SHP Cap Question

[paradisecs]

The original part of this post was lost but here goes the most recent update from where I stand now:

SHP3 chasis, driver, and preamp lytics replaced.  Q15/25, Q16/26 replaced as well as all four 100Ohm Flameproofs.  C46 moved to Q25.  Major damage to are of Q14 so R71, Q14, R74, & C42 replaced.  Q14 was broken and the other parts in the area looked like charcoal.  

Following the procedure on page 39 of the SHP Amp book for continuity tests these three points failed continuity:

C65 + terminal to Q17 Collector - (Now fixed, bad resistor R77) pulled one from another amp that tested out spec wise same as remaining three.  Will order replacements in future.

C66 - terminal to Q16 Collector
C66 - terminal to Q26 Collector

Both of the above fail continuity but test out at 100 ohms.  Continuity works up to the PCB Side of R99 and R76 which are both 100 ohm FP resistors.  Does that mean that it is fine even if no tone?

[paradisecs]

I assume that the reading is correct so no tone is not an issue.

The next and final step is to check +Q15 Base to -Q16 Base and you should get between 75-150 ohm. Same thing for +25 Base to -Q26 Base. I get the following:

Q15 base to Q25 Base - 11.3 Ohms
Q16 base to Q26 Base - .915 KOhms

Both readings seem really odd to me. Does that mean there is another resistor out of whack in the line between?

[robnyc]

"Both of the above fail continuity but test out at 100 ohms. Continuity works up to the PCB Side of R99 and R76 which are both 100 ohm FP resistors. Does that mean that it is fine even if no tone?"

This is due to the meter beep ckt not being able to deal with the 100 Ohms in series. All meters have this limitation to some degree. The one i use most (1988 B&K) has a limit of approx 190 Ohms.

RobNYC

[paradisecs]

I've traced back to the heatsinks and I'm confused on what is supposed to be there. All three of the SHPs I've checked today have a 3300 resistor which I think is R86. The amp book shows R84 & R85 in that spot. Schematics show a 10 ohm R62/85 there but I read the 3.3 as being for the SHP1. I know I'm just confused here. What are the correct part numbers and ratings for the two resistors under heatsinks? Is one of them a thermistor? I've having a hard time reading the color stripes on the ones in the unit I have.

[robnyc]

Ok, I don't know what you are seeing in that book, this is the SHP power stage schematic that I've used:

www.flickr.com/photos/90641375@N06/42575714551/

I don't have a schematic for the SHP-1, but this is what in under there: www.flickr.com/photos/90641375@N06/42575714921/

RobNYC

[paradisecs]

Thanks Rob. I'm certain my problem is just blockhead interpretation. I read the schematics for "under heatsink" and don't see what I'm trying to find but I find similar numbers on the chasis.  My guess is because it relates to a different circuit, SOS maybe? Here is a picture of what I seem stuck on. I see the 3.3k resistor but not sure what part# its supposed to be on the schematic. The other one I'm not sure what it is. I read one schematic as a 10 ohm resistor but the bands come out at 920 if I read the colors correctly.

[Ron Rich]

Steve,
I don't think your "block head" has anything to do with this ! What I think is happening is that you are reading a schematic for an amplifier that has the "thirmister" bias, rather then what is in that amp--a "transistor based" bias. What you have marked as "ca 17/16", is the transistor, "Q-11", &/or, "21". The 3.3 k, is R-61, &/or, 84, and the last, 820 resistor, is R-62, &/or 85.
QUESTION: I believe you said earlier that you had replaced the bias pot(s?) with a fixed resistor--what value(s) did you use ?
Ron Rich

[paradisecs]

Thanks Ron. That has to be the issue. I didn't know there were two sets of schematics, that has to be the hang up and if it wasn't so expensive to replace parts it might be kind of fun to put the wrong parts in and see what happens. And now that you pointed that out I do recall reading somewhere about the two different bias but I don't remember where.

I haven't replaced any bias pots yet. That's next on my list. I'm trying to not jump around to much. I've seen several times Robs posts about replacing with a fixed resistor but I'm not sure I understand the method yet.

Now that I can identify Q11, R61, and R62 I can refocus my readings that got me looking under the heatsinks to start with.

Q15 base to Q25 Base - 11.3 Ohms
Q16 base to Q26 Base - .915 KOhms

The book says they should by 75-150 ohms.

[Ron Rich]

Steve,
There are at least 4 sets to that board--need to go by the number and -letter on the individual board,or you can up-date it to the later style thirmister bias. (IMHO, there is no real point in doing so, as the transistor based one works just fine). Ron Rich

[paradisecs]

Its an SHP3 Code B with 70-31295-B & a 70-319269 Pre-Amp board. Now I need to find a copy of both schematics to see where the differences are. I have both your guides as well as the amp book and black box book and a handful of other stuff I found on the web. I know I've seen it, just don't remember where. Projects for later this week.

[robnyc]

Jun 5, 2018 17:30:34 GMT paradisecs said:

Thanks Rob. I'm certain my problem is just blockhead interpretation. I read the schematics for "under heatsink" and don't see what I'm trying to find but I find similar numbers on the chasis.  My guess is because it relates to a different circuit, SOS maybe? Here is a picture of what I seem stuck on. I see the 3.3k resistor but not sure what part# its supposed to be on the schematic. The other one I'm not sure what it is. I read one schematic as a 10 ohm resistor but the bands come out at 920 if I read the colors correctly.

You have the original bias ckt using a transistor. I suspected that given the 3.3k you mentioned. Since I don't have that schematic I posted a pic. I agree w/Ron that as long as it works there is no need to replace it.

On several of those amps that I "fixed" for  3 retro bar locations in the early 90's I did a crude hack where I eliminated all the bias components and just used a fixed resistor of a value that offered the least idle bias w/out crossover distortion. This was done because the amps were repeatedly failing due to being fully loaded and run "flat out". After much testing the real culprits turned out to be inferior 2N3055's and hot switching of the speakers between the house PA and juke.

RobNYC

[paradisecs]

It all makes sense now. Thanks for the clarification.

So the big question is then are the ohm readings as stated in the amp book wrong for this bias setup for 15/16 base to base and q25/26 base to base? Or do I still have a major problem?

[Ron Rich]

Good question, I don't know since I have never used this method of repairing one of these -- Ron Rich

[paradisecs]

Some further testing shows that my left channel bias pot is ranging from .9k to 2.6k. The right channel doesn't adjust at all and is stuck and 1.7 ohms. Huge difference is readings (both sides checked at the solder points on the pots. So it looks like I need to replace the pots or perform the "update" and remove the pots and use a fixed resistor.

Replacing pots with other pots is simple. Replacing the variable pots with resistors I think goes like this, please correct me if I'm wrong:

a) Set bias to where you want it
b) Cut the pot out of the circuit and measure its resistance
c) Replace with 1/4 watt resistor of measure value and insulate

From what I can tell this assumes you are working on an amp with a thermistor in the bias circuit.  How then do you make this modification if you are working on an amp with the transistor instead?

How do you do this if the pots are bad to start with?

I know I need a 100 ohm potentiometer, but at what power?  It says in the amp book that power dissipated by the pot is minuscule but that doesn't really tell me what wattage to use and I'd rather not burn up yet another set of transistors.

[Ron Rich]

Steve,
Should work with either type circuit-
Attsa why my question, of what you (or someone?) used (before).
1/4 watt or better --I would assume !
Ron Rich