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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 23rd Jun 2024, 11:21 am   #1
Linnovice
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Join Date: Jun 2014
Location: Chelmsford, Essex, UK
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Default PR99 MkII Output Problems

Good morning all.

I have now nearly completed the refurbishment of my PR99 MkII but have a couple of questions.

It is working fine and sounding extremely good.

Question 1. When setting up the repro from tape I notice a slight wobble to the signal on channel One. It's oscillating about .5dB. Only Channel One, channel two is a solid as a rock so I don't think its wow. When recording its still there but slightly more pronounced, about 1.5dB again, only on channel one. The input signal is solid on both channels. I'm thinking it may be a problem with the output amp. Would the consensus agree with this? This wobble is not audible, the sound is really great but on a meter/scope its a bit unsettling.


Question 2. I do have a couple of spare boards but one (and I can't remember where it came from) has some odd additions to the board. Not shown on the service manual diagrams. There appears to be a couple of capacitors strapped over IC3 and IC4 (see pictures). Would anyone have any idea what these are for? Or the resistor on the back of the board? There looks to be a bit of bodging going on but in my ignorance it may well be a logical upgrade. I have no idea.


Any guidance would be gratefully recieved, as always. Mike.
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Old 23rd Jun 2024, 11:46 am   #2
TIMTAPE
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Default Re: PR99 MkII Output Problems

Humans generally have trouble hearing a level difference of only 0.5db. Also it depends on the frequencies. The higher the frequency the more noticeable are tape dropouts. The slower the tape speed the more noticeable are tape dropouts.

I'd check the record and play heads are clean and in top shape (flat area in the centre not more than a couple of mm), the alignment and the tape is in excellent condition. The top edge of track 1 is easier to view in a good light and with an inspection mirror.

To test the tape itself reverse the tape and see if the problem swaps. To test the heads, swap tracks 1 and 2 leads at the back of the head and see if the problem swaps.
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Old 23rd Jun 2024, 9:30 pm   #3
jamesperrett
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Default Re: PR99 MkII Output Problems

I'd suspect something mechanical that is causing the head to tape contact to be less than optimal.
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Old 25th Jun 2024, 2:36 am   #4
TIMTAPE
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Default Re: PR99 MkII Output Problems

Quote:
Originally Posted by jamesperrett View Post
I'd suspect something mechanical that is causing the head to tape contact to be less than optimal.
Yes that same suspicion was behind my reply.
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Old 25th Jun 2024, 5:20 pm   #5
Linnovice
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Default Re: PR99 MkII Output Problems

I checked out the tape path this morning and can see nothing untoward happening. I've also taken another couple of photos of the heads, this time through an illuminated magnifying glass. I cleaned them previously with acetone. The wear pattern on the playback head is not dead straight but it is parallel. It doesn't look wildly worn. What do you guys think? Bear in mind that they are significantly magnified.
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Old 25th Jun 2024, 5:22 pm   #6
Linnovice
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Default Re: PR99 MkII Output Problems

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Old 26th Jun 2024, 5:30 am   #7
TIMTAPE
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Default Re: PR99 MkII Output Problems

The picture clarity isnt great but enough to see the heads are worn but fortunately were set up correctly so wear is even. No keystoning or uneven wear due to rotation on the vertical axis. Still, that amount of wear can weaken tape to gap contact pressure especially at the beginning of a full reel of tape.

In addition to the other tests mentioned earlier you could try recording a high frequency such as 7 kHz or whatever higher tone you can hear, and see if gently pressing the tape onto the central gap area with the bristles of a small artist's brush decreases the unsteadiness of playback. Or just try the tape towards the end of a reel where back tension, producing tape to head pressure, is stronger.
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