I am going to do exactly that tomorrow in the tuner. I want to see how close the voltages there are as well. A friend of mine, badrestorer on ARF who lives about 35 miles north of me, suggested that I clean the volume, treble and bass controls again. He said he's seen radios that sounded awful because of dirty controls. I have sprayed the controls once, but I'm going to do it again just to be on the safe side.

BTW, the 11 NOS tubes I ordered for the set should be here Monday. Ordered them from The Tube Center in Orlando for $48, which was $18 less than what AES charges for the same tubes. The tubes in this set are almost 60 years old, and this set is worth installing brand new tubes in it. The new tubes should outlast me!!!!

Thanks for the warning. I'll keep that in mind if I ever come to your shop so it won't hurt my virgin ears!!!

Hey Virge…I mean Larry,

Now that you know that the power amp is not going to self destruct, no smells, excessive heat or glowing plates, you need to eliminate the power amplifier before you troubleshoot the tuner. It seems odd that multiple problems would arise at the same time. Does your solid state stereo receiver have bridging jacks that connect the pre-amp out to the power amp in? If it does you can use it as a signal generator / signal tracer. You can remove one of the bridging jacks and run the pre-amp output to the RCA input jack on the power amp chassis of the HF-1. The controls and tuner on the stereo receiver would all work normally driving the power amp in the HF-1. If the sound is clean and powerful you know that the power amp is not at fault and can move on.

The tape jack on the HF-1 tuner is a fixed line level output when in AM or FM. The signal comes from the plate of the 6AV6 AF amp stage, before the volume and tone control stage. You can run a cable from the tape jack on the HF-1 to the aux input on your solid state receiver and determine if the AM-FM tuner signal is distorted at that point.

You can then isolate and check the control amplifier stage in the tuner by connecting the cable to the tape output jack on your solid state receiver to the tape input on the back of the HF-1 tuner chassis. Set the selector switch to tape playback on the HF-1. In this case the incoming signal from the solid state receiver is constant and the volume and tone controls on the HF-1 would work normally. This would tell you something about how well the 6C4 AF amp stage on the tuner chassis is functioning.

That .22uf capacitor that broke off the selector switch goes to a loudness tap on the volume control. I don’t have a clear schematic but it looks like it may be used only in the phono position. It would affect frequency response.

Keep in mind that a good tube is a good tube, whether it has been stored on a shelf or in a tube socket. Don’t expect miracles from a “new” set of tubes. You could be buying a whole new set of problems.

Doug, there are no bridging jacks on the back of the stereo receiver. This receiver is a mid 90s Pioneer and it doesn't have them. How about hooking up the Magnecord reel to reel straight to the input jack on the Mark I amplifier and playing a tape through it that way? The Magnecord has a preamp in it.

I spent all Sunday afternoon and evening working on the tuner. I did a couple of voltage checks on two of the tubes and didn't like what I saw, so I decided to check every single resistor in the tuner, compared to the parts list or schematic, with an ohm meter. That shorted 6AU6 tube did quite a number on all the parts around it. A 56K ohm 1-watt resistor was burned nearly up internally, and it ohm'd out at 6.6K ohms instead of 56K ohms. I pushed gently on the middle of that resistor and it broke right into. In addition, two resistors that go across that tube were way, way out of tolerance as well. I found one other out of tolerance resistor on the 6AV6 tube and replaced that, and tonight I found 4 more badly out of tolerance resistors in front of the rotating switch, some of which are in the phono circuitry. I'll replace all 4 of those today. I want you to know, there must be over 50 resistors in this tuner. It takes quite a while to check them all. I'm tired.

I tested it awhile ago and the FM sounds fairly decent but there's still distortion in the phono. I didn't have time to check for distortion in tape mode. Will get after it again Monday afternoon. New tubes are scheduled to arrive later today too.

Note to Doug VanCleave-- I used a 56K 1/2 watt resistor in place of the burned out 1-watt resistor, and will replace it with a 1-watt resistor when my parts order comes in. I don't plan to use the set much at all until everything is done and it tests great in all modes. It seems to be working just fine on FM with the 1/2 watt resistor in there.

I have now replaced 7 resistors in the tuner, installed brand new old stock tubes, and the set still has a distortion problem. I really need an old fashioned single FM tuner to plug into the power amp so I could determine if the distortion problem is in the power amp or the tuner. I have nothing with a volume control on it that I could plug into the amp to test it that way. I really need to be able to rule out the power amp as the source of the problem.

Looks like I very well may wind up taking the entire system to someone with a signal generator who can trace the signal through the circuitry and find the problem.

