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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Sun 20, 2021 6:06 pm 
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Location: Davenport, Iowa
Here is the same schematic but with some more resistance measurements added. One interesting thing is R84, R85, and R114 and R115. These are all supposed to be 0.5Ω. Three of them measure 1.1Ω but R115 measures 1100Ω. Could this be that R115 is failing? Or could this mean Q21 has a short when it shouldn't, so I'm measuring parallel resistances? This is all I had time to measure right now, but there are a few others added that check out OK.

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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Sun 20, 2021 6:33 pm 
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When you measure across a resistor while it's in the circuit, you shouldn't measure more resistance than what the value of the resistor is. And if you measure a wildly lower resistance it's worth examining the circuit to see why if it's a potential problem area. In your case, Q15 did, in fact, appear to be bad. Looking at the circuit and the possible current flow through R91, IMHO the reason why it measured such a drastically low resistance (0.47k) is because the base to collector junction in Q15 had broken down -- and your meter saw the transistor as a much lower resistance than R91. Same thing when considering the 1.25k reading on R92 -- there's really nowhere for the current to go except back through Q15 to ground. Replacing Q15 was a must -- now let's see what else is cooking. Looking over more of the schematic and the measurements -- BRB.

Steve, KW4H


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Sun 20, 2021 6:43 pm 
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OK -- looking at the other readings. Let's start with Q18:

Q18: C92, C24, and C25 are all blocking DC current, so grab an in-circuit reading of R95 and R97. Also, the voltage reading at point 4 is two volts lower than expected -- it's 8.4 volts instead of 10.4. Shoot over a copy of the power supply schematic so we can see where that voltage is coming from.

Steve, KW4H


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Sun 20, 2021 6:52 pm 
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pioneer07 wrote:
Here is the same schematic but with some more resistance measurements added. One interesting thing is R84, R85, and R114 and R115. These are all supposed to be 0.5Ω. Three of them measure 1.1Ω but R115 measures 1100Ω. Could this be that R115 is failing? Or could this mean Q21 has a short when it shouldn't, so I'm measuring parallel resistances? This is all I had time to measure right now, but there are a few others added that check out OK.


Regarding any resistor that measures significantly more than its rated resistance when it's in the circuit -- it's most likely bad. I'd suggest lifting a leg on those resistors and check again to be sure -- but they'll probably need to be replaced. R114 and R115 are more than double their intended values, which could definitely cause the screwy voltage readings in that area. I'd suggest replacing those resistors before we even look at the power supply voltage on point 4. And consider replacing R84 and R85 while you're at it.

One additional note -- since we're finding resistors with significant drift, you might want to check all the resistors.

Steve.


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Sun 20, 2021 6:57 pm 
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OK so Q16 is the confusing one because I believe they mis-labeled the E and C on that in the pictures.

R95 goes from Q16B to Q16E. That one is jumping all over the place and never settles on a resistance.

R97 is measuring 1.32kΩ, about 1K lower than it should.

Here is the power schematic.

Attachment:
Power.JPG
Power.JPG [ 271.06 KiB | Viewed 453 times ]


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Sun 20, 2021 7:07 pm 
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pioneer07 wrote:
Here is the power schematic.


It's too blurry to make out resistor part numbers and the ratings -- however it does appear to be a voltage divider circuit using resistors. I would suggest checking all of the resistors in the divider circuit -- it's all of the resistors past the rectifier and filter capacitor.

Regarding the potential error on the schematic for R95 -- have a look at R74 (the left channel). Is it in the same position? If so, then the schematic might have an error. If it's a Sams Photofact it wouldn't be the first time. ;-)

Steve.


Last edited by KW4H on Jun Sun 20, 2021 7:25 pm, edited 1 time in total.

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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Sun 20, 2021 7:21 pm 
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pioneer07 wrote:
OK so Q16 is the confusing one because I believe they mis-labeled the E and C on that in the pictures.

R95 goes from Q16B to Q16E. That one is jumping all over the place and never settles on a resistance.

R97 is measuring 1.32kΩ, about 1K lower than it should.


Hmm. It's not out of the realm of possibility that Q16 is having problems, but on the other hand the transistor might be reacting to your multimeter when reading R95 in-circuit. I wouldn't worry about the 1.32k reading on R97 for now -- it might be good. Take identical readings in the working channel -- if the multimeter reacts the same, then those resistors are probably fine. What I'd do at this point if I were you is to go ahead and jump over to troubleshooting the power supply -- the voltage at point 4 indicates it might be a source of trouble. Check the power supply resistors in-circuit and look for any that have significant drift. Address the potential power supply issue before continuing the march through the amplifier circuit.

Steve.


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Mon 21, 2021 2:44 am 
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Thanks so much for the help! I may go ahead and replace R115 just because it looks bad, and it would make me feel better to to finally hear some noise from this channel.

