Sony TC-377 Erase head burned my finger

[beans]

Got this 377 as a parts unit S/N 24318 (early model) and got it all working, or so I thought. Made some recordings but noticed the old tape signal wasn't being erased completely. Then I noticed the erase head was getting hot enough to burn my finger. The problem is with the left channel only. I found an OEM erase head and put that in but that was of no help with the overheating, but the new head was necessary since the old one was trashed after running so hot. On the schematic it appears the head is connected to the oscillator transformer unless I'm reading it wrong and I think C505 is involved.

Anyone familiar with this?

 

[beans]

Bump

 

[safebet]

Typically a pair of transistors drive the bias transformer in a classic a-stable multi-vibrator configuration. Not sure how DC would end up on the other side of the transformer. The secondary should also connect to each of the record channels via a trap circuit.

Have you measured for DC flowing through the erase head?

 

[dankik]

Also, if you haven't already, clean slide switch S105-2 and check coil L501. There is a procedure for Dummy Coil Adjustment in the SM, but I'm not sure if it will help.

 

[beans]

Thanks guys. I'll look into that and Check the voltages at the EH again but I have to be careful not to burn up the EH... or desolder it.

 

[beans]

UPDATE: There is a rotary switch on the back side of the main control shaft and one of the wipers was all bent out of shape to where it was not making contact. So I fixed it up as best I could. I haven't mentioned that this rig had been traumatized, probably dropped, so when I got it there were 3 wooden blocks rattling around inside. Also I am re-capping everything. I have tested the caps that I've removed and the values on these 50-year-old caps are all over the place. The new ones are a random assortment of MUSE and Panasonic FM, and the rest are Nichicon UKL. Unfortunately I can't test it yet because I'm waiting on parts.

 

[gadget73]

I have a somewhat older TC-777 and found it also had a lot of off-value resistors. Mine were carbon comp though, hopefully yours is using carbon film which ought to be a lot more stable.

 

[beans]

I'm likely in the same boat with you. They're listed in the SM as "carbon". We'll see.

 

[gadget73]

well if they are dark brown and squared off you may be SOL. Hopefully they are the tan ones with the slightly curvy body. Mine is probably a decade older than the 377 though, its still rocking germanium transistors.

 

[teletor]

it will be wise to check the oscillator frequency. A lot wrong and it will go hot

 

[beans]

it will be wise to check the oscillator frequency. A lot wrong and it will go hot

I'd love to but I can't find instructions in the SM on how to check it. And I don't have an oscilloscope. Once I finish re-capping I'll test it again using my finger, both on the EH and on the board. I took out C505 and tested it, and it was right on the button so I put it back.

I don't think there's a problem with recording. I was able to do some recording before I knew about the EH overheating, but the signal already on the tape wasn't being erased completely.

 

[teletor]

I give you an advice. Its your choice to not follow it.
It is not possible to repair a cassette deck without correct test equipment , parts and experience. I will propose to deliver it to a professional one for repair . Hopefully he still will receive it after your own trying to repair and modify it.
Recapping by non professionals is one of the most frequent reason for unit to be sold as parts deck on ebay etc.

 

[dankik]

@beans, one thing that you can check is the REC/PLAY slide switch, S105 & S106. It is probably a long switch with multiple contacts that is connected to the record switch levers on the front of the deck. Here is the board they are on. With power off (unplugged), compare the resistances for both L & R channels to see if there is a difference. Also, clean the switch contacts with Deoxit and make sure both are sliding correctly. There are also some voltages on the layout, I assume that is what you should measure at those pads. It won't hurt to reflow the 15 solder joints to each switch connection.

If your issue is with the left channel only, I don't think it is the oscillator. If you record on a new tape, does the audio signal get recorded on both L & R tracks OK... is it only the erase function on the L channel that is not working?

If you are adventurous, you could unsolder and swap the RED and WHT wires and see if the erase problem moves to the other channel.

 

[beans]

Finished replacing all electrolytics and the mylar caps in the oscillator, but it still won't record in the left channel. No signal at all from LINE IN and MIC. Right channel seems fine. Playback sounds great after the re-cap.

As for the erase function, getting 168 VAC to the left. Could be bad transformer(?). I'm starting to think this thing was zapped by a power surge/lightning strike, That makes sense because I haven't been using a surge protector and we have had serious power glitches. Here's something I can afford:

 

[teletor]

have you checked the 23 Volt on pin 2 on the bias transformer. If too high the psu regulator not work

 

[dankik]

As for the erase function, getting 168 VAC to the left.

Is this correct, 168VAC? Where did you measure this?
I'm a little confused. First you said that the problem was that the L channel did not erase (and the erase head was hot). But now are you saying that you cannot also Record any signal on the L channel? When you say "no signal for LINE IN or MIC" where are you looking for the signal, only the VU meters?

Sounds like the problem is in the REC circuit only now. (see block diagram)

1. If, on REC, you cannot record both Line In AND MIC IN, then look for a problem in RECORD AMP 3 (Q103). See schematic below.
2. Check voltages around Q103.

 

[beans]

^^^ 168 VAC is way too high which explains the overheating. I have two of the machines, and the other one works ...sort of. On that one I get 38 VAC on the left channel and 70 VAC on the right. It erases the signal, for the most part, but it leaves some noise behind.

 

[beans]

As for the tranny, I get about 64VAC across the red pair and about 7.6 VAC across the blue.

 

[beans]

Thanks guys for all the help. I have suspected the rec amp, but this gives me more specifics. I failed to mentioned that I have already replaced the 4 transistors on the left with 2N4401.

To your first question, I am looking for the signal both at the meter and into the play amp and out the LINE OUT, and then through the left speaker.

 

[dankik]

Look for a signal coming out of the Q103 collector when in Record. If it is there, follow it down to the Source/Tape selector switch.