Rx 5250 Woes

[XDJ659]

Hi all, newbie to boomboxery here.
I have two rx5250's.
#1 Rx has been in my hands for many years, tape belt broke and started to get a loud humming sound when I try to turn the radio on. Found out it goes away after some minutes (thinking bad cap somewhere).
I bought #2 Rx recently and turned out to be in excellent condition for it's age. I had bought this for parts as to fix #1 Rx. This one gets a distorting sound through the tape if I switch to stereo or ambience. This doesn't happen in line in.
The original problem with #2 Rx is that no sound comes from am/fm, zilch, nada.
I have the schematics for the model but I am no expert on electronics just some basic no how. I have decided to remodel #1 Rx , paint, new knobs, etc.
My question is this: has anyone just bypassed the internal amp and use a small 12v bt amp on these? Looking into the block diagram it seems that the line signal goes into pin #5 on the amps ic.
There are two ic's, left and right.
Would it make sense that I disconnect the speaker output of the ic's, solder a wire from pin #5 from left and right and use this line signal as an input to a stand alone amp? I believe all the buttons on the radio would still work as intended, any thoughts? Perhaps it's already been done and I just typed up a lot of mumbo jumbo?

 

[XDJ659]

Here they are, left is #1.

 

[Reli]

If the tape head got stuck in the up position when the belt broke, just go in and turn the big metal flywheel behind the tape deck until the head goes back down. This disengages a circuit intended for recording that might be messing up your sound somewhat.

 

[Superduper]

There’s probably a 70% chance that operational condition can be addressed with a simple deoxizing treatment of all switches and controls including the dreaded record bar. These Panasonics are very prone to these.

 

[XDJ659]

Thanks for the replies. I had read about the recording bars on these but neither units have that issue.
#1 Rx has a loud frequency pop that starts to speed up into a rumble until it goes away after a minute or so.
I had checked the tape head and it's where it's supposed to be.
#2 Rx is a higher frequency distortion with the cassette player in stereo or ambience only, maybe the tape head is disaligned?

 

[Superduper]

I’ll double down and repeat my suggestion. And AFTER a very rigorous and thorough cleaning, if the motorboating sound persists, then you’ll likely need to recap it.

 

[Tinman]

Agree, cleaning all switches and pots is a fairly easy process that most of us do with a "new" box.
After 40+ years, those things get pretty gunked up.

 

[XDJ659]

Ok, I didn't write everything as I thought it would be too lengthy. I had opened up both boxes and sprayed everything I could find. But then again, I had only crc 2-26 available which, I believe, is more of a lubricant than a cleaner. I'll redo them once I can find a decent cleaner here (Puerto Rico) and test them again. I'll chime back in once they are done and post the results.
If #1 doesn't work out right, I am still keen on bypassing the internal amp if possible and customize that one. Thanks for the advice.

 

[Superduper]

Well you can customize your box if you like, but if you don’t address the issues first, the new amplifier will perform exactly the same since it’s very unlikely the issues you speak of originates in the amplifier section. If it’s a more powerful amplifier, then it’ll behave exactly the same except louder.

 

[Tinman]

Just throwing this out there for your first box.
I recapped an alarm clock radio today.
It's from 1980 and had a fairly loud hum/buzz from the radio.
I tested most of the caps I removed.
There was a mix of Rubycon, Panasonic, Nippon/Chemi Con and a few Marcons.
The Rubycons and Nippons I tested were all reading within 10%.
A couple of the Panasonics read within 10 or 20% but a few were way out including the 1000uf in the pic.
I recapped both of my 7700s because I don't trust the vintage Panasonic caps.
Hopefully you get both of yours sorted out.

 

[XDJ659]

Well you can customize your box if you like, but if you don’t address the issues first, the new amplifier will perform exactly the same since it’s very unlikely the issues you speak of originates in the amplifier section. If it’s a more powerful amplifier, then it’ll behave exactly the same except louder.

Gotcha, I was thinking more in the lines that it could be an amp issue and not a line in issue. If the noise is generated before the amp than I would achieve nothing.

 

[XDJ659]

I just finished cleaning everything I could find in RX #1 after finding a CRC cleaner here. Sadly it made no difference. I really got into the record switch as suggested but no luck. Are there a number of specific caps for everything audio related or do you guys just replace every single cap in there?
I know that if a cap is bad the others will surely follow but is there a good place to start?

 

[XDJ659]

Just throwing this out there for your first box.
I recapped an alarm clock radio today.
It's from 1980 and had a fairly loud hum/buzz from the radio.
I tested most of the caps I removed.
There was a mix of Rubycon, Panasonic, Nippon/Chemi Con and a few Marcons.
The Rubycons and Nippons I tested were all reading within 10%.
A couple of the Panasonics read within 10 or 20% but a few were way out including the 1000uf in the pic.
I recapped both of my 7700s because I don't trust the vintage Panasonic caps.
Hopefully you get both of yours sorted out.

View attachment 59977

Yeah, I'm sure there are a couple of bad caps in there.