I plan to do detailed ohm readings on every resistor in the power amplifier tonight.

In what way does it distort, what does it sound like ?

Szzzcracklecracklezzzzzzftzftzftzsptstpsptzzzzzcracklecrackle....POP!

It sounds like an FM radio station off frequency.

I checked every resistor in the power amp tonight, and all were within tolerance, so no problem there. Then, I noticed that the receptacle for the phono plug seemed to be a little dirty, so I took pipe cleaners and dipped them in alcohol and cleaned the inside of that jack, also the jack for the 8 pin plug which connects the amp to the tuner, and the four pin speaker receptacle as well. I also noticed the pins on the speaker plug appeared rather tarnished, so I polished those pins. I tested it again, and it sounded great! FM is perfect, and I thought phono was perfect too since I played a Cars LP and it sounded real good. I turned it off awhile, turned it back on and tried a couple of other records, and they didn't sound quite so good. I still believe there is something in the phono circuitry that isn't quite right. I'm going to continue to do more testing tomorrow.

Is that what your restorations do when a customer gets them back ?

For troubleshooting the power amplifier, you could use an iPod, Discman-type portable CD player, or a cassette boombox. Really anything with a headphone jack. Of course you could use the tape output from your Pioneer receiver. In a pinch, even an old transistor radio with an earphone jack.

Is the distortion at all volume levels, just at very low volume, or just at high volume?

If the distortion is at all volume levels and is on phono only, check that the tonearm isn't dragging, causing the stylus to be pushed over to one side. That could cause the type of distortion you describe. You probably can't see the stylus very well on that set while a record is playing.

-David

No. What I generally restore is simple record players, such as a 7HF5, SHF6/7, etc. Those don't have these kind of problems. The Mark I is a very sophisticated and complex piece of equipment with two parts (power amplifier and tuner) connected together by a huge 8 wire connector and cable. The Mark I was the most professional piece of equipment RCA made for home use in the 1950s. The thing has developed a complex set of problems all at once, and I'm slowly but surely chipping away on it and getting them resolved. The set sounds a lot better now than it sounded a week ago. But it's been one helluva lot of work....

The distortion is mainly at higher volume levels on phono and tape. At very low volume, or with the treble turned down a bit, it sounds better. I'm going to pull the record changer today and check for loose phono connections underneath, and substitute another Stanton stylus as well to see if that makes any difference. It is entirely possible that my needle on the stylus has been damaged. I need to try another stylus to find out. I need to be able to rule out the record changer as affecting the problem.

moses_007 i was poking fun at RT not you, he was the one who put a silly comment when i asked you about distortion.
sorry if you thought i meant you, and RT your a trouble causer LOL.

Sorry, I misunderstood what you said. I thought you were talking about MY restorations.

I can tell you this much... regardless of how good a restoration anybody does on one of these sets, it can develop problems later on because of the age of the equipment. Tubes can fail, resistors that are 50 to 60 years old can fail, other parts can fail. Also consider the fact that most of these sets have been put up and stored for 30 or more years, and the new owner starts playing them every day after restoration. That's like taking a vehicle that's set up for years and driving it daily...parts are going to start to fail.

If the distortion is on the tape input then that probably eliminates the turntable/cartridge/stylus/preamp as the problem. If it were phono-only, I was going to suggest that you inspect the cartridge terminals to be sure the clips are clean and tight, in addition to the checks you plan to make. Also check all your connections to your preamp. The input side runs at an extremely low level, and a bad or oxidized connection here can cause a fuzzy sound.

This may or may not have anything to do with it -- you noticed the problem after spending several days listening to tapes. Tapes sound cleaner than records at their best. Could it be that you are now more sensitive to the distortion that often comes up on records?

-David

Like I said, I'm going to pull the changer and check everything. Could be that the RCA phono plug coming from the changer is a bit oxidized (like the phono plug from the tuner to the power amp was), and that needs polished and roughed up a bit with 0000 steel wool to make a better connection with the preamp. I tried a different preamp a couple of days ago and it sounded the same.

I agree, some of my solid state consoles are over 40 years old now, i recapped one and a few weeks later the sound wasnt as it should be, i got frustrated checking everything, it turned out to be a bad mica cap, they dont usually give trouble.
Age catches up with them all in the end, why i dont understand some people who say just turn it on and use it

Sorry if I'm pointing out the obvious, but have you tried the set with a different speaker?

Ken D.

No, I haven't tried that. I did check the speakers, connections to the speakers, crossover capacitor connections, etc., and didn't find any problems. The distortion a week ago was so bad it sounded like it might have been a blown speaker. The FM sounds good now, so don't believe that's the case.