Marantz 2500 Refurb

[catrafter]

Randy,
Good luck with your beast, I have great confidence in you finding the FM problem.
I have the 2500 I was working on finished and in my listening room.
These should not be worked on by someone who gives up easily.
I don't want to hijack your thread, but I will post an after picture.

Tom

 

[rBuckner]

Thanks Tom for the encouragement. You're always welcome to comment on my posts, especially if I say something wrong! It's sure nice to see a 2500 with the front panel on too. To me the 2500 has a very nice sound, I never got tired of listening to it for a few hours one evening.

I'm nowhere near giving up! I was just sitting here at work studying the schematic and trying to locate (unsucessfully) some info on the TA7301P IC that's in the "box", I think it's the 10.7MHz osc. maybe. At any rate it's time to do some resistance checks, continuity checks and voltage readings. Onwards and upwards! :thmbsp:

 

[rBuckner]

Well, found that the first FET was shorted, one of the gates to source was 5 ohms. So I installed one of the precious new ones and got the DC voltages I expected then but still no improvement. I had previously replaced the third one and didn't cut the leads. When I reassembled the tuner last round that shorted out the FET leads so I removed that FET (which was one of the new ones) and put the old one back with no improvement. I tested the removed FET and it passes the simple DMM test so it's likely OK.

I may have blown out the TA7301P chip which I cannot find or a find a substitute for it either. It is the local oscillator amp. I may be able to use a UPC555H IC (if I had one) and you can get an NTE sub for that (NTE703A). This is so frustrating!! :gigglemad

 

[rBuckner]

Upon further examination last night I found that there was a 330 ohm resistor in line with the TA7301's output (I think) on pin 7, the schematic shows a direct connection. Also, moving the tuner drum does allow tuning in and out of the station I had it on so the IC is doing something so I'm going on the premise that it's at least partially functioning and maybe even totally OK. I also found that the resistor values in the first stage were all 10X the schematic's values. It looks factory so likely they found better performance by upping the impedance.

I see RF all over the place on my scope but I think what I need to do is scope out another working front end out of something else to get an idea of what is "normal". Of course I don't have anything with that *^(*@##$ IC in it which would be ideal.

Stay tuned! (oooooh, bad pun!)

 

[catrafter]

Randy,
I doubt you would pick up much of anything if the 2500's LO was dead.
To see if the local oscillator in the 2500 is running, set the 2500's tuning dial to say, 88MHz, then take a known working receiver, connect a lead wire to the FM antenna terminal and lay it near the 2500's front end. As you tune receiver #2 around 98.7MHz, you should hear receiver #2's audio quiet as it picks up the LO from the 2500.
BTW,
Also, have you metered the AGC voltage on the 2500? Pin 1 of J101 should be connected to J202 of the IF amp and should go more negative as the incoming RF gets stronger.

Tom

 

[rBuckner]

Randy,
I doubt you would pick up much of anything if the 2500's LO was dead.

Exactly what I was thinking when I remembered it would "tune".

To see if the local oscillator in the 2500 is running, set the 2500's tuning dial to say, 88MHz, then take a known working receiver, connect a lead wire to the FM antenna terminal and lay it near the 2500's front end. As you tune receiver #2 around 98.7MHz, you should hear receiver #2's audio quiet as it picks up the LO from the 2500.

Ooh, forgot about this trick! I'll try it.

BTW,
Also, have you metered the AGC voltage on the 2500? Pin 1 of J101 should be connected to J202 of the IF amp and should go more negative as the incoming RF gets stronger.

I have looked at the AFC but it basically sits around 2.5Vdc. I have a Sencore generator that puts out 10.7MHZ IF so I'll hook it up to the IF input and vary the level and see what the RF does. That was one of the things I was going to observe.

Edit: I could get brave and hook a pot up between ground and the V+ to supply the AGC, being careful to not go all the way to the V+ end and see what effect it has. I'd be just as happy if it was an IF board issue!

Thanks Tom!! :thmbsp:

 

[catrafter]

Watch your polarity! I believe the ACG is a negative voltage. Less negative=more FE gain.
I think that Q211, Q212, Q213 and/or something in the associated circuitry may be bad. Q211 amplifies the 10.7MHz IF signal, Q212 and Q213 detect (rectify) the 10.7 MHz IF signal to a negative DC voltage. The AGC should vary as you tune through a strong station.

Edit:
OK, the top was still off the 2500 I was working on, so I took my DMM and measured the AGC. When I tuned in a strong station, the AGC voltage was -0.5VDC and with no station tuned in it was -0.01VDC.
Hope this helps!

Tom

 

[rBuckner]

Thanks for looking under the hood! I do have good news at last, it's working very well again! The AGC was stuck at +2.5Vdc and never moved. I traced the IF to the first transistor base and no signal was there nor on the resistor right off the pin. With all of the cable wiggling around the trace was cracked at the pin on the back of the board. A bus wire bridge later it came up working strong and in stereo. So the blasted FET at the first (antenna) stage was the only real issue.

On this 2500 the AGC range is 2.5Vdc off tune, usually about 2.35Vdc and 1.25Vdc at the lowest with the strongest station and antenna optimized. So I bet you have the tuner that matches the schematic! Here's a staion from about 70 air miles away, scope on multipath display:

So it's time for.....

Thanks so much for the assistance Tom and your clue got me to the solution. Ya gotta love AK and the knowledge bank here. :thmbsp:

Randy

 

[marantzfan]

Thanks for looking under the hood! I do have good news at last, it's working very well again! The AGC was stuck at +2.5Vdc and never moved. I traced the IF to the first transistor base and no signal was there nor on the resistor right off the pin. With all of the cable wiggling around the trace was cracked at the pin on the back of the board. A bus wire bridge later it came up working strong and in stereo. So the blasted FET at the first (antenna) stage was the only real issue.

