Distorted playback on reel to reel - what could it be?

cjacek

cjacek

Analogue Enthusiast
Hey guys,

I'd love to get some opinions from you on the most plausible cause of something I've found with an old Roberts 450A (same as Akai 4000D) 1/4 track deck from the early 70's... I've had it packed for a long time but upon unpacking, I first found that it wouldn't power up. After several tries of operating the power switch, the VU meters started to slowly come back to life and then the rest of the machine. That was odd I thought. Never happened before. I let it warm up and then played back some factory pre-recorded tape with some music on it. It was fine at first but then when it got louder, there was terrible distortion. (Note that this recorder doesn't have output level controls). I then did a test on brand new tape by recording and monitoring some music. The source and monitoring off tape sounded fine, even when pushed to the red on the VU meters. I then stopped recording, rewound and pressed play to hear again what I recorded. It sounded distorted at mid to higher levels. The lower volume parts sounded fine (low on the VU) but when when the music started to get louder (higher on the VU) it sounded all distorted. Odd, especially that it sounded fine when monitoring off tape while recording. I check the sound from the RCA output and also headphones out. Same thing. What could this be?

Thanks.
 
cjacek said:
Hey guys,

I'd love to get some opinions from you on the most plausible cause of something I've found with an old Roberts 450A (same as Akai 4000D) 1/4 track deck from the early 70's... I've had it packed for a long time but upon unpacking, I first found that it wouldn't power up. After several tries of operating the power switch, the VU meters started to slowly come back to life and then the rest of the machine. That was odd I thought. Never happened before. I let it warm up and then played back some factory pre-recorded tape with some music on it. It was fine at first but then when it got louder, there was terrible distortion. (Note that this recorder doesn't have output level controls). I then did a test on brand new tape by recording and monitoring some music. The source and monitoring off tape sounded fine, even when pushed to the red on the VU meters. I then stopped recording, rewound and pressed play to hear again what I recorded. It sounded distorted at mid to higher levels. The lower volume parts sounded fine (low on the VU) but when when the music started to get louder (higher on the VU) it sounded all distorted. Odd, especially that it sounded fine when monitoring off tape while recording. I check the sound from the RCA output and also headphones out. Same thing. What could this be?

Thanks.
Click to expand...

Not sure what is going on there but the earlier trouble, the meters slowly getting working, is almost certainly the electrolytic capacitors slowly re-forming.

Some of these caps could also be leaking corrosive fluid so if you intend to keep and use the machine get it to a repair tech. There is also a strong possibility that a PSU cap could go bang!

N.B. This could be expensive!

Dave.
 
Amplification problem:...

I'm venturing a wild guess, that just as the caps may be worn out in the power section, similarly worn out caps in the audio section may be playing havoc with the proper biasing of the amp tubes (or transistors). Likewise, any variability or inadequacy in power supply voltages or current may cause the same problem, uncontrollable variability & distortion in the audio channels.

:spank::eek:;)
 
Thank you very much. I doubt I'll put any money or time into it. It'll probably go into recycling. I'm in the process of downsizing so this helps my decision. Thanks again.
 
Well I'll be damned... I've never seen this happen before. The unit "fixed itself" it seems. Actually, not an accurate statement. Here's what happened: When I was testing the unit (as before), you recall I said it lacked output control levels. I've also hooked it up (via RCA cables) to an audio receiver that I use for all my music playing. I hooked it up just like I do any component. I then, as you recall, listened through the receiver, via the speakers, then the headphone jack on the receiver and finally through the headphone jack of the reel to reel. I was getting distortion in all of these situations. After all these tests I removed the RCA cable connection between the receiver and reel to reel, turned everything off and planned on giving the recorder away. The other day, just for the heck of it, I listened back to the reel to reel again using its headphone jack, as I did before. It sounded fine, no distortion. I wasn't sure what was going on. I then connected it again to the receiver via RCA and the problem started up again. I unplugged the RCA and listened again. No distortion. I was really scratching my head. Why would an RCA cable, going "RCA OUT" to RCA INPUT of receiver be causing distortion when I was listening via headphones on the actual reel to reel? That didn't make sense. I then thought, "doesn't the receiver control signal?". It seemed to be distorting but why would the reel to reel be distorting too (via its headphone jack)? How could going "RCA OUT" be influencing the reel to reel too? I then was tempted to go from the reel to reel to the RCA INPUT of a cassette deck with level control adjustment.... and then went from cassette deck OUT and into IN of the receiver. The signal was very hot on the cassette deck. I had to set it at 2 or 3 (out of 10). I usually keep it at 6 or 7 using. No distortion in any case. Has anyone ever seen a scenario that going RCA OUT from a component to a receiver would create distortion of the component itself (via directly listening through its headphone jack?).
 
I suspect bad caps' again.
The RCA out blocking capacitors are probably leaky or a complete short (might be the old, dreaded "blue Tants"!) The input to the receiver probably has a DC path to earth, likely 10 or 20 kOhms and this is enough to shift the working point of the RCA output devices.
Easy enough to prove with a $20 DMM!

Dave.
 
Found "a" schematic. C211 shows the idea.

Dave.
 

Attachments

  • Akai 4000d op circ.png
    Akai 4000d op circ.png
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BLLX! Can't be that if the schematic is right!
The headphone matching traff would only be a few 100Ohms and would cause distortion if the cap were short.

You will just have to get systematically diagnostically with it!

Dave.
 
Thanks for your reply, much appreciated but maybe either I misunderstood what you are saying or I have not clearly explained what happened. The reel to reel works fine now when first connected to a cassette deck with an input signal level adjust knob (reel to reel RCA OUTS to cassette deck RCA INS) and then the cassette deck RCA OUTS to stereo receiver RCA INS. It is only when the reel to reel is connected to a stereo receiver first (via RCA) that the playback distorts (on the receiver and also on the reel to reel headphone jack). The reel to reel doesn't have any output signal level adjustment. Why would it be fine in the above configuration and not another?
 
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