Rhodes 4 pin power amp transistor troubleshooting

[Skiroy]

Okay guys I have a 1975 4 pin suitcase rhodes. It is 100W and 4 speaker version so Im not sure if it is the peterson or not but both left and right power amps use 2 Delco GM 120725 Transistors.

The thing is the left side has the writing in Green and the bottom number says"7539", which I am guessing is the batch number or something. But the right set is in red writing and the bottom number is " 7515".

So how can you tell if they are Germanium or Silicon?

The reason I am asking is because the Green ones stay cool while the red ones get really hot. I am also getting intermittent distortion when I hit the keys harder at times and the distortion is alot more frequent through the head phones vs through the speakers.


I ruled out the power supply because I swapped the power connectors and RCAs and the power amp with the red writing still gets hot. Also when I disconnect the preamp and run straight off the harp to the power amps they dont get hot. But if it was a problem with the preamp on one side I would think when I swapped the power connectors and RCAs to the Power amps the other power amp with the green transistors would get hot. Also I checked the resistor on the power amp boards and they both have the same values for all resistors.



I tested the green for shorting and it doesnt seem to be shorted.  But when on the multimeter setting to test for shorts it reads 67(which I dont know what this reading is because I have been told too many things from current,to beat,to ohms) but the the resistance does measure 68ohms. Also a note is I can only get a reading for a short or resistance when the positive probe is on the transistor's leg marked "E" and the Neg probe is on the leg marked "B".

Now the Red transistor is weird. It wasnt reading as shorted and the resistance changes every time I test it. It was at 96, then 67 and now its at 61. It didnt beep(indicating a short) but now it is and the reading on the multimeters short position says 79. This is why I am confused of this value because the other transistors reading on this position and ohms were very close but now here they are not.

So here are my questions.

1. Are these red transistors bad because of the reading and is it normal for them to get this hot?

2. How do I tell if they are silicon or germanium because why would the other be written in green that stay cool? And if the greens are germanium and I have to change the red ones to silicon will the Left and Right sides sound different?

3. The power amps on this rhodes have a adjustable pot like on the the power supply. Is this used to set bias on these as opposed to having to soldier in addition resistors? And could this be my heat problem?

4. If this pot is for bias or even if I have to do it the old school way where am I measuring voltage while setting the bias? What terminals,wires ect?

Thankx,
Colin

 

[abbey road d enfer]

The thing is the left side has the writing in Green and the bottom number says"7539", which I am guessing is the batch number or something. But the right set is in red writing and the bottom number is " 7515".

Year-week of production in both cases. Color difference? They ran out of green at some time.

So how can you tell if they are Germanium or Silicon?

Year of production says Si, Ge was long gone at the time.

The reason I am asking is because the Green ones stay cool while the red ones get really hot. I am also getting intermittent distortion when I hit the keys harder at times and the distortion is alot more frequent through the head phones vs through the speakers.

That's because one of the power amp modules has gone out of normal operation. I don't have the schemo of this older version, but I think there may be a quiescent current trimmer.

1. Are these red transistors bad because of the reading

Erratic reading is certainly strange. I can't say more.

and is it normal for them to get this hot?

Definitely not.

2. How do I tell if they are silicon or germanium because why would the other be written in green that stay cool? And if the greens are germanium and I have to change the red ones to silicon will the Left and Right sides sound different?

Already answered.

3. The power amps on this rhodes have a adjustable pot like on the the power supply. Is this used to set bias on these as opposed to having to soldier in addition resistors? And could this be my heat problem?

Very likely.

4. If this pot is for bias or even if I have to do it the old school way where am I measuring voltage while setting the bias? What terminals,wires ect?

You have to measure the voltage at one of the emitter resistors. You need the maintenance manual. If not, one must retro-engineer the thing.