On this 2500 the AGC range is 2.5Vdc off tune, usually about 2.35Vdc and 1.25Vdc at the lowest with the strongest station and antenna optimized. So I bet you have the tuner that matches the schematic! Here's a staion from about 70 air miles away, scope on multipath display:

So it's time for.....

Thanks so much for the assistance Tom and your clue got me to the solution. Ya gotta love AK and the knowledge bank here. :thmbsp:

Randy

Congratulations on a well fought battle and epic thread! I haven't said much but I've been following along since the start. I hope to own a 2500 someday, till then I'll just be content with my 4400.

PS - Next time your in the valley let me know!

 

[catrafter]

Great work, Randy!
Now you can put the face plate on it and get on with your life.
This thread was a great help to me during my particular 2500 restoration journey.

Thanks again,
Tom

 

[bktheking]

Awesome work!

 

[rBuckner]

The company is all gone so it's time to finish this sucka up! Not all in this post though. First was the conversion of the scope illumination to a warm white LED. I used one of my pure DC fuse lamp pc boards for AC conversion as getting to DC would have been messy.

Then it was time to pull out the old power resistors for the slow start and fan low speed operation. Good thing I did, the wires had been only tack soldered on by a previous tech. The ends of the cut off wires were still on some legs.

The slow start one had blown apart before and the replacement was just piggybacked on.

The tacked on one (now on the bottom of this picture) doesn't look so hot either!

Here's the wires, notice the flash burn on the red wire from the blown out resistor.

I decided on 25W power resistors and had to drill two 1/8" holes for some 4-40 screws. The standoffs are for the other resistor to mount to.

All done. I took the opportunity to polish up the plate. I combined the wires with heavy duty (18 ga) bus wire, soldered them and put on some heatshrink. Then the bus wire attached nicely to the resistor lead. The blue and red wires were soldered directly to the fat bus wire common connection. I did cut out the toasted part of the red wire and spliced in some fresh wire.

Next was the LED display board. I thought all red LEDs was rather boring even though it's technically correct so I wanted different colors. I only had red, green and yellow in 2mm cylindrical types so I took a T1-3/4 size one and turned a down a new "snout" on it. Also done was an orange and a pink one.

Looks more fun already! I have some mods to do to for even lighting. I'll select the dropping resistor for the peak LEDs after it's all together and the one common one for the others (only one is on at a time unless the tape monitor is on) will be selected for the orange LED as it's the least bright. The others will all have to have the low side trace cut and bridged with individually selected resistors.

That's all for tonight.

 

[marantzfan]

You tease!!!

You know we all wanted to see a full frontal with the new LED's blazing away in defiance.

 

[rBuckner]

You shall be satisfied my friend! I got no further last night and now I'm wasting my time being at work (!). Well, except for the income part.

I have some damage repair work to attempt on the front panel before that goes back on so it'll be a bit. -- sigh -- :sigh:

 

[rBuckner]

The dropping resistors for the LED board are all selected and installed. I forgot to take a shot of that but it wasn't all that exciting anyway! Tonight I unmasked the repaired front panel and bezel. It had some nasty scratches and I thought it would look better overall if I sanded them out and coated the repaired area with semi-gloss laquer. Then the LED board has been installed and reglued on the left side. It's a bit warped so I epoxied in the ones that sat flat. This is the way this board was originally installed.

The other task for the evening was to start the rework of the bottom cover. The screening was a bit torn up and the cover scratched. The screen was removed and the adhesive, which looked like double sided tape, was scraped off. Both sides were spot sanded at the chips and scratches with 180 grit paper and feathered out with some 220. Then an overall sanding with a DA sander and 220 pad, cleaning and dusting. One side is primed and the other will get done before bed.

That looks better already!

 

[bktheking]

Must be nice to be able to paint in December, -24c tonight here in CP.

 

[rBuckner]

It is nice! The garage is well insulated and was likely about 55F / 13C. Our low outside the other morning tied a record at 10F/-12C. The brass monkeys were in hiding!

 

[bktheking]

Sounds like the great white north temperatures, there are no brass monkey's round here lol.

 

[rBuckner]

----- Done!!! -----

Yep, done at last! I'll see if the owner can come by and discuss marketing this bad boy although it wouldn't surprise me to see him keep it. I did the best I could on the faceplate damage repair. It can be seen, in some photos it's worse looking than others but still it looks way better than it was. Just go back to the early posts to see what I mean. It's purdy at the distance you'd be when listening to it!

Here's the inside view of the bottom cover, repainted with Krylon semi-gloss black (maybe they call it satin these days). I used 1/4" wide double sided craft tape from the scrapbooking section of the crafts store to stick down the new screening. (Thanks for that tip Tom/Catrafter!) I used charcoal color fiberglass screen for window screens. The old beat up stuff looked just like that and was fuzzed like fiberglass would do.

Top cover all repainted and rescreened. I wasn't sure I wanted to trust the tape so I added glue dots for insurance. I used a glue called Weldbond which is a super duty type of white glue. The inside corners were too small for the tape and Marantz used a glue themselves here. The right side is a wooden ruler with sockets for weights to hold the screen down while that side dried. The punches on the left were to also hold the screen down, apparently I didn't get that end section taut. Looks great from the outside

The top wasn't on yet in this picture I took last night so that's why you can see the background at the top of the dial.

The peak meters lit up, sorry it's out of focus. No the speakers weren't connected!!

Angled views

Back panel with that nice ball bearing fan.

Here's the function LEDs:

Yet to do is to hook up to the audio analyzer and check out the tone controls, noise and distortion stuff and max power out. I'll post that info.

 

[marantzfan]

That is pretty wild with the different colored LED's. I like it. :yes: