Akai X-200D reel to reel excessively high bias voltage

[michael_w]

Hi all,

Briefly, I picked up a "for parts" X-200D and decided to see if I could get it back in service. It was in decent mechanical shape but needed a fair bit of clean up. Still it has had its fair share of surprises. It had a weird 29.4kHz circuit oscillation in playback mode which pegged the VU meters (only at 15ips equalization!) but a small pf cap shorting the input to ground eliminated that. There was also a small step down audio transformer (for the headphone output) which had a broken primary coil which necessitated a cumbersome workaround. And the playback head really should be resurfaced...

The funny thing I can't reconcile is the measured bias voltage. According to the service manual it is supposed to be about 100V at 95kHz. With my DVM I measure an AC rms voltage of about 600V!!! I can't tell what the frequency is because my scope's test leads are only rated to 150V. It isn't a meter issue because my AC outlets sit at the expected 120V. Recording works well enough (except for the high frequency drop off due to head wear).

So, before I even consider adjusting the tunable inductor I thought that maybe someone could chime in and suggest why it should be so high.

Below is the reference page in the service manual.



Thanks in advance, M.

 

[dhnash]

According to the service manual it is supposed to be about 100V at 95kHz. With my DVM I measure an AC rms voltage of about 600V!!! I can't tell what the frequency is because my scope's test leads are only rated to 150V. It isn't a meter issue because my AC outlets sit at the expected 120V.

This measurement is supposed to be performed with a VTVM (vacuum tube voltmeter) as indicated in the diagram you posted. The DVM you are using may be unable to accurately measure AC voltages at frequencies of 95kHz while being perfectly capable of doing that for AC line voltages at 60Hz. BTW - There are 10 to 1 (10:1) divider probes available that would allow you to measure voltages higher than your scope's 150V limit.

 

[michael_w]

Thanks for the quick response. The mystery looks to have a simple solution, live and learn. The DVM I'm using is an older and reputable handheld meter (a Keithley) and not some cheap knock off and so I made an offhand assumption. I presume then that my scope's current probes are up to the task at hand because I don't have a VTVM on the shelf. But just to be safe I can borrow a high voltage probe from an electronics shop I have access to. Until just this week the building it is situated at was off limits because of the pandemic. Its been over a year since I've had access to it or the machine shop I typically rely on.

As for the worn head, if I go to the trouble of relapping it then I will have to locate an alignment tape. Another corollary of Murphy's Law, every task has a non empty set of prerequisite tasks ad infinitum.