John Leary SP-600 mk IV
These are fairly hi-res. pix, I have even higher, and many closeups.
Received in Sept. 2012
It has a onsiderably different layout on topside from Leary's No. 28, although it appears to have all the same features.
It has 2 xtal calibrators, one 200kc, one 1000kc; a product detector, a modified audio section like Collins in the S-line: a frame grid tube as the 2nd mixer, to reduce noise, and more. Everything rewired, home done new front panel, etc. Said to be very sensitive.
I wish I had some documentation on it, lots of ckts are different from factory, and some things are hard to get to without some dis-assembly. But I should be used to that. It is very interesting to work on, I'll trace things out a bit and make sketches.
Will need some investigation into AVC system, it isn't providing enough negative voltage as received, but several bias adjustments are provided. When set as told, the RF gain still must be set at ~50% to avoid at least one stage overloading. I've done some changing of settings, have improved things quite a bit.
The 3955 IF coupling xfmrs were considerably off, and overall gain on high bands suffered.
Everything was completely rewired. All solder connections are full and well made.
I'm taking my time at getting into it, making notes, learning what's what on the way.
At first I was underwhelmed, but it's getting better. At least one stage is overloading greatly unless the RF gain is reduced, I need to play with the several bias adjustments. Have set them as they are labeled. The prev. owner said it had an AVC problem, and he has a small tape arrow where the RF gain should be set, at about 50%.
There's a "3rd IF" with tuned output, which is in place of the normal driver. It just feeds the Prod. Det., plus a cap off of the plate ckt to the IF Out jack, which gets loaded down even by the scope probe. I need to check the plate voltage on that stage, as it starts to flat-top at around 50v p-p. I notice that most plate voltages are low, 190 or so.
Still fighting a tendency to oscillate, particularly in the 3kc sel. posn. Am able to have the RF gain wide open now tho'. The 455kc IF's were all right on the xtal freq., only one slug was a turn or 2 off. It does sound a bit tight, I know many do stagger them a bit, sometimes I do.
Wire dress is good, but there are some places that are quite inaccessible.
I'm surprized, with his comments about mechanical stability, that he used trim washers under all the front panel screws, even the ones that go to the side panels, which need to be very tight to reduce twist. Can't get them tight with those washers. I'll put a flat one under them.
I've been poking at this rcvr off & on, an hr or so at a time, for 3 wks now. My main complaints were lousy AGC/AVC range, and then gurgly/gargly/gravely voice (and CW) signals. I haven't fixed it yet, but I think I'm close.
The gurgle's been my biggest concern lately. I've suspected leaky tubes, leaky sockets, lousy electrolytics, lousy audio section (the 75A-4 thing), etc, etc. 2 things finally sank into my thick head this week - it FM's on strong signals (or not very strong even), and the audio secn was not the problem. Plus, signals above 7.4mc, the double conversion bands, seemed OK, no gargle. I had taken the audio output of the prod. det. and fed another SP-600 thru a "phono" tap.
The only ckt that is used for sgl conv. and not dbl conv. is the 2nd IF gate, V-7. Plus, I remember that FM-ing can make signals gurgle/grumble, whatever you call it. I got looking at the gate ckt, it's close to the orig., with one maybe non-significant dif., the suppressor grid is tied to the cathode, and the cathode has a res. to gnd, 390 ohm IIRC. Also it, as most stages, has very low plate and screen voltages, this one ~175 plate, and ~125 screen. Looking at those voltages with the digi-mtr the reading wasn't stable, I did have an incoming signal on 40 or 20 mtrs while I looked. Looking at them with the scope, in AC mode, they really jumped around, so I put the meter back on them and varied the RF gain. Wow, 15-25 volt variation.
I suspect that is the main gurgle problem. This stage does not have regulated screen voltage (there are 2 VR tubes, one 110v, one 125v), but the IF stages all do (110v), and also they have lower plate voltages (~150 IIRC). This stage, the gate, may not be the only source of the problem, but I think it's pointing me to the cause. It's a pain not having a schematic, I need to ck on osc. stage voltages and just what is regulated, etc. I think the regulated voltages are also changing, so maybe the dropping resistors to the VR tubes aren't right. Tracing is tough, but at least he did use some different size and/or color wires in a pattern of sorts.
I have been playing with the 4 AVC adjustments, may be finding a reasonable combination, just by ear, but I'm still a bit worried that it doesn't have sufficient range, similar to the R-620/JX-28.
The Leary SP-600mk-IV is almost squared away now.
Hard to get the AVC evened out, there are at least 4 adjustments for it at different stages, I think it's pretty good now.
Finally resolved the gargling audio - Had a heater-cathode leakage in the 1st mixer. I had swapped tubes with no change, and I'm sure I did that stage, but after going all around, came back and swapped it out again and fixed it. The 150v reg. tube had too large a dropping resistor and the reg. voltage was from 135 to 120, made things really weird. I pulled the cover off the bottom of the turret and found he'd never changed the BBOD's in the modules that have them. You figure, he completely rewired everything else (not sure abt the RF deck, but expect so).
One thing remaining is an oscillation in the 2nd mixer, 6EJ7 stage. There is a gain adj. in the cathode, but it must be way down to get rid of the osc., which is at abt 4kc. My ears quit at abt 3500 so I can adj. the 455kc IF's so I don't hear it, but it's still there. Leary did bypass everything that Hammarlund did, and it's well bp'd there, tho' I did add an .01 right at the cathode, with no change. If I can beat that, it's ready to be put to use. Maybe it needs a "squeeg cutter" 10 ohm resistor at the grid of the 6EV7, I'll try that. The IF's all have it, as original.
It is very stable, after about 10 min. warmup I can park on 14.300 MM net and rarely make any change. The proper dropping resistor in the 150v VR tube made that difference I think, and stopped the FMing on strong signals.
I have found the oscillation problem! Can't tell you yet just why it is, as I don't know just what the internal wiring is in T2. But one terminal of that can should be grounded, and it wasn't, it was not connected to anything. I discovered it just by listening to the oscillation and probing around with a small screwdriver. It really took off when I hit that terminal, so I looked at the SP-600 physical wiring diagram and saw that it should be grounded. This one should have a wire to a close by ground lug. It does now and things sure are a lot quieter.
I think I'm done! Except -- this PM I just discovered it won't fit into a standard cabinet without some trimming. I was/am going to use it as the shop rcvr, but don't want to trim the cabinet. But I might, just needs a little nibbling on the front panel mounting flanges, invisible when installed. The braces added to the top of the side panels stick out a little too far at the front, so they hang up about 3 inches short of going in all the way.