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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Nov Thu 02, 2017 7:50 am 
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Location: Stephenson County IL. 61032
Sorry guys for disappearing. I needed some time to cool off from this. I ended up putting the receiver away(actually, just moved to my worktable in the basement), and have worked on and fixed a few other "little" things in the mean time. One of them being my meter... I guess the problems it had were just symptoms to a blown fuse :oops: I thought I killed it, and even purchased a (same model) replacement. So, now I have 2 of them digital multimeter's, along with my old analog mm.

The receiver is back on my "normal" workspace and i've already started with some trouble shooting. Too bad I didn't come back to this thread first. I would have tested what radiotechnicion posted. No more working on the unit past midnight :lol: ...and its after 1:30am now so :roll:
"Tape up the test prods, so only the tips are exposed"... That is a GREAT idea. I'm going to do that from now on. I'll probably just leave the tape on them.

I checked what my little stock had, and I found I need to order them little adjustment pots if I replace them. The ones I have are of the same exact type, and they look like they were used...

There are no fuses to my knowledge, except for the main fuse in the back of the receiver.

"how were you measuring the bias?" I was measuring it across those resistors.

Well, here's where I'm at guy's: I pulled the main output transistors and re-ohm'd them and compared to what I had written previously in my notebook. They were all about the same. None of them dead shorted. For S&G's I swapped them from one side to the other and tested the receiver. No change.
Since I remembered someone talking about the driver transistors and possible issues, I decided to ohm them out too.
Well, I found one that leads into the left channel's circuitry that has a dead short. I'm figuring that is/may be the main culprit. Like said above, I still need to check other component's. but, Is it likely that the dead channel is out because of that transistor?

BTW, that transistor is a 2SA546A has an E in a circle after that part number and is also labeled MATSUSHITA, with another E underneath that.


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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Nov Thu 02, 2017 7:05 pm 
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If you replace just one driver transistor unless you find an exact replacement you will need to replace all the driver transistors.


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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Nov Mon 13, 2017 9:49 pm 
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Well, I would like it best if I could find an original to replace with. But, I noticed that the one that blew, in accordance with the schematic is not original. So, id like to find a same model replacement as what blew, or 2 of the original transistors that were installed in factory.

Well, I finally got to testing those requested parts. My results are positive. 8)
R521(100ohm) is at 105ohm,
Q507 I could not find... couldn't even match the transistor. Schematic shows a BF3 in place, but none to be found in actual circuit.
R647(.5ohm) tested at .7 (Same as the rest of them)
Q603(DF2)-allready tested as good. Output transistor
D503(15446)-tested good
Q505(25L4588) tested the exact same as the one on the other channel, so I figure:Good

So far, it seems other parts have all survived. Should I put a wanted add up for my needed transistor, or would that just be a waste of time? I don't know of any other units this transistor could be scarfed from if someone else happens to have some junk receivers sitting around.

I did find a source for "new" Matsushita 2SA546A Transistor's.
http://www.ebay.co.uk/itm/2SA546A-Trans ... 25e0c6a8f3

hmm, fake or fraud :roll:


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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Nov Tue 14, 2017 12:30 am 
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Location: Sayreville, NJ 08872
ZackN920 wrote:
Well, I would like it best if I could find an original to replace with. But, I noticed that the one that blew, in accordance with the schematic is not original. So, id like to find a same model replacement as what blew, or 2 of the original transistors that were installed in factory.

Well, I finally got to testing those requested parts. My results are positive. 8)
R521(100ohm) is at 105ohm,
Q507 I could not find... couldn't even match the transistor. Schematic shows a BF3 in place, but none to be found in actual circuit.
R647(.5ohm) tested at .7 (Same as the rest of them)
Q603(DF2)-allready tested as good. Output transistor
D503(15446)-tested good
Q505(25L4588) tested the exact same as the one on the other channel, so I figure:Good

So far, it seems other parts have all survived. Should I put a wanted add up for my needed transistor, or would that just be a waste of time? I don't know of any other units this transistor could be scarfed from if someone else happens to have some junk receivers sitting around.

I did find a source for "new" Matsushita 2SA546A Transistor's.
http://www.ebay.co.uk/itm/2SA546A-Trans ... 25e0c6a8f3

hmm, fake or fraud :roll:

I hope you're using a LR-1500T schematic. There are differences between the LR-1500T and 1500TA schematics and board layouts.
The LR-1500T doesn't use any 2SA546A transistors.
The BF3 and DF3 are the driver transistors. You'll never find them listed on the market with these identifiers. The BF3 (PNP) (Q508, Q516) is 2SA485; the DF3 (NPN) (Q507, Q515) is 2SC485.
Here's the picture again:
Image

In your text above, Q505 is 2SC458B.

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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Nov Tue 14, 2017 3:28 am 
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Location: Stephenson County IL. 61032
Why do they have to make things difficult!!!

Anyway's, yes I am using a schematic for the 1500T, not the TA version.

This receiver may not originally use 2SA546A transistors, but that's what's in here. It still has the 2SC485 in the other half.

Here's the picture in better clarity so everyone can read and decipher what those top 2 drivers read on them.
Image

:|


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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Nov Sat 18, 2017 6:56 am 
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So...

