacing all electrolytics and the tantalum, I found the voltages to be a bit low, at +23.4, +8, +5, and -3.8. By adjusting R2103, the +10V line has come up to spec, but it is near its physical limit; the pot was close to mid-range before. The -5V line remains at -3.8. The low-voltage output of the transformer looks a bit low, at about 3.8VAC, but the voltage feeding the bridge is at about 35VAC, which seems about right.
Any thoughts on bringing up the values on the +24V and -5v lines? (The schematic says it's a 26V line, but the text says 24V, and the regulator is a 78L24).

Thanks,

Bob
K7CY

I agree with Jeff, that Tantalum capacitors are a bad choice for some applications. Just ask anyone who has had problems with a Collins WM-380. That radio was filled with them and they caused multiple maladies, most commonly in the primitive synthesizer in the radio. This radio was also built before anyone became familiar with the problem of not overrating Tantalums.

They do have their place. They give a lot of capacitance for their sizes, and, once successfully installed, they offer high reliability (key here - once installed and not thermally shocked).

They also can operate at higher temperatures than electrolytic can, 125C. This is the reason we used them in our safety equipment, which had to operate reliably up to 80C.

This seems contradictory to the commentary about soldering but soldering involves temperatures far greater than 125C and the rate of heating that high becomes quite important).

Tantalum capacitors also do not allow high ripple current. Thus they are ill-suited for use in power supply filter applications. That may be the reason why the 5 volt supply in the TR7 shows such low voltage - this line has the highest current draw in the radio from this combination switching supply.

Pull the Tantalums and put in electrolytics…

Gary

W0DVN

Early Fosgate car stereo amps were another example. They used tants in a power supply where they should not have. Almost killed the brand.

73

-Jim
NU0C

On Sun, 19 Jan 2020 13:06:56 -0600
"Gary Follett" <dukeshifi@...> wrote:

I agree with Jeff, that Tantalum capacitors are a bad choice for some applications. Just ask anyone who has had problems with a Collins WM-380.

Thanks for all of the comments. I replaced electrolytics with the same, and the single tantalum with a tantalum. Should I replace that with an electrolytic as well? I take back what I said about the 10v pot. It was initially flakey, but after exercising it a few times it settled down to mid-range for a 10v reading.? Must have been dirty. Sorting through a pile of 1N4148s, I picked out one with very low forward resistance. Now I'm seeing -4v on the -5v line. Still rather low, on this unregulated line. I'll search for the QST article on bypassing the 1723. Do you happen to know when it was published?

No, do not replace a tantalum with an electrolytic. It can be assumed that the Drake engineers used a tantalum in that spot for a specific reason. In my experience, tantalums don't seem to degrade the way electrolytics do. They are either good, shorted, or exploded.

The sole purpose of the -5V line is to provide some back bias to the PIN diodes on the IF Filter board to turn them more off. It is a very low current load. As such it is not critical and the voltage you are seeing is OK. You could replace the rectifiers with a Schottky diode. These days Schottky is the preferred part for high frequency DC-DC converters anyway so in theory you could make that hole circuit a little quieter. But that may be a solution in search of a problem. I'd love to have a lab (and the time) where I could measure this stuff out to quantum levels.....

I should have the QST note in my archives, I'll see if I can turn it up.

73

-Jim
NU0C


On Sun, 19 Jan 2020 13:45:14 -0800
"Robert Berg" <Bob.B@...> wrote:

Thanks for all of the comments. I replaced electrolytics with the same, and the single tantalum with a tantalum. Should I replace that with an electrolytic as well? I take back what I said about the 10v pot. It was initially flakey, but after exercising it a few times it settled down to mid-range for a 10v reading.? Must have been dirty. Sorting through a pile of 1N4148s, I picked out one with very low forward resistance. Now I'm seeing -4v on the -5v line. Still rather low, on this unregulated line. I'll search for the QST article on bypassing the 1723. Do you happen to know when it was published?

Thanks, Jim!
And... it's alive!
So far, my only complaint is a flaky bandswitch.

I have a few mods in mind, including an HP balanced mixer from a 3586A Selective Level Meter. This radio has the NB-7A noise blanker, evidently from an R-7 receiver. Was that ever a factory option?? I've wondered if it's possible to mod the NB-7 in my late TR-7 to the NB-7A configuration. Looks like it just needs a couple of parts added--a choke and a crystal, IIRC.

73,
Bob
K7CY

Congrats! Some DeOxit would probably help out the band switch (D100 needle applicator, NOT spray!) and it might pay to check for cracked solder joints on the boards the wafers are mounted to.

The NB-7A is an "improved" version of the NB-7. I'm not sure how much improved. My NB-7 is nothing short of amazing on impulse noise. My R-7A has a NB-7A but I've never really compared them A/B. I just use them.

Mini-Circuits makes drop-in parts that will replace U401 on the upconverter board and should perform a little better in theory. Don't let the data sheet pinouts fool you, those clever Drake engineers actually swapped IIRC the IF and LO ports around to get RX coverage down to essentially DC. I've used 1N5711 Schottky diodes for the other diode mixers.

73

-Jim
NU0C

On Sun, 19 Jan 2020 19:15:59 -0800
"Robert Berg" <Bob.B@...> wrote:

Thanks, Jim!
And... it's alive!
So far, my only complaint is a flaky bandswitch.

I have a few mods in mind, including an HP balanced mixer from a 3586A Selective Level Meter. This radio has the NB-7A noise blanker, evidently from an R-7 receiver. Was that ever a factory option?? I've wondered if it's possible to mod the NB-7 in my late TR-7 to the NB-7A configuration. Looks like it just needs a couple of parts added--a choke and a crystal, IRRC.