I gave the power circuit a closer look and yeah there are tons of issues. There's only 4-5 resistors that are measuring correctly. I zoomed in on the right side where all but 1 of the resistors are. The only one not shown is R117. That is supposed to be a 2MΩ resistor, and it's measuring 5.5MΩ! Hopefully you can read this version for the rest of them. I went ahead and measured voltages again, too:

Attachment:
Power resistor measurements.JPG
Power resistor measurements.JPG [ 230.82 KiB | Viewed 443 times ]



Looks like I'll be ordering a lot of resistors here soon.


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Mon 21, 2021 2:48 am 
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KW4H wrote:
pioneer07 wrote:
OK so Q16 is the confusing one because I believe they mis-labeled the E and C on that in the pictures.

R95 goes from Q16B to Q16E. That one is jumping all over the place and never settles on a resistance.

R97 is measuring 1.32kΩ, about 1K lower than it should.


Hmm. It's not out of the realm of possibility that Q16 is having problems, but on the other hand the transistor might be reacting to your multimeter when reading R95 in-circuit. I wouldn't worry about the 1.32k reading on R97 for now -- it might be good. Take identical readings in the working channel -- if the multimeter reacts the same, then those resistors are probably fine. What I'd do at this point if I were you is to go ahead and jump over to troubleshooting the power supply -- the voltage at point 4 indicates it might be a source of trouble. Check the power supply resistors in-circuit and look for any that have significant drift. Address the potential power supply issue before continuing the march through the amplifier circuit.

Steve.



Regarding this issue - R74 is the equivalent resistor in the other channel, and it's also measuring right around 1.27kΩ. So those two are measuring really similar so they're probably OK.


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Mon 21, 2021 2:52 am 
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One thing I am slightly concerned about is taking the schematic at face value. For the power circuit, it's probably OK. But other parts of the circuit, I'm a little nervous about. For example, one of the electrolytic caps I pulled out said 200uF on the side of it, but the schematic called for 100uF (I replaced it with a 220uF). I'm worried resistors could be the same way, but I'll need a refresher on deciphering these old color codes.


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Mon 21, 2021 3:17 am 
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pioneer07 wrote:
One thing I am slightly concerned about is taking the schematic at face value. For the power circuit, it's probably OK. But other parts of the circuit, I'm a little nervous about. For example, one of the electrolytic caps I pulled out said 200uF on the side of it, but the schematic called for 100uF (I replaced it with a 220uF). I'm worried resistors could be the same way, but I'll need a refresher on deciphering these old color codes.


You're getting close! Suggest staying focused on the power supply first -- replacing the old resistors that have drifted and bringing the power supply up to snuff should have a positive effect on resolving everything else. In the meantime -- on the schematic issues -- there may be slight differences between the unit you have and the schematic in your possession. Before proceeding further -- what markings are on the cabinet and/or chassis that might give us a lead to confirm or deny that the schematic you have is the correct one? I believe that Morse/Electrophonic was, back in the day, an entry-level system sold at retailers like K-Mart. There could have been a number of variations.

73, Steve


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Mon 21, 2021 3:31 am 
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The marking on the chassis says "74T-1 / 69" and then "02293"

I'm not sure where I got 74C from when I started this process, but this is looking like it is a 74T.

Also, what kind of power ratings should the resistors in the power circuit be? This manual has no info on the resistors, and all I've got to go by is what's on the schematic, which only has resistance values. I know those 0.5Ω resistors need to be rated fairly high, like maybe 2-3W but not sure about these others. Probably similar to that?


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Mon 21, 2021 3:45 am 
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pioneer07 wrote:
The marking on the chassis says "74T-1 / 69" and then "02293"

I'm not sure where I got 74C from when I started this process, but this is looking like it is a 74T.


The 74T obviously has a lot in common with the 74C chassis -- but there appear to be differences. Can you locate a schematic and/or a Photofact for the 74T?


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Mon 21, 2021 4:01 am 
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KW4H wrote:
pioneer07 wrote:
The marking on the chassis says "74T-1 / 69" and then "02293"

I'm not sure where I got 74C from when I started this process, but this is looking like it is a 74T.


The 74T obviously has a lot in common with the 74C chassis -- but there appear to be differences. Can you locate a schematic and/or a Photofact for the 74T?



The only one I see at the Sams web site is a 74TN, not sure if that would be a match for mine. And then there is the 74C.


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Mon 21, 2021 4:23 am 
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pioneer07 wrote:
The only one I see at the Sams web site is a 74TN, not sure if that would be a match for mine. And then there is the 74C.


Hmm -- well, there is no Photofact for just a 74T, so most likely the 74TN is correct. However -- don't take my word for it. Although there are differences, I think the 74C is close enough that you can proceed with repairs for now, unless you don't mind purchasing the Photofact.