No thoughts or anything on what's actually in my receiver?... :roll:


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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Nov Sat 18, 2017 6:46 pm 
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Joined: Nov Wed 14, 2007 11:37 pm
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Looks like "Matsushita"


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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Nov Mon 20, 2017 5:58 am 
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"oh, hardy-har-har" :lol:




Hey, what could be causing that very light hiss in my right channel after the recap? Since I need to purchase some transistors, if I could find out what's causing it, I can see about finding a replacement for that too.


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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Nov Mon 20, 2017 12:56 pm 
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Location: Warner Robins, GA
The hiss would be one or several transistors.

If this has a tape monitor switch turn it on and see if the hiss goes away.

If it does then you know it's before the tape monitor.

If it does not then you know it is after the tape monitor.


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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Jan Sat 06, 2018 1:32 am 
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Did you ever get yours working? I'm working on one now, that according to the owner "had no output". I blew the dust out of it and turned the volume control all the way down,which supposedly resets the overload protection circuit, and it now works. The multiplexer was not producing stereo so I replaced the coupling caps at the two 19kc transformers and one at the 38kc transformer. That fixed it. I notice this receiver has a "pop" at each speaker when its turned on . According to the owners manual this is normal. But it doesn't say how loud the pop is. It seems fairly loud and powerful. It also pops when turned off. I also have a 1000ta model that doesnt pop at all and doesn't mention that it should in the manual either. What would cause such a "built in" pop?


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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Jan Sat 06, 2018 4:32 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Boston, MA USA
This is an older design using a unipolar power supply that requires the use of large (2000uF in this case) output coupling capacitors. There will be a loud pop as the capacitors charge on power-up, and another one as they discharge on power down. Nothing to be done.

-David


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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Jan Sat 06, 2018 5:20 am 
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Quote:
The LR-1500T doesn't use any 2SA546A transistors.



The 1500T I'm working on has the exact same transistors in the exact same positions as the OP's. Two 2SA546A and two 2SC485.


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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Jan Sat 06, 2018 8:57 am 
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metzman wrote:
Quote:
The LR-1500T doesn't use any 2SA546A transistors.

The 1500T I'm working on has the exact same transistors in the exact same positions as the OP's. Two 2SA546A and two 2SC485.


Lafayette did issue a service bulletin for the LR-1500T indicating that 2SA546A's could be used as a sub for the 2SA485's.

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Pete, WA2CWA - "A cluttered desk is a sign of genius"
http://www.classicradiomanuals.com


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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Jan Mon 08, 2018 4:52 pm 
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metzman wrote:
Did you ever get yours working? I'm working on one now, that according to the owner "had no output". I blew the dust out of it and turned the volume control all the way down,which supposedly resets the overload protection circuit, and it now works. The multiplexer was not producing stereo so I replaced the coupling caps at the two 19kc transformers and one at the 38kc transformer. That fixed it. I notice this receiver has a "pop" at each speaker when its turned on . According to the owners manual this is normal. But it doesn't say how loud the pop is. It seems fairly loud and powerful. It also pops when turned off. I also have a 1000ta model that doesnt pop at all and doesn't mention that it should in the manual either. What would cause such a "built in" pop?


The pop can be taken care of.

Will require a DPDT relay with built in timer circuit capable of operating off 120Vac. House it in an external box connected between the amp and speakers.

The NC contacts connect to two 100 ohm resistors and the NO contacts connect to the speakers. The delay is set for 1-5 seconds. The relay unit is plugged into the outlet on the receiver if it has one.

Upon turn on the relay is delayed by 5 seconds which allows the caps to charge through the 100 ohm resistors. Upon turn off the caps discharge through the 100 ohm resistors.


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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Jan Tue 23, 2018 8:46 pm 
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The preamp circuit in the 1500T is a tricky circuit to work on. As you switch inputs you are also switching in and out different feedback networks to match the input. The switch wafers have to be absolutely clean in order to work properly. I used a Chemtronics product called Tuno-Power. It actually reacted with tarnish and dissolved it away. Be careful with Deoxit products. They use solvents that can damage some components. Use sparingly and always test on a small area first.The other problem was the germanium transistors would go noisy. I have replaced them with silicon types like the 2N3904 and 2N3906 depending on polarity. The lR1500TA had all silicon devices. The amplifier had a 70 volt supply that was capacitive coupled. When you turned the unit on you would hear this tremendous thud as 35 volts was thrown across the speakers.

A couple of years back I was ripping out the whole amplifier section and installing these Yuansing amplifier boards from China. It requires rewinding the power transformer to create a center tap in the 50 volt winding so the output can swing negative. The boards have a speaker protection relay built in and are a pretty good match gain wise. This is extreme solution when the amp is blown and the parts are NLA. You loose the muting function since that was implemented on the original amplifier board but other than that you can't tell the difference. And of course no more thud. People who paid me to do this pointed out the price in 1969 for a 1500T was $299 so to them it was worth it.


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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Jan Wed 24, 2018 6:34 pm 
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Location: Warner Robins, GA
That thud can be easily taken care of with a relay and delay circuit.

The relay is DPDT and its NC contacts are connected to a 100 ohm resistor for each channel. The delay circuit energizes the relay after 2-5 seconds which disconnects the resistor and connects the speaker jacks.


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 Post subject: Re: Lafayette LR-1500T repair/restoration
PostPosted: Jan Fri 26, 2018 11:28 pm 
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There are speaker protection boards on Ebay with all the time delay electronics built in. There is already a bleed down resistor so the 100 ohm is not necessary. The amp may not react well to switching the impedances abruptly. Best to let the amp stabilize and then make the connection to the speakers.


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