Rt. Channel Recording issue with newly acquired Pioneer RT-707

[Ricktptman]

I just got around to checking the recording capability after installing a new motor in a new to me RT-707.

Here's the symptom list:
All pots and switches Deoxited.
Pre-recorded tape playback is fine in both directions.
Alignment is fine in both directions. (Screws still locked in place from when it was new.)
I've cleaned the heads and they appear to be very clean and they show very little wear.
The stop spring is clean and properly positioned.

The preamp input and throughput are fine.

But when recording (with level and bias exercised throughout their travel-level left at maximum at both the input control and the record/oscillator adjustment) I get a low level but very distorted signal that DOES print on the tape. It sounds a little like it'd LIKE to "break on through to the other side". If I turn the level down the signal drops out or nearly so. On my very first 707 (a pawn shop machine that had heavy wear from 1991) I had to buy a new Record/Oscillator board (still available then) because both of the relays were bad/intermittent, but it made a similar noise, sometimes even when record was not engaged. It would spuriously print signal onto tapes I wanted to keep. (Those were tapes that came with the machine. It was well traveled.) (If the input levels were high enough unless I defeated the record enable switches. It's very distorted and much lower in level than the left channel which records perfectly.

I thought I was hard to stump, but this Rt. Channel thing has done it. Any assistance would be greatly appreciated.

 

[zebulon1]

Got scope?
You know you were going to get asked this question.
Switches/jacks cleaned and lubed? Had to ask this as well.

 

[Ricktptman]

Thanks, Zebulon. Yes, I have a scope. (It's a new one that I'm not really used to yet.) Yes, I've Deoxited it stem to stern, including the top controls. (All of them, some twice.) This one isn't going to be simple. You don't know how much I wish it could be. I do my best to try everything before I post an SOS here. I have a bad feeling it's either a Tranny or a Cap, but the innards in this machine looked deceptively "driven on Sunday"-like. The motor issue kept me from checking anything other than engaging the Rec circuit when I was picking it up from a CL ad. It's been one surprise after another, some of it just too weird to bother with relaying. But I will say I think I'm going to swear off working on anything in July and December. Not good months for me, restoration-wise, and I'm not particularly superstitious.

Here's what I've discovered thus far: I'm fairly certain it's not the heads. (PB's ultrafine, and I left the machine on and in record while brought my Head Demagnetizer to within about 6 inches of the record heads and both channels slowly increased (and printed to tape, which is interesting) by almost the exact same amount, both up and down. When played back, the distortion was present in the Right channel just like when I sent Music (with a FiiO X5 I keep on my bench), just at a much lower level than the unaffected right channel. (Once more, it records fine on the Left channel.) I don't think it's the PB board because pre-recorded tapes play back great. I think it's on the Record amp/Oscillator board somewhere. The board shows no trauma, no swollen caps, no burn marks or CSJ's, ring type or otherwise. Any help's appreciated...

 

[zebulon1]

I looked at the transistor list wondering about any problematic ones. A ton of the 945's in the oscillator which are not on the list.
I never worked on one but I would try swap things and isolate if easy.
The scope will help out immensely. Maybe a perfect time to warm up to the scope.
If you need someone to say "Dive in head first" I'll do it.

Maybe the guys in the tape forum can help?

 

[Ricktptman]

Thanks, Zebulon. I didn't think to post this there. But yes, I appreciate your advice and encouragement. I just need to "do it". I'll post something in the tape forum about it.

 

[Ricktptman]

Interesting night. A few solder burns later, I managed to get a very nice MONO signal out of both sides. At least part of the problem is the relay. Or relays. I lost the left channel after I cleaned the relays, and had to jumper the contact from underneath the board to get it back. While down there I began to notice a few weird things. One of those coils was actually loose. I tried to put it back, but I'm not sure I succeeded. I TRIED to jumper the relay on the Right side and no combination would get the Right Channel signal back. I did get the right side to come up for a fraction of a second or so and then collapse again. Annoying...

 

[Ricktptman]

I think it was two left channels and no right channels. So much for "poking around". I'm going to try to take measurements off of Q207. That (in the circuit description section of the SM) seems to be where the amplification on that board starts. I have current to both sides of the record head, so I don't think it's the oscillator trannys, but I've been wrong about this stuff before. Wish me luck...

 

[Ricktptman]

It's fixed. Had to make that L. Channel jumper permanent. Everything's aligned now, with the exception of the bias/level being just a bit low, but only at the 3.75ips speed on the Left Channel. I know that coil with the problem is the prime candidate, but has anybody else ever had a problem with the bias differing between SPEEDS? TIA...