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 Post subject: Drake TR-4 Issues
PostPosted: Feb Thu 22, 2018 7:19 am 
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Joined: Nov Mon 09, 2015 2:55 am
Posts: 238
Location: St. Louis, MO
I recently acquired a TR-4.

I have a few issues with it, but bear in mind I am brand new to Drake gear. So some may be operator error.

The power supply came with the AC-4R kit already installed. I checked the voltage and they all check out within spec per http://www.wb4hfn.com/DRAKE/DrakeArticl ... ooting.pdf

Note that the bias voltage goes from -48V to about -95V with no load. (I replaced the bias pot which only went to 7K when the schematic for the AC-4 indicates a 10K. This let it go to -48V, when it previously would only go to -54V.)


Issue #1
Unable to obtain .1 amp idle current when adjusting the bias control on the power supply.

The most I can get with the bias pot at full stop is about 80-90 mA. Interestingly this was about 65mA until I started playing with tuning. In other words it changed.. (Makes me think relay contacts??)

I verified the current meter using a power supply and a good Fluke DMM to measure the current. I did have to modify the shunt resistance a little to get it accurate. It now reads spot on at 100mA and at 400mA. So I don't think it is a meter issue.

I verified the values of the resistors marked with red in the attached schematic.


Issue #2
Sometimes there is a static crashing type sound even when on a dummy load. This almost sounds like the old Silver Mica Disease on old Zeniths. However, sometimes it goes away. It seems worst right after transmitting.

When the crashing occurs, if I turn down the volume, I can here a very soft corresponding ticking type sound coming from somewhere in the radio. I can't yet determine where that is coming from.

Adjusting the RF Tune seems to have some effect on the crashing. A certain spot causes the crashing. At least at one point tuning the RF Tune off made it go away.

I wonder if I should have a look for foreign matter in the RF Tune can. I also wonder if this isn't relay contacts since it seems to get worse after transmitting.

I tried lightly tapping all transformers, switches, and tubes. This did not seem to control the crashing sound at all.



Issue #3 (at least partially related to Issue #1 think)
I don't get more than about 300mA max plate current when tuning (I tried 40m). Also, the Load control does not seem to increase RF output at all. I am tuning into a 50 Ohm dummy load.

I think I need to work out the idle current first.

Any thoughts on any of these issues (or if they are related)?

*** If you call out any parts, and you know where they are physically in a TR-4 *please* let me know where they are physically, as I am new to this radio.

Relay wise it looks like I have two blue covered relays down by the Upper and Lower lights. I think there is also the main K1 relay at the back of the radio, right next to the sidetone pot.

NOTE: I did clean the wafer switches and the pots (other than sidetone and carrier balance). I have not cleaned the relays.

Thanks in advance.


Attachments:
TR-4.GIF
TR-4.GIF [ 67.95 KiB | Viewed 583 times ]

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 Post subject: Re: Drake TR-4 Issues
PostPosted: Feb Thu 22, 2018 9:22 am 
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Joined: Nov Mon 09, 2015 2:55 am
Posts: 238
Location: St. Louis, MO
(My apologies to anyone also following my thread I had already started on QRZ...)

I was able to measure the current through the cathode resistors:

Back: 16.67 Ohms 0.35V drop = ~21 mA
Mid: 16.90 Ohms 0.26V drop = ~15.4 mA
Front: 16.01 Ohms 0.43V Drop = ~25.42 mA

So I think at least the middle tube needs to be replaced, and perhaps the back tube. The plate caps are well connected, and the plate chokes read less than 0.02 Ohms across them.

So I think this should take care of issue #1 and likely issue #3

Now I need to figure out what this static crash type sound is, even on a dummy load.

Note I have not replaced most of the tubes because I do not have spares.

I am thinking start at the output with the scope (through a small cap) and work my way backwards through the signal path and see when the noise stops. Any thoughts are appreciated. Thanks.