73,
Bob
K7CY

I haven't been able to find it yet.... It was more a generic note of bypassing regulator ICs per manufacturer specs and referenced two radios, one of them being the TR-7. I thought I had a digital copy of it but I haven't been able to locate it yet and my hardcopy is with my 723 notes, which also seem to have gone walkabout. I'm guessing it was sometime in the 80s, maybe late 70s, but that's just a guess. Likely in the technical correspondence or H&K section.

73

-Jim
NU0C

On Sun, 19 Jan 2020 13:45:14 -0800
"Robert Berg" <Bob.B@...> wrote:

I'll search for the QST article on bypassing the 1723. Do you happen to know when it was published?

And wouldn't you know, I just found it.

Feb. 1989 page 47 (Technical Correspondence), "Zener Diode Noise".

BTW, you don't really want to set the TR-7 IF Gain to max as the author recommends. That will make your S-Meter very generous and your system noise will be higher than it needs to be.

73

-Jim
NU0C

On Sun, 19 Jan 2020 23:58:49 -0600
"Jim Shorney" <jshorney@...> wrote:

I haven't been able to find it yet.... It was more a generic note of bypassing regulator ICs per manufacturer specs and referenced two radios, one of them being the TR-7. I thought I had a digital copy of it but I haven't been able to locate it yet and my hardcopy is with my 723 notes, which also seem to have gone walkabout. I'm guessing it was sometime in the 80s, maybe late 70s, but that's just a guess. Likely in the technical correspondence or H&K section.

73

-Jim
NU0C

On Sun, 19 Jan 2020 13:45:14 -0800
"Robert Berg" <Bob.B@...> wrote:

I'll search for the QST article on bypassing the 1723. Do you happen to know when it was published?

Jim,

I found the article. Thanks!

Bob
K7CY

The drama continues...

The radio performed the same after replacing the electrolytics and the single tantalum, and adjusting the 10v pot. I also added some 0.1 ceramics to the locations suggested in the article Jim mentioned. This didn't affect output voltages as measured on the bench. Cleaning the switch contacts didn't help. Swapping the VCO card didn't help.

Then I measured the voltages at the power supply card when installed in the radio. What a surprise! On the bench, I'm seeing -4.2, +23.3, +10.0, and +4.8. That's with a +13.8v supply. In the radio, I see -3.5, +23.3, +10.0, and +8.3! And the mighty PS-7 seems to be providing only +13.2 at the power supply board. So the 24v line is a bit low--maybe I should swap out the regulator. But how does the +5v regulated line go up to +8.3? That seems problematic. And the radio emits a squeal of about 1100 Hz, although occasionally the receiver seems to work well upon a cold start (and without a squeal) on the selected band, as long as I don't switch to a different band.

Any suggestions about next steps? Thinking the 7805 might be suspect.

Bob
K7CY

Bob,

13.2V input should be fine, that's not far enough off to affect anything much. 23.3 is probably not unreasonable for the +24, it is within 10%. What it most important is that it is clean and stable. +5 should definitely be +5. You probably either have a bad regulator IC or a grounding problem. Note that the PS board has two ground pins and they are not connected together on the board, just on the chassis - at least in some units.

The squeal is probably either a bad decoupling capacitor or a grounding problem. Check any screws that are a grounding point. I'm going to sing the Molex song again, the most common cause of strange 7-line problems is dirty/oxidized Molex connectors that connect the boards together. Often just re-seating them will clear the problem. Also check the male Molex pins for stressed/cracked solder connections and check the female contacts to ensure that the are not deformed enough to cause poor connections. And don't overlook that big Jones power plug on the back, they can get grungy too. The ON/OFF switch can cause erratic operation also if it is gunked up too badly.

That's enough to keep you busy for a while... :)

73

-Jim
NU0C

On Sat, 25 Jan 2020 12:40:49 -0800
"Robert Berg" <Bob.B@...> wrote:

[Edited Message Follows]

The drama continues...

The radio performed the same after replacing the electrolytics and the single tantalum, and adjusting the 10v pot. I also added some 0.1 ceramics to the locations suggested in the article Jim mentioned. This didn't affect output voltages as measured on the bench. Cleaning the switch contacts didn't help. Swapping the VCO card didn't help.

Then I measured the voltages at the power supply card when installed in the radio. What a surprise! On the bench, I'm seeing -4.2, +23.3, +10.0, and +4.8. That's with a +13.8v supply. In the radio, I see -3.5, +23.3, +10.0, and +8.3! And the mighty PS-7 seems to be providing only +13.2 at the power supply board. So the 24v line is a bit low--maybe I should swap out the regulator. But how does the +5v regulated line go up to +8.3? That seems problematic. And the radio emits a squeal of about 1100 Hz, although occasionally the receiver seems to work well upon a cold start (and without a squeal) on the selected band, as long as I don't switch to a different band.

Any suggestions about next steps? Thinking the 7805 might be suspect.

Bob
K7CY

Jim,
Yes, that will keep me busy.
Thanks!
Bob
K7CY

Jim, et al,

With the digital control board removed, the +5v line drops down to 5 volts, from about +8.5 with the board installed. I have not tried substituting a known working digital board yet. Does this suggest a chip with an internal short?

Bob

开云体育

Do you have an oscilloscope?

Gary,

I do have a scope. I swapped in a known good digital board with no change in results. Where would you be looking for oscillation?

Thanks,
Bob

开云体育

I would look on the 5 volt line. When you get activity like that which you are experiencing, oscillation on the power supply is a common cause.