Just realize that we're going to have to go with gut on a few things and use the schematic of the 74C as a guide, not as a definitive solution. Console systems and other entry-level systems like this were kind of "one-offs" back in the day -- not a whole lot were made, and a whole lot of what was manufactured eventually ended up in the dump, which is why Googling for the schematic brings up pretty much nothing -- few people have these and have a need to repair them. That doesn't mean that it's not worthy of repair, of course.

You need to circle back around to the power supply section, and the resistors that have drifted need to be replaced. The wattage of those resistors can be judged by their size -- however -- modern resistors are smaller for a given power rating. If those resistors were 1/2 watt carbon composition, you should replace them with possibly 1-watt versions today. Higher in rating is good. If you can post a photo of the power supply circuit board, showing the resistors, we can tell by the size relative to the other components.


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Thu 24, 2021 2:52 am 
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I did a few things in the power supply circuit but nothing made a difference.

- I replaced R108 which had drifted up a lot (measured 233Ω when it was in the circuit, supposed to be 100Ω). Replacing it did not make a difference in the voltages though.
- I lifted R107 and R119 to get a more accurate measurement on them. When I lifted them, they measured right in spec, so I put them back.
- I tried replacing R117. It measures 2.6MΩ, but the schematic says it's supposed to be 2.0MΩ. When I replaced it, that made all 3 of the top voltages measure 16.6V (they are supposed to be 16V, 14V, and 10.4V). So I am guessing that this is a difference in the schematic. I put the 2.6MΩ resistor back in for R117 and it brought the voltages back to where they were.
- I just double-checked that I got the main power capacitors correct. All match the schematic except for C3 and C5. The schematic calls for 100uF, but the caps I pulled out were 200uF for those two. I replaced them with 220uF caps.

So, all the other resistors are measuring pretty close to spec, except maybe R110. I'd have to lift that one to get an accurate measurement.
I do not believe I have R118 and C94 on my unit (the schematic has a * and says some may not have them).

These are the only things really left:
- Capacitor C93 on the far left by the 2.6MΩ resistor
- The un-named 100pF capacitor below R104
- The X1 diode
- The transformer itself


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Thu 24, 2021 2:59 am 
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Here is the power schematic with some updated resistor values:
Attachment:
Morse 06232021.JPG
Morse 06232021.JPG [ 278.62 KiB | Viewed 401 times ]


Here is a link to the Sams that has a schematic you can zoom in on and might be more readable.


I happen to have a diode that could replace X1 on hand. My Zenith uses the same replacement part as this one. I'm tempted to try it. Just as a sanity, check, this is how it would go, right? I'm 99% sure but I want to be 100%
Attachment:
X1 diode.JPG
X1 diode.JPG [ 67.73 KiB | Viewed 401 times ]




EDIT: I realized my error and I had that diode backwards in the photo. I did go ahead and replace it. The only change is that the 8.4V is now measuring 8.8V. Everything else is the same.


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Thu 24, 2021 4:25 pm 
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So I'm going to chalk up any difference between what my power circuit has and the schematic has due to the fact that I don't have the exact schematic for this model.

Since the only thing I see different between the two channels is the one voltage at Q20 is wrong, I'm going to replace Q20. I have the part on order. It will get here in a couple days.


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Thu 24, 2021 5:20 pm 
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pioneer07 wrote:
So I'm going to chalk up any difference between what my power circuit has and the schematic has due to the fact that I don't have the exact schematic for this model.

Since the only thing I see different between the two channels is the one voltage at Q20 is wrong, I'm going to replace Q20. I have the part on order. It will get here in a couple days.


Sounds familiar in a way. This reminds me of how I (sometimes, but not very often) ended up doing repairs on electronics back in 80s and 90s. I was in the business of repairing electronics, and you sometimes DO reach the point where your time is more valuable than a 50 cent resistor or a $1 transistor, especially if those parts are common and on the shelf. I would do my best to narrow things down to a specific stage and group of parts, and then replace the likely parts rather than get all cerebral about things and spend hours trying to prove whether the problem was Part A or Part B. Of course, this tactic is not always practical with antique electronics, where parts can be scarce -- and, of course, most of us are here because this is a hobby and not a business -- and the journey is part of the fun. Your tactic here certainly won't hurt the radio and I do agree that Q20 is most likely in the mix somewhere.


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 Post subject: Re: Morse/electrophonic 74C (1969)
PostPosted: Jun Thu 24, 2021 6:31 pm 
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It was painful but I did go ahead and drop the $22 to get the manual for 74TN. That one also does not match what I have exactly. The circuit boards in mine look more like the 74C. However, the top 4 voltages in the power circuit match pretty much exactly what I'm measuring. See photo.

The other voltages in the power circuit don't quite have an equivalent to mine. From a schematic standpoint, mine is closer to the 74C as far as I can tell, but the values of some components are a little different (such as the C3 and C5 capacitors previously mentioned). I think my power circuit is probably good.

Attachment:
74TN voltages.JPG
74TN voltages.JPG [ 364.73 KiB | Viewed 384 times ]


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