Discussion in 'Fisher' started by audmod01, Nov 12, 2018.
Serial number of my 800c (for what it’s worth is: 22481C.
Out of curiosity I just checked my 800-C. It has that hole plugged. The unit came from a console I picked up off the street years ago.
I checked mine y'day. The Executive's 800c hole is plugged, while the stand alone is open.
Well someone is plugging those holes, at least in the factory consoles.
I steered off of Fisher console work due to their size. Otherwise, I would have needed another outbuilding to house them all.
So I will qualify my earlier statement as to consoles.
I’d still not plug it. The more ventilation back there, the better.
With the addition of the EFB modification an 800-C output circuitry will run considerably cooler. This was true on my 500-C I modified. It is a great modification as it reduces internal heat dissipation while simultaneously improving power output level and lowering distortion. I may also add a cooling fan right behind the output tubes. We used to recommend the use of a Rotron fan behind the output tubes of 400, 500-B or C and 800-B or C when I worked for Berkman's Hi-Fi in Austin, TX. It is easy to see how significant heat is trapped inside when the chassis is installed in one of the OEM type cabinets that was available at the time.
It appears that the hole plug was applied in some cases. That would explain why we see some on the internet that have the plug and it appears just as old as the chassis itself.
The factory cabinets and consoles are fine, provided the room ambients are not obscene (ie, don’t place a console on top of a register). The downsized cabinet used at the end ran a little warmer, but still ok with enough rear airflow space.
It’s the homebrew consoles and in-wall installations where the factory ventilation recommendations needed to be observed. And sometimes weren’t. In-wall installs were very popular with these, especially in ‘modern’ construction for that time. I saw a couple that were buried in the walls of small enclosed closets, and it’s a miracle the house didn't burn down. And Fisher expressly recommended forced-air cooling for all vertical installs.
These don’t run that all that hot in stock form on an absolute scale. Some hot running Dynas made these look like igloos. Bumping down the fixed bias to preserve the outputs, as has been de rigeur for 40 years, cools them considerably. Just observe common sense airflow clearances and you will be a-ok. Don’t jam one already in a factory cabinet in an enclosed rack or behind a door, or start stacking books all around them. And if you are not spinning vinyl, pull the phono stage tubes. . . . and if you already have open chassis holes smaller than a fingertip, leave them open.
Just a question: If one was only using the aux input, and not phono or AM/FM reception, are there any other tubes in the receiver that could be pulled without causing any adverse effect or potential damage to other components of the receiver?
Leave the tuner tubes in. The supplies are designed for a load, and if you excessively de-tube it, the operating voltages, which are already slightly high due to today's higher mains voltages, could drift too high. Besides, these receivers do have nice tuner sections. The C series AM section is not as nice as that in the B, but it is still a nice tuner.
Just pull the phono stage tubes (V18 & 19 on an 800C if I recall right) if you do not intend to spin vinyl or play other low-level sources. This will reduce the heat output of the instrument, save two nice 12AX7s from burning away unnecessarily, and relieve the load on the bias supply (which is used to power their heaters).
Leave everything else in.
Always thought it wasn't good to run these without signal tubes in the phono section, am I wrong?
Won't hurt it a bit.
Hi there! Wondering why the Orange Drops should not be used with an unpolarized cord?
Line capacitors should only be XY rated as those will fail open. XY did not exist when these units were built.
Ditto Fred's comments about the AC Line capacitors.
I received my AES order yesterday which included some 2.7K ohm 2W resistors to replace R120 and a new silicon bridge rectifier for the negative bias/preamp filament circuitry. The bridge is rated at 400VPIV and 3.0 Amps so it is more than adequate for the task. It has a center mounting hole to permit securing to the chassis with machine screw and nut. Here is a picture of the parts:
I hope to get these installed today and get the area cleaned up. I will work on installing IBAM controls for the output tubes before proceeding to EFB installation. I want to check overall performance before doing more changes.
Correct on the suppressor caps. Anything running mains to ground must use a class Y. Line to line can get by with a class X.
Especially if using the original line cord, which is UNPOLARIZED, you must use a class Y capacitor in there. The originals are wax/paper types. They, and most other capacitor types, tend to fail shorted. Very few have adequate AC endurance in any event. So when they fail, there is a 50/50 chance (depending on the orientation of the plug in the outlet) you will have hot mains voltage on the chassis 24/7 (and a 100% chance whenever the instrument is turned on). Because this is before the power switch, the chassis will be hot whenever the instrument is plugged in if you pluged it in unluckily. Not good, especially for the elderly or young children. At a minimum, it will definitely get your attention when you touch the faceplate.
Same rough scenario for the suppressor off the doubler, although it it is not strictly a line-to-mains connection, but an ac to ground one off the switched secondary. All the originals are DONE. I cannot overstress this. Get them out of there. The guitarheads call them ‘deathcaps’ for a reason.
Using a polarized cord helps a little, but is not a proper solution. What I will do with the OE line sets is put a little black marker dot next to the hot leg blade on the inside face, and remember to align the blades correctly to the outlet (hot should be the shorter slot). That keeps this suppressor on the neutral side of the power switch, so it is not exposed to full mains 24/7.
The best solution is a grounded cord set. I will always install them on equipment that will be around rug rats or someone with a heart condition. But many purists, me included, like to stick with the original cord sets.
If you ever install a polarized or grounded line set on these, be sure to orientate the suppressor on the neutral leg side of the switch. The cap will last much longer.
When you replace those 2, opt for a metallized film type. Metallized film class Y caps will always fail OPEN, which is what you want. There are too many reports of ceramic class Ys fusing closed on failure (the ceramics sometimes literally melt down, like a MOV) for me to trust them. A 250 or 300 vac rating is fine on the mains.