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 Post subject: Re: Drake TR-4 Issues
PostPosted: Feb Thu 22, 2018 1:05 pm 
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I was going to suggest weak finals as the most likely cause of the low idling current and lack of current under drive. Although the tubes don't have to be perfectly matched I would replace all three tubes. Sylvania were the preferred brand by Drake but RCA tubes have also worked well in my experience. Avoid Servicemaster International brand like the plague-I think those were the quality model for the early Chinese tube efforts and they are largely junk.

I have used 6GJ5 replacements in a couple of my Drake 4 line transmitters and they worked fine. They used to be far less expensive than 6JB6 tubes but I haven't compared the price recently.

No matter what tubes you use do them a favor and add a small muffin fan to help cool the tubes, either top or back mount works fine. It doesn't have to be a lot of air and anything is better than the "nothing" approach used by Drake and many other users of these sweep tubes for finals. When designed these tubes were dirt cheap due to their ready availability from use in TV sets but now they are anything but cheap and are generally more expensive than "real" transmitting tubes. Sweep tubes used in ham rigs can last a long time if treated well so there are fine and I would NOT modify to use 6146 or other such invasive mods because it is not needed and detracts from the rig in many ways.

The sound you are hearing in receive could be the relay or a tube issue. But hearing a ticking sound sure makes it sound like something is arcing which is more likely a capacitor.

The way the transceiver is configured it is possible for a problem in a final tube to be heard in the receiver so you may even get lucky and cure all three problems with one fix. Because the adjustment of the RF tune affects this issue the problem is most likely going to be found in the receiver RF stage, crystal calibrator stage, or one of the transmitter stages that can interact with these parts (and unfortunately that is a large number of stages).

Troubleshooting is always more of a pain in a transceiver and the ticking sound with crackling being worse after transmit makes me wonder about something in the HV circuit in the final compartment. Normally you would just pull the finals (to protect the screens) and disconnect the HV lead to the rig to check but in this case the tube filaments are part of the series filament chain so to do this check the tubes have to remain in place and you will have to disconnect the screen grid feed to the finals and then you can disconnect the HV lead at the power supply connector to test. If the screens were left connected then they will behave like the plate and burn themselves out immediately. In receive the cutoff bias should protect the tubes but I wouldn't risk a set of finals to a testing accident so also disconnect the screen feed to the finals (where it enters the final compartment via the feedthrough capacitor is a convenient spot).

Rodger WQ9E


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 Post subject: Re: Drake TR-4 Issues
PostPosted: Feb Thu 22, 2018 3:59 pm 
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Location: Stephenson, MI USA
"Relay wise it looks like I have two blue covered relays down by the Upper and Lower lights. I think there is also the main K1 relay at the back of the radio, right next to the sidetone pot"

The blue ones are not relays. If they are behind the front panel near the sideband select switch, they are the upper and lower sideband filters. Do not attempt to open them. They are not relays and there is nothing in them to clean.

The main relay is inside the final cage if I remember correctly, and there is another small relay under the chassis, that controls the tx offset if I remember correctly.

73,
Noel, K8NB


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 Post subject: Re: Drake TR-4 Issues
PostPosted: Feb Thu 22, 2018 6:30 pm 
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Joined: Nov Mon 09, 2015 2:55 am
Posts: 238
Location: St. Louis, MO
UpNorth wrote:
"Relay wise it looks like I have two blue covered relays down by the Upper and Lower lights. I think there is also the main K1 relay at the back of the radio, right next to the sidetone pot"

The blue ones are not relays. If they are behind the front panel near the sideband select switch, they are the upper and lower sideband filters. Do not attempt to open them. They are not relays and there is nothing in them to clean.

The main relay is inside the final cage if I remember correctly, and there is another small relay under the chassis, that controls the tx offset if I remember correctly.


Ah, duh! Yes, those blue things are the filters.

I did find the K1 in the finals cage, and there is another relay back by the sidetone pot. Given further testing I don't think the crackling is coming from relay contacts, though.

Sounds like the best bet is a new set of finals first. Do these need to be "matched," or does the neutralization pretty much make the irrelevant?