 

[Superduper]

I have an RX-5150 (same basic unit but lacking the stereo meters) and it had all kinds of gremlins. Not wanting to mess around, I cleaned it thoroughly and then recapped the entire box. After that, all of the gremlins disappeared and it performed like new again. I also have many of the RX-7700 units which are like their top of the line set and they likewise suffer terribly by oxidation in their controls. I can tell you that Panasonic radios back in that era were very prone to oxidation in the controls which will cause motorboating due to poor connections, drop outs, roaring static whenever a control was touched. And the record bar (playblack/record switch) is notoriously difficult to clean. It is shrouded by the tin shell and the opening slit on top and if you just spray there, you'll just waste cleaner since the contacts are actually nestled under that nylon block (like a mushroom umbrella). The only way to clean those is to spray into one of the ends with the bar vertical, then the solution will drip down via gravity and that is the point where you will vigorously actuate the switch like 50, 100 times. Blot off the solvent that ran off and wait for a bit. Try the radio. If there is improvement, any at all, it basically confirms that the switch is dirty and likely needs further cleaning or disassembly.

 

[XDJ659]

Looks like you know your stuff, I will definitely try this tomorrow on Rx #1.
Rx#2 started working without problem when switched to stereo today, go figure. I will open this one also and clean it . The only thing wrong with this one is that the radio does nothing in am or fm, no static noise or anything.

 

[Superduper]

99% it started working again because flipping the switches enough times has mechanically scrubbed off enough oxidation in the contacts to pass current again. This is exactly what I was speaking to. Used regularly, oxidation doesn’t get a chance to accumulate enough to block current but when they’ve been sitting for years or decades, that’s something that almost surely occurs and needs addressing before anything else.

 

[XDJ659]

Ok, started from scratch for rx#1.
Followed the advice on the record switch and gave it a good cleaning. Put the rx on its side, sprayed it with cleaner, made sure cleaner came out the other side and must have moved the switch hundreds of times. While I was at it, I resprayed every pot switch and moved them all around also.
Noticed that the power led was on even in the off/tape position, disconnected the tape player and light is now off (will deal with that later on).
Results: Same noise after the cleaning but, I found out that if I move the volume switch I can get it to eliminate the noise and can here the radio. I left the volume alone and switched to line in and no distortion from the speakers. I am guessing that the volume potentiometer is the culprit in this box? If so, is there a good source where I can order a new one?

If it is actually the pot only, I would fix the cassette assembly and try to restore this one to its former glory.

 

[XDJ659]

Ok, performed a micro surgery on the volume pot today. Was able to dissect it and clean it. Got the same results.
Next I wanted to confirm that the distortion was somewhere before the amp so I connected the radios' line out to my mixer and amp and the radio sounds perfect, no weird noises or distortion. So, following the manuals' block diagram, the only other parts or components going into the amp ic (after the line out rca) are the volume/bass/treble controls.
I am by far no expert but it would seem possible that the amp section is where the problem lies or the 3 pots just before it.
Btw, I do get the distortion thru the headphones also.

 

[Superduper]

You are convoluting everything, hard to follow along anymore. Initially, you said that RX#1 had a humming sound that went away with time.
Then you said RX#2 had distortion with tape when using the ambiance mode. Then you spoke about wanting to bypass the amp, but it wasn't articulated whether this is for #1 or #2, presuming #2.

Then on post #17, you said you are working on #1 (humming noise?) which you thought might be volume control related. Then on #18, you spoke about getting distortion out of the headphone socket. Distortion was mentioned for #2 but not for #1. I can't follow along.

About the block diagram, you should know that it is a simplistic flow chart and not necessarily indicative of the actual signal flow. For example, if you look at the schematic diagram, there's quite a bit of circuitry between the line out junction and the power amp, which is just an IC. Additionally, the ambience IC isn't even indicated in the block diagram but it's there, notably between line-out and the amp chips.

Sounds like you really want to condemn the amp section. Which side? There isn't really an amp "section", as the output amplifier circuitry is all encased within the amplifier IC module. And again, there's 2 of them. Both sides bad? Exact same issue?

Also the noise you referenced... you did not specify if this is on both channels or on one channel only. And is it a humming that goes away, or is it distortion or is it motorboating?

There are generic fixes of the most common issues. I think we touched on some of those here. After that, you'll have to do some signal tracing. If you want to confirm or exclude the amplifier chip, you can inject an audio signal source directly into the amplifier input. Make sure that it is a source that can be attenuated, otherwise the amp will blow out full tilt. But you have 2 parallel channels and amplifiers and it's extremely unlikely that both channels/amps will have suffered the exact same issue so you should be looking at a part of the circuit that is common to both channels (unless it's only happening on one side -- you didn't clarify). Just a note: the ambience IC is common to both channels. Also an oscilloscope probing along the audio signal path while injecting a known sinewave into the line-in may be a good way to identify where the distortion starts if you compare the input waveform to the waveform along the way. If you don't have the ability to do those, then you can touch up all of the solder joints to ensure cold solder joints aren't the issue. Then there is recapping. Do them all since you probably aren't able to diagnose the exact culprit. Furthermore, if you buy a cheap tester, best practice is to remove the capacitor to test. But honestly, why even bother buying the cheap tester? If you remove a capacitor from the pcb, they are so cheap you might as well replace it instead of stuffing a 45 year old cap back onto the board. Besides, a cap can test within the 20% capacitance but have high ESR or leakage, etc.

 

[XDJ659]

Ok, just to be clear, I am only working on #1. Post #18 was an update or continuation of #17. I should have been more clear, sorry about that.
Like I said, I am no electronics expert, I am just trying to pinpoint the area where the problem might be.
In my train of thought, if I have no distortion coming from line out (#1), what is left are the vol/bass/treble pots and amp circuitry.
I mentioned that I got distortion from the headphones because I see that it is also from the amp output.