The ONLY reason orange drops were being used in that photo example was because a grounded line set was being installed, and it was what was on my parts inventory for that job. The AC rating was ok, but they were not certiffed for application under either CE or UL rules. It was a temporary install to get through some other tests. We later replaced them with proper Ys.
I posted a very long article on this about 15-20 years ago on AA. All the commercial resto kits at that time were packing plain jane dc rated caps for these locations, and I was concerned that someone was going to get hurt. It may still be in the archives there, and contains all the then applicable standards and references.
This is safety related stuff, so it demands a stronger caveat.
Years ago when I worked for RCA Service Co., many of RCA Color & B&W TV sets used some TRW line caps that had a red-brown outer case while some had caps that were in a white ceramic sleeve (Cornell Dublier). In the TV sets these were often directly above a wooden cabinet with no metal between them. Lightning strikes involving the AC Line would explode the Line caps in the TV set. The TRW caps external covering would actually burn in some cases and resulted in fires that could destroy people's homes. AC on-off switches were another possible fire hazard if the switch contacts were embedded in a black plastic material. To make matters even worse, there was a dab of grease inside the switch. Needless to say, if the contacts became pitted and any arcing took place another source of a fire existed. RCA had us going to all known customer's homes replacing the on-off switch which could be removed from the rear of the volume control. Those were on push-pull type on-off switches. Fortunately Fisher Radio Corp. used AC switch contacts embedded in a different material that was far less likely to burn and also covered the connecting wires at the switch with a metal cap. This is an item that needs to be put back on when changing the on-off switch so that fire safety is maintained - very important! Be sure to put the bottom cover back on the chassis before installing in a wooden cabinet!
Back to the negative bias/preamp tube filament supply, some early serial number sequences used a 5.6K ohm 1/2W resistor for R119 and a 15K ohm 1/2W resistor for R122. This set came with R119 as an 8.2K ohm 1/2W resistor and R122 as a 27K ohm 1/2W resistor. The kit of electrolytics from Hayseed Hamfest included 1000uF@35VDC radial capacitors to replace C95, so I am in the process of installing them. Due to the location I chose to mount the bridge rectifier (closer to R127 the 15 ohm 5W resistor) I had to extend one wire coming from the secondary winding that feeds AC to the bridge rectifier. I used a short piece of orange stranded wire to maintain the color code of the factory transformer wiring. Some clear heat shrink went over that lead and then a larger clear heat shrink went over both wires after making the extension. With that done the wires easily reach the bridge rectifier without any strain on the wires. I am mounting the two 1000uF radial electrolytics close by and will provide some pictures when I am done.
I finished replacing C95 and the Bridge Rectifier SR1 and also replaced R120 the 2.7K ohm 1W resistor with a 2.7K ohm 2W metal film resistor. This cleared out considerable room under the power transformer where I plan to install a perf-board with most of the EFB components. I decided to stop, eat lunch then rest in my recliner. Mr. Leo, our cat, is desperate for some lap time and a nap, so we may both have a nap.
The two snubber capacitors were replaced during the process, so I will not have to visit that situation again. I used two .01uF metalized polypropylene capacitors rated at 300VAC type Y safety rating.
Today I reached the point of checking for B+, negative supply and negative bias voltage at each output tube signal grid socket pin 6. First I discovered that the original 3.2A slo-blo fuse was open. This must have happened when I ran my first power-on test looking for B+. I did not have another 3.2A slo-blo fuse, so I installed a quick blow 3A fuse for tests. The output tubes were not installed. With power applied I saw B+ rise to +500VDC until the tubes in other circuits of the receiver began to draw current. The B+ settled at about +453VDC. The negative voltage developed for the preamp filaments was -26.7VDC. Then I measured the feed on the other side of R119, which is an 8.2K 1/2 watt resistor in this unit. I saw no voltage. Some quick checks showed that the white wire carrying the negative voltage toward the 330K ohm 1/2 watt grid isolation resistors of the output tubes was shorted to ground. I managed to do this while installing the output transformers. I replaced the wire after loosening the offending nut under the chassis, then secured the transformer again making sure I did not short the wire out again.
More tests looking at pin 6 of V10, V11, V13 and V14 showed the following:
V10-6 = -16.8V
V11-6 = +156V (C17 coupling capacitor is shorted!)
V13-6 = -16.8V
V14-6 = -16.8V
Therefore, the coupling capacitors to each output tube signal grid will be replaced before any output tube is installed. In this set, R119 is an 8.2K value and R122 is a 27K ohm 1/2W resistor that checks at 26.7K ohms. I would rather have a greater negative voltage to work from than -16.8VDC at the grids of the output tubes, so I may reduce the value of R119 closer to say 4.7K ohms and install IBAM controls and associated parts to get the negative bias voltage adjustable to a higher negative voltage swing than this before installing output tubes. The coupling capacitors will be increased in value and the grid isolation resistors will be changed from 330K ohms to 220K ohms.
I need to get some 3.2A slo-blo fuses on hand, so it is time to order parts again.
Joe. When you install the IBAM Board, R119 and R122 become Redundant, and are removed. Same thing with Dave's IBBA board. You should have close to -22V to -24V at the Bias caps. If you run Tung-Sol ReIssues, you'll need that -24v. Bump the coupling caps to .068uf and the resistors to 220K.. This will give you the closest R/C timing compared to the originals. 3.0A S.B. are fine. Hard to find 3.2A S.B.'s.
After searching diligently I realized that I had used up all the bread-board material that I had, so I have ordered two more from Mouser which is not far away from my location. So, in the meantime, I have to deal with two repair projects - replacing our garage door opener and replacing a jet pump that provides water pressure to our house from our water storage tank. Those are necessary things that have to be done, so they take precedence.
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