[EdHolland]

I have read quite a bit about the JJ tubes and will avoid them.

I agree about the time constant/preservation of amp character. Always wondered why this is not maintained in favor of a larger cap. An audiophile notion to extend bass response perhaps?

I really appreciate the part numbers and other details. Not that I can't do it, but nice to save the leg work.

Another big thank you

Mike, the 1k resistors are already in circuit, I think as grid stoppers for the feed from the inverter stage.

[Ron Ramirez]

Agree about JJ tubes...avoid...

Regarding the Russian output tubes, I have a new quad of Tung-Sol (Russian made) 7591A tubes. I had originally bought them for my 500-B but I think I will use them in my TA-800 instead. Since neither receiver is working (well, the 500-B sort of works but I don't want to press my luck before restoring it properly), I can't comment yet on how good, bad, or indifferent the Russian Tung-Sols are.

I do know that Soviet mil surplus 6П14П-EB tubes are great replacements for 7189 tubes. I think they sound just fine in my TA-600.

[morzh]

I had Russian Tung-Sols recommended to me by name by Jim McShane and I trust his judgement.
So I bought them from him (he sells other name lines, also Russians from the same factory).
They are in my Eico monoblocks.
They sound just fine.

[EdHolland]

Ordered parts to build an IBAM board today. Progress in a couple of weekends, as there won't be a chance this week.

Also, I've been thinking about FM alignment. No scope or proper FM generator here, but I do have an accurate signal generator. Here is my question - in adjusting the discriminator, can I check linearity by zeroing at 10.7 MHz, and then look at the DC voltages generated for equal shifts in frequency above and below this center point, out to the +/-75 kHz max?

Just a thought, as I have some odd issues with distortion and weird artifacts in the stereo reception - Especially on busy music with a lot of tonal range. With limited test equipment, and no opportunity to find and get this to a pro, and an overwhelming desire to over-think the process in order to have more fun, I want to explore the options . Whatever the outcome, I can always get back to where I am now. Meanwhile, I'm learning more about multplex (And the Fisher MPX unit) and FM receivers in general.

Comments welcome!

Ed

[EdHolland]

OK, no worries. what I was trying to say is that the original 0.047 uF coupling caps form a high pass filter. The AC output signal from each inverter passes through the cap, and drives not only the grid of its output tube, but also works in to the resistive bias network which is eventually decoupled to ground in the power supply by the caps there. That means there's an inherent bass roll-off in the signal reaching the grid. More coupling capacitance could, in theory, lower the corner frequency of that roll-off.

...And of course, among the hi-fi tinkerers, more is better, even if some of them don't know what they are doing.

I do hope I'm not coming across as difficult or contrary, it isn't my intention at all. Above all, I appreciate the advice and discussion.

Lastly, I am happy to report that before the receiver was ever plugged in, all electrolytics were replaced - top to bottom through PSU, output stage, and FM circuits. They were not in good shape at all.

Cheers and good night,

Ed

[EdHolland]

Returning to thoughts on FM alignment, here is something that is making me scratch my head:

Instructions for finalising the IF alignment ahead of the stereo decoder are confusing... The alignment instructions http://www.fisherconsoles.com/non%20cons...9%20sm.pdf are as follows...

• After aligning the IF passband, step one of aligning the discriminator calls for measurement at TP3 (total discriminator voltage) and adjusting top and bottom slugs for maximum voltage, with a 10.7 MHz unmodulated signal applied
  
• Second step in this calls for a DC measurement which is very poorly defined. Positive lead goes "To MPX output". Negative lead goes to "The junction of 47 k resistors connected between TSP3 and ground. Well, TSP3 doesn't exist. There is a TP3, used above, and a TSP103 in the Multiplex module (Right channel MPX detection circuit). It is hard to see how the latter would be the correct point.
  

Perhaps by "MPX output" they meant the discriminator output, TP4, which would have MPX (or mono) signal prior to DC decouping?  Perhaps the resistors are to be added at TP3, also in the discriminator circuit and the mention of the non-existent TSP3 is a mistake. I understand the intent of the adjustment - DC output of the discriminator should be zero for an unmodulated carrier at the centre of the IF - and that is the only effect the requested adjustment could have. It is instructions for the way that this is measured which are confusing.

Presumably the 47k resistors are to be tacked in by the tech doing the alignment - there is nothing on the schematic of that nature.

If anyone followed along my ramblings - thanks

[Ron Ramirez]

Ed

I looked at a 500-C service manual (S/N 30001 - 49999, not knowing the S/N of your unit) and I found the instructions as confusing as you did.

So I looked at the procedure for the 500-B. The procedure appears to be the same, but the text is worded in a more understandable manner:

Hot lead of DC VTVM to Test Point 4. Ground lead of DC VTVM to junction of two series connected resistors (47K), wired between Test Point 3 and ground (emphasis mine).

Adjust: Z5 top

Indication: Zero indication on zero center dial.

