[Phlogiston]

I might play with re-forming, just for the learning as I've never tried it. As for the amp, the new caps are paid for and very nice quality, no doubt they will see good service.

Don't waste your time. The results are usually very short term. Even then ESR is likely to go up. The problem is usually being the electrolyte has become un-evenly distributed and this causes the oxide layer to also be uneven IF you can get it to reform.

We used to turn them upside down before reforming - actually makes sense - also a waste of time.

After you have a "bad" cap removed, don't invest the time putting it back.

[EdHolland]

Russ - I won't place any original electrolytic caps back in this receiver. I sprung for the Hayseed kit and am glad I took this route - extracting the old cans is fiddly enough that the added effort of restuffing them on top of it all is not a welcome thought. If there were one or two, I might have considered it, but with four to deal with, plus figuring out how to squeeze new caps in, it was an easy choice.

The new parts are configured as mechanically identical replacements, with the added bonus that the temp and voltage ratings are improved over the original caps.

I wondered about reforming, just out of curiosity, and am prepared to do the experiment with absolutely no intention of re-using the components. At least one of the cans I replaced has obviously bad sections, even using the (low voltage) capacitance tester.

Cheers

Ed

[Phlogiston]

Here is an example of the caps that I have found best suited to restuff those cans from the 1950s. I am not sure how high they go (1000uf?). They are good quality and not too expensive.

https://www.mouser.com/ProductDetail/Nic...%2F71GQ%3D

That one is only 10mm in diameter. UPZ and UCS are the first 3 digits of the part no. Be sure and check the ripple current (mArms) since these can vary from 180ma to nearly an amp.

P.S. again - -I looked and did not find any of these larger than 200uf at voltages over 400V.

[EdHolland]

Changed out the last can for its Hayseed replacement. Thought it would be easy, with just one section, simple wiring. However, it was the 200 uF unit with the floating can. It simply did not want to part company with the insulated phenolic mounting plate, and took lots of persuasion to flatten, twist and push the four twist tabs before even a little movement was achieved. All the while, I was mindful that the phenolic could easily be wrecked if too much force was applied. The old twist tabs have cuts in them to create extra lips for more holding power, and it is hard to get these flat again with the clearance required to pass back out of the mounting holes. After about 1/2 hour of persuasion, it finally came free, although the phenolic chipped slighty - fortunately an area hidden by the can. The new cap was installed, and I replaced the HT rectifier diodes. I should be ready for a test tomorrow, see what we have, and get ready for the next steps.

Cheers

Ed

[EdHolland]

Something forgotten in my last post, was that the inner workings of the Hayseed cans were revealed to me yesterday. The insulating cardboard sleeve of the 200 uF unit detached from the metal body as I withdrew it from the box. The can has an open top, and inside, I could see the tops of two smaller electrolytic capacitors, sitting in some type of brittle cream colored potting compound.

Since the tag end of the can appeared to have the same construction as the other cans, I assume that all the kits are made in the same manner, essentially a professional stuffing job. Not that there is anything wrong with this, assuming that the parts inside each can have the appropriate electrical parameters.

[EdHolland]

Life!

Last checks of wiring and I found some speakers to make sure there was a load on the output. I turned on quickly and checked the HT and bias supplies before the valves warmed up. All looked as expected - a bit high before the heaters warmed up. Since the chassis was upside-down and resting on the power transformer, there was a little bit of buzz from within. Power on again and left on. All the valves were lit up, and with the selector set to the Aux input with a phono lead connected ready for the finger test, I advanced the volume a little and was pleased to hear some hiss and hum from the speakers.

Switching to FM (no antenna) brought the hoped-for white noise, but not unexpectedly, there were no stations. Placing a test lead on the antenna input fixed that, and stations came in, weakly, across the dial. I'll try a better ribbon antenna soon. The transformer buzz seemed to change slightly as I switched between FM and tape inputs, but was also very sensitive to how the unit was positioned on the wooden blocks used to support it.

Bias on the output valves measured -17.5v, and the all seemed to be behaving properly, no signs of gross overheating. I doubt they are matched as 3 are branded RCA and just one, presumably original, as Fisher.

More evaluation, and some pictures to follow soon, but that is step one reached with success. A nice little Easter Egg

[EdHolland]

I poked around a little more, and found the folded dipole that I've been keeping for no apparent reason. FM comes in pretty well across the dial in mono, but I am sure would benefit from alignment.

Selecting Stereo, or FM automatic indicates that there is some issue here. On "Stereo" the relay clicks (switching the MPX module I think) but the signal is cut out. On "FM Auto", careful tuning on strong signals will cause the relay to activate.. and the sound to cut out. It seems to me that there is an issue somewhere in the MPX circuitry, but that it might be detecting that a stereo signal is present - enough to switch the signal path.

More fun detective work

EDIT: Progress!

Had a few minutes to look things over. I checked, and very carefully cleaned the relay contacts, using a piece of clean card, on to which a little De-Oxit was applied. Proper connections re-established. Also checked the bulb in the stereo indicator - and it was open circuit. I exchanged it temporarily for the bulb behind the signal meter. A new set is in order all round, as the tuning dial lamps are not working either.

