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Thread: Pioneer RT-701 Adjustment Procedure Question

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    Member parentheticalfact's Avatar
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    Pioneer RT-701 Adjustment Procedure Question

    I have a Pioneer RT-701 on the bench. I've aligned and adjusted several reel to reel machines in the past but this one (and it's service manual) have left me with some questions.

    First issue is that the VU meters (when mechanically zero'd) read differently from one another while injecting a 1k .316v sine wave into the line input jacks with the inputs bridged. This tells me I need to make sure my line input and vu calibration is correct before proceeding.

    In the service manual there is a level diagram with some "test points" that reference the block diagram. It seems to state that .316v at the line input should show 1.99mv at point 8. Point 8 shows to be after VR903 and at the input of the FLAT AMP's first transistor base, labeled as Q205. This is a block diagram so it omits certain things that are present in the schematic.
    The schematic shows VR903 as a trim pot on the Servo Amplifier Assembly and does not appear to be used as any sort of input trim but rather relating to the motor speed control. The boards are not marked for their component placement so the service manual must be used to identify test points and component placement.

    If I'm not mistaken (and I certainly could be) it seems VR903 is NOT the input trim. Does anyone have any insight as to where to properly measure voltages to get the input side and VU's trimmed out before working on other aspects of the machine?

    The machine was modified by a radio station for stereo half track use with different heads and then nearly abused in it's improper storage for years so I cannot assume any trim pots are in their correct positions.

    I love the sound of the 707 and 701 machines when they work well, any help would certainly be appreciated!

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    Member parentheticalfact's Avatar
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    I hate to do this but I am not able to upload the service manual pdf for some reason. There is a link to the service manual here --- https://www.hifiengine.com/manual_li...r/rt-701.shtml

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    Supporting Member The Dude's Avatar
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    For those who don't want to register at hifiengine, here's a direct link to the schematic.

    https://freeservicemanuals.info/en/s...eer/rt-701.pdf

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    Lifetime Member Enzo's Avatar
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    Isolate the problem. You have the two inputs shorted together so they are using the same identical signal, right?

    The meters are driven by the headphone amps. Apply your test signal, and measure accurately the signal voltage at either the line out jacks or even at the L and R contacts of the phones jack. Are they the same, or is there a difference. That tells you if the amps have the imbalance. If the voltages are the same, then the meter circuits or the meters themselves are the issue.

    The signal to the meter is rectified and filtered to make a DC voltage that follows the signal level. Each has an adjustment.

    The record amp takes either mic or line in, the line in bypasses the mic pre. CLearly your VR903 was a typo and they meant VR203 - look at the schematic. That is the only adjustment I see before the tap off to S3 to the playback amp, which drives the phones and meters. And there are no trimmers in that.

    When reading schematics like this, often they use different series of numbers for parts. The playback amps are in the 100s, like Q101, R101, C101, etc. The record amps are in the 200s, like Q201, R201, etc. The bias oscillator and related circuits are in the 300s. Power supply 400s, etc. That should be a warning that a 900 series part number is suspicious.

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    Member parentheticalfact's Avatar
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    Thanks to The Dude for posting a non-register schematic and Enzo who is constantly to the rescue! Another issue I realize is that the voltage measurements are dbV, which admittedly I do not work with often.
    Taking voltage measurements I noticed an odd anomaly where if I turned the line in gain inputs all the way up I would read the same voltage accross the 100k series resistors after the RCA line inputs. When turning down the line input to halfway there would be more voltage drop across the left input resistor, which was the weaker channel. I re-capped the mic preamplifier card (which also has the line input circuitry as well) as I noticed some caps did not test well as far as leakage and this did nothing. Thinking harder I checked the resistance of the line input potentiometer and bingo - the dual 10k pot reads differently between both sides with the same position and reads higher on the hotter right channel. I removed the dual pot and made matched voltage dividers for fixed gain and the issue was straightened out in regards to the preamp voltage output per side. To remedy this without sourcing a replacement pot I added parallel resistors across the higher resistance side of the pot to help balance things out. Now voltage between both sides sits nearly identical.

    Enzo, you are very correct about the typo and warning of the 900 series part number. When first trying to rush my way through this (time is money until you've under-thought your way into a hole of ignorance) I said aloud "WHY ON EARTH DID THEY PUT THE INPUT CALIBRATION ON THE SERVO BOARD!!????". I was having someone shadow me to teach them a bit about what crosses the bench here day to day and felt like a bumbling idiot. I much prefer quiet time and solitude when working through a set of schematics like this.

    Next time it hits the bench I'll spend a little time adjusting bias and record/playback levels to further calibrate things. It's recording a little hot currently but sounds incredible.

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    Lifetime Member Enzo's Avatar
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    I almost mentioned turning the controls all the way up. Those controls are not precision parts, so you cannot assume that "half way up" is the same on both. NAtural variation in the parts can account for the level differences. That is why we mix by levels and not control settings.

    I had a 707 in the shop years ago, and I could get it all balanced for playback, and one side would record a little hotter, or I could balance up the records, and the playback would tilt a hair. (Meter level indications) I'd get it close, but the owner wanted it all exact. I finally had to ask him, "How many hundreds of dollars do you want to spend on this $400 tape deck? It was after all a consumer deck, not studio gear. Oh they are very nice decks, but still consumer goods.

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