Teac A-4070 reel to reel hot chassis (bad transformer?)

I bought this Teac A-4070 reel to reel a few months ago for almost nothing and have been trying to refurbish it just for fun (I was a CET in the 70s but am rusty at troubleshooting at times)
When I first noticed the hot chassis I moved wires around from the transformer and fixed a few dodgy connections and it went away, Now the hot chassis is back.

Of course the chassis is not grounded and has a non-polarized 2 prong power cord (1971).

From the chassis to the house earth (ground on the 3 prong socket) I get 89 to 92 VAC. From chassis to the 0V transformer tap I get 89 to 92VAC.

I replaced all of the electrolytic capacitors as they were off spec by 80% to over 100%, the hot chassis did not start after that work.

If you look at the attached schematic I put readings (In Red) from the transformer taps to the chassis.

I replaced the bridge rectifier even though it tested good because the voltage to chassis on that tap was so low compared to the others.

Now I pulled the transformer and tried some inductance readings on the secondary (with a cheap component tester) hoping that leakage would show up.
With the primary shorted:
40V tap to 0V tap = 0.39mH
50V tap to 0V tap = 0.56mH
60V tap to 0V tap = 0.73mH
80V tap to 0Vtap = 1.02mH
With the primary open I got 3.6mH on all taps to 0V tap.

What do you guys think?

Trans VAC 5-27-23.jpg
You can connect let"s say 12 or 24 V AC between the 0 V tap and the 240 V tap and the other voltages will be around one tenth of their nominal voltage.
I do not think that the transformer is the culprit.
I suspect that it was only the effect of stray inductance that you measured around 90 V.
Maybe you could connect an incandescent lamp between the chassis and the house ground.
If the lamp does not light, then this is the case.
The voltages are correct. I connected it to the 120vac line (I am in the USA) with the transformer out of the chassis and they were all correct.

What strikes me as odd is that the voltage from the 0v tap to chassis and the chassis to house ground are the same. Could this be due to stray inductance?
I hooked up the transformer the 120vac line again (0V and 120V taps) and tested the rest of the taps voltage to house ground.

The A and B taps were 184 and 209vac, the C and D taps were 153 and 159vac, to house ground. To the transformer chassis it was A and B= 130/109vac, C and D= 91/96vac.

Is this normal for an autoformer?
I can´t believe what I´m seeing, the guy who designed that tape recorder is CRAZY.
Forget inductance or any secondary effect, THAT TAPE RECORDER INTERNALS, INCLUDING CHASSIS, IS LIVE MAINS.

Absolutely unsafe and unusable under any circumstance.
What were they thinking?

Please post the whole schematic, there is the slight chance that direct mains feeds only the capstan motor and maybe some auxiliary circuit, but not the Audio section ..... which you do not show.

Maybe "0V" must NOT be connected to chassis, but some uninformed tinkerer did.

But in any case, we need the FULL schematic.

FWIW googling "Teac A-4070 hot chassis" brings many posts mentioning that, so it clearly is a thing, not only yours.

I suggest for now you unplug that suspect tape recorder and abstain from working on it until this mystery clears up.
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Here is the PDF of the service manual. Actually chassis on the main was common in the early 70s and before. This isn't the first refurb I have done on a device designed like this and I remember being lit up a time or two in electronics school in the mid 70s working on them.


  • hfe_teac_a-4070_service_interim_en.pdf
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At Tapeheads.net:
"The small pcb that holds the capstan motor caps had dodgy traces and bad connections. I removed it and wired point to point, I replaced the motor run caps too. Moving the wires going to the board dropped the chassis voltage to 28vac. Removing it dropped it to less than 100mv."