So it appears that you add two 47K resistors in series between Test Point 3 and ground, just long enough to make that adjustment, then remove the 47K resistors.

[EdHolland]

Thanks Ron!

That makes much more sense. I'll give this a try as soon as I can get back to the unit. I'm away this weekend, so no Fisher work... but I do get to enjoy our Zenith console instead

Cheers

Ed

[EdHolland]

Meanwhile, 10 KOhm pots arrived over the weekend, so I can begin construction of the IBAM circuitry. I did note the alternative Bias-balance adjustment, said to be a little easier to tweak, and wondered about implementing it. It was claimed that at least one design eliminated the risk of failure of bias supply, should one of the pots lose wiper contact. A couple of things put me off. First, I could not find a schematic, second, it was noted that the bias voltage range was limited, and not necessarily sufficient for all replacement 7591 valves.

Pondering a little more, it seems that with the addition of one extra resistor, one might be able to configure a fail-safe IBAM, but I want to sketch something out and run the numbers to be sure. I'll write up here if it looks like there is any progress on that.

Cheers

Ed

[EdHolland]

Ron - that was a great tip. I looked at the rest of the instructions for the 500B, found at fisherconsoles.com. The tuner is pretty much the same circuit as the C. This helped me understand something else that I had not read correctly - most of the transformers, not only Z5, have top and bottom slugs. I confirmed this with a quick check yesterday evening.

Since I had only been adjusting the tops of the other units, it is quite possible that the IF is still not aligned properly. I'll go through it all again this weekend and see if FM reception improves any further.

Cheers

Ed

[EdHolland]

I did get to the electronics bench today, and took another look at IF alignment up to the discriminator. It turned out to be an adventure. First off, I warmed up the generator and set it to 10.7 MHz, using a portable shortwave set to check frequency was accurate to a few hundred Hz. I made a coil of 22 gauge wire, probably 5-8 turns, and large enough to slip over the RF amplifier (6HA5/EC900). This made for a good injection that made it's way through the IF train, and level adjustable to suit any requirements.

Before making adjustments to the receiver, I thought to sweep the generator (by hand) across the IF passband. Even the AM on my old Heathkit has quite a warble, making for a useable FM tone at 400 Hz. It was evident that the Fisher's response was quite unsymmetrical as the generator changed either side of best audible reception, and measuring discriminator voltage at the IF test point. On to a repeat performance of alignment

This time, I attended to both top and bottom slug adjustment of the IF cans - Why I had not realised they nearly all have two adjustments is another matter for discussion... This went well, peaking up the voltage at the discriminator until I got to Z3. All of a sudden, the adjustment felt odd, and the meter showed the test point voltage had dropped suddenly. More gentle tweaks, but there was still an odd feeling to adjustment and more drop in signal. I feared something awful, and there was nothing for it but to unsolder Z3 and investigate. Wiring and orientations were noted and marked, and the transformer pulled from its can.

The construction is of a small coil, perhaps 3/16" diameter, with a large core has a central rod, flat top with a slot, and a wide skirt with a fine exterior thread. This rides in a two sided plastic frame. What I suspect, is that the top slug had jumped it's fine threads, to become cross threaded. I removed it carefully, cleaned the threads and applied just a little graphite using a pencil point. All back together, both cores adjusted freely, and Z3 was returned to the chassis.

Alignment resumed, now, of course way out of whack, but Z3 was brought quickly in to resonance, top and bottom, in a couple of iterations. On through to Z5 and a sound check was in order.

Much better already. Cleaner mono and stereo, with much less distortion and intermod artifacts.. Tomorrow, I'll finish the discriminator and detector alignments, now that I understand how to proceed (thank you again Ron).

Cheers

Ed

[Ron Ramirez]

Good job...and good save on Z3.

[EdHolland]

Thanks Ron. Happy to report that the alignment instructions made sense. With the added resistors, and meter connected between their junction and TP4, it was easy to obtain a zero voltage as required.

That is about as far as I got today.

[EdHolland]

Starting to think about the IBAM mod today. I have suitable prototype "perf-board" and the first order of business is working out a scheme to mount it. I'll probably fabricate a bracket that will support the board vertically in the space between the power supply and output circuit. There's room here, and it is in-line with the wiring of the existing bias circuit. I also sorted through the components needed, so am ready to go once I have the board planned out.

[EdHolland]

I forgot how frustrating working with perforated board can be. Things end up bigger than hoped-for as components cover holes that could be used. Fortunately I caught this at the design stage.

Also, fixturing is tricky, and this slowed progress quite a bit. Rather than drill new holes, I only want to make mods on this unit that are 100% reversible. The board needs to be secure, otherwise something nasty might happen, but it needs more thought. I mocked up a number of possible support bracket designs using card, but they will either be hard to install, or too flimsy for comfort. A few ideas are brewing, but time ran out yesterday.

Cheers

Ed