On a re-test, I found that V101 in the multiplex section was not heating. I jiggled it, and then got one stereo channel - Good, not a dead tube. Pulled all the small valves and cleaned the pins. Success! Working stereo. In fixed "stereo" mode, it seems to demultiplex on all channels, but tends to sound a bit scratchy, hissy and distorted, which could be my dodgy basement signal or alignment. The "Auto Stereo" will click in on some stations, but even on strong signals, seems a bit reluctant and is very picky about tuning.

I need to think about alignment, as I don't have much gear for this. First thing is to read the procedure thoroughly.

[EdHolland]

Step 1 alignment went well. Peaked up the 10.7 MHz if and the tracking of the front end. That brought a nice improvement in sensitivity and put stations where they belong on the dial. Also it improved the auto switching to stereo mode. A small portable receiver at the same location shows the signals are not very good in the test location, so I will try running in a better spot.

Next, I am starting to think over the "IBAM" grid bias adjustment and resistor/ cap changes needed on the output stage.

Fun progress!

[EdHolland]

More progress

Lamps for the signal meter and FM stereo indicator came in, as did dial illuminator "Festoon" bulbs. The latter are modern LED replacements for 41 mm, 6.3V bulbs, but seem to do a nice job. Pics soon, as this thread has become too wordy.

I also ordered a couple of 6CW4 Nuvistors from the auction site. If I swap the two original devices that came with the receiver (V2 and V18) there is no reception, and after a bit of extra warm-up, the FM "hash" is silenced, and the S meter pegs at S5. Switched back, it seems to work well. I got lucky with the gamble and one of the replacement Nuvistors is a goog replacement if needed. I do wonder what fault/failure in the bad 6CW4 would mean it works in one location, and not the other?

Cheers

Ed

[EdHolland]

At first, I did have contact issues. Wiggling the Nuvistors showed that up. Cleaning their pins very carefully, and the sockets helped a lot. However, the pattern I see now when swapping the original 6CW4 pair between sockets, or substituting for replacements seems to be consistent. I'll take another look though, per your comments.

Today has seen little activity, save for cleaning the front panel and polishing the brass caps on the controls.

Cheers,

Ed

[EdHolland]

Replacement brass caps to replace those lost from a couple of the controlas arrived in the post. Cosmetic work should be concluded soon for a Photo shoot

10 Ohm 1% tolerance resistors are also here, which can be fitted ahead of making any changes to the bias circuit. It will be interesting to see how much current each output valve draws.

Then, on to configuring the IBAM circuitry. I am debating how much range is needed in bias. Ergo, whether to use a multi-turn pot for the whole span of the 22 v supply, or use a pot in combination with a fixed resistor to limit the range of adjustment. This could also be a useful safety feature, as it would prevent (or limit) bias settings that could cause over-current conditions.

Any thoughts?

Ed

[Ron Ramirez]

Hmmm. I hadn't considered multi-trim trimpots or adding a fixed resistor in combination with a pot, although the added fixed resistor does sound like a good idea.

I used single turn trimpots when I constructed the IBAM for my TA-600, and after I worked out a bug or two it seemed to work quite well.

[EdHolland]

Thanks Ron.

I suppose the obvious thing to do is to start out the setup with pots at maximum voltage - least current in the valves - and advance slowly until the 30 mA operating point is reached.

I'll sketch this out once I've thought about it a bit more.

Ed

[EdHolland]

Thank you gentlemen for posting the circuit details, and persevering through everything else to get them into this thread

Next, an apology for not keeping up with the thread when folks were trying to offer help and advice

Is there a preferred value specified for the potentiometers? I can certainly calculate what is needed, based on the target output and range of voltage adjustment needed.

Today, I took the first step in implementing bias control, and fitted a 10 ohm, 1% tolerance resistor in the cathode circuit of each 7591 valve. Also added was a twist of wire on each solder tag to make it easier to hold a meter probe for measurements. I'll order parts for the rest of the bias modification this week. However, it was still possible to operate the receiver, and measure the voltages to determine the corresponding quiescent currents in the output stage when running with the old valves.

I have V8: 35 mA, V9: 35 mA, V10: 24 mA, -V11: 42mA, give or take a few minutes warm-up..

Given that B+ measured at the anodes is about 438 V (430 V specified on the schematic) the valves are working a bit hard, except for V10 which must be quite tired. It is perhaps not a coincidence that this low current tube appears to be an original Fisher branded part, whereas the remaining three are RCA branded. Switching V10 and V11 proved that the low current followed the Fisher valve.

Grid bias voltage at this point was measured at approx -17.6 V relative to chassis - it seems the factory circuit is working as intended, but could use some help, and the independent adjustments will be added, along with lower value (220K) resistors and 0.1 uF coupling caps. This will enable me to save up for, and fit a new set of valves.

Having fun with this!

Ed

[morzh]

Any reason 200K resistor is in series with 1k one?