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TU-9900 resistors / ceramics

hopjohn

hopjohn

Silver Face
Subscriber
Hi,

I'm doing a major overhaul on a dirty, abused TU-9900 I found. All the main boards are going to need cleaning. This will require major parts removal. The unit needs new resistors pretty much throughout because so many of the stock carbpn comps measure well out of tolerance. What boards can take metal film resistors (in place of carbon comps) and polyester caps (in place of the larger ceramics), and what boards need to remain as stock due to inductance issues? Specifically I'm asking about the AM IF Board (F-2539), the FM IF Board (F-2524), the MPX amp (small board on the bottom side), and the Discriminator board (F-2525) These are the ones I need to know about.

I'm pretty sure I can use metal films throughout the MPX/Audio board (F-2526) unless someone says different and I rebuilt the power supply using one value higher than stock wattage rated carbon films.
 
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hopjohn said:
Hi,

I'm doing a major overhaul on a dirty, abused TU-9900 I found. All the main boards are going to need cleaning. This will require major parts removal. The unit needs new resistors pretty much throughout because so many of the stock carbpn comps measure well out of tolerance. What boards can take metal film resistors (in place of carbon comps) and polyester caps (in place of the larger ceramics), and what boards need to remain as stock due to inductance issues? Specifically I'm asking about the AM IF Board (F-2539), the FM IF Board (F-2524), the MPX amp (small board on the bottom side), and the Discriminator board (F-2525) These are the ones I need to know about.

I'm pretty sure I can use metal films throughout the MPX/Audio board (F-2526) unless someone says different and I rebuilt the power supply using one value higher than stock wattage rated carbon films.
Click to expand...

Do not replace Ceramic capacitors with Polyester capacitors - replace Ceramics with Ceramics.

I have also heard that metal films are not a good choice for use in RF carrying circuits.
 
Yes, I've heard this as well, but I need to know for certain that these boards on this unit qualify as such.
 
Well, if I get this wrong someone (Punker X) is sure to jump on me from a very great height and put you & me straight.... I would say....

AM IF Board (F-2539)
MPX amp
Discriminator board (F-2525)
MPX/Audio board (F-2526)

All OK for resistor change, only change ceramics if damaged.

FM IF Board (F-2524) = Dont change resistors, and definitely dont change ceramics. (unless damaged)

Unless you really know what you are doing you should never change the dielectric type of a capacitor - especially not in a tuner.
 
Personally I would advise against wholesale swap out of resistors and caps other than electrolytic. The more you do the bigger the chance you have of making a mistake.

If you proceed, what you have will not sound like a TU-9900 and odds are it will not sound as good. A few years ago at a customers urging I replaced all the resistors in the audio section with PRP metal film resistors. The customer was happy and it sounded ok, but to me to took away a bit of the 9900 magic and would not have necessarily classified it as an improvement.
 
I'll keep that under advisement moving forward. Though, understand this is a victim of circumstance given the condition the unit arrived in. some part swaps are required given their state.

Punker, if not metal films in the audio section, what other changes do you recommend for a TU-9900 beyond alignment, opamps, and power supply upgrades?
 
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