 

[moamps]

Hi,

GM120725 are germanium. What are the DMM's readings between legs B (base) and E (emitter) and transistor's chassis (collector)?

links:
how to check transistors (reading for GE transistors is about 0,1V)
http://byunghyunkimttec4847.blogspot.com/2011/08/transistors.html

good info about servicing:
http://www.fenderrhodes.com/service/amp-failures.php

schematics:
http://www.fenderrhodes.com/org/ch11.zip

Regards,
Milan

 

[abbey road d enfer]

Hi,

GM120725 are germanium.

My mistake. Thanks. Who would have thought they would continue using Ge when the 2N3055 was commonly available?

 

[PRR]

> Who would have thought they would continue using Ge

I suspected they might.

The 1950s Delco power transistors were VERY good.

The Rhodes amp dates to the 1960s.

This amp has a base driver transformer... clue to age.

Proven Ge designs were favored over extremely expensive Si parts.

As Si captured new markets, the Ge parts dropped in price.

The amp worked (in Fender's opinion). It was tooled-up. Parts were on hand. Why change it?

IMHO the amp should have been replaced long before it fell into Skiroy's hands. It was antiquated then and we have MUCH better (more robust) amps now. Strip to the power supply and audio connections, install a nice module or chip. Blowout-proof.

 

[abbey road d enfer]

According to schemo, there is no quiescent current adjustment.
Current can be measured by measuring voltage across the 0.5ohm/5W emitter resistors. I suspect it should measure anything between 5-20mV; a maintenance manual seems necessary there. It must be measured with the load (loudspeaker) disconnected and the output voltage must be measured too - should be ideally zero, but in practice less than 0.1V is acceptable.
Probably replacing the output transistors with a NOS matched pait would solve th eproblem, but these transistors seem to work somewhat but may have drifted. It is probably possible by tweaking the values of surrounding resistors to solve the problem, but it takes some understanding of transistor biasing.

 

[Skiroy]

Hi,

GM120725 are germanium. What are the DMM's readings between legs B (base) and E (emitter) and transistor's chassis (collector)?

links:
how to check transistors (reading for GE transistors is about 0,1V)
http://byunghyunkimttec4847.blogspot.com/2011/08/transistors.html

good info about servicing:
http://www.fenderrhodes.com/service/amp-failures.php

schematics:
http://www.fenderrhodes.com/org/ch11.zip

Regards,
Milan

I am assuming you are talking about the reading in the diode position on the DMM? Its weird because it has fluctuated. It was 79 and beeped(indicating short) but I tested it again(just E and B legs) and it said 81. Then I tested it this morning and it says 83 and didnt beep like the 81 reading.

So out of curiousity I blew on it to warm it up and sure enough its now 77 and beeping. So I heated it slightly with a hair dryer and it then read 50 and beeped. Should the DMM ever beep?

Whats going on here?The transistor is seeming to short or not out depending on the temp. I did the same thing with the green one. I heated up the green one with a hair dryer and the DMM diode position ( I am assuming is current) read 057 and beeped. As the transistor cooled down the reading got higher and the DMM stopped beeping when it got around 083.

So now I am more confused than ever on how to test these things.

 

[Skiroy]

According to schemo, there is no quiescent current adjustment.
Current can be measured by measuring voltage across the 0.5ohm/5W emitter resistors. I suspect it should measure anything between 5-20mV; a maintenance manual seems necessary there. It must be measured with the load (loudspeaker) disconnected and the output voltage must be measured too - should be ideally zero, but in practice less than 0.1V is acceptable.
Probably replacing the output transistors with a NOS matched pait would solve th eproblem, but these transistors seem to work somewhat but may have drifted. It is probably possible by tweaking the values of surrounding resistors to solve the problem, but it takes some understanding of transistor biasing.

Yes that trimmer is for the DC Offset which I dont really know what that means but this is what the guy in the power amp repair video says on youtube from vintage vibes. But would this Offset not being at zero cause them to get hot?


First of all I really need to be able deffinately figure out if these are bad before I move on. I am just not understanding the reading on my DMM in diode position and what it means and if it should ever beep indicating a short?