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 Post subject: Re: Drake TR-4 Issues
PostPosted: Feb Thu 22, 2018 7:01 pm 
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Location: St. Louis, MO
Rodger:

Great info, thank you.

I am having trouble identifying the "feed through capacitor" for the screen grids in the schematic. Is that C75 ?

Forgive my naïveté in this regard, but that is one of those little terminal post looking devices that is mounted to the copper divider into the underside of the finals section, right? I have not encountered these before.

Just desoldering it there and tape it off for good measure, right?

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 Post subject: Re: Drake TR-4 Issues
PostPosted: Feb Thu 22, 2018 7:07 pm 
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C75 is correct and you have correctly described how they look. Those types of capacitors are the most effective method of bypassing RF to ground to keep RF generated in the final compartment from escaping via connecting leads to the rest of the radio. Desoldering it and taping it off will be fine and make sure to exercise the same care with the disconnected HV lead (preferably in the power supply itself).

Matching is used to ensure that the three tubes reasonably share the load between them. The separate cathode resistors used in this rig help a little bit since the "hotter" tubes will try to draw more current thus increasing their bias. You don't need perfect matching and so called matched tubes are generally matched for idle current rather than peak current which is more important. And three tubes that start out perfectly matched won't stay that way.

If you get three NOS tubes from the same manufacturer they are very likely going to be close enough not to worry. If you buy three different unknown used tubes then it is a different story and you may end up with 2 weak and 1 good tube and the good tube will soon become a bad tube from overloading. The series/parallel filament wiring used in this rig preclude a simple check of idle current by tube but you can measure this by setting the three tubes to draw proper total idle current and then the voltage developed across each individual cathode resistor will tell you how closely they are matched (remember to allow for any differences in the three resistor values from each other). A better check would be with the tubes producing around 1/3 to 1/2 normal output but this would require making rapid measurements to avoid stressing the tubes-again you would measure this by measuring cathode resistor voltage drop.

Rodger WQ9E


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 Post subject: Re: Drake TR-4 Issues
PostPosted: Feb Thu 22, 2018 7:21 pm 
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Joined: Nov Mon 09, 2015 2:55 am
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Location: St. Louis, MO
Again, thank you, Rodger.

I did use that cathode resistor measurement method (posted the values above). I think I will just get three NOS tubes and then go from there.

If the noise persists then I will trace it through the circuit with a scope. Though, if I get bored waiting for the tubes then I shall use that method you describe to test the finals to see if they are the noise source.

It kind of sounded like the noise was coming from closer to the front of the rig, near the SSB filter area. But I didn't want to get my head too close in operation, and I did not listen to it on its side. So if the sound came from underneath than it could be almost anywhere.

Thanks again. Eager to get this guy working. The receive is very clear and quite selective.

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 Post subject: Re: Drake TR-4 Issues
PostPosted: Feb Thu 22, 2018 9:07 pm 
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Location: St. Louis, MO
So from a cold state today I was listening for about a half and hour with pretty much no noise (there may have been a couple tiny ticks, but those could be atmospheric or local noise sources).

Then I tried for about 2 secs to do that bias adjustment (Turned on to X-CW with sideband switch set counter-clock). Right after that immediately static noise, though it seems to have faded down quite a bit. So could be heat related in terms of the tubes or somewhere else. Just gathering info now. I still think replacing the finals is the logical next step.

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 Post subject: Re: Drake TR-4 Issues
PostPosted: Feb Thu 22, 2018 9:39 pm 
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If you had noise after the bias setting, it sounds like the relay has a poor contact. You may need to remove the finals cage and clean the relay contacts. I spray contact cleaner or alcohol on a small piece of paper and slide that between the contacts as I manually press them together. Don't use an abrasive. Have you tested the finals with a tube tester? What about the driver tube?

Norm

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 Post subject: Re: Drake TR-4 Issues
PostPosted: Feb Fri 23, 2018 1:45 am 
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Thanks for the input, Norm. I definitely plan on cleaning